The VLJ Price Fantasy

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Surveying the wreckage that is what's left of the very light jet market—remember them?—one thing is obvious. If this market exists at all—and I'm not convinced it does—no one has come even close to either proving it or getting the delicate relationship between investment, sale price of the airplane and potential volume just right. Although there are still true believers out there, every VLJ effort has been the equivalent of a dry hole. There must be oil out there somewhere, reason those who continue to strive.

I'm not so sure. The most spectacular failure has been Eclipse, which tanked for a multitude of reasons related to hubris, out-of-control costs, inappropriate technology and just ordinary mismanagement, in my estimation. But looming over the entire project was Eclipse's concept of building jets and selling them at or right around the $1 million mark. Among other things, Eclipse proved that this can't be done, or at least it can't be done the way Eclipse tried it.

Now comes Cirrus promising initial buy-in owners a $1.39 million price and a maximum price of $1.55 million. These are higher, of course, than Eclipse's initial offer of around $850,000. As it marched steadily toward bankruptcy, Eclipse kept raising the price in increments through a contorted arrangement of escalators that position holders frequently complained about.

So, with its simpler single-engine design, has Cirrus got the price right? We don't really know and there's almost no way to predict such things. Even the companies who have attempted to build such airplanes have got the numbers consistently wrong and have paid the price for the error. With the exception of Cessna, whose successful Mustang isn't really a VLJ according to most people in the industry, all have been failures.

My own view is that Cirrus is, once again, too low on its price. My guess is that even a modest, simple single-engine jet should hover around the $2 million mark—a little less, a little more, maybe. That's because these projects have always proven more difficult and time consuming to certify and more expensive to build than the companies originally estimated. The idea here is not to just deliver a couple of hundred airplanes to the market—which Eclipse, in fact, did—but to build a business that is sustainable over the long term. You know…one that stays in business, answers the phone, has parts and customer service, that sort of thing.

Coming into the market with a loss-leader price does no one any favors—not the initial buyers, not the company and certainly not the industry as a whole. Some 250 Eclipse owners may yet be stuck with an expensive airplane that won't be sustainably airworthy. Several people in the industry have told me that the only way to make these projects work is to prime the pump with an initial cheap offer, then raise the price later.

That may be true, but it hasn't worked yet. VLJ projects have thus far proven to be a one-way ticket to bankruptcy. They are consumptive of cash and resources and represent a dangerous, dark hole for companies who attempt them. That's not an argument that they shouldn't be attempted, but I think companies need to build their business plans on realistic prices, even if such prices make the sales staff wince.

I think it's critical for Cirrus (and Diamond) to succeed with their single-engine jets. I'm just not sure they're asking customers for enough money to make that happen.

Comments (246)

Perhaps suggest that the Piper Jet at $2.4MM may well be a more thoughtful and realistic approach.

Posted by: Seth Rudnick | September 7, 2009 6:15 AM    Report this comment

But look at it another way : maybe the proliferation of VLJ-wannabes was what killed Eclipse in the first place. What if all the others had never even tried, and all orders would have gone to Eclipse ? Life would have been much easier for them. How do you keep others from crossing your path ? Keeping your price very low is one way to scare them off. Making sure your design is efficient (1 engine, 1 boom,...) is another. Really, a VLJ is just some composites, a simple engine, and a bit of fancy electronics and snappy hardware tossed together. Why should that cost over $2m ? Cirrus do have a jolly good track record, I hope they will maintain it. Mind you, some 5 years ago I was defending the Eclipse proposition, and the Epic's beauty still raises my pulse. Stratos anyone ?

Posted by: Peter De Ceulaer | September 7, 2009 7:06 AM    Report this comment

It never made sense that an aircraft with two turbine engines could sell for less than a single engine turboprop (Piper). Usually a cost thing that does not make sense has a basic economic flaw.

Posted by: Richard Jenkins | September 7, 2009 7:15 AM    Report this comment

"...maybe the proliferation of VLJ-wannabes was what killed Eclipse in the first place..."

Nope. It was Eclipse that killed Eclipse. The spending was out of control and upper management alienated more suppliers than anyone thought possible -- a flight instructor with 700 hours total time and NO previous jet time, an employee just graduated from UND with NO jet time in charge of the Avio avionics and negotiation with Avidyne.

Add to that assests squandered in devoping another aircraft -- the single engine Eclipse 400 -- when Vern hadn't figured out how to successfully build his first jet.

The company had a lot of talent at the beginning, most of which was purged in the final years. If you didn't believe everything Vern said, you were gone. (He actually thought you could train a low-time pilot to fly a jet in one week. You may be able to teach him how to manipulate the controls, but he won't be a jet pilot. Experience DOES count. Folks like Captain Sullenberger have again demonstrated that.


Posted by: Linda Pendleton | September 7, 2009 8:51 AM    Report this comment

Maybe it could be done if the performance was limited? Many of us would be quite happy with a 4 place jet that cruised at 250kts or less with a ceiling of 30,000 ft. That would make it more of a baby step up to jets, and perhaps minimize the other problems inherent with this aviation genre - insurance, training, mentor pilots, etc..

Posted by: CRAIG DOW | September 7, 2009 9:43 AM    Report this comment

Rappers, drug dealers, Pro-ballers, Execs, 'n such want to "go in style" and prove they can afford it. For real speed there are lots of ex-warbirds. What's left is a Very Light Joke.

Posted by: Larry Fries | September 7, 2009 12:17 PM    Report this comment

I believe that companies purposely set to low a price in order to secure a large number of orders, providing working capital through deposits, as well as instilling confidence in suppliers, lenders, and potential equity investors.

As manufacturers approach production and realize they cannot possibly satisfy the demand at the low price, they can raise the price for the early delivery positions to make profits to pay off the debt. later on, as they move up the learning curve, they can retard price increases. But it just makes no sense to actually deliver the early positions at too cheap a price.

Posted by: CHRIS HYNES | September 7, 2009 12:24 PM    Report this comment

"Maybe it could be done if the performance was limited? Many of us would be quite happy with a 4 place jet that cruised at 250kts or less with a ceiling of 30,000 ft."


The business of flying a jet is not difficult -- in fact it's far easier and safer than a 421 -- it's the experience at the higher altitudes that you need. It really is different up there -- you cannot trust your eyes for traffic avoidance because the horizon is not where your mind tells you it is. A fanjet has 40% LESS range at FL290 as it does at 410. Flying at FL300 would require RVSM equipment and training. New jet pilots flying in the sunshine above the clouds tend to ramain oblivious to the fact that their destination has gone down rather late in the flight when options are narrower. A jet can experience large airspeed excursions without the pilot getting the sound clues he's used to. Jets don't respond immediately to power like a piston engine does. You don't want to get into a high sink rate and find yourself 4 seconds from the ground with a 7 second spool-up time (like one Yankee pitcher).

It's different, not difficult and since primacy is going to try to force you back to your piston habits in an emergency, you just need to build experience to counter that.


Posted by: Linda Pendleton | September 7, 2009 1:10 PM    Report this comment

First I would agree with Linda 100% (or more) that the first major problem with the VLJ concept has always been the nut holding the stick. The faster you go the more hours it takes to stay sharp. A minimum of 50-75 hours a years for something simple like a 172 isn't really enough to keep you safe. Go four times the speed and you need to be sharp and have experience to react properly. You can teach almost anyone to fly straight and level in a week but add traffic or weather or an emergency and they are in deep trouble. Whens the last time you threw the hood on and did a minimum approach? Or even something simple like a forced landing? Probably not since you took your training. And the problem is there are lots of people with the cash to go buy one thinking that a few hours of training is all they need. The end result is going to be high speed lawn darts all over.

As far as Vern the man is a good salesmen. Probably will make a fortune selling used cars some day. I had the misfortune to get involved with one of his proposed regional centers.Obviously wasn't quite in favour of it.

The one thing that still doesn't add up for most of these is the production cost. Unless the price is a basic steam gauge model there is no way they can do the avionics suite. That alone can cost as much as the engine not including labor. If Piper says 2.4 then I'd say that's the right price. They have been around long enough to know better. Obviously a lot of the new boys haven't and won't be.

Posted by: Dave King | September 7, 2009 3:20 PM    Report this comment

All good points. My thought (just blue-sky dreaming really) was the concept of a VLJ operating below RVSM and at speeds not much faster than pistons, which would be a step-up aircraft well within the skill range of most proficient high performance piston pilots. Granted, you'd lose a lot of the fuel economy and range operating in such a restricted flight environment, but the benefits of jet reliability, high TBO, pressurization and abundant hot air for ice protection would sure be nice. On second thought, I suppose Meridians or TBMs fill that niche quite nicely and they're well over $1.5M too!

Posted by: CRAIG DOW | September 7, 2009 6:23 PM    Report this comment

"...abundant hot air for ice protection would sure be nice..."


Well, with a small fanjet you really don't have abundant air for ice protectoin. Did you ever notice that the Citation 500 series has wing boots? They had to make a choice between heating the wings and going forward. You see, a jet will always have enough air to burn the fuel. When you add extra bleed air requirements for heating wings, etc., with air taken off the compressor you limit the amount available to swirl around the inside of the burner can to keep the flame centered and keep it from burning a hole in the can. The performanc figures for the jet is figured with "normal bleeds" -- pressurization, and a few others. Anything else -- anti-icing for the engines will require you to reduce power. Besides, the only real safe way to handle ice is to get out of it immediately. You don't ask ATC, you tell them.

Another thought is that there is a radical difference between flying at FL290 and FL410 or 430. at 410 and 430 you're usually above the tropopause and into the stratosphere. It's a bit different than the troposphere we're all used to here on the ground and up to 330 or 350. Lose pressurization up there and you have about 3 seconds to get your oxygen on and working and believe me a small cabin will decompress a lot faster than a small one.

Capt. Sully said it when he said he'd been making small deposits to the bank of experience all these years and he finally could make a large withdrawal.

Posted by: Linda Pendleton | September 7, 2009 9:36 PM    Report this comment

Maybe it could be done if the performance was limited? Many of us would be quite happy with a 4 place jet that cruised at 250kts or less with a ceiling of 30,000 ft. That would make it more of a baby step up to jets, and perhaps minimize the other problems inherent with this aviation genre - insurance, training, mentor pilots, etc..

Sounds like Diamond's D-Jet. 315kt (advertised) up to FL250 would be a nice step up from a piston twin or slower turboprop, and it'd keep you out of RVSM airspace with the simplicity & lower cost of single engine operation.

Posted by: Will Alibrandi | September 8, 2009 8:29 AM    Report this comment

"Now comes Cirrus promising initial buy-in owners a $1.39 million price and a maximum price of $1.55 million"

The Cirrus press release concerning their jet pricing mentions that the $1.39 mil price point is for those who already had deposits in, the $1.55 mil price point is for anyone putting down a $100k non-refundable deposit before the end of 2009, and a $1.72 mil price point will then be established for any new orders beginning in 2010, all prices being in 2009 dollars.

I think Cirrus may be able to do it for $1.72 million.

Posted by: Jens Torell | September 8, 2009 11:15 AM    Report this comment

I think Paul is right here. Recently Diamond announced a price increase to 1.89mil for its D-Jet for new position holders. Existing position holders will stay at the 1.38mil price point (July 06 $$) adjusted by CPI-W. Why would Cirrus price their Jet 500K lower than Diamond's similar offering?
The price of admission is not the only obsticle to overcome. In July of 09 Diamond released the latest D-Jet flyer and they finally published some performance figures. At it's max cruise of 315kts the fuel burn is 74 gal/hr. Even reducing the cuise speed to 300kts results in a fuel burn of 67 gal/hr. Fuel burn drops to the low 40's at its best economy speed of 220kts. During take off and initial climb the fuel flow will be over 110 gal/hr.
The problem with this is that it results in a dismal range payload tradeoff as well as a rather high DOC. The range at its normal cruise of 300kts is less than 900nm. DOC for just fuel and engine reserves will be approaching $400/hr at todays fuel prices.
The problem is the low service ceiling of 25K feet as well as the very spacious (width and height)cabin It takes a lot of thrust to punch that big of a hole through the air.
I think Piper got it right by targeting 35K feet for their jet. At 35K feet you can still fly single pilot with only a quick donning face canula. The efficiency of a Turbofan will be much better at that altitude. It requires RVSM certification but the increased utility and reduced fuel flow would be worth it.

Posted by: GREGORY WROCLAWSKI | September 8, 2009 11:26 AM    Report this comment

"Even reducing the cuise speed to 300kts results in a fuel burn of 67 gal/hr."

"The range at its normal cruise of 300kts is less than 900nm"

Well, Gregory, these are just some of the realities of jet flying. You will find that almost every fanjet is gets the best performance at 330-350. Above that, because the temperature in the strasphere does not drop with altitude like it does in the tropopause. I think that if you run DOC numbers on the increased time on the airframe will more than eat up the savings in fuel.

Also, jet engines are most efficient when they are run at 100% or as close as possible. Also, from the beginning of the takeoff run until the airplane achieves about 0.3M you're losing power. The ram recovery begins at about 0.3M or about 200 KIAS in most cases.

Personally, I wouldn't fly at FL350 with only a face canula for all the money in the world. I lost a cabin at FL350, and believe me, a facial canula won't do it. You'll be so punch drunk when you get to the end of the flight, you're likely to blow the approach. Per 14CFR25, one of the things you have to do to certify a maximum cabin altitude of 8000 feet. I have many times been coming back to VNY with green first officers after we had been at 8000 feet for a couple of hours. I asked them to look at the dessert and tell me what they saw. Then I had them take a few puffs of oxygen and look again. They were amazed at what they hadn't seen the first time.

Posted by: Linda Pendleton | September 8, 2009 11:45 AM    Report this comment

Like I said, "Very Light Joke"!

Posted by: Larry Fries | September 8, 2009 12:33 PM    Report this comment

Linda replies...
Personally, I wouldn't fly at FL350 with only a face canula for all the money in the world.

Linda, I assume that the quick donning full face canula is for emergency purposes only. If you lose cabin pressurization, put on the face canula and immeadiately start a very rapid descent down to under 10K feet and find a place to land and fix the problem.

Linda writes:
I have many times been coming back to VNY with green first officers after we had been at 8000 feet for a couple of hours.

I have one of those sports watches with a built in altimeter and I've noticed that on commercial flights the cabin is close to 8000ft when in level cruise at FL350-370. Do the airlines provide oxygen for the crews to suck on if they start feeling punchy?
Maybe I'm better physiologically than most people but when I flew an unpressurized airplane I used a pulse oximeter and my oxygen saturation was over 90% when level at 11K feet for a while. At 8K feet my saturation level was about 95%. My current plane, a Piper Malibu, has a pressurization differential of 5.5psi and provides a 8K ft cabin at FL250

Posted by: GREGORY WROCLAWSKI | September 8, 2009 12:43 PM    Report this comment

If I were making the buy decision for a corporation, I think it would be an easy sell to pay 2.55 mil for a Citation Mustang as opposed to 2 mil for a new Eclipse or other start up. Aviation in general has been very unfriendly to start up companies - who really wants to risk having an orphan airplane when the manufacturer fails? I think the Piperjet will be an alright machine, but it's almost the price of the Mustang - for $150,000 more I'll have two engines, thank you! That being said, if I were ever in the position to purchase a jet, I'd look for a well-maintained, used Citation I myself.

Posted by: Josh Johnson | September 8, 2009 1:55 PM    Report this comment

While most of the VLJ comments have focused on the (evidently) unrealistic cost estimates, I believe Elipse's (and other VLJ producers) fatal error was their misreadng of the realistic size of the VLJ market. Their core belief was that price alone was all that separated the high-performance GA pilot from a jet - and their huge order book in 2006 seemed to support that belief. However, that number (approximately 2,400!?) consisted of a) a couple of charter company block orders, b) some actual owner/operator orders, and c)a whole bunch of spec orders that were trading on delivery dates. And all of that was before it became clear the underwriters would want pt. 135 level initial and recurrent training to fly these lawn darts. I sold fractional shares in Eclipses and it was clear that the majority of the purchasers would have to hire professional pilots, even if they were already licensed. Bottom line: If you could afford one of these toys, you will be too busy with your day job to stay legal and/or sharp. Thus, even if Eclipse had had its manufacturing act together, the deal would have collapsed within a couple of years due to insufficient sales volume. Cirrus please take note.

Posted by: ROBERT HAWLEY | September 9, 2009 10:55 AM    Report this comment

I tend to agree with you, Mr. Johnson. Although I cannot provide specific examples without doing a bit of research, it seems there have been plenty of start up companies that failed, only to be bought out of bankruptcy after a majority of the initial developement was finished that have gone on to be successful. When you look at the production of one of these machines, you have to consider the cost of the actual materials, the cost of the labor, and the cost of the developement. Once you shed the cost of the developement through bankruptcy, you can sell them much cheaper. You also have to compare costs from start up companies to those who have sustained production of other models. For an established company you already have in place and paid for, the cost of administration (office buildings, upper level management salaries, etc). Eclipse was fighting a tremendous uphill battle. I find it quite amazing they made it as far as they did. Now that the assets have been purchased with no surviving debt, there is a real chance this can become a popular type. Too bad for the original vendors and investors they won't be able to reap the benefits.

Posted by: WILLIAM BEEVER JR | September 9, 2009 11:34 AM    Report this comment

And all of that was before it became clear the underwriters would want pt. 135 level initial and recurrent training to fly these lawn darts.""

Actually, it wasn't the underwriters. That was the training I wrote -- the training required for a type rating under FARs. The only difference between a type rating and a 135 PIC check is that for the type rating the outcome of a maneuver can never be seriously in doubt. For a 135 PIC the outcome can never be in doubt, period.

Vern didn't like that training program. It was compreshensive -- as it should have been to step up to a jet. I was also scenario based. Instead, they hired a company that said they could do the training in one week which I consider impossible unless the student were an experienced jet PIC.

Robert -- are you a jet pilot. I've had over 24 years in the jet business and have been a FlightSafety program manager and a Citation DPE. I know what it takes. We had a mentoring program tied into the training.

Posted by: Linda Pendleton | September 9, 2009 1:49 PM    Report this comment

Kind of an interesting parallel to Eclipse is the development of the Learjet - as I understand the story, Bill Lear was a multimillionaire who nearly lost it all building his little jet - and when he found his expected market wasn't as big as he originally thought, he sold his shares in Learjet to Gates Rubber, then to Bombardier. Perhaps they should have called the Eclipse the "Raburn 500" - at least he would have some bragging rights! Even Cessna (sold to Textron) and Piper (sold who knows how many times) ended up being investments of huge corporations. If Eclipse stands a chance, sadly the bankruptcy just about had to happen.

Posted by: Josh Johnson | September 9, 2009 6:15 PM    Report this comment

"Many of us would be quite happy with a 4 place jet that cruised at 250kts or less with a ceiling of 30,000 ft."

Guess what, you don't get 30,000 ft, you get 25,000 ft. The reason all those turboprop singles were certified at 31,000 is that 23.841a didn't apply until 31,000 ft. This changed some time ago to 25,000 ft, which means that you get a 25,000 ft ceiling if you want a cheap(ish) airplane.

Posted by: Colin Hildinger | September 10, 2009 8:50 AM    Report this comment

I own an early (s/n 19) Eclipse and have a few observations. Yes, Vern was totally the wrong person to run the company after he raised all the capital. Way overstaffed especially with those who would agree with his latest whim.
My comments re the plane I own. Paid 1.1M and have none of the upgrades ie Avio NG, ETT or FIKI. Current value is about half what I paid BUT: As the new owners slowly restore the fleet and the economy improves I expect that my unmodified plane will be worth closer to my initial cost. The plane is fantastic to fly typically at FL350 or so watching the weather below often to 25K. Mine is a plane worth the same as a new Cirrus G3 turbo that is pressurized, two jet engines,etc...I know which I had rather have. Training was tough especially since Vern wanted to control it but wold not support his own employees doing the training. This was training in the plane not in the simulators that were not running (or paid for). Overall a great plane for my personal use which is good weather and light IFR. I have a G400W for enroute,arrivals and GPS (WAAS) approaches. G496 for XM weather. In summary I have lost more $$$ in stoks without nearly this much fun.

Posted by: BOB YARBROUGH | September 10, 2009 9:27 AM    Report this comment


Why all the comment about Cirrus to make your point? It is as if Diamond did not exist when, in fact, Christian Dries' "vision" is spot-on.

Diamond prudently delayed certification of the D-Jet for another year to conserve cash flow during a cash-intensive phase of certification. And the program is on schedule with nearly 4 flying prototypes, one of which is a conformity model.

DAI selected the best engine (and engine manufacturer) in Williams since the FJ33 is a "downsized" version of the tried and true FJ44, 25K foot service ceiling for all of the reasons stated above, and has the pre-eminent airframe design and composite materials technology. In addtion, Christian has been prescient in his establishment of manufacturing plants in Europe, Asia and North America, realizing that the Asian and American markets are vital for Diamond's success. Add to that the re-establishment of the DA42's preeminence as the very best piston twin in the world (Yes, I am still unhappy with DAI's response--or lack thereof-- to the Thielert engine insolvency which has caused a huge debacle for them. Hey, not every business decision is going to be correct) and you have a company, though still privately held, poised to become the major player in the 21st century general aviation world.

You would have done better to make Diamond your example and not Cirrus.


Todd House

Posted by: Todd House | September 10, 2009 2:06 PM    Report this comment


The Cirrus SF50 matches the D-jet's price of $1.38 million (on the low end) so I'm guessing that's why it got so much attention in the article. I suspect Paul would have the same reservations about the D-Jet's price as the Cirrus.

Posted by: Will Alibrandi | September 10, 2009 2:51 PM    Report this comment

I made the comments because it related to a news announcement last week from Cirrus. Our blogs are often timely and topical to news we publish.

I also believe Diamond's D-jet pricing is too low. We will find out, eventually. But high-dollar development projects put these companies at great risk for rewards that absolutely no one has demonstrated even exist. That's the salient point.

Posted by: Paul Bertorelli | September 10, 2009 3:27 PM    Report this comment

No one had ever demonstrated that flight by humans was possible until the Wrights did it long ago. I'm glad that entrepreneurs and visionaries dream and ACT!

I see this as a progression: Eclipse and Vern Raburn provided the vision and first prototype with disaterous results (for investors and their business). But, their failure has not yet proven to be the harbinger of impossibility that so many observers write about. Others have followed suit and picked up the mantle, as it were. Diamond, Adams, Cirrus, Piper, ATG, Spectrum, and on and on the list goes. Only Diamond has the track record, technology and finances (seemingly) to prevail. It is no coincidence that Diamond is an innovator and disruptor in an anachronistic GA industry that marches inexorably to the tar pits.

New engines, new materials, new airframes, new avionics and a new 21st century mentality are mandatory if the industry that begat our passion is to survive.

Thanks, Paul, for your insightful columns--pretty clear on these pages that you stimulate productive conversation.



Posted by: Todd House | September 10, 2009 6:09 PM    Report this comment


Perhaps you missed my post earlier in this blog. But Diamond has raised the D-Jet pricing to 1.89mil in March 09 economics adjusted to CPI-W change from that point to delivery. The Legacy deposit holders (roughly the first 70 positions) are currently locked into 1.38mil in July 06 economics adjusted for CPI-W PLUS 5% wiggle room. That makes the price currently about 1.55mil for those position holders.
Diamond delayed delivery of the D-Jet (yet again) for two reasons, development of the Austro engine and re-certification of the DA-42 with it, sucked up too much money AND a number of early position holders are much poorer now(yours truly is one of them) and are hoping their situation improves so they can afford to take delivery. When I put my money down for my position in June of 03 I was told a delivey date of mid 07 at a price of 850K in April 03 economics adjusted by CPI-W. Now I will have to take delivery of it in tremendously deflated dollars(most of my investments are in equities and realestate) at a much higher price.

Posted by: GREGORY WROCLAWSKI | September 11, 2009 7:09 AM    Report this comment


Your point is at once well-made and duly noted. I hold 6 DJet positions, one of which is a legacy position.

Of course delay means a retroactive price increase for us early adopters when, instead, we should be rewarded for signing on the the vision and its inherent risks.

I am pleased with the Williams engine issue and the delay suits my current economic situation though I do realize that I have to pay for this opportunity to survive til next year.


Posted by: Todd House | September 11, 2009 8:03 AM    Report this comment

The problem with personal jets is that there is not a suitable engine. All of the light and very light jets use engines designed for high-altitude, high-speed flight.

They are simply sclaed-dwon, big-plane turbofans with a fairly low bypass ratio, so you have to fly them up high and fairly fast in order to get any sort of tolerable fuel consumption.

What is needed for lower-altitude, lower-speed flight is a much higher fan bypass ratio and a low overall pressure ratio.

What a lot of airplane owners would really like to step up to is a small jet that can fly in the 20's and burn about as much fuel as a piston twin.

That's not a pipe dream, but it requires a turbofan engine specifically built for this kind of flying. Gerry Merrill is an ex GE engine guy who has done the math on this kind of engine and, as an engineer, I believe it is perfectly doable.

The problem is the turbine engine makers don't need to bother with this small "private" market.

Without the right engine, all of the everyman jets that are dreamed up by entrepreneurs are bound to fail.

Neither the Diamond nor the Cirrus, if they ever get to market, can succeed because they have the wrong engine for the job.

Posted by: Gordon Arnaut | September 12, 2009 10:00 PM    Report this comment

As an engineer I've done some research into this turbine efficiency issue. I believe you are wrong in that you do want a high pressure ratio in the turbine portion since that improves efficiency. Maybe you are referring to a low fan pressure ratio. Yes you do want a high bypass ratio. A nearly infinite bypass ratio is already available with a turbine. It's called a turboprop engine. And they aren't nearly as efficient as pistons engines.
Williams which is the most advanced and successful small jet engine manufacturer started out making small turbofans for cruise missles. They progressed to small jet engines in the late 80's. In the late 90's and early 2000s they burned through several 10's of millions of GAP technology money developing the FJ-22. A small clean sheet design turbo fan that weighed 85lbs with 750lbs thrust. This was the original engine that was to be used in the Eclipse. After one official flight(there were many test flights) Eclipse scrapped it. It wasn't close to meeting target performance(thrust or fuel consumption) and wasn't reliable. They chose a new clean sheet design P&W engine. P&W actually built a new state of the art factory in Canada to build the huge quantities of this new small 900lbs thrust engine for Eclipse.
Maybe a highly efficient at low speed and altitudes turbofan is theoretically possible but practical tolerance limits and internal airflow which doesn't scale well limits whats practically possible.

Posted by: GREGORY WROCLAWSKI | September 14, 2009 6:58 AM    Report this comment

"cheap" and "high performance jet" should never be used together when talking about production aircraft. The structure and complexity alone at that performance level should put it over $1M even before thinking about what engine(s) are used.

Only way to keep costs down are jet variants of home built designs. No if RV could do "a jet" then that would be interesting news in the VLJ arena!

Posted by: Mark Fraser | September 14, 2009 1:29 PM    Report this comment

Anybody remember Jim Bede?

Posted by: Linda Pendleton | September 14, 2009 9:54 PM    Report this comment

"Anybody remember Jim Bede?"
Probably all those who lost their deposits on the -5 remember him intensely. Those also who lost family members in the ill-designed -10 probably do as well. His is the one name I'd avoid if he started into any aeronautical design or business venture...

Posted by: Mark Fraser | September 15, 2009 8:34 AM    Report this comment

VLJ's are just not anywhere near the utility of a good turbo prop. Take the Diamond Jet at 300KTS at 60gph and 25K vs. the Conquest II doing 295 at 35K on the same 60 gph. You get a total of 10 seats in the Conquest vs 6 or so in the VLJ. The Conquest has enough useful load to fill those seats and still go somewhere. Sure the Eclipse is faster, but with any passenger load at all the Conquest will beat it on a trip of any distance because the Eclipse will have to stop for gas. The current VLJ designs lack the utility to compete with a good turbo prop. The only reason any have sold is so that the owners get to say "jet" instead of "prop".

Posted by: Barrett Roessler | September 15, 2009 9:05 AM    Report this comment

Barrett has a good point. I'd go one step further and make another comparison. Slick composite turboprop singles rival VLJs in speed, range and payload. Epic's certified Dynasty has a 340kt max cruise & 1,200nm range. Their experimental LT is 10kt faster, and the smaller Escape is 15kt higher than that. (Economy cruise in the Escape is 300kt) That kind of speed with short field capability and the ability to slow the plane down quickly with the prop makes it very attractive. Hopefully Epic's recent troubles will be resolved by their hookup with Vijay Mallya, as I think their aircraft are phenomenal.

Posted by: Will Alibrandi | September 15, 2009 9:37 AM    Report this comment

That Yankee who learned the hard way about being 4 seconds from the ground with a 7-second spool-up time was Thurman Munson, and he was a catcher, not a pitcher. Pardon my pointing out the nit.

Posted by: John Lloyd | September 15, 2009 6:08 PM    Report this comment

From about 1999 I followed the fortunes of the biggest and smallest of jets with great interest - the eclipse 500 and the a380. Now that some time has passed since one failed and the other has (maybe) started to succeed, I reckon I was in love with the idea of the VLJ at the expense of the reality. No, make that STILL in love with the idea! But VLJs now seem to me to be a bit like what I call the "nothing cars" we have here in Australia - I think you guys call em SUVs - they are not good off road, they suck on road and they just suck gas, reduce everyone else's visibility and safety, are seldom full of people and are often driven by people without adequate skills to control them! VLJs generally seem to have marginal performance benefits over TBMs etc, they suck more gas, will encourage low hour people who will have trouble maintaining truly adequate currency and, yeah, seem like "nothing aircraft". I still love the idea but I think the scale required is not there. I think they need better propulsion (whatever happened to the that prompted the fj22 and diesel motor that dissappeared into TCM? - a scandal I reckon), more and better automation and a larger pool of people world wide who train up and (maybe) enter into a buy-a-share arrangement where they book one from a large, dispersed pool and hop into the nearest one to them. That's my head talking. Then I see a lonely eclipse in the pattern at YBAF, barely audible, and I am almost convinced again.
thanks John.

Posted by: John Hogan | September 16, 2009 12:30 PM    Report this comment

Thanks, John. I guess I should proofread before I hit submit. I know Thurman Munson, the Yankee CATCHER, was the one involved. He died because he wouldn't wear the shoulder harness -- he thought it was too uncomfortable. His neck was broken and he was too big to drag out.

Posted by: Linda Pendleton | September 16, 2009 12:48 PM    Report this comment

Thanks, John. I guess I should proofread before I hit submit. I know Thurman Munson, the Yankee CATCHER, was the one involved. He died because he wouldn't wear the shoulder harness -- he thought it was too uncomfortable. His neck was broken and he was too big to drag out.

Posted by: Linda Pendleton | September 16, 2009 12:48 PM    Report this comment

Linda; My AA1C will, forever, remain the high point of my life. Jim's concepts and designs are remarkable! My opinion of of aviation in general is that the brain can't get around need vs. dream as a profit motive. If we all had "Cubs" at our beck and call, we might consider "Longer, Faster, Higher" less attractive. If ship captains can day sail or truck drivers bash about in the dirt, they seem to clear the cobwebs. A simple question, If we could afford a personal "Jet" how long would it take to become a hangar queen. A more complex question, How much money can be generated because no one wants to answer the first question? Reality is illusive. Example; India is aviation rich and automotive poor. Howz that going to work? China can build 747 and BMW parts but can't put it's own together. So, dream on but watch the success or failure of LSA's as an industry. Wish to fly. Wish to buy. Wish to make a buck. What will our brains do next?

Posted by: Larry Fries | September 16, 2009 3:28 PM    Report this comment

And who's going to fly them? How many pilots do you think are out there that have the competence to fly a jet a the flight levels, in all weather, and that would be willing to do it for the meager salary someone who paid less than a million for a new jet would want to pay? The owners fly them? Yes, on blue sky days with no weather on their route maybe. That's when I'd always hear Piper Malibu's on the radio at the lower flight levels. The VLJ's? they'd be flying in the same weather. Lots of potential customers like to dream about having their own jet that they could afford to buy, such as one of the low cost VLJ's, but when the day comes to file flight plan at the upper flight levels, with weather along their route, reality comes crashing in on them and they'd never leave the ground.

Posted by: Richard May | September 21, 2009 7:39 AM    Report this comment

I meant to say that a low fan pressure ratio is desirable, not a low "overall" pressure ratio, which is a measure of engine air compression.

You are correct that overall engine pressure ratio is better for efficiency. This is like the compression ratio in piston engines. The latest large turbofans have pressure ratios over 40 and very high fuel efficiency.

Such high ratios are not actually required for a low and slow light airplane. A lot of the propulsive efficiency would come from using a larger fan. But having an efficient engine core is needed too.

The Pratt PT6 has a pressure ratio of about 6 or 7 and manages SFC of about 0.6, not that much worse than a piston. The more efficient Honeywell TPE 331 has a modeslty higher pressure ratio, about 11 or 12, and improved SFC of about 0.55.

The recipe for a fuel efficient turbofan for low altitude work is a modest pressure ratio, of about 10 or 15, and a very high fan bypass ratio..

This is very easily doable with today's manufacturing. So the ingredients are all there to make a very efficient turbofan engine that could replace piston singles or twins.

Some engineers, myself included, believe that the engine cost could be quite reasonable.The overall airplane cost could then be kept reasonable also because it is the high-altitude flight that demands very robust systems for pressurization, instrumentation, etc.

Posted by: Gordon Arnaut | September 26, 2009 2:50 PM    Report this comment


Sorry, the above post was meant as a reply to your comment.



Posted by: Gordon Arnaut | September 26, 2009 2:53 PM    Report this comment

About the turboprop versus jet question, the big attraction of a jet (turbofan) is the low noise signature and overall civility.

A prop is still a prop, even when it's attached to a jet (turbine) engine. There noise presence is a big factr, and there is also slipstream buffeting from the prop.

A turbofan with turboprop fuel efficiency is very possible.

And as far as cost is concerned, jets cost a lot because they have to fly very high.

A turbofan airplane that is designed to fly like a Baron, or even an SR-20 does not need those very expensive systems.

Posted by: Gordon Arnaut | September 26, 2009 3:01 PM    Report this comment

For the non-engineers who are wondering why a turboprop is more fuel efficient than a jet (turbofan), the reason (mostly) is due to the fact that it takes more energy to make a lot of thrust with a small chunk of air.

A big prop works on a large mass of air, so it does not need to accelerate that air as much as a jet engine, or even a turbofan engine with a fairly small fan. The faster you need to speed up the air, the more energy you are wasting.

On the Williams FJ-33 and the small Pratt 600 series turbofans, the fan diameter is less than 20 inches. A prop on the TBM is closer to 90 inches. It can make thrust much more efficiently, so the engine does not need to put out as much power or burn as much fuel.

If you were to take the same engine core as the FJ-33 and increase fan size to about 30 inches, you would have an engine with hugely improved fuel efficiency, especially at slower speeds and lower altitudes.

It is entirely possible to build a turbofan engine meant to cruise in the teens at about 250 mph, at efficiency matching todays turboprops.

Posted by: Gordon Arnaut | September 26, 2009 4:32 PM    Report this comment

I'm just going to add a little here because turbine engine efficiency is a fairly complex subject that depends very much on both speed and altitude.

If you look at the latest large transport turbofan engines, these are the most efficient engines yet devised -- more fuel efficient than diesel piston engines.

They do this with very high pressure (compression) ratios (approaching 50 to 1), and very large diameter fans, the biggest of which are now about 12 ft.

The high pressure ratios cause a lot of heat in the compressor sections and require very fancy metallurgy and machining. Very expensive.

If the goal is to make a turbofan engine that will replace piston engines, this approach costs too much.

But todays small turbofan engines are designed for flying above FL400 and at 400 knots. They do a good job with this flight profile, but that requires a small airplane with all the critical safety and high-altitude capability of a large plane.

This costs a lot too.

Building a suitable high-bypass, large fan engine for low-altitude work at 200 to 300 mph is technically very doable and need not cost a whole lot more than a turbocharged piston engine.

And if you want to fly at piston altitudes, the airplane need not cost much more than a piston craft either.

Posted by: Gordon Arnaut | September 26, 2009 4:48 PM    Report this comment

Just one more comment for those who insist that jets have to cost a lot of money.

I mentioned Gerry Merrill and he had proposed just such a high efficiency, low-speed low-altitude turbofan engine for NASA's next-generation GA propulsion program a few years ago.

This program should have resulted in just the kind of engine I've been describing here, but the program got sidetracked and went in the other direction -- high and fast.

There was a good writeup on Merrill in Air and Space magazine last year. I think the article is called "Who says jets can't be cheap?"

thatAny engine guy will tell you such an engine is perfectly feasible technically. The problem is that the large engine makers don't want to step down into the light plane category, fo rany number of reasons.

These are large companies that make a lot of money in the transport and military aviation. Why would they bother with a market that sells maybe a 1000 planes a year, worth maybe $500 million?

Compared to the real aerospace industry, light planes for the private market is really a cottage industry.

Still, the day will probably come when small planes will get the quiet and efficient turbofans they really need. Then maybe that will spark a new growth in private aviation. Jets do have that certain something...

Posted by: Gordon Arnaut | September 26, 2009 5:01 PM    Report this comment

Gregory, I just want to respond to your specific point about "what's possible."

You are correct that internal limits and airflow play a more significant role in smaller engines.

This has to do a couple of things. First is Reynolds number, which is much higher for the large airplane wings and fans, so scaling down means losing efficiency due to lower Re.

Likewise with the internal tolerances, such as the gap between fan tips and duct wall, which are relatively bigger on a smaller engine.

Airflow is likewise not in our favor as we go smaller. The ratio of surface area to volume inside the smaller engine is going to be bigger which means more friction drag from moving the working fluid (air) around.

So yes, a scaled down engine will be less efficient. But these losses are not that big.

A GE CF6 which is on the B-747 and makes about 60,000 pounds of thrust has a cruise SFC of about 0.6.

A good bizjet engine of the same generation is the Honeywell (Garrett) TFE731 which is used in Citations and Falcons. It has an order of magnitude less thrust, 5000 lb for hte largest engine in the family, but still has a cruise SFC of 0.68. That's little more than 10 percent difference.

Both have comparable bypass ratios (4.3 for the small engine and 5.7 for hte big engine) and pressure ratios in the low 20s.

Btw, those SFC numbers are pounds of thrust per pound of fuel burned. If we convert the thrust at cruise speeds into power, the SFC would be about half that.

Posted by: Gordon Arnaut | September 27, 2009 4:45 PM    Report this comment

Thanks for those posts - very interesting and informative. I have wondered previously if a different compromise between a turbo prop and a turbofan might be more appropriate in this segment - something that is more like a ducted turbo-prop with more blades and possibly with a ducting arrangement that also provides some lift to help negate its presumable drag penalty. Maybe that would mess with the airflow into the fan too much. Thanks John

Posted by: John Hogan | September 28, 2009 5:01 AM    Report this comment


That small Hoenywell (Garrett) turbofan I mentioned above is an instructive example.

If we look closely at its design and performance and then compare that to say the PT6, we see exactly what is going on. And what is going on is that the big engine makers are not interested in small private airplanes.

Look at the TFE731. This is '70s technology but it has a good engine pressure ratio of about 24, so fuel efficiency is pretty good.

It uses a gearbox to drive the fan, unlike most turbofans which are direct driven from one of the turbines.

It's takeoff threust is 5000 lb, so two of these are used on bizjets of about 25 to 30k lb gross weight.

Cruise thrust is about 1100 lb and it takes 0.68 lb of fuel for each pound of thrust so consumption in cruise is about 748 lb/hr, or about 100 gal/hr.

In order to compare apples to apples we convert thrust to hp (power = T * V). Cruising at about 450 knots, and assuming propulsive efficiency of 0.8, it takes about 2000 hp to make 1100 lb of thrust.

750 lb of fuel divided by 2000 hp per hour = an SFC of 0.375 lb of fuel for each hp, per hour. That is about 20 percent better than you are going to get with a piston Lycoming.

GA pilots have a msiconception that all turbines are fuel thirsty.

I will continue this in the next post due to space.

Posted by: Gordon Arnaut | September 28, 2009 7:22 AM    Report this comment

Now let's look at the numbers for a PT6 or the small Allison.

These have engine pressure ratios of only about 6 to 1. This is 1940s technology.

Lots of hobbyists buid turbines from car or truck turbochargers, all you have to do is squirt fuel into them. If you take a good lsized truck turbo, the compressor will give a pressure ratio of about 3 or 4. It will make lots of power but it will use a lot of fuel.

Now think of how much more efficient a modern turboprop engine would be if we increased the pressure ratio to about 20 to 1 -- like the TFE we mentioned above.

That engine with a prop on it would beat any piston enngine in fuel economy.

Now if we wanted to have the quietness and civility of a turbofan, we simply put a fairly large, geared fan on the front instead of a geared prop.

having a duct greatly increases propulsive efficiency because it mostly eliminates the induced drag from the open prop tips. In fact, given the same power, it takes a prop of double the disk area to equal the thrust of a ducted fan.

Remember that's area, not diameter. So a prop of about 90 inches would equal a fan of about 60 inches.

That's really too big for drag and stability reasons, but a fan of 30 to 40 inches is still going to give very good efficiency.

This is not rocket science. All the ingredients to make a very efficient turbofan engine that could replace piston power on even the smallest certified planes have been there since the 1960s.

Posted by: Gordon Arnaut | September 28, 2009 7:37 AM    Report this comment

I'm just going to comment a little on the reason why we don't have good, fuel efficient turbine engines for small planes of the private variety.

Recently Rolls Royce came out with a marketing push on their "improved" Allison 250 engine (or whatever it's called now).

This is basically the same '40s technology with the same very low pressure ratio of about 7. The Pratt PT6 is the same deal.

At the same time Rolls bought the Czech Walter engine company which made a very rugged PT6 clone for Russian GA. A Polish company PZL made an Allison 250 clone, the Isotov GTD350, which powered small Russian helos.

PZL was bought by EADS, the European aerospace conglomerate.

Lots of my colleagues in the Eastern Bloc were excited at the prospect of welathy new owners. They figured there was going to be new development to modernize these engines.

Not a chance. Instead, the new owneres simply shut these plants down. Oh sure they make a few parts and pieces, but that's it. A lot of these very good engineers are now in the unemployment line rather than working on the nextgen GA powerplant.

Why? Because the big companies do not want to bother with the roughly $100 million small airplane engine market.

Rolls bought Allison and has invested nothing to modernize this engine. Yet their marketing hype is that this '40s technology is the "next" step for small planes.

EADS likewise has its existing turbine lines and they shut down Isotov so as not to disturb that.

Posted by: Gordon Arnaut | September 28, 2009 10:31 AM    Report this comment

I'm also going to comment briefly on the Williams EJ22, which was the original engine for the Eclipse -- and was originally funded by the Nasa GAP program.

Williams never released any technical info on this engine, but most engine guys figure it is a 3-spool design. (Maybe Linda could confirm if original Eclipse prototype had an N3 gauge, alongside the usual N1 and N2. This would indicate a third turbine spool).

This was no doubt a jewel of a little engine, but 3-spool engines are very complex. Only Rolls and a couple of the Russian engine makers have bothered with 3-spoolers.

Efficiency can be better on a 3-spool, but it is not necessary for a low and slow turbofan engine.

Sam Williams sold NASA on this idea and the goal of GAP morphed from what Gerry Merrill had in mind, which was a low and slow turbofan for existing small planes -- to a small turbofan that would go 400 mph.

To go 400 mph you need to go well into the flight levels. The airplane you need is a mini bizjet, with the same kind of engine and systems.

If you want to go 200 to 300 mph under about FL250, with or without pressurization, you need a different kind of turbofan. You need an efficient engine core with pressure ratio of 15 to 20. And you need a very high bypass ratio of 20 or more. This means a geared fan like the small Honeywell turbofan.

This is the engine that Merrill proposed to NASA, but they went for pie in the sky instead. Too bad. A lot of taxpayer dough wasted.

Posted by: Gordon Arnaut | September 28, 2009 10:55 AM    Report this comment

Gordon, I have enjoyed your technical posts on the small turbofan engine possibility for low/slow GA aircraft.

As evidenced by this blog and others, there seems to be a significant level of interest in such a powerplant. I would also imagine there are some folks in this crowd with the business,technical, and financial ability to shepherd such a dream into reality, maybe with the help of NASA or another organization.

Maybe we should all get together and do it ourselves as a user community?

Posted by: CRAIG DOW | September 28, 2009 11:37 AM    Report this comment

Not sure how much interest there is when the last 11 posts came from the same person. BTW, GE bought Walter Engines, not Rolls. And Rolls' new RR500 turboprop is much improved over the Model 250 turboprop, even if its EPR is low.

Posted by: Will Alibrandi | September 28, 2009 12:27 PM    Report this comment

Your statement that Rolls bought Walter Engines was incorrect, and that's what I responded to. The RR500 has similar architecture and airflow path to the M250, but has a Ti centrifugal compressor, no accel bleed air valve, electronic engine monitoring system, and a whole bunch of other differences that RR isn't releasing just yet. It also has a higher maintenance interval vs the 250 and is multifuel capable. While none of those things increase the EPR, to say the two engines are practically the same is not accurate. You were correct about the EJ22, it was a three-spool engine. Pretty amazing considering its fan was barely 15in in diameter.

Posted by: Will Alibrandi | September 28, 2009 2:03 PM    Report this comment

Will, the problem with a 3-spool is you have 3 concentric shafts (one passing through the other, for the non-technical people). The complexity in lubrication, bearings and seals is an engineering nightmare.

Like I said it is not necessary. Again for the non-techs, the efficiency of a turbofan engine depends on just two things -- how effectively the engine turns fuel into power, and how effectively the fan turns that power into thrust.

It's the same for a turboprop. The first one depends almost entirely on the engine pressure ratio. Just like a diesel engine with a CR of 24 is going to be a lot more efficient than a gas engine with a CR of 8. That's the law of thermodynamics for any heat engine.

The propulsive efficiency depends on how fast we want to fly. For anything under supersonic, the efficiency is better the bigger our fan (or prop). That's why the latest engines have such huge fans.

The slower you go the bigger your fan should be. Of course for a small plane there are limits due to stability and drag.

So if you take a turbine engine that with a pretty good EPR and use a fan that is a workable size for a small plane, you will have a powerplant that will be as efficient as a piston plane.

You are losing some from the smallish fan (compared to a prop), but you are gaining due to the engine efficiency of the high EPR. (High compared to today's small plane turboprops, which are '40s technology; it is still low compared to the latest large turbofans).

Posted by: Gordon Arnaut | September 28, 2009 4:39 PM    Report this comment

Guys, The current state of turbine technology is not what's keeping VLJ's out of the sky. One more time, there's no market. The initial training, recurrent training, and general sharpness needed to stay safe in one of these darts, puts them beyond the practical reach of most GA pilots - even if they could afford the $1-2MM price tag.

Posted by: ROBERT HAWLEY | September 28, 2009 6:58 PM    Report this comment

Robert, a turbine airplane does not have to cost a million dollars. You can buy a brand new turboprop airplane for less than half that. It's called a Maule.

And a VLJ does not have to be a "dart" that flies 400 mph at FL400 -- and requires the robust pilot training that goes with that.

What we are talking about here is an airplane similar in size, speed and operating altitude as today's high-performance singles. The only difference is the turbofan power.

Posted by: Gordon Arnaut | September 28, 2009 7:18 PM    Report this comment

>>a turbine airplane does not have to cost a million dollars

As Ronald Reagan famously said, there you go again. It's true that conversions based on existing designs don't have to cost a million or more. But for GA, turbines are predominantly about speed first, efficiency somewhere further down the list. The Maule exits in tiny boutique shred of the market because the airplane doesn't really do that much that the piston can't do. It's certainly not more efficient. If it did more, it would sell more.

Eclipse proved rather harshly that the price-depressed large volume model doesn't work. Had Eclipse not been disastrously mismanaged, would the numbers have worked? I submit that they would not have, because I believe the 2500 orders Eclipse once claimed was a fantasy.

I further believe that for a new design, if you plot the curves for realistic volume against cost of cert/production and a desired ROI, they don't come together until at least the $2M mark.

Posted by: Paul Bertorelli | September 29, 2009 4:21 AM    Report this comment

Paul, I do not agree that speed is the paramount consideration.

Airplane owners own airplanes for many reasons -- and for most private GA owners, speed -- especially once we get past 200 knots or so -- is not that high on the list.

If I were to offer buyers an airplane of about the size, weight and general capability of an SR22 -- but with a very quiet, efficient turbofan engine, do you think the interest would be minimal?

I don't think so. I think people would be beating down my door.

I agree that a mini-bizjet like the Eclipse or Mustang cannot be offered for under $2 million. There can be no debate on that.

Both of the single-engine jets from Cirrus and Diamnond fall into the mini-bizjet category because they have mini-bizjet engines that must be flown high.

I am merely pointing out that a turbofan engine need not, by necessity, be a high and fast machine. It can be a low and slow machine just as well. The only reason it isn't right now is because no one is building such an engine.

What I am saying is that

Posted by: Gordon Arnaut | September 29, 2009 5:17 AM    Report this comment

Gordon, nobody is building the engine you are describing because like Paul said, the cost to design, certify & produce such an engine (and eventually make some kind of profit) is the biggest barrier. A 200kt 6 seat turboprop with an equivalent fuel burn of a piston at a competitive price would be great, but the mfr would never recover its costs.

Posted by: Will Alibrandi | September 29, 2009 8:08 AM    Report this comment

I do not agree, Will.

I mentioned Walter and PZL because I know their technical capability from past experience.

The Isotov engine is actually a Russian design that was built by PZL under license. It is the same layout and size as the Allison, with about the same performance. Like most of the Russian engines it is quite rugged for Siberian operation in harsh conditions and few and far between maintenance facilities.

This engine could be manufactured in existing plants for about the cost of the Continental in the SR22.

A program to update this engine would be well within the reach of the engineers employed by Isotov. The investment would certainly return a handsome profit.

This engine, like the small Allison and Pratt are reverse-flow so they would need to be reconfigured for axial-flow for a turbofan.

Another possiblity is to keep the engine as a turboshaft, and driving one or two ducted fans by chain or belt. The engine would be located in the aft fuselage with a duct on either side.

The main investment would be to update the engine with an additional compressor stage. This is not pocket change, but it is definitely recoverable even in a $100 million market.

I will continue in the next post.

Posted by: Gordon Arnaut | September 29, 2009 10:02 AM    Report this comment


I don't know why you'd want to tie your selves down to a propeller. That add a level of complexity that makes it more expensive to maintain and, harder to fly.

JETS ARE NOT HARD TO FLY. Even a girl can do it!


Posted by: Linda Pendleton | September 29, 2009 10:15 AM    Report this comment


I don't know why you'd want to tie your selves down to a propeller. That add a level of complexity that makes it more expensive to maintain and, harder to fly.

JETS ARE NOT HARD TO FLY. Even a girl can do it!


Posted by: Linda Pendleton | September 29, 2009 10:16 AM    Report this comment

Rolls, GE or Pratt could do the same thing.

The reason they do not want to do this is not because there is no money to be made, but because the money to be made is not big enough to bother with. There is a difference and it is not semantics.

In case you don't realize this yet, personal aviation is seen as the problem child in the entire aviation sector -- right from the political and regulatory level, to the business and manufacturing end.

All of the big companies got out of personal aviation 40 years ago. Cessna and Beech are not big companies, but even for them personal planes are a sideline to their bizjets, which account for 90 percent of revenues.

These companies see the spotty training and unprofessionalism in personal aviation as a big problem -- and a potential legal liability. Why bother?

That's just the kind of world we live in. We are not in the '40s anymore. (Unfortunately, in a lot of ways).

Posted by: Gordon Arnaut | September 29, 2009 10:17 AM    Report this comment

Bold statements here, how do you know what it would cost to re-engineer/certify/produce this engine? How do you know how big the market is? The simple fact is any engine designed & built in Russia will have a tough time being accepted in the West - this is a major reason why PowerJet teamed up with Snecma to build the SaM146 engine for the Superjet 100.

A chain or belt-driven ducted fan is an interesting idea, but that drive would be the weakest link in the powertrain and probably not feasible for the market you're considering. If it hasn't been done before, there's probably a good reason for it. I'm an aero turbine analyst for a consulting company, and I deal with this topic daily. There's nothing keeping any engine OEM from doing what you're suggesting, other than the fact that there's a very limited market and the costs wouldn't be recoverable, regardless of what you say.

Posted by: Will Alibrandi | September 29, 2009 10:26 AM    Report this comment

Right on, Linda!

Let's talk them down from their defeatist attitude.

I will just add that there is going to be training component because even an SR22 clone with a turbofan will fly a little differently.

Turbines do take a little longer to spool up,as has been mentioned here, although not nearly like the old turbojets.

Managing the energy of the airplane becomes more important, although with the relatively low wing loading the transition from an SR22 piston to an SR22 turbofan is not going to be that hard.

I really believe this is the airplane most piston owners really want and need. And is the airplane they can afford and handle.

Some may still want to step up to the flight levels and go 400 mph. Fine. But that's not for everyone, even if you can afford it.

Posted by: Gordon Arnaut | September 29, 2009 10:32 AM    Report this comment

Okay Will, I'm biting.

How much WILL it cost to reengineer these engines? And how long will it take to recoup the investment? Talk about Bold. Let's see the numbers that back up your statements.

Russian engines will be accepted just fine by anybody who wakes up one day and realizes they can buy a nice 200 knot turbofan for the price of a piston. They will be humming the Russian anthem as they fly happily (and noiselessly and vibrationlessly) along.

How do I know how big the market is? How many high-end singles are sold each year? Do a survey and see if you can find even one single buyer who would not choose a turbofan for a little bit more money.

Posted by: Gordon Arnaut | September 29, 2009 10:44 AM    Report this comment

Linda, I'm a sucker for props because I fly PC12s & Caravans p/t for a charter company. While a prop may add a little complexity to the powertrain, it's hung on a much less costly aircraft. The fast turboprop singles from Socata and Epic offer cruise speeds in the 320-360kt range for considerably less dough than any new jet I know of. That may be an apples & oranges comparison, but the bottom line is you can go pretty fast for a lower initial buy-in and lower ongoing costs. (Never mind the type rating req cost for the jet)

I don't doubt what you're saying - I have friends that fly everything from CJs to 747s and they all say the same thing "Thrust levers forward - go fast. Thrust levers back - go slow" That may be a goofy oversimplification, but in essence it's accurate. When flying approaches into TEB in the Pilatus with jets behind, it's nice to be able to maintain 180 to the marker then pull the power back and settle in at a solid 120. That big air brake out front makes energy management pretty simple IMHO :)

Posted by: Will Alibrandi | September 29, 2009 10:45 AM    Report this comment

Will, about the drive belts: Next time you go up in a Robinson helo make sure to tell them that drive belts won't work.

Posted by: Gordon Arnaut | September 29, 2009 10:57 AM    Report this comment


You seem to be very knowledgeable about turbine engines. Could you explain to me why on a turboprop engine the efficiency of the engine decreases with altitude.

I have the power setting tables for a PT6-35.

For example at the same torque and rpm( e.g. I used 800ftlbs and 2190rpm which calculates to 334HP) the fuel consumption at 27K ft is 203 lbs/hr, at 18K ft the fuel consumption is 210lbs/hr and at 9K ft it goes up dramatically to 243 lbs/hr.

Calculating BSFC it comes to .61 lbs/hp-hr at 27K ft but increases dramatically to .73lbs/hp-hr.

Thye best BSFC I've seen on turboprops is the Garrett TPE331 series fixed shaft turbines which have a BSFCof around .56 at high altitudes.

The Continental IO-550 series engines when operated LOP have a BSFC of .39lb/hp-hr. Even the lower compression ratio TSIO-550 engines have a much better than turboprop BSFC of around .42.

This illustrates my point that even though a turboprop cannot not match the BSFC of a piston engine the other mitigating factors are the fact that an equivalent HP at altitude a turboprop is much lighter than a piston and produces much less drag due to lack of cooling drag and a much narrower profile which aides aerodynamics.

continued in next post

Posted by: GREGORY WROCLAWSKI | September 29, 2009 11:02 AM    Report this comment

"This engine could be manufactured in existing plants for about the cost of the Continental in the SR22. A program to update this engine would be well within the reach of the engineers employed by Isotov. The investment would certainly return a handsome profit."

This was your assertion, not mine. If anybody should provide numbers to back up a claim it should be you. You present your own opinions as facts, which does not mean that you are right or wrong, just that they are only your opinions.
If any engine OEM thought the market was big enough and they could make a worthwhile profit on a highly efficient small turbofan like you mentioned (and find a cost-competitive airframe to bolt it to) I'm sure they'd sell a whole bunch. Until then, I ain't holdin' my breath.

Posted by: Will Alibrandi | September 29, 2009 11:06 AM    Report this comment

In the previous post my opening sentence should have read: "decreases with decreasing altitude"

A good illustration of this is the Jetprop conversion of a Malibu/Mirage airframe. The 350HP piston is replaced with 750HP PT6-35 (flat rated to 560HP)and gains no wieght. At altitude due to temp limits the max availble HP on the PT-6-35 is about 375HP where the piston can cruise at 75% power(~262HP) in the mid 20's. The turbine PA-46 cruise sabout 25% faster on about twice the fuel flow.

Proponents of the Jetprop conversion say their firewall forward maintenance is a fraction of what it was on their piston engines before conversion and the TBO of the PT6-35 is 4000hrs with no HSI required. The piston engined PA-46 with either a Lycoming or Continental will typically require a mid time top overhaul and has a TBO of only 2000hrs.Of course the PT-6 overhaul wil be more than twice the cost of the piston overhaul.

Posted by: GREGORY WROCLAWSKI | September 29, 2009 11:07 AM    Report this comment

Will, on the basis of logic it is impossible to disprove a negative.

I made a statement. You countered with a negative disputing my statement.

I cannot disprove your negative statement on the basis of logic. However, you can use logic to disprove my statement, so go ahead. Let's see the proof.

The great thing about taking the negative position is you can always counter anything that is proposed. There are still people who will tell you that man will never go to the moon.

Posted by: Gordon Arnaut | September 29, 2009 11:18 AM    Report this comment

"Next time you go up in a Robinson helo make sure to tell them that drive belts won't work."

Drive belts work well for that application, but there's a big difference between an R22 whose rotor turns at +/- 530 rpm and the small ducted fan application you suggested that would run at several times that speed. I'd guess the belts would go flying off the pulleys.

Posted by: Will Alibrandi | September 29, 2009 11:19 AM    Report this comment

Gordon, your statements were your own opinions presented as facts. I called you out on this and you told me to back up my statements with numbers. The burden of proof falls on you, as you made the original statements, which suggest to me that you're not very familiar with the obstacles engine OEMs have to contend with when designing and building an engine for production, (both engineering-wise and marketing-wise). That's ultimately what drives their decisions.

Posted by: Will Alibrandi | September 29, 2009 11:33 AM    Report this comment

Gregory, the turbine engine is more efficient at altitude for the same reason as a piston engine. While the piston engine is leaned manually, the fuel controller on a turbine does essentially the same thing by reducing fuel flow. The PC12 burns 450pph at cruise setting @ 22k but we can get it down to 400pph by climbing to FL270, although this is partially due to the slight power reduction necessary to keep TIT within limits.

Posted by: Will Alibrandi | September 29, 2009 11:47 AM    Report this comment

Will, I'm guessing you're not an engineer.

The R44 uses belt drive too. That is a machine that weighs 2500 lb. That means the rotor must be making more than 2500 lb of thrust to lift that machine.

We are talking at most 500 lb of thrust per belt. The load is the important factor not the rpm.

Rotational speed is not an issue. Belt limits are based on translational speed, which depends on pulley diameter.

And yes I stand by my statements as fact until you can provide convincing arguments otherwise.

As for doing a feasibility study for you? I am too busy for that. I certainly invite you to go ahead and spend your time and do a real appraisal of this possiblity rather than blowing hot air on a chat room.

Posted by: Gordon Arnaut | September 29, 2009 12:00 PM    Report this comment

Gregory, the explanation Will gave is correct.

As the airplane climbs higher, the air density is lower, which means less power is required to propel the plane.

It is a nice coincidence of physics and thermodynamics that engine power due to the thinner air decreases somewhat less quickly than the drag on the airframe.

Hence, we can reduce fuel flow and fly faster on less power.

I have to disagree with your blanket statement that turboprops are less efficent than pistons. If you look at my comments on the SFE731 turbofan you will see that a Falcon jet with these two babies will cruise at an BSFC of 0.375.

That is well below even the leanest piston engine.

The reason is basic engine efficiency due to a high pressure ratio. The TPE331 like you said is more efficient than the Pratt because its pressure ratio is 11 or 12 compared to 6 or 7.

Posted by: Gordon Arnaut | September 29, 2009 12:13 PM    Report this comment

The piston engined PA-46 with either a Lycoming or Continental will typically require a mid time top overhaul and has a TBO of only 2000hrs.Of course the PT-6 overhaul wil be more than twice the cost of the piston overhaul.

Posted by: Paul Bertorelli | September 29, 2009 12:22 PM    Report this comment

Whoa, Paul.

How can you say this is not a technical problem? The problem is exactly technical. The small turboprop scraps we are thrown by the big engine boys are ancient technology and not fuel efficient.

More than 70 years ago the very first production turbine airplane took wing over Germany using an engine with a pressure ratio of about 5.

Seventy years later GA pilots are flying turboprops with an EPR of 6 or 7.

The big turbofans are now at over 50 to 1 and get twice the gas mileage of the best diesel car or truck.

The reason these dinosaur turboprops even work at all for the bigger GA planes is that their mission profile is to fly longer legs than the typical small private plane and they can thus climb higher and lower fuel burn, as we just discussed.

And then there is the question of why even bother with a prop? Even a King Air sounds like a boat compared to a jet.

The honest to god truth is that a small turbofan engine with a pressure ratio of 20 and a good sized fan or two can match or surpass piston fuel economy above say 10,000.

Any one of the engine makers could build that engine tomorrow and price it about the same as the small turboprops. They just don't want to. Heck they haven't even bothered to improve the '40s technology turboprops we ahve had for years.

Posted by: Gordon Arnaut | September 29, 2009 12:44 PM    Report this comment

Should'a, would'a, could'a...Come on guys n' gal, let's just wait 'till Tomahawks, and such, hit the war surplus market. Then we'll all have cheap burners to stick anywhere we like! The key word is "wait". Quiet, affordable high speed flight sounds GREAT! Buy a ticket.

Posted by: Larry Fries | September 29, 2009 1:03 PM    Report this comment

Gordon, you are correct - I am not an engineer but an aero turbine analyst that deals with the realities of the engine market. Engineers, I've found, are often too focused on their specialities to see the big picture. While the engine you describe is technically feasible, economically it is not for all the reasons already given. The fact that it is not in production should be proof of this. Your statement that speed is not a major consideration in purchasing a turbine aircraft simply shows that you do not understand the general aviation market. I am a commercial pilot & instructor and fly turboprops for a charter company. I am very familiar with this segment and am constantly in contact with reps from the major engine OEMs as part of my work. I'd love to see a turbine that burns 16gph on a GA aircraft while tooling along at 200kt. It just won't happen until there's an impetus for an engine mfr to do so.
As for your "factual" statements, Paul and I have already explained through argument as to why you are wrong. If you don't believe it, or just don't want to hear it, that's fine. No appraisal necesary, the market has spoken.

As far as your "blowing hot air" crack, you certainly have had enough time to post on this forum all day long - you'd posted no less than 11 times before I jumped in.

Posted by: Will Alibrandi | September 29, 2009 1:51 PM    Report this comment

>>Any one of the engine makers could build that engine tomorrow and price it about the same as the small turboprops. They just don't want to.

Posted by: Paul Bertorelli | September 29, 2009 1:54 PM    Report this comment

Will, I see you don't mind telling people bluntly that they are wrong. And then not backing that up with anything solid.

I am a commercial pilot with lots flight test experience for a major airframer.

I'm not interesting in lots of flapping about "markets" and other such speculation.

I've been around the industry long enough to know what side is up. If you want to go poo-pooing people's ideas by saying "well nobody has done it yet, so there must be a good reason" then that only makes me question your seriousness.

Posted by: Gordon Arnaut | September 29, 2009 2:08 PM    Report this comment

Paul, you have hit it on the head.

There is not much of a business there. But I believe there is a busines, small as it is.

It is not worth it obviously to the big engine guys, they have voted with their feet.

But I have been saying all along that there may be other smaller companies that could bre interested. Unfortunately many of these smaller companies have been swallowed up.

But if someone builds it, they will come. That's my point. I absolutely disagree with a statement that says "Nobody is is building it because they already know nobody will come."

Posted by: Gordon Arnaut | September 29, 2009 2:18 PM    Report this comment

Gordon, if you're not interested in "lots of flapping about markets" then this whole thing is just an exercise, because the market ultimately controls what gets built. Economics 101. I'm not poo-pooing your idea, I actually like it a lot (and have said so repeatedly) my point (and Paul's) is that there is a reason why this advanced engine has not been built. The engine OEMs can't make a business case for it. Peace.

Posted by: Will Alibrandi | September 29, 2009 2:18 PM    Report this comment

This thread has come a long way from Paul's original post, and it seems that there is consensus that VLJs are really more like mini-bizjets for which a $2.4M price is more in line with reality than Eclipse and the other players perhaps thought. And, the marketing impediments of high altitude operations, significant training, mentors, insurance, recurrency, etc., are such that it would be difficult to generate the requisite market size to make this class of aircraft profitable and sustainable even at the higher, more realistic, price.

I find the current discussion, though, on the feasibility of developing a modern, piston competitive, turbofan for the new and retrofit market of high performance single engine GA aircraft more intriguing. The fact that it hasn't been done before means only that..that it hasn't been done before! Same as cars and electric lights before their time.

What if those of us so interested formed a group to research and objectively examine the realistic feasibility, both technical and economic, of such a new powerplant? Might be a good article for Paul's magazine if nothing else!!

Posted by: CRAIG DOW | September 29, 2009 2:22 PM    Report this comment

Yep, there definitely was some thread creep here (sorry, G) It'd probably take a small company like Innodyn to build the kind of small, efficient turbofan Gordon has referred to. Then again, a small company might not have the resources to develop and certify such an engine without an application for it.

Posted by: Will Alibrandi | September 29, 2009 2:40 PM    Report this comment

Just as an aside here. Remember the V-jet? It was a proof-of-concept for what ultimately became the Eclipse. I wrote an article about it around about 1997 or so. Burt Rutan designed the V-jet and in the course of an interview, he said the coming VLJs would sell for well south of $1M and he thought the volume would get to 20,000 units in five years and be sustainable. The reason for this was a major shift in the economics of manufacturing due to widespread and accellerating CNC. NASA said this, Pratt said it. Williams said it..everyone did.

And they were all correct. What they appear to have gotten wrong was the magnitude effect. CNC wasn't efficient enough to ignite a buying frenzy due to low cost. It might yet get there. But it's not there yet.

Posted by: Paul Bertorelli | September 29, 2009 2:40 PM    Report this comment

Gordon and Will,

You obviously misunderstood my question. It is quite obvious why an airplane will go faster in the thinner air at higher altitudes on the same power. What I was asking is why is the power a turboprop engine produces on the SAME amount of fuel LESS at lower altitudes than at higher altitudes. The example I used was for the SAME POWER to the prop (334HP) the PT6 has a BSFC of .73 at 9000ft vs a BSFC of .61 PRODUCING THE SAME POWER at 27000ft. This has nothing to do with the the airframe.

Lets remove the airplane entirely from this discussion and say for sake of argument that the turbo prop engine is driving a generator. For the generator to produce the same amount of power at 9000 ft (e.g. the base of many western ski areas) the turbo prop engine will consume much more fuel than if we moved the engine/generator combination up to the top of Mt Everest and had the turboprop engine drive the generator to produce the same amount of output power.

Continued in next box

Posted by: GREGORY WROCLAWSKI | September 29, 2009 3:00 PM    Report this comment

Your comment about leaning a piston engine is oversimplied. A TURBOCHARGED piston engine can produce the same amount of power at 25K ft as it can at 10K ft. provided its below its critical altitude. It fuel consumption will be the same when producing the same power at 10K ft and at 25K ft. Of course the plane the engine is propelling will be going quite a bit faster at 25K ft. than it will at 10K ft. Although I have no turbine time I have about 1500 hours flying turbocharged airplanes. From my research on air density and it lapse rate of air density the theoretical speed gain is 1.15% per 1000 ft of altitude increase (compounded every 1000 ft)when power is constant.

Yes a normally aspirated engine will lose max power with altitude as soon as you move it above S.L. and most normally aspirated engines need to be leaned since manifold pressure is dropping due to ambient air pressure dropping. Some Continental engines have an aneroid on the fuel inection pump that does this automatically.

My Turbocharged Continental TSIO-550 due to its slope controller on its wastegate will maintain constant climb MP (35") from SL to about 20K ft and it fuel flow of course is constant. Same thing when I descend I usually keep normal cruise MAP and RPM from the flight levels to near airport patern altitude. The slope controller keeps MAP constant during the descent without touching the throttle and of course fuel flow is constant too without touching the mixture control.

Posted by: GREGORY WROCLAWSKI | September 29, 2009 3:19 PM    Report this comment

Craig, sign me up. I've been kicking this idea around for a while. The best bet is to partner or acquire an eastern bloc company.

Innodyn is not the solution, Will. Go talk to some RV guys and see what they tell you. This kind of project is way beyond the ken of a shade-tree operation.

I've got some feelers out now in Serbia, which had some pretty good homegrown turbine design and manufacturing capability. This is about the best bet now.

I can tell you having traveled extensively in the Soviet bloc and seen their planes and their people, they are top notch.

People don't realize the aerospace industry is political. It is about protecting turf.

Look at the war between Airbus and Boeing. It is fought at the highest political levels. That's why the Russian stuff is kept out.

There is some pretty fine eastern bloc technology and people that could do something like this. That's about the best bet as I can see.

Posted by: Gordon Arnaut | September 29, 2009 4:00 PM    Report this comment

Gregory, I did not say that an airplane will go faster in thinner air on the SAME power.

I said it will go faster in thinner air on LESS power.

Anyway, I see what you are referring to now. There is a slight decrease in BSFC as you fly higher in a turboprop. At least until you reach the tropopause.

The reason is that the power lapse is not proportional to the density decrease -- due to decreasing temperature.

The decreasing temp gives an increase in thermal efficiency and the BSFC decreases. It is the same for turbofan engines only we measure SFC in thrust not power.

Posted by: Gordon Arnaut | September 29, 2009 4:13 PM    Report this comment

Gregory, for a given fuel flow a turboprop will make more power at lower altitudes than at higher altitudes. The example you gave of the two BSFCs values shows a lower fuel burn at FL270. This is due to the fuel controller unit taking the ambient pressure and temp inputs and adjusting the fuel metering accordingly. Down low, the engine is limited by torque, and up high it's limited by max ITT (intra-turbine temp). On takeoff (assumed near sea level) you can't just firewall the PT6A's power lever as the engine will over-torque and overstress the reduction gearbox. The PC12 has a torque limiter that will automatically reduce power, but the Caravan doesn't. At altitude, firewalling the power will cause the ITT to run in the red, and that's not healthy for the turbine section. So, your generator example will burn less fuel at 27,000 feet because the fuel controller senses the lower pressure (thinner air) and adjusts the fuel supply to the engine.

Posted by: Will Alibrandi | September 29, 2009 7:04 PM    Report this comment

I've read about the problems Innodyn has run into. They're way behind on their development and were supposed to have their engine certified & flying years ago. If increasing the efficiency is a matter of raising a turbine's EPR couldn't Pratt Canada just add a stage or two to the compressor on the PT6A and adjust fuel flows accordingly? I realize that's probably oversimplified, but isn't that essentially what needs to be accomplished?

Posted by: Will Alibrandi | September 29, 2009 7:11 PM    Report this comment

Will, having more compressor stages would raise the pressure ratio which is what is needed.

But it is not quite so simple. You can't just add more stages without redesigning the entire compressor section.

And then you will have to make changes to the turbine section (LP) that is driving that compressor because it will take more power, so you may have to add a turbine stage.

It's not a huge deal. This kind of work would be considered an update not a redesign. But an engine guy will tell you it is not exactly chicken scratch either.

Again it boils down to commitment. The big engine guys don`t need to bother with this little stuff.

It was mentioned also that surplus cruise missile turbofans could be used for small planes. This is not going to work because these are designed for high-subsonic snd transonic flight.

Their fans are way too small for low and slow. Another problem is that they are not man-rated engines. They are designed to make one flight so their durability for repeated flights is not going to be very good.

Posted by: Gordon Arnaut | September 30, 2009 5:14 AM    Report this comment

Interesting that P&WC hasn't addressed this, seeing as their PT6A is the dominant GA turboprop engine. (It's good to be the king) The difference between the PT6A small, medium and large is airflow and # of turbine stages. They're developing FADEC in-house for the PT6A which should help a bit, but I've read the PW600 may eventually supply the core for a new TP derivative like it did for the PW210S turboshaft. Then again the PW600 only has two compressor stages and two turbine stages, so that might not be such a big improvement(?)

Posted by: Will Alibrandi | September 30, 2009 8:02 AM    Report this comment

Will, if you want to see what a modern turboprop looks like have a look at the GE CT7. 1700 shp, PR of 17, SFC just over 0.4 in cruise (0.47 T/O).

This engine is used on the new Sukhoi twin-boom STOL light transport now in production. Nice plane.

Why should Pratt update the PT6 when they are selling all they can make now? New engines are driven by new airframe designs.

Since no one is building anything new of any significance in GA light turboprop, no new engines will come along.

Anyway even if Pratt did modernize the PT6, it's not going to be a game changer for the high end single. It's still a prop plane.

Only a turbofan can be a game changer on the high end single. There is a world of difference.

Posted by: Gordon Arnaut | September 30, 2009 10:23 AM    Report this comment

Pratt Canada is updating the PT6A with FADEC, but I doubt it'll be much more than an electronic nanny, keeping temps and torque within safe ranges. Maybe it wouldn't be a game changer as there's only so much that can be done with a prop, but a significant improvement in fuel burn for existing applications would be nice.

Posted by: Will Alibrandi | September 30, 2009 11:40 AM    Report this comment

>>Only a turbofan can be a game changer on the high end single.

Posted by: Paul Bertorelli | September 30, 2009 1:35 PM    Report this comment

Paul, I'm all for learning lessons.

Unfortunately I see the wrong lessons being learned here.

Raburn thought you could build a mini-bizjet for under a million bucks and he was wrong. That's a no-brainer really. Why? because it is dumb to think that thousands of new bizjet pilots will appear out of thin air.

Most of the people who can afford a bizjet prefer to be chauffered around in it, not fly it themselves. Even most ofaverage Skylane and Bonanza owner who lusts after a jet

What he should have done was go into the engine business.

Posted by: Gordon Arnaut | September 30, 2009 2:16 PM    Report this comment

Sorry, that message is not complete. Keystroke mishap.

What I meant to say is that very few current owners of singles are able to step up to that much higher level of commitment -- both financial and skill.

What many of these owners would like and could handle is a turbofan airplane that fits in the high-end single category. The Diamond and Cirrus do not fit the bill.

With a piston replacement turbofan available, it would be different.

And don't tell me the engine makers see that there is no "business case" here. Of course there isn't. Not for these large players.

Is there a business case for Donald Trump to make money building strip malls in Iowa? He could make a million bucks easy. So why doesn't he do it?

My point is that one man's "there is no business case here" is another man's opportunity.

And no I have no interest in going into any kind of business, aviation or otherwise. I just think it is too bad we have to pay $80k for a piston engine when $100 k will get you a turbofan.

Posted by: Gordon Arnaut | September 30, 2009 2:33 PM    Report this comment

I'm not really sure what would constitute a game-changer in GA at this point. However, I think a new turbofan could have applications beyond aviation which would might be substantial. Such as marine, power generation, or even automotive. Much like Garmin navigators cover a broad spectrum of users, not just aviation. For s/low GA, I think the main benefits would be higher TBO, improved reliability, less dependence upon avgas, reduced noise, and a modest boost in overall aircraft performance, all for a cost comparable to our current piston choices. While perhaps not a game-changer, still a good leap forward.

Posted by: CRAIG DOW | September 30, 2009 2:58 PM    Report this comment

There's a test case for the upstart enginemaker already underway and that's the Austro diesel. It's not a turbofan, but if what you say is true, the business case may be similar. It's far from a sure deal, but looks promising.

Eclipse, Adam, Safire and some half dozen others launched into the VLJ market because Cessna, Gulfstream and Bombardier saw no market there. Eclipse was gonna show how it's done...

I wouldn't say it can't be done. Same with an upstart engine company. But it will take good market timing, shrewd technical acumen and exceptional management to make it work. The fact that these so rarely come together explains why so many plainly obvious gamechangers end up as smoking holes.

Strategic partnerships may be the way to go. A clean sheet small turbofan would cost at least $200M I'd guess to bring to market. Probably more. (The Austro was $60M and it's based on an existing engine.)

You're gonna need to sell a *hell* of a lot of $100,000 engines to pencil out enough ROI to keep investors from laughing you out of the board room.

Posted by: Paul Bertorelli | September 30, 2009 5:05 PM    Report this comment

At that price point it'd have to come from Russia, China or India (or some other place with low production costs). Maybe it wouldn't have to be a clean-sheet design? Just like the Walter 601 was similar to the PT6A maybe the new engine could use the architecture from an existing engine and make the necessary changes to boost efficiency. That's pretty much what Klimov did when they reverse-engineered the VK-1 from the Rolls-Royce Nene turbojet.

Posted by: Will Alibrandi | September 30, 2009 6:34 PM    Report this comment

I don't think a diesel prop can be compared to a turbofan.

I'd be interested to know the EPR and thrust cruise on the PW600 and FJ33. They're not publishing those specs as far as I can find. The takeoff TSFC is about 0.5, but this increases quite a bit at cruise on a turbofan. (Takeoff TSFC on a good large turbofan is under 0.3)

The Honda engine which is about the same size -- and has the same takeoff TSFC of about 0.5 -- has a cruise TSFC of 0.75. on cruise thrust of 420 lb. Converting thrust to hp that works out to an equivalent BSFC of just under 0.5, assuming a cruise of 400 knots and a propulsive efficiency of 0.8.

That's not bad for this kind of fast and high flying, but if down low and slow it's going to burn half again as much fuel.

I would guess the EPR on this engine (and the small Pratt and Williams is in the low teens at best. This is a very simple engine with few compressor stages and no fancy stuff (like single-crystal turbine blades).

That's why Pratt said cheap and efficient aren't on the table. This kind of engine is an afterthought for them. Their R&D money goes on the big stuff. that's where the money is.

But even this kind of fairly low compression engine could work if it had a good size geared fan. It wouldn't match the efficiency of a piston, but it would come close to a turboprop.

But really what's needed is something else altogether. A good high-compression engine.


Posted by: Gordon Arnaut | September 30, 2009 8:19 PM    Report this comment

A piston replacement turbofan will need to match piston SFC in order to keep range and payload without increasing fuel load.

Two hundred million to develop an engine like this? Maybe that's what Pratt will tell you. I know it would take a roomful of good engineers and a factory that knows how to build turbines.

China and India? Forget it. They don't have the engineering or manufacturing base. All their capable people and plant are going full bore for military and transport.

Even Russia's excess capacity is quickly disappearing. Their aerospace industry is reviving and a lot of the plants are humming.

It is pointless to speculate about dollars if basic assumptions do not make sense. My guess is that something like $20 million would be a pretty solid foundation IF you have the right people and plant. Especially if the host government were to match funds.

Posted by: Gordon Arnaut | September 30, 2009 9:10 PM    Report this comment

Just for reference, GE paid $70 million for Walter.

Posted by: Gordon Arnaut | September 30, 2009 9:23 PM    Report this comment

Oh yeah, and then the Czech government threw GE a few million in research grants.

Posted by: Gordon Arnaut | September 30, 2009 9:54 PM    Report this comment

As you can see fromm the graph on page 8 the cruise fuel flow at the max cruise is of 315kts at 25K feet is 500lbs/hr or 74gals/hr.

If the cruise TSFC number Williams told me is accurate then the engine is producing a little over 700lbs of thrust at 315kts.

Lets convert that to equivalent HP. We know that 1 HP is defined as 550lbs-ft/sec. And 315 kts is 532 ft/sec. So that means that the HP equivalent is (532 x 700)/550 = 677HP. Lets assume a shaft engine (either turboprop or piston) would be used to drive a propeller with an efficiency of 85% and that means that turboprop or piston engine would need to produce a propeller flange HP of nearly 800HP.

So the euqivalent BSFC would be 500lbs/hr/800HP or about .62. That is nearly identical to what a large PT-6 gets for BSFC at altitude. It is amazing how the numbers come together.

What is troubling is that at the same speed and altitude a TBM-850 is using about 70 lbs/hr LESS fuel. The TBM is a much larger airplane and weighs about 2000lbs more. SO why is the D-Jet so inefficient. One reason which I mentioned earlier is the width and height of the cabin. Although much shorter than a TBM the D-Jet cabin is about 9 inches wider and 7 inches taller.

That large of a cabin is one reason detriment to the D-Jet compared to the TBM. But it still cannot account for all of the in-efficiency difference.

Posted by: GREGORY WROCLAWSKI | September 30, 2009 10:27 PM    Report this comment

So if you wanted to use a very efficient 800HP piston to propel the D-Jet you would need an engine that weighs upwards of 1200lbs compared to the svelte 350lbs the FJ-33 weighs. Much of that weight difference can be saved by the reduced wieght of the fuel needed. But, the size of the piston engine would dwarf the FJ-33 and the added size plus added cooling drag would be another hit to a piston implimentation.

So Gordon, how could you package a turbofan with a ~35" diameter fan into a single engine fuselage. I think it would result in a very unusual looking airplane.

Posted by: GREGORY WROCLAWSKI | September 30, 2009 10:37 PM    Report this comment

Gregory, the dynamic pressure at 315 kt and FL250 is 150 lb/ft^2.

If we take Williams at their word and the engine is making 700 lb of thrust, then we can divide the thrust by the dynamic pressure and get the drag area of the airplane, which would be about 4.7 ft^2.

That's not bad. The reason a TBM is burning less fuel is because its drag area is about 4 ft^2 if you run the numbers. A Meridian will be about 5.

I would say the Williams numbers are legit. A drag area of 4.55 for a plane the size of the D-jet is not bad.

Actually a cruise SFC of 0.7 is not so bad. This is close to the TFE731 which I quoted earlier. If you take that plane up to FL400 it will go 400 knots and at that point its equivalent BSFC would be well under 0.5.

I would say Williams has improved the engine in the latest versions. Based on this I would say its compression is probably closer to 20.

But now we can see how the design of this engine does not fit well with its mission profile.


Posted by: Gordon Arnaut | September 30, 2009 10:51 PM    Report this comment

Thanks again – I don’t mind you using a few postings to explain this. I thought that turbines had better thermodynamic efficiency so your comments about the possibility of scaling down turbines fits in with that. When the FJ22 was around I wondered about using it or something like it as the basis for a small turbo prop. I also thought it would suit a two seat tandem personal jet-style aircraft.
I agree with Craig from up further – I reckon a lot of people would chip in a few bucks to quietly beaver away at something like this. I would.
I reckon that a pretty major “grass-roots” improvement in the relevance of GA needs to co-occur with some visionary and risky efforts before a serious market could exist to create the scale really needed for this type of development. Most people see GA aircraft as toys for the rich, I believe. I reckon that there would have to be concerted push to get kids and other interested people up flying, at least to so they have cross the imaginary boundary and “been there, done that”. Here in Oz most aircraft sit around most of the time just getting old. Guess that’s a whole other topic though.
This is an excellent forum, by the way – thanks to avweb/Paul etc!

Posted by: John Hogan | September 30, 2009 11:08 PM    Report this comment

This is an engine that will do well at FL400. Here it will be as efficient as a piston.

You will still be burning a lot of fuel per hour, but you are going a long distance in that hour.

But if the ceiling is FL250, this plane is not very fuel efficient. That is why I said this airplane has the wrong engine.

Flying at only 315 kt, the propulsive efficiency would increase quite a bit with a bigger streamtube (bigger fan)

If we put two good size fans on either side and drove them both from this same Williams engine (with a shaft on it) we could probably reduce fuel flows by 10 percent at least because this is how much propulsive efficiency we would gain.

Btw, just to explain the math for the non-techs, thrust equals drag in straight and level unaccelerated flight, so if our thrust is 700 lb, it means the drag of the airframe against the air at that speed will be 700 lb.

If we divide the plane's drag area (also called the equivalent flat plate area) by its wing area we will get the airplane's coefficient of drag. I would guess wing area is under 200 ft^2, so CD would be 0.023.

That's pretty good. Diamond did a pretty good job. Too bad they don't have an optimal engine.

Posted by: Gordon Arnaut | September 30, 2009 11:13 PM    Report this comment

Gregory, just a thought but you could get close to that power from a turbocharged 4-rotor rotary like Mazda used to win Le Mans. That engine put out just under 700 hp.

I think that engine weighed under 400 lb.

Posted by: Gordon Arnaut | September 30, 2009 11:33 PM    Report this comment

I forgot to mention that Williams' number are probably for the gnine only. Installed performance is going to be less, by anywhere from 3 to 5 percent usually for transports and bizjets.

For this plane it could be even more because that bifurcated inlet is changing the direction of the flow and taking away efficiency.

Posted by: Gordon Arnaut | October 1, 2009 12:04 AM    Report this comment

The VLJ's in concept that I have seen are nothing short of lip-licking sexy. If scaled down to LSA configured specs the market would explode. I think still hold that the dream of .8MACH at FL40 could be left for a few more years. There are many Dudes out there with "Big Bucks" that buy jet powered speedboats and yachts with helicopter pads. As a business you only have to sell one. PLEASE, let me dream but do not mix me up with the care and feeding of a super model.

Posted by: Larry Fries | October 1, 2009 1:22 AM    Report this comment

think still? I get too excited to write right.

Posted by: Larry Fries | October 1, 2009 1:25 AM    Report this comment

Gregory, just to get back to your original question about why turbine BSFC is better at altitude.

The TBM pt6 puts out 1825 shp at sl. If we just do a straight conversion based on air density, its max hp at 26,000 ft will be 650 hp. However, the fuel burn and SFC numbers tell us the engine is burning 430 lb at a rate of 0.6 lb/hp*hr, so that means the actual power of the engine is more like 700 hp.

Like I said, the power lapse is not as great as the density lapse. The lower temps at altitude mean better thermal efficiency because compressing air causes heat. If we start with colder air we lose less heat and more goes to doing work.

This applies equally to any internal combustion engine, piston, turboprop or turbofan.

Posted by: Gordon Arnaut | October 1, 2009 6:45 AM    Report this comment

Just to tie up a couple of loose ends. That TBM cruising at 320 kt at FL260, burning 430 lb will be making

430 lb fuel / 0.6 sfc = 700 shp.

700 * 0.85 prop eff = 600 thrust horsepower

600 * 550 / 540 fps airspeed = 600 lb of thrust. Compared to your D-jet cruise thrust of 700 lb.

In summary your FJ33 is a more powerful engine than the pt6. The airframe is a little more draggy because it is wider and taller. The extra wetted area on the TBM does not factor as big, Neither does the weight because weight is supported by lift and lift creates induced drag, but at high speed the induced drag is a very small component of total drag. Most is parasite.

Btw, your FJ33 engine making 800 hp at cruise is going to be making over 2000 hp at sea level.

From 350 lb weight that is pretty impressive.

Posted by: Gordon Arnaut | October 1, 2009 6:56 AM    Report this comment

lw, I agree. A 2-seater with a quiet fan engine (could be turbine, could be rotary) would ignite the general public`s interest in flying. Especially if you can give them that plane for $50,000. Which is what a small plane cost in the postwar boom years of personal aviation, if we adjust for inflation in today`s dollars.

It was possible then, why not now?

The fact of the matter is that any brand new airplane that you buy is a terrible value proposition based on value add. A half million dollar Cirrus has far less value add than a $100 k mercedes.

It is a very simple machine with very little sophistication if we take away the comm and nav equipment. The aerodynamics and engine are 1930s technology.

A ten million Falcon is actually a far better proposition based on value add. Here you are actually getting a lot for your money. Very sophisticated aerodynamics, propulsion and avionics. The manufacturing is very sophisticated and there is a lot of value add.

If you saw how a single-crystal turbine blade is manufactured you would realize how much human effort and ingenuity this takes.

A half million dollar personal airplane is basically a fiberglass boat with simple wings.

Posted by: Gordon Arnaut | October 1, 2009 7:11 AM    Report this comment


I've seen kit airplanes where individuals built them with mazda rotories. The problem with them and any automobile derived engine is that aircraft engines are supposed to last thousands of hours developing a high percentage of their max HP. In automobiles the engines are developing a very small percebtage of their max power most of the time.

The Lemans winnng Mazda rotary only had to last 24hours.

The other issue with rotaries or using small automobile based engines is that they turn too fast to drive a prop directly. They need a gearbox negating a good portion of their weight advantage. A gearbox saps 3-5% of the efficiency.

Another rotary specific issue is that compared to a piston a rotary has a very large surface to combustion volume ratio wwhich results in flame quencing and reduced thermodynamic effciency.

Any tiny automobile based internal combustion engine needs some seriously large turbochargers associated plumbing (intercoolers)to maintain power at altitude.

That is one of the problems with the Thielert/Austro diesels. Being only 120cu-in they have relatively heavy gearboxes and even with relatively large turbocharger and intercooler (adding cooling drag) their critical altitude is much lower than the large slow turning aviation specific turbocharged engines from Continental and Lycoming.

Both the tiny 120 cu-in Theilert and Austro diesels are heavier firewall forward than a 50 year old 360 cu -in Lycoming developing more HP.

Posted by: GREGORY WROCLAWSKI | October 1, 2009 7:26 AM    Report this comment

My next point is that Christian Dries (the owner of Diamond)is a pretty savvy businessman. He must have seen the Thielert train wreck coming 3 years ago and started development of his own diesel engine. He needed his won engine to keep the brisk selling DA-42 production line going.

If it cost him 64 mil to develop, certify and build a factory to build the engine I doubt it could have been done much cheaper.

And this is a engine based on an existing mature Mercedes automotive design. Many of the major componens are source to Austro by Mercedes reducing much development and manufacturing startup costs.

With all due respect, At 200mil Pauls SWAG of the cost to develop a clean sheet turbo fan may be high but I think he is closer to the right number than your estimate.

Posted by: GREGORY WROCLAWSKI | October 1, 2009 7:31 AM    Report this comment

Gordon, I wouldn't write off China so quickly. They're making strides to develop their indigenous aerospace capability. Besides AVIC's ARJ21, COMAC is working on the C919 narrowbody, which initially will have western engines but eventually will fly with engines built in China (maybe under license?). At any rate it won't be long before they have the capacity to engineer and produce engines - how well they hold up is another story. I agree about India though, they have a long history of engine and aircraft programs that remain in development for decades. I'd think Russia has the best potential for producting a next-gen efficient GA turbine.

Posted by: Will Alibrandi | October 1, 2009 8:41 AM    Report this comment

Gregory, the rotary is not as good a turbine. No argument there. But it is much better than a piston.

The surface area to volume issue is not that big of a deal. The new Mazda design addresses this with a bigger displacement and longer "stroke."

For a small cheap plane (2-seater) a rotary ducted fan would be more practical than a turbine.

Nasa did a study with industry partners back in the `70s on just his idea. Do a keyword search on "propfan" and you will find the report.

The engine-fan units were conceived as direct replacements for piston twins and even with the bigger ducts the sketches look very nice.

There is a high wing single engine with the engine-duct mounted under the tailcone. Looks pretty good, even with a 2.5 ft duct.


Posted by: Gordon Arnaut | October 1, 2009 8:51 AM    Report this comment

Gerry Merrill`s idea is to develop a clean-sheet, high-efficiency small turbofan as a piston replacement.

My idea is to update an existing small turboshaft engine and drive dual fans from a fuse-mounted engine.

There have been something like 50,000 of the small Isotov`s built. Russia built more small helicopters than the West during the cold war decades.

That infrastructure is still there. What is needed to make a twin-fan engine a reality is to update this engine to a compression of 15 to 20, like the GE CT7.

This would not be a whole redesign. A redesigned axial compressor section and LP turbine section.

Designing the belt drive and ducts is not much. The belts cost a little power loss, but it`s less than 5 percent, about the same as gears on a gear driven prop or fan.

The nice thing about this configuration is it would not require clean-sheet airframes. Existing fiberglass planes could be redesigned with wing moved back a bit for cg and tailcone adapted for mounting propulsion unit. Not that big of a deal.

If you look at Gerry`s idea, you need V-tail airplanes. Plus with two fans, they can be smaller and still have the required disk area.

Two fans of 27 in will equal the disk area of a 36 in single fan.

Posted by: Gordon Arnaut | October 1, 2009 9:01 AM    Report this comment

The good part about my plan is that the same updated small turboshafts will have a ready market for small helos.

Do you think Frank Robinson would not be interested in an engine for his new turbine model with better fuel specifics than what is currently available?

The numbers look a lot better when we can use the same engine in more places.

Posted by: Gordon Arnaut | October 1, 2009 9:05 AM    Report this comment

Will, I`m not writing off China. The problem with China is they are at full employment. They are funnelling each and every engineering grad they can chhurn out into military and transport.

They are very ambitious like you said. They want to go to the moon. There may be a possibility there, but it is not a ready made thing like Russia.

Posted by: Gordon Arnaut | October 1, 2009 9:08 AM    Report this comment

Looking at those fuel flow numbers for the D-jet, we see that slowing down to 220 knots gives a flow of 280 lb/hr, about 40 ga/hr.

The ram pressure (dynamic pressure) at 220 is about half that at 315 kt., so thrust required will be about half too. We find our thrust by multiplying the plane's drag area of 4.7 by the dynamic pressure 73.4 lb/ft^2 * 4.7 ft^2 = 350 lb thrust.

Converting to hp this gives 350 lb * 371 fps airspeed / 550 = 240 thp. Assuming prop efficiency of 0.85, we divide 240 hp by 0.85 and get 280 shp.

This gives an equivalent BSFC of 1. One hp an hour for each pound of fuel burned.

This is why I say it is the wrong engine. You are burning 40 gph to go 250 mph, which you can do in a turbo single on less than half the fuel.

Plus you have to fly at FL250 and burn all the fuel required to climb to that altitude. For private pilots that make short hops this will get old fast.

Good luck D-jet.

Posted by: Gordon Arnaut | October 1, 2009 10:30 AM    Report this comment

Gregory, can you get a copy of hte drag polar from Diamond?

This will give us the drag data to verify Williams' numbers. They've been known to fudge a little here and there. I still think the bigger cabin is making a little difference, but it would be nice to know for sure.

Posted by: Gordon Arnaut | October 1, 2009 11:36 AM    Report this comment

Just to sum up, we see that the Djet would be a very good airplane at FL400 and 400 knots, where its fuel economy would be as good or better than a piston.

But even at FL250 this plane is at the bottom edge of what I would consider its design envelope, and that's only if you go max cruise. If you slow down the fuel specifics go down the drain.

And if you go lower than FL250, well that gets pretty ugly I'm sure. Bottom line is this plane is a huge compromise. It would have made more sense just to make it a 400 knot, FL400 airplane. At least with that engine.

Posted by: Gordon Arnaut | October 1, 2009 11:40 AM    Report this comment

Just a little more technical stuff here. If there is one problem with turbine engines is that they are not as flexible. They have a pretty narrow power range, compared to a piston.

That is why the fuel economy plummets as we slow this engine down. It is designed to ruun well at a particular design point.

The reason is grasped if we think of a turbine engine as consisting of a whole bunch of little propellers turning in there. That's basically what it is. The conmpressors are little props (fans technically) with little airfoils and so are the turbines.

Now we know that fixed pitch prop is not going to work well on a plane that has to fly fast. The reason is that the angle of the blades will not be optimum at all speeds from takeoff to cruise.

This is because a rotary wing's angle of attack, changes its angle of attack as the aircraft airspeed changes. That's why we use variable pitch props to get good performance.

A turbine engine would give good performance and different speeds too i we could change the angles of all those little airfoils in there. Unfortunately that is not really technically feasible.

On the big engines we have movable stator vanes that redirect the angle of airflow and accomplish the same thing as movable blades. But this will not be seen on a small engine.


Posted by: Gordon Arnaut | October 1, 2009 11:48 AM    Report this comment

So when we want to design a small jet plane we have to keep in mind that the engine is not going to be as flexible.

This is not really a problem though if we have an engine that is the right size. In that Djet we see that the airframe makes 350 lb of drag to cruise at 250 mph. So it only takes 350 lb of thrust.

So what we really need is an engine whose maximum cruise thrust is about 350 lb, not 700 lb.

Unfortunately there is a gotcha with this plan too. If we were to put a small engine with only 350 lb max cruise thrust, this plane would need a 2 mile takeoff roll. (Exaggerating just a little)

The reason is that the small engine has half the power of the Fj33 so its takeoff thrust with thhe same size fan will be about half too.


Posted by: Gordon Arnaut | October 1, 2009 11:52 AM    Report this comment

So what we need is a bigger fan because that will give us a lot more thrust on the same power.

We do not need a huge fan. One thing to remember here is that takeoff performance is very sensitive to weight. And it actually scales down in our favor.

If we cut our thrust in half we can actually get the same takeoff performance as before if we reduce our weight by ONE THIRD.

So if we have a Djet that weighs 4500 lb and has a 1500 lb thrust engine, we can get the same takeoff performance with 750 lb thrust if we lower weight to 3000 lb.

Since we have to carry less fuel for the smaller engine this will work.

All of this is to say that the personal jet idea is very nice but the righht engine is not there for it. It is not hard to do the math and come up with what is needed.

Posted by: Gordon Arnaut | October 1, 2009 11:58 AM    Report this comment

Just to wrap up this technical outline. If we want a turbofan airplane that has a max cruise of 250 mph with good economy, we need only about 300 lb of thrust in cruise.

A plane like the Columbia (Cessna) has a drag area of about 3.5 square feet. (Just for comparison a Bonanza with retracts is only a bit better at 3.3; a Skylane is about 5).

If we want to fly at a celing of 20,000 which is realistic for an unpressurized plane, the ram pressure at 250 mph will be just under 90 lb/ft^2, so 300 lb of cruise thrust will get us there.

That's going to require engine power of about 250 hp, if we assume a propulsive efficiency of 0.8. That's at 20,000 feet. The sea level horsepower would need to be about double that because a turbine engine is a natrually aspirated engine that loses power with altitude.

The Isotov engine makes about 400 to 500 hp in its crrent low compression state. Even in its present form it has enough power. Updating this engine with higher compression would give us easily enough power without working the engine very hard.

Now what about fan size? For a 3000 lb airplane, 1000 lb of thrust would give a weight to thrust ratio of 3 and a very brisk takeoff and climb.

Two fans of just 22 in diameter would give about 1000 lb of static thrust, without counting the engine exhaust thrust, which would add at least another 100 lb. maybe more.

The Fj33 fan is only 17 in diameter. With the same 500 hp we would have about half the thrust.

Posted by: Gordon Arnaut | October 2, 2009 3:29 AM    Report this comment

There is already a good high-compression small turboshaft engine, the LHTEC T800 engine that was developed for the military Comacnhe attack helo. The helo program was canceled but the engine is used in other light helos.

It is about the size as the small pt6, 315 lb, 30 in long. Compression is 15 and SFC is 0.45. Max power is close to 1500 hp at sl.

This is bigger than the small Allison and Isotov, which are the right engine for a 3000 lb 4-seater.

This size engine would be good for a slightly bigger plane.

So just to sum everything up here, we see that the Djet, while a fine airplane, is not economical or practical for the typical single pilot.

Flown at max cruise of 315 knots and design altitide of FL250 Djet can match the economy of a pt6 turboprop, which is not bad. But if you slow down or go lower, the fuel numbers go through the roof.

It's not a very flexible airplane and that's what private owners will not like about it. It could well be successful as a low-end charter plane because it is still a lot more gas thrifty than the present alternatives, all of which are twin-jets.

If you can offer a ride to business people for the same cost as a hop in a King Air, people will choose the jet every time.

Posted by: Gordon Arnaut | October 2, 2009 3:42 AM    Report this comment

What is the market really for the D-Jet or the Cirrus Jet or the Piper Jet if they ever become reality.

For Part 135 ops like you suggest the problem is the D-Jet has a poor range payload tradeoff compared to the King Air line. So your market will be limited to 2 people with a decent amount of luggage for a avg 500nm trip.

With a full fuel payload of only 500 lbs in the D-Jet, three FAA size adults(that's two paying customers and a pilot with no luggage) will put you over the 500lbs mark so you will always be flying with reduced fuel unless you have got only one passenger.

The other issue is how will the paying traveler perceive the safety of a single, even if it is a jet, to a twin with props.

Posted by: GREGORY WROCLAWSKI | October 2, 2009 8:29 AM    Report this comment

In the Northeast where I do most of my flying there is a fractional operator (Plane Sense) which has PC-12s as there only airplane. They currently have 38 of them in their fleet and I see them at most of the major GA airports I frequent in the Northeast. They seem to have plenty of business and family travelers. So I think that there is a large enough portion of the population that is not resistant to single engine aircraft.

My wife thinks that the sheer size of the PC-12 placates people enough to overcome the apprehension of single fan up front.

Being an MMOPA (Malibu Mirage Owners and Pilots Assoc) member I see that a majority of members are small business owners and use their aircraft for business a large percentage of the time. I see members transition through a Malibu/Mirage to a turbine. Many moving to a Meridian or Jetprop but also quite a few move to a TBM or PC-12. Some have even transitioned to Eclipse or Mustage or even a CJ1. The CJ1 being about as high up the food chain you can go and still be owner flown.

I think the market for the D-Jet will be mostly the owner flown bussines folks who could afford a Meridian or even a TBM but don't need the payload of the TBM. The Meridian range payload is very similar to the D-Jet. The TBM is far better.

continued ....

Posted by: GREGORY WROCLAWSKI | October 2, 2009 8:33 AM    Report this comment

The DOC of the D-Jet will be quite a bit higher than the Meridian but very close to the TBM.

With a Sea Level TSFC of .45 and 1700lbs of T.O. thrust the Fuel burn at T.O and intial climb will be over 110gal/hr for the D-Jet.

I am told that the bottom line TAP program from Williams will be $90-100/hr for the engine. Figure a block fuel burn of 70gal/hr and just those two items come to about $400/hr at today's fuel prices. Add in another $100/hr for maintenance once the two year warranty is up and you are at $500/hr DOC.

That is palatable to many of the MMOPA members I rub elebows with at their annual conventions.

Unfortunately the market for this type of use is quite limited. Diamond has priced thier D-Jet for a 100 airplane a year market. A lot less than the Eclipse pricing model which assummed a 1000 airplane per year.

Only time will tell if the cachet of flying a jet will attract enough owner operators that would have gone the turboprop route.

Posted by: GREGORY WROCLAWSKI | October 2, 2009 8:33 AM    Report this comment

Which engine are you referring to? The only one I've found that comes close to your specs is the Klimov (Isotov) GTD-350 turboshaft.

Posted by: Will Alibrandi | October 2, 2009 9:41 AM    Report this comment

Will, that's the one. You'll notice it has a compression ratio of just 6. But it has 7 axial compressor stages and one centrifugal compressor.

The significant thing here is that in order to update the compressor, you do not actually need to add any stages. Seven stages is a lot, that's probably less than the GE CT7 has, which has a compression of close to 20.

Btw for the non-techs, an axial compressor stage is just a wheel with little airfoils on it -- the little props I mentioned previously.

With 7 stages each one is compressing at a rate of just over 1.1. That's very early technology. Today's axial compressors are getting 1.5 per stage. That is all a result of better aerodynamics -- better airfoils.

We don't need to go that high, but you can see that making this engine a 15 compression ratio engine would not mean rearchitecting it. You could basically just design and build new compressor and turbine wheels and bolt them right in.

Btw, you can buuy an airworthy GTD350 for $5000. You'll see them in tractor pulls.

Posted by: Gordon Arnaut | October 2, 2009 12:24 PM    Report this comment

Btw, that LHTEC engine I mentioned has compression of 15 using two centrifugal compressors and no axial stages at all.

A centrifugal compressor is what you have on a turbocharger and it gives a compression of about 3 to 4. So if you have two in series you get compression of 16. The nice thing about this is they are much smaller and lighter than an axial.

But you cannot do much to adjust the airflow angles in a centrifugal pump, like you can in an axial. So they are more prone to compressor stall.

That's one of the reasons these simple engines, like the Isotov, use such low stage ratios. They wanted this engine to be flexible. Once you start getting into the high ratios the efficiency band of the engine becomes narrower.

But a compression of 15 on the Isotov should be quite easy to achieve, along with a nice wide operating envelope. Compressor aerodynamics has advanced quite a bit since the 1940s.

Posted by: Gordon Arnaut | October 2, 2009 12:34 PM    Report this comment

Yep, just looked it up. The CT7 has 5 axial stages and one centrifugal.

Posted by: Gordon Arnaut | October 2, 2009 1:03 PM    Report this comment

In my earlier comment I meant to say the GTD350 has more axial stages than the CT7. And it does.

With the 7 existing wheels you could push the compression past 20 and not really redesign anything. Just bolt in new wheels.

Posted by: Gordon Arnaut | October 2, 2009 1:06 PM    Report this comment

In searching for info about the GTD-350, I found a quote from Dave Sutton of Red Star Aviation in the rec.aviation.homebuilt newgroup. He mentioned the GTD-350 and the Walter 601B as potential powerplants for hi-perf homebuilts. He even suggested an unspecified Russian APU that gives 400lb thrust @ takeoff used to power an electic motor turning a prop. A turboprop hybrid? That's different...

Posted by: Will Alibrandi | October 2, 2009 1:24 PM    Report this comment

Just to give some numbers here. On the CT7 with 5 wheels, the compression ratio on each wheel (stage) would be over 1.4.

For example if we assume the contrifugal compressor is 3:1 and the overall compression ratio is 18, then the axial compressor is 6:1.

With 5 stages, that means 1.4^5 = 6.

With the isotov, new wheels with a ratio of just 1.25 would give 1.25^7 = 4.8, times centrifugal compression ratio of 3 = overall compression of over 14.

Going from 1.1 to 1.25 is a cakewalk in aerodynamic terms.

The engine will still be very flexible at off-design speeds.

Posted by: Gordon Arnaut | October 2, 2009 1:28 PM    Report this comment

Would this be the core for a turbofan or the turboshaft turning ducted fans thru belt/chain drive?

Posted by: Will Alibrandi | October 2, 2009 1:31 PM    Report this comment

Btw the small allison has 6 axial wheels and 1 centrifugal. The Isotov is a much better base to start with if you are going to update an engine with modern aerodynamics.

Posted by: Gordon Arnaut | October 2, 2009 1:32 PM    Report this comment

Will, this would be suitable only for the twin-fan belt-driven.

Today's synchronous belts are as strong as chains and are direct replacement for a chain of the same width.

A turbofan would need to be axial flow. These small shaft engines (pt6, walter, allison, isotov) are all reverse flow. The intake air is funneled to the back and turned around to go back up front.

The exhaust would still give useful thrust, but the stacks would need to be oriented properly.

The twin-fan concept has some real advantages. One is bird strikes. That's one of the weak point's of Merril's idea. The single fan is quite big and the engine power is not that great.

Even a small bird ingestion could stop the engine. The rules for Part 23 bird strikes are not that tough, but still...

With my twin fan design, a bird strike is not likely to end up with ingestion in both fans. Plus it won't kill the engine. Even if it takes out both fans you still have a little thrust from the exhaust.

You will be landing sure, but at least you have more options.

Posted by: Gordon Arnaut | October 2, 2009 1:45 PM    Report this comment

Twin fan also has packaging advantages. The whole propulsion unit with fans could just be stuck inside the tailcone, with the fans sticking out of course.

A single turbofan will have a large fan and the mounting options are limited.

I've done the thrust numbers and two 22 in fans will give 1000 lb thrust, plus engine thrust. This will literally rocket a 3000 lb plane.

The propulsion unit will also be quite a bit lighter than a piston engine, so payload increase will be very welcome.

Cruising speed will be better than piston even on the same power because of elimination of cooling drag, which is about 10 percent of total drag on a piston.

A fixed gear twin fan the size of a Columbia could have a drag area of closer to 3, just with the savings on cooling drag.

Posted by: Gordon Arnaut | October 2, 2009 1:58 PM    Report this comment

How would that compare to the efficiecy of an APU powering an electric motor to turn a prop? No cooling drag and I'd have to assume an APU would have a significantly lower fuel burn than a propulsion engine.

Posted by: Will Alibrandi | October 2, 2009 2:34 PM    Report this comment

Will, APUs have much lower fuel efficiency. At least on the small ones. Big planes will actually use some of the high efficiency turboshafts I've mentioned here as an apu.

A popular small apu is the Solar T62, which has SFC of 1.3 lb/hp*hr. Less than half the efficiency of a pt6. Btw, this is the core on which the Innodyn is based.

Then there is the loss in generating electrical power. Each watt of power that comes out of the engine shaft will make less than a watt of electric power. I'm not sure what the efficiency of small generators is, could be close to 90 percent. But still you are losing efficiency in the power conversion.

And then you have one more power conversion loss when the motor converts that power back to kinetic energy to turn the prop or fans. Electric motors are pretty efficient nowadays, close to 90 percent on the good ones.

So you have lost at least 20 percent from all the power conversion steps. Even if you start out with a very efficient small engine -- if there was such a thing -- it would be very inefficient. Not to mention the extra weight of the electric generators and electric motors, which would be significant.

Posted by: Gordon Arnaut | October 2, 2009 3:13 PM    Report this comment

I see the new RR500 has an EPR of 9.3 (compared to the Model 250 turboprop's 7.9) and Rolls says it's 4% more efficient than a 300hp piston. This doesn't seem like a huge difference to me. What do you suggest would be an optimal ratio for efficiency in that motor?

Posted by: Will Alibrandi | October 5, 2009 11:42 AM    Report this comment

Will, I don't see how it could be more efficient than a piston if it's SFC is close to 0.6, maybe even higher. That sounds like marketing fluff.

The Walter 602, which is an updated version of the 601 with higher compression (I think it's around 12) has pretty good SFC, closer to 0.5.

If you want to be competitve with pistons you have to get the SFC down below 0.5. That means a compression of at least 15.

Posted by: Gordon Arnaut | October 5, 2009 10:39 PM    Report this comment

I don't believe the BSFC of the Walther 602 will be that good. The current 601 has a BSFC of .67 at high altitude. I find it hard to believe they could improve its efficiency more than 10%. If they get it to .60-.61 it will match the PT-6 series.

The 601 has been a favorite with home builders of the pressurized Lancair IV-P. A friend of mine who built one contemplated using the Walther 601E in his kit but went the Continental TSIO-550 route after doing the numbers. Yes, the tiny IV-P will scream along at 330kts at FL250-270 on 33gph but the piston powered IV-P does 270kts at FL250 on 17gph. Even if you pull the power back on the Walther powered IV-P to piston speeds it will still burn over 24gal/hr at altitude. Almost 50% more the piston powered in gph and about 60% more in pph due to the density difference of the fuels.

You also lose flexibility with a turbine vs a piston. The BSFC of a piston stays nearly constant whether at low or high altitude while as I noted earlier a turbine BSFC goes way up at low altitude.

I usually fly lower when flying westbound because even though I will lose airspeed at lower altitudes, the winds aloft drop at lower altitudes will out weigh the airspeed drop resulting in a ground speed increase.

The airframe of a turbine will exhibit those same characteristics but the BSFC increase at the lower altitude of a turbine results in no net range increase even though your ground speed would increase.

Posted by: GREGORY WROCLAWSKI | October 6, 2009 7:07 AM    Report this comment

The biggest problem detriment to my friend was the poor range the 601 powered IV-P has. At idle on the ground its guzzling 20gph and it low altitude efficiecy drop (BSFC increase for the same power) is even worse than the PT6 series.

A IV-P with a TSIO-550 has a 1000nm range while pushing the Walther 601 powered one for more than 600nm can get you into trouble if you have to go missed at your original destination and go to your alternate due to its low altitude inefficiency.

BTW a PT-6-35 (Jetprop conversion) guzzles 14gph on the ground idling and at PT6-66D (TBM850) guzzles 33-34gph idling. The RR/Allison 250 only uses 12gph on the ground because its a much smaller engine. By comparison the Continental TSIO-550 uses on 2.5gph idling.

The cross section off the RR500 or RR/Allison 250 shows only a single centrifugal compressor stage. Gordon, how can they get that high of a pressure ratio out of it?

The RR/Allison 250 was a favorite with turbine conversions of the Beech Bonanza. It best cruise speeds are at 15k ft in that airframe. A friend of mine has one at my home base and he generally does not go above oxygen altitudes so he loses a few knots. But my point is most people who fly unpressurized airframe ususally do not go above 12.5K ft. In order for a turbine to even approach the efficiency of a piston you need fly it high which pretty much means they only make sense in a pressurized airframe.

Posted by: GREGORY WROCLAWSKI | October 6, 2009 7:15 AM    Report this comment


I wanted to clarify and respond to your comments about turbine efficiency at low altitude. As an example I used numbers for a PT-6-35 from the Jetprop POH cruise power setting tables.

I used 800ftlbs of torque (which is 334HP at 2190rpm) because it is listed as the highest torque in the cruise power at the 6K ft table and is available all the way to 27K ft where temps will become an issue at higher torques.

The max torque at lower altitudes where ITT temps are not an issue is 1343ft-lbs (that is 560HP at 2190rpm - the max flat rated HP in the PA-46 airframe) That torque level is available to about 15Kft.

The reason that max torque of 1363ft-lbs is not shown in the cruise tables is that it will cause the PA-46 airframe to exceed its Vneat lower altitudes. You can use all that torque (and power) for climb but as soon as you level off you will over speed the airframe.

That is the problem when you STC a turbine into a piston airframe. The FAA regs make you lower Vne to the top of the green arc (Vno). Which in the PA-46 is 173KIAS.

The Vne in the piston powered PA-46 is 203KIAS.

So I surmise and paraphrase what Gordon explained and the increase in BSFC at lower altitudes on a PT-6-35 (from .60 at FL270 to .73 at 6K ft) is due to inefficient airflow in the engine caused by the much denser air at lower altitudes.

Posted by: GREGORY WROCLAWSKI | October 6, 2009 8:40 AM    Report this comment

Gordon, RR says the RR500 has 4% better fuel consumption in nm/gal compared to a piston. I suppose that's not quite the same as saying it's 4% more efficient? (marketing dept trickery?) GE advertises the new M601H as having an SFC of .585 to .65

Greg, earlier versions of the M250 have a multistage axial and single-stage centrifugal compressor. The RR300 and RR500 have a new centrifugal compressor with a CR of 9.2
The idle consumption numbers for the PT6-66D sound a bit high to me. (is that at flight idle or ground idle?) Where did those figures come from?

Posted by: Will Alibrandi | October 6, 2009 8:54 AM    Report this comment

"...But you cannot do much to adjust the airflow angles in a centrifugal pump, like you can in an axial. So they are more prone to compressor stall..."

Really, Gordon? the small Pratts I'm very familiar with all have centrifugal compressors and are therefore practically imune to compressor stall. True, I suppose you could stall any compressor if you tried hard enough, but the small Pratts have always been insensitive (relatively) to cross winds and can be started with winds up the tailpipe.

Of course, I'm not an aerodynamic engineer -- or any kind of engineer for that matter -- but I've been strapped between two Pratts for a lot of years and a lot of hours and that's been my experience.


Posted by: Linda Pendleton | October 6, 2009 8:54 AM    Report this comment

Linda, the small Pratts (and most small turbofans) have the centrifugal compressor AFTER the axial compressor. The axial is the LP and the centrifugal is the HP stage.

You will NOT see a turbofan or even a turbojet with only centrifugal compressors for exactly the reason I said, they are more prone to stall.

Some of the early turbojets had only cetrifugal compressors and they would flame out quite easily.

I was talking specifically about a turboshaft engine, the Honeywell-Rolls LHTEC, which uses two centrifugal pumps in series and no axial pump. This okay on a helo but not on a turbofan.

Posted by: Gordon Arnaut | October 6, 2009 9:23 AM    Report this comment

Gregoy, the Walter M602 was developed in the 1980s for the new LET-610 light civil transport.

It has a compression ratio of 13 and an SFC of .56. The Pratt PT6 is not comparable. It has a compression of only about 7 or 8 in somne versions and an SFC well over 0.6.

Btw, the Russians eventually chose to use the GE CT7 engine instead of the M602. This engine is evend better, as I have previously mentioned with compression of 18 and SFC of 0.45.

It is not correct that turboprops see improved SFC at high altitudes, while pistons do not. The same is true for both.

I mentioned already that power does not decrease with altitude as wuickly as density decreases. The reason is better thermodynamic efficiency at higher altitudes.

It is the same for a piston. It will have better SFC at higher altitudes too. This does not apply to a turbocharged piston below its critical altitude because its air density dows not change.

Posted by: Gordon Arnaut | October 6, 2009 9:35 AM    Report this comment

Gregory, I do not agree that a turbine airplane must be a high-altitude airplane.

That is mostly true with today's very low efficiency small turbines such as the PT6 or 601. These have compression ratios fromt he 1940s.

A small turbine with a compression like the CT7 would be just as efficient as a piston in the teen altitudes.

No argument that a turbine is not as flexible. By its nature it has a very narrow operating speed range, we all know that.

A piston can work very well at a much wider range of engine speeds.

Posted by: Gordon Arnaut | October 6, 2009 9:49 AM    Report this comment

Gregory, the small Allison has an axial comnpressor with 6 wheels (stages) and then a single centrifugal compressor.

A centrifugal compressor is what you have on your turbocharger. The flow comes from the radial driection of the wheel (like a water wheel, or a squirrel cage fan), that's why they are called radial compressors.

An axial compressor (or turbine) is like a windmill or a prop with the flow coming in the axial direction.

A centrifugal compressor is more sensitive to changes in speed. Also it is more difficult to alter the flow in a centrifugal pump in order to give wider speed range.

An axial pump can have variable guide vanes to give a suitable blade alpha (AOA) at wider speeds, kind of like a CS prop.

Some new centrifugal pumps for car turbocharges have come up with variable vanes to accomplish a similar thing and give broader performance.

Most of the larger turbofans do not have a centrifugal pump at all. Only the small engines will have a centrifugal pump, but always AFTER the axial pump.

Posted by: Gordon Arnaut | October 6, 2009 10:15 AM    Report this comment

About the thermodynamic efficiency, it increases at higher alittudes due to the falling temperature.

At least until the tropopause, where the temps stabilize. Once you get past the stratosphere temps start going up again. But I don't think you will be flying that high with a pt6.

Posted by: Gordon Arnaut | October 6, 2009 10:23 AM    Report this comment

Gregory, just to be precise about the efficiency at altitude question. It is actually a matter of heat dynamics not fluid dynamics (airflow).

The reason is that when you start with colder air, the compression process works at a higher isentropic efficiency. So the engine is more efficienct.

And Will's answer was not incorrect. The fuel controller takes inputs form pressure AND temp sensors and adjusts fuel flow accordingly. The result is less flow at the same power at higher altitude.

But that explanation does not tell us why.

Posted by: Gordon Arnaut | October 6, 2009 10:40 AM    Report this comment

The early M250s had axial/centrifugal compressors, but beginning with the 250-C28 they have only a single Ti centrifugal wheel

Posted by: Will Alibrandi | October 6, 2009 10:50 AM    Report this comment

Yep and these are used only on helos.

Posted by: Gordon Arnaut | October 6, 2009 10:54 AM    Report this comment

Gregory, a compression ratio of 9 or 10 for a single cnnetrifugal puump is indeed very high.

I'm not an engine guy, but my understanding is that 10 is about as high as you can go before stress becomes a limiting factor.

The aerodynamics of it are not difficult to understand. If you look at a typical centrifugal pump (car turbo or compressor on a small turbine), the compression is about 3 or 4.

The fluid is flung around the circumference of the housing, while the hosuing cross section decreases gradually, like a snail shell.

Now those centrifugal compressors we are all familiar with have about a full turn of the "shell." You can obviously get more compression if you keep twisting that sheall around and making its inside smaller.

Theoretically you could go very high this way. But practical limits of both stress and aerodynamics do not make it feaisble.

I would be interested to see the compressor map of this unit. I would bet that the stall and surge limits are much closer together.

Posted by: Gordon Arnaut | October 6, 2009 11:07 AM    Report this comment

And speaking of compressor maps, if you look at a turbocharger map, those contour lines show the isentropic efficiency versus the pressure ratio, mass air flow and rpm.

The line at the bottom of hte contours is the surge/stall line.

Those maps are taken at one specific temperature. If your ambient temp goes lower, your isentropic efficiency increases.

Posted by: Gordon Arnaut | October 6, 2009 11:23 AM    Report this comment

First of all I guess whenever I talk about about piston airplane engines I only am referring to turbocharged ones. I haven't owned a non turbocharged airplane in about 15 years. Notice that when I refer to a piston engine I usually use a TSIO-550. The "TS" in the Continental moniker TSIO refers to "Turbo Superchaged" Continental prefixes their non turbocharged engines with prefix of simply "IO"

I think one can only compare turbocharged airplane with turboprops simply to be in the same performance ballpark at higher altitudes.

The BSFC numbers for the PT-6 I arrived at from published numbers or the -35 and -21 POH supplements for the PA-46 airframe. I convert torque to HP and calculate the BSFC in lbs/hp/hr from the published fuel flows.

The BSFC of a TSIO-550 is very constant at around .41 -.42 lbs/hp/hr over a very wide range of altitudes. This is always leaned to about 70-80F Lean of Peak at around 65-75% power.

This is from in flight data I've seen gathered by Malibu Aerospace with a Lebow torque hub mounted between the crankshft and propeller as part of their STC process.

BSFC of a piston starts going up significantly below about 50% power and above 80% power. Also above 80% power it is not recommended to operate LOP which really causes a tremendous increase in BSFC.

Posted by: GREGORY WROCLAWSKI | October 6, 2009 11:58 AM    Report this comment

The induction temperature of a Turbocharged piston engine increases with altitude even as OAT decreases. I have this instrumented on my engine and recorded by my EDM 800 engine monitor.

I have both CDT(Compressor Discharge Temperature) and IAT(Induction Air Temp) instrumented. JPI provides this option on their EDM series but I don't know anyone else who has added these options to it during installation.

The CDT is right after the turbochargers and the IAT is after the intercoolers.

I've seen CDTs of over 300F at FL230 at my typical cruise MAP of 30"Hg during ISA +10C days. Of course the lower the altitudes the lower the CDT rise from ambient OAT.

The reason is that as the turbochargers compress the cold ambient air it heats up. The intercoolers reduce the temps but their efficiency is only about 55% so under the above conditions I see IAT of over 170F. In order to not see a IAT rise after the intercoolers at standard OAT lapse rates the intercoolers would need to have an efficiency of about 85%. This would result in IAT of temps near S.L.

But intercoolers that efficient would be prohibitively large and create too much cooling drag.

Posted by: GREGORY WROCLAWSKI | October 6, 2009 12:23 PM    Report this comment

Gordon, you seem to know quite a bit for someone who's "not an engine guy" :)

Posted by: Will Alibrandi | October 6, 2009 12:30 PM    Report this comment

But even with IAT increasing significantly the power output of a turbocharged piston actually rises slightly from S.L. to about 10K feet when MAP and RPM are held constant. The reason for this is because the Volumetric efficiency goes up due to a reduction in exhaust back pressure because ambient air pressure is decreasing.

From about 10K ft to 20K feet the power remains fairly constant (Again constant MAP and RPM)since the decrease in exhaust back pressure is offset by the increase in IAT.

Then above 20K feet the power starts decreasing with constant MAP and RPM because IAT is increasing rather dramatically.

Theoretically a 10F rise in IAT results in 1% decrease in power assuming everything else stays the same.

Continental only published S.L. power charts for their engines(They are very conservative I may add).

Lycoming has very nice HP vs MAP, RPM and altitude charts in their Operators manuals for their turbocharged engines.

Posted by: GREGORY WROCLAWSKI | October 6, 2009 12:35 PM    Report this comment

>>It is the same for a piston. It will have better SFC at higher altitudes too. This does not apply to a turbocharged piston below its critical altitude because its air density dows not change.

Posted by: Paul Bertorelli | October 6, 2009 12:37 PM    Report this comment

Gregory, a turbocharged piston's induction temp continues to increase with altitude because the pressure ratio continues to increase.

In order to keep the same MAP, the compressor will need to compress the air by 2:1 in the teens and by more than 3 to 1 past FL200.

The ambient temp is going down, but that is not enough to make up for the large increase in temp as the compression is increasing.

A turbine engine is a naturally aspirated engine, so comparing a supercharged piston to a turbine is an apples to oranges thing.

You are right about the intercoolers. A few years ago I helped some hobbyists with a turbo rotary flight engine and I built a program to quickly find the right size turbo for different mission profiles.

I quickly realized that an intercooler is a big efficiency waste. Besides the cooling drag it also lowers MAP by a few inches.

Here again we see that the turbo pistons are planes designed to fly high. They are like turboprops in that regard.

I have been saying all along that flying high is not what the typical single pilot is interested in. Nor is it good for him in terms of affordability or skill.

I have said all along that what we need is a small turbine that can be flown efficiently down low. A Cirrus can do close to 200 knots down low. Are you really gaining all that much with a few extra knots up high?

Posted by: Gordon Arnaut | October 6, 2009 12:44 PM    Report this comment


The fuel consumption numbers I published above are at ground idle. The Jetprop -35 is what I observed from the right seat.

By the way a Piper Meridian with a PT6-42 is using about 20gal/hr at ground idle.

The PT6-66D number is what a demo pilot for the TBM850 told me to expect when at an open house.

These numbers are of importance to me because of where I am based (MMU). On busy IFR only days (like the Wednesday before thanksgiving)I waited as long as an hour for my release. You can't shut down because you have to be ready to go right now when you get your release or you will lose your turn.

That is one of the joys of being based underneath the NY class B airspace.

Burning 10% of your fuel while waiting for release can seriously affect you flight planning.

Posted by: GREGORY WROCLAWSKI | October 6, 2009 12:47 PM    Report this comment

Paul, what book is that? I have Taylor's classic work on internal combustion engines (2 volumes) and will check into it.

About the IO360, there is not a huge temp decrease at 8,000. Any increases in isentropic efficiency may well be offset by other factors, such as the engine running at lower power.

This does not hold true for all NA pistons. I've heard guys who retrofitted the IO550 for smaller engines and they get better SFC in the teens.

This is because the smaller engine just did not have the reserve power to do well up there.

Posted by: Gordon Arnaut | October 6, 2009 12:58 PM    Report this comment

Paul, the LHTEC has SFC of 0.45 on 14 to 1 compression.

I would say that is competitive with pistons. Running lean will decrease SFC a bit but I think under 0.4 may be stretching it.

Posted by: Gordon Arnaut | October 6, 2009 1:00 PM    Report this comment

It's Taylor's The Internal Combustion Engine Theory and Practice, second edition. Great standard text for this kind of stuff.

I'd be surprised if the SFC relationships are displacement sensitive enough to upset the basic pattern described here. Temp and density changes seem to affect this less than do throttle position (pumping efficiency) and, of course, lean state.

I've run the calcs on bigger engines and don't see much detectable difference. It is true that the large displacement Continentals are the SFC champs, but this relates to induction design and other factors. They will get better SFCs than an O-360, but I don't think it's demonstrable that they'll do higher SFCs in the teens than at 6000 feet.

Posted by: Paul Bertorelli | October 6, 2009 1:06 PM    Report this comment

Gordon, I recall my pal telling me the intercoolers in his Navajo give him an extra 1" or 2" MP, although I'm not sure in which flight regime that was. I recognize the associated cooling drag, but isn't the reduction in intake air temp compensating somewhat?

Greg, I fly into HPN and TEB all summer long in the Caravan - yeah NY airspace is a PITA when the WX goes south. I never noticed the idle burn rate because I guess we were just lucky to not get held up for too long. Then again, we were only flying 1-1.5hr legs out to MVY/ACK.

Posted by: Will Alibrandi | October 6, 2009 1:55 PM    Report this comment

>>LHTEC has SFC of 0.45 on 14 to 1 compression.

Posted by: Paul Bertorelli | October 6, 2009 2:02 PM    Report this comment

Will, the intercooler allows more boost and therefore higher MAP.

But at the same boost or pressure ratio inside the compressor, adding an intercooler will drop the MAP by a good two inches.

The reason is that the air in the intake manifold is funneled from one big pipe into lots of very small tubes. This creates lots of drag on the airflow inside the manifold due to the very large surface area to volume ratio of a small tube versus a big tube.

The result is that your engnien has to pump harder just to pull the air through those little tubes. That costs efficiency obviously.

Of course if you want to fly higher than about the teens you need more boost if you want to keep your SL MAP. That extra boost adds heat and your engine will detonate.

There is a way to lower the temp coming out of the compressor and that is evaporative cooling from the fuel.

When you squirt fuel into a mist it absorbs the heat around it. That's why you get carb icing. The trick is to squirt it right into the combustion chamber, or as close to it as possible to get the full effect.

Btw ethanol fuel has a much higher latent heat than gasoline so you can get a much bigger cooling effect. Running your turbo piston on ethanol you could go well into the flight levels without an intercooler.

Posted by: Gordon Arnaut | October 6, 2009 2:18 PM    Report this comment

Gregory, you made an interesting observation about your turbo engine making more power at 10,000 than at SL.

You chalk this up to the reduction in ambient pressure, which is mostly correct. It is useful to think about the pressure ratio across the turbine wheel of that turbocharger.

When you are at sea level, the pressure inside your intake manifold is about the ambient pressure. So the pressure ratio across the turbine wheel is about 1.

When you get up to FL180 where ambient pressure is about half, the pressure ratio across your turbine will be 2 to 1 (assuming your compressor keeps the intake pressure constant).

This speeds up the rotation of the turbine and creates more power. That turbine is there to spin a compressor, so the extra power spins up the compressor to a higher speed so you get more boost (which you need in the thinner air).

The key thing is that the compressor has a better efficiency (isentropic) at the higher spinning speed. Which means it is compressing the air without causing as much heat.

This is good for engine efficiency. With your turbo the sweet spot seems to be at 10,000. This is where isentropic efficiency of the compressor would seem to be best. As compressor pressure increases after that, the efficiency goes down and your intake heat goes up.

The thing that puzzles me is why at 10,000? You could as easily choose a turbo that would put this sweet spot at higher altitude, where this plane was really designed to live.

Posted by: Gordon Arnaut | October 6, 2009 3:09 PM    Report this comment

Adjusting the turbo controller for more/less boost (a given MP setting at X altitude) shouldn't be too complicated, if that's what you're after.

Posted by: Will Alibrandi | October 6, 2009 3:28 PM    Report this comment

That's not it, Will. It has to do with the design of the compressor.

Go to the Garrett site and look at some compressor maps. The contour lines showing isentropic efficiency form an "island" right in the middle of the map.

This is where efficiency is highest. Now if you look to the left you see the pressure ratio at which you get this highest efficiency. You can see that if you operate your engine at a pressure ratio outside of this island, then you are losing efficiency.

Point is you can choose a compressor that will give the best efficiency at a certain pressure ratio. If you are flying high your pressure ratio will need to be high, well over 2, maybe even 3. So you want a compressor with efficiency at these ratios.

If you want to fly at 10,000 you choose a compressor which will give best efficiency at lower pressure ratio. Sounds to me like Gregory's engine has a compressor that is most efficient at a ratio of about 1.5. At higher altitudes where pressure ratio is over 2, efficiency has dropped off.

Posted by: Gordon Arnaut | October 6, 2009 4:21 PM    Report this comment


You are probably right about the turbochargers. I know that Malibu Aerospace has been doing a lot of testing in conjunction with Continental for the FADEC version of the TSIO-550. Rather than just adding FADEC to the existing engine they decided to make some improvements. One of them was to change to, I was told, large turbochargers. Malibu Aerospace claims they increase critical altitude and lower CDTs. So I assume they are more efficient than the original turbos especially at higher altitudes.

Sir Stanley Hooker was the lead engineer at Rolls Royce during WWII and designed the superchargers for the Merlin and Griffon engines which we all know were very successful. After the war he became chief engineer at Rolls Royce and led them into the jet age as a premier turbine engine company.

One of his most memorable quotes as chief engineer was:

"The four stroke engine, that's one stroke for producing power and three for wearing the engine out."

Posted by: GREGORY WROCLAWSKI | October 6, 2009 9:28 PM    Report this comment

Are you suggesting swapping out turbos? Replacing a turbo may be the best technical solution, but with a list price of $3,000-5,000 it sure isn't the best financially. That's just not realistic, when the boost controller can be adjusted to provide best power or efficiency for the typical altitude the aircraft is being operated at.

Posted by: Will Alibrandi | October 6, 2009 9:38 PM    Report this comment

Greg's comment provided a little clarity. Replacement makes sense when the original turbo eats its bearing for lunch, although I'm not sure how a larger displacement turbo would make any more difference than changing the impeller profile for a higher flow, but I don't design these things. At any rate, isn't adjusting the wastegate via the boost controller doing essentially the same thing?

Posted by: Will Alibrandi | October 6, 2009 9:48 PM    Report this comment

Will, you can't just stick in another turbo even if you wanted to. The whole system would have to be reengineered and STC'd.

And adjusting the boost controller is not going to give that turbo more efficiency. It may give more boost, but efficiency will be reduced even more.

Posted by: Gordon Arnaut | October 7, 2009 5:52 AM    Report this comment

Gregory, yes the lower CDT's indicate better isentropic efficiency, which is defined as compressing air with as little heat increase as possible.

The higher you go the bigger your turbo should be. One of the things I found when crunching the numbers for the rotary was that much bigger turbo was needed than we figured at first.

My Turbochart program plots the engine's mass flow and pressure ratio requirements (at every altitude in its operating range) onto several compressor maps for turbos of different sizes. It's hard to argue when you see it in black and white.

Choosing the right turbo for a piston flight engine is not as easy as it seems. A one-size fits all approach is not going to be optimal.

Posted by: Gordon Arnaut | October 7, 2009 6:14 AM    Report this comment


I guess I didn't make myself clear. Continental decided to make the switch to larger turbos for the FADEC version of the TSIO-550. It was done with input from Malibu Aerospace who is doing the flight testing of the engine. Its been a couple year process so far with Continental making lots of software changes and some hardware changes along the way. So the engine will have the larger turbos right fom the factory.

To get this discussion back on track. Gordon, what can be done to reduce the ground idle fuel consumption of these turboprop engines. Why is it so high to begin with?

Posted by: GREGORY WROCLAWSKI | October 7, 2009 7:32 AM    Report this comment

Gordon, How much better can the efficiency of the ducted fan be than a propeller. I have seen efficiecny plots of some propellers and they claim around 85% in cruise for a well matched and designed constant speed prop.

Posted by: GREGORY WROCLAWSKI | October 7, 2009 7:36 AM    Report this comment

Luc, that is very interesting news. I was not aware of this engine program.

The website is Listen to what they say:

"Observation: The VLJs are not adapted to the sector of General Aviation...the certification cost of a Mach 0.6 aircraft flying above 35,000 ft will cost between five and eight times that of an unpressurized aircraft designed ot fly at lower speeds and altitudes."

They actually give the pertinent numbers, which is a good sign. They have two engines, 575 lb and 740 lb thrust. Bypass ratios are fairly high, about 7.

Most important are is the cruise SFC, which is given as just under 0.7. This is pretty good, indicating an engine with a compression close to 20.

This is at a cruise thrust of 350 lb for hte big engine and 270 for the small one. Cruise condition is M 0.34 at SL, which is about 260 mph (220 knots).

At this speed and altitude the ram pressure is about 170 lb/ft^2. If we take a plane like the Columbia with a drag area of 3.5 ft^2, we can see that it will take 3.5 * 170 = 600 lb of thrust to achieve this speed at this altitude (SL)

So two of the small engines would give not quite enough thrust, and two of the big ones would give a little more. With the two small engines, this plane could do almost 250 mph at SL, which is pretty impressive anyway.


Posted by: Gordon Arnaut | October 7, 2009 9:21 AM    Report this comment

I guess 10 is not the CR limit for centrifugal compressors as you suggested? I thought it interesting that it uses a planetary gearbox for reduction. I guess Pratt & Whitney isn't the only company that thinks it's a good solution?

Posted by: Will Alibrandi | October 7, 2009 10:14 AM    Report this comment

They must have worked out compressor stall issues to go with a single stage centrifugal pump?

Addendum: IIRC Honeywell's TFE 731 is also geared

Posted by: Will Alibrandi | October 7, 2009 10:20 AM    Report this comment

So let's work out a comparion between piston and turbofan for a Columbia-type airplane that has enough power to go just under 250 mph at SL.

Power = Thrust * Velocity, so it would take about 210 hp to make 270 lb thrust at 250 mph, at SL.

Fuel flow for a piston would be 210 *.45 = 95 lb/hr = 16 gph.

We have two of these engines on our Columbia because that is how much power we need to go 250 mph at SL. So total fuel flow is 32 gph.

Now the turbofan. The smaller engine has TSFC of 0.73, so 270 lb thrust * 0.73 = 200 lb/hr = 28 gph.

This is not bad but nearly twice as much as the piston.

I like that they are giving the cruise numbers at SL though, because this is the worst we can expect. Points for honesty.

Above 10,000 we will see considerably better efficiency. Again, points for honesty because they are not speculating. We don't know exactly what the numbers will be until an airplane flies with these engines.

Now flying in the teens, the ram pressure, due to thinner air, is almost half. Which means we need only about half the thrust to fly at the same 250 mph.

Our Columbia will need only about 300 lb thrust to fly 250 mph at FL180, the maximum you can go without oxygen face mask.

So 300 lb of thrust * TSFC of 0.73 = 230 lb fuel total for both engines. That is just over 30 gph.


Posted by: Gordon Arnaut | October 7, 2009 10:30 AM    Report this comment

And more good news. This is assuming TSFC remains constant at the SL number of 0.732 (small engine).

The fact is that TSFC goes doesn with altitude (as we have discussed) because engine thermodynamic efficiency goes up. Looking at some charts in a turbine book will show about a 20 percent drop between SL and FL200 on a high-bypass turbofan.

So we can lower that 30 gph by about 20 percent and get about 25 gph. I think this is a reasonable interpretation of the numbers.

This is conmpetitive with piston performance. A Columbia 400 turbo will burn over 20 gph at similar speed and altitude.

Just on first impressions, I would say this outfit looks serious.

Posted by: Gordon Arnaut | October 7, 2009 10:31 AM    Report this comment

Will, are you saying this engine uses a single centrifugal compressor?

Posted by: Gordon Arnaut | October 7, 2009 10:33 AM    Report this comment

Looking at the cutaway on the website it looks like there is a small axial compressor with 3 or 4 stages ahead of the big centrifugal pump. Hard to tell for sure.

Posted by: Gordon Arnaut | October 7, 2009 10:57 AM    Report this comment

That looks like a bearing surface for the spool to me, not a flow path. They're far too small to be axial stages, methinks.

Posted by: Will Alibrandi | October 7, 2009 11:04 AM    Report this comment

Could be just a bearing. If that one radial pump is all there is I would be much more skeptical of the whole thing.

Like I said, theoretically you can get a pretty high compression form a single pump, but it has not been done before. Maybe on a small engine like this? Who knows?

The compressor stall issues have to do with the flight speeds of the aircraft. That's why radial pumps are okay on helos, they don't have a wide speed range.

The blades on the compressors are fixed, just like on a fixed-pitch prop. A fixed pitch prop will work okay on a plane that goes maybe 200 mph tops. After that the blade angle for cruise would be too high for the standing still (static) condition, which would mean the blade would be operating stalled.

Same with the blades on a compressor. If your plane is slow enough a radial pump can work. I think a 250 mph airplane is pushing it a bit for just a radial pump. Even if it can work, efficiency will be poor at takeoff and climb.

Posted by: Gordon Arnaut | October 7, 2009 11:23 AM    Report this comment

I did find an article that referenced the engine's architecture.
"The DGEN 380 engine configuration is believed to be a single stage fan supercharging a centrifugal HP compressor, the whole being driven by a 4-stage turbine. A reverse flow combustor is featured. The separate jets exhaust system has a plugged primary nozzle.

At Sea Level Static, ISA the net thrust is 562lbf, at a (very competitive) specific fuel consumption of 0.402lb/hr/lbf. The corresponding figures at Mach 0.35, 12000ft, ISA are 193lbf and 0.745lb/hr/lbf respectively.

A bypass ratio of 8.0 is claimed, which explains the need for 4 turbine stages. The absence of any axial core compression stages indicates a low overall pressure ratio."

Sounds about right, except for the reference to a 4 stage turbine and low EPR. They were at the Paris Air Show back in June and have raised the funds for a certification program with the help of Turbomeca. Back in late 2005 they were considering a turboprop variant of the engine, but I haven't found any other mention of that more recently. Very interesting that they're focusing on that lower & slower niche you talked about.

Posted by: Will Alibrandi | October 7, 2009 11:29 AM    Report this comment

Yep, and I think we might as well forget about this one.

I had taken them at their word and assumed the cruise SFC was at SL, which is what it says for the cruise thrust. But they are actually extrapolating the cruise SFC number for 12,000 ft.

The fan can compress the air a little bit and that's what it looks like. But this is not going to do much for the overall compression ratio.

This engine is not going to have a compression much above 6 or 7. I think those numbers are just wishful thinking, like Innodyn and their 0.45 SFC from an APU that burns 1.3.

Posted by: Gordon Arnaut | October 7, 2009 11:46 AM    Report this comment

Getting back to reality here.

When it comes to fuel efficiency, it's all about the compression ratio. The efficiency of Brayton cycle engine (gas turbine)is given by the relation: 1 - 1/PR^k

Where PR is the pressure ratio of the engine and k is the ratio of specific heats of the working fluid. Looking at this equation we see that the efficiency varies in direct proportion to only two things, the Pressure Ratio, and the ratio of specific heats of the fluid.

Since we cannot choose our working fluid (it has to be air), the ONLY thing that determines the efficiency of a gas turbine is the compression ratio.

To quote from an introductory thermodynamics text: "the thermal efficiency of an ideal Brayton cycle depends on the pressure ratio of the gas turbine and the specific heat ratio of the working fluid." -- Thermodynamics: An Engineering Approach, by Cengel.

(Btw, gas turbines on nuclear plants and subs will run on byproducts of the nuclear process, like helium, which greatly increases their efficiency.)

Posted by: Gordon Arnaut | October 7, 2009 12:03 PM    Report this comment

It makes me wonder why people bother to go through all the motions to build an engine with a low pressuure ratio and then expect that it will work as a piston replacement.

Especially when you put a farily small fan on it, which reduces propulsive efficiency even further. I'm not going to redo the numbers I just did, but the bottom line is this engine will burn nearly twice as much fuel as a piston.

Now some people may choose to live with that because a jet does offer huge advantages in noise and smoothness. But most will find the tradeoff not worthwhile.

Posted by: Gordon Arnaut | October 7, 2009 12:10 PM    Report this comment

Gordon, you are correct in saying the engine will burn twice as much. I don't think anyone expected this engine to be a piston replacement.

I also agree with you that most will find the tradeoff not worthwhile. But some will, and that is the market. Is there a business case for it? Personally I do not think so for the 380 alone, but when you consider the growth path towards more performance in my opinion it is worth having a shot at it.

Posted by: Luc Van Bavel | October 7, 2009 12:27 PM    Report this comment

Oops. Made a mistake in the efficiency equation. Efficieny = 1 - 1/PR^(k-1/k)

Just to give a numerical example of the efficiency equation.

The ratio of specific heats for air is 1.4. If our compression ratio is 5 then we get efficiency of : 1 - 1/5^((1.4-1)/1.4) = 0.37

If we raise our compression to 15, we get: 1 - 1/15^((1.4-1)/1.4) = 0.54

So our thermal efficiency has gone from 37 percent to 54 percent. This is only the thermal efficiency not the actual fuel efficiency which will be less.

But there is no getting around it. If you want efficiency you need high compression.

Posted by: Gordon Arnaut | October 7, 2009 12:31 PM    Report this comment

Luc, nice drawings of the concept airplanes.

Since you have a working rleationship with this outfit, tell me this: Why did they not just bite the bullet and design a proper high compression engine?

Yes it is going to require an additional compressor and more turbine stages. But they already have a 2-spool engine, which is most of it right there. And they have the gearbox, which is necessary too.

This is a pretty simple engine with low compression. When you go to higher compression you will need pretty good bleed air cooling for the blades, but up to about 15 to 1 you will not need fancy metallurgy.

Still as a first effort it is pretty good. I applaud them and don't want to take anything away. A bunch of young fellows building even a 5 to 1 engine is an accomplishment. A good learning experience I'm sure.

Posted by: Gordon Arnaut | October 7, 2009 12:48 PM    Report this comment

The fan was designed and built by Snecma, and other key parts of the engine by Turbomeca. The engine designer is the retired head of design at Snecma.

A good learning experience by a bunch of young fellows is therefore not exactly the right way to characterize what is going on there, although there are a bunch of young fellows involved as well (and bright ones).

The engine design is driven primarily for low acquisition cost therefore choices had to be made. The first step needs to be prudent, you can always add more advanced technologies later on.

Posted by: Luc Van Bavel | October 7, 2009 1:01 PM    Report this comment

Can you tell me what the overall pressure ratio is? And how much compression is coming from that fan?

Posted by: Gordon Arnaut | October 7, 2009 1:20 PM    Report this comment

Sorry I can't. What will really matter are the hard numbers coming from flight test next year. I am not the only one looking forward to that.

Posted by: Luc Van Bavel | October 7, 2009 1:30 PM    Report this comment

What airframe will the engines go on?

Posted by: Gordon Arnaut | October 7, 2009 2:13 PM    Report this comment

I don't believe that has been made public yet.

Posted by: Luc Van Bavel | October 7, 2009 2:18 PM    Report this comment

Gregory, propulsive efficiency is another whole big subject.

To start with some generalizations, a prop is the efficiency king until about 300 mph. From there the ducted fan (turbofan) starts taking over.

Aircraft are propelled by accelerating a fluid in the direction opposite to motion. This is accomplished by either slightly accelerating a large mass of fluid (prop) or greatly accelerating a small mass of fluid (jet).

The problem is that accelerating the air incolves a loss of energy. The more you need to speed up the air, the more energy is wasted. So a big prop is the most efficient.

(The math is actually quite simple but I won't get into it here, unless someone really needs to know).

However, if you compare a prop and a ducted fan of roughly similar size, the prop will need to have an area twice as big as the fan to make the same thrust from the same power.

So the thrust from a 6 ft prop will equal the thrust from a 4.2 ft ducted fan. Of course 4.2 ft for a ducted fan is too big for a small airplane. Even if you split that total swept area over two separate fans, each would need to be 3 ft diameter.

Still too big.


Posted by: Gordon Arnaut | October 7, 2009 2:39 PM    Report this comment

The previous assumed equal power going into both the fan and prop.

All of that applied only to static thrust also. So if we wonder what would happen if we took our piston plane of 300 hp and replaced the prop with a 24 inch ducted fan, the asnwer is that we would not have enough static thrust to launch the airplane for takeoff.

But assuming we could somehow launch the plane (if we had a 2 mile long runway), once we got up to cruising speed of say 200 mph, our 24 inch fan would not be giving up all that much in efficiency, perhaps ten percent.

On a 300 hp engine cruisng at 200 hp, this is maybe 20 hp. This would certainly be attractive trade for the low noise and vibration.

The problem is the takeoff and climb.


Posted by: Gordon Arnaut | October 7, 2009 2:47 PM    Report this comment

So what you need with that 24 inch fan is about twice the engine power. Then you will be okay for takeoff and climb.

But now you have a huge piston engine that weighs a lot more. Either that or you can get a jet, which will have the power and low weight, but will burn a lot of gas.

So that is why ducted fans have not broken through for small planes. Nasa did a study back in the '70s with turbocharged rotary engines (Curtiss-Wright) and flat piston engines (Lyc and Cont). The rotary engines looked pretty good, almost turbine small and much lighter than piston.

This could work on 2-seaters, especially with one engine driving 2 fans.

It could even work for 4-seaters, but here you would be just as well having a small turboshaft driving the fans (Isotov or Allison).

Posted by: Gordon Arnaut | October 7, 2009 2:55 PM    Report this comment

Okay, so if the prop is more efficient until +/- 300mph (260kt) then wouldn't the best solution be a turboprop single powered by a high-efficiency turbine engine? Something with a cruise speed below 260kts that flies in the 15k-20k altitudes to keep the insurance companies placated?

Posted by: Will Alibrandi | October 7, 2009 2:59 PM    Report this comment

I guess if your idea of the best solution includes a lot of tiresome prop noise and vibration.

For me it is worth it to give up 20 hp at cruise for the civility of a turbofan. As you get past 200 mph cruise the gap is even less. At 300 mph the fan is giving up very little, if anything.

Posted by: Gordon Arnaut | October 7, 2009 3:12 PM    Report this comment

Gregory, about the high fuel flow at idle...

There are a few things involved here. One is the fact that a turbine is a machine with a fairly narrow operating range, in terms of rotational speed.

A piston engine will idle at 500 rpm, less than 1/5 of full power. I don't know what the speeds are for the Pt6, but on a big fan engine idle N1 (which drives the fan) will be about 25 percent of full speed, and N2 (the high pressure turbine that drives the compressor) will be about 60 percent.

Clearly this takes a lot of fuel. Why? because it takes a lot of power to drive that big compressor, particularly if we have a high pressure ratio.

This is analogous to the pumping losses of a piston engine, but the difference is that turbine machinery cannot slow down very much... and still run.

The reason for this is mostly aerodynamic. All those compressor and turbine blades in there are airfoils with a fixed angle or pitch. Just like a fixed pitch prop, it means they can only operate over a narrow speed range without stalling or making no lift.

So even without a load on it, the engine has to run at a pretty high speed and do all the work of compressing all the air it is ingesting.


Posted by: Gordon Arnaut | October 7, 2009 4:10 PM    Report this comment

The other part of it involves thermodynamics. Here the engine is at a disadvantage because it was designed to optimal thermodynamic efficiency at a certain condition of rotational speed, pressure and temperature.

Just like the turbos we have been disucssing. One turbo can have its best isentropic efficiency at a certain rotational speed, pressure ratio, etc. Another will be different.

This can be varieed by the designer of course. That is why the pt6 you mentioned has a 20 percent difference in SFC between FL260 and 6000. It was designed for peak efficiency at the pressure and temp conditions of FL260.

If we wanted to, we could design it to give best isentropic efficiency at 12,000. And some engines do. I'm sure the Pt6 in the Caravan has a different compressor and turbine map than the pt6 in the TBM.

Btw, isentropic efficiency also applies to the turbine, not just the compressor.

Bottom line is a turbine can never be as thrifty at idle as a piston. But if the engine is designed for efficiency in the teens idle will be better than one designed for the flight levels.

Posted by: Gordon Arnaut | October 7, 2009 4:18 PM    Report this comment

Just to clarify here. If we designed a turbine engine to give best isentropic efficiency at 12,000 ft (or even SL), the engine would still see a slight increase in efficiency as temps decrease at altitude.

But it would not be very much. It would certainly not be 20 percent.

Posted by: Gordon Arnaut | October 7, 2009 7:22 PM    Report this comment

And just another tidbit here on the whole question of why turbine efficiency improves with altitude.

A good rule of thumb for figuring how much your SFC will decrease with altitude is to take the square of the ratio of temperatures and multiply that by your SL SFC.

So if your SL SFC is 0.7, and you go up to FL270 where the temp is only 80 percent of what it is at SL (in absolute temp scale), then the square root of 0.8 is 0.9.

So you multiply that by by your SL SFC of 0.7 and you get 0.63. Your SFC has decreased by 11 percent.

Going back to Gregory's example, where SFC at 9000 ft was 0.73 and SFC at FL270 was 0.61, the ratio of temps is 0.88, so the square toot of that is 0.94.

We multiply the 9000 ft SFC of 0.73 by 0.94 and get 0.68, an improvement of 7 percent.

So that is how much SFC improvement is caused by the altitude and attending low temperature. The rest of the improvement is caused by the fact that the engine is optimized for isentropic efficiency at this operating condition.

Also important to note that the reverse is true. When temps are high, efficiency goes down by the same temperature ratio squared.

Posted by: Gordon Arnaut | October 7, 2009 8:52 PM    Report this comment

I think you're overstating the noise and vibration issue, but comparatively speaking there is of course a difference between prop & fan. If we're talking about the lower end of the speed range, say 200-220kt where the prop's efficiency over the fan is greatest, then wouldn't a high CR turbine turning a prop be the most efficient?

Posted by: Will Alibrandi | October 7, 2009 8:56 PM    Report this comment

In the 200 to 220 knot cruise range the fan's propulsive efficiency will be about 5 to 10 percent less than a prop. It is not much.

Yes for the absolute best efficiency you could have a small high-compression turboprop that would be absolutely in piston territory for fuel specifics.

I guess some people would find that okay. Some people have a problem with prop singles due to the eye strain caused by the whirling prop. It's not something you really notice and it kind of goes away like background noise, but it does have a phsyiological effect.

Posted by: Gordon Arnaut | October 8, 2009 5:55 AM    Report this comment

I'm going to revisit the Dgen numbers, based on what Will found for thrust and SFC at 12,000. (Which is an estimate because the plane hasn't flown yet).

The fact that they have a serious engine guy on board certainly lends credibility to this outfit.

So we have an engine with 562 lb static thrust on SFC of 0.402. At cruise of M 0.35 (255 mph) at 12,000 ft, the thrust is 193 lb on SFC of 0.745.

The ram pressure at this speed and altitude is 111 lb/ft^2. With two of these engines making a total thrust of 386 lb, it would be enough to propel an airplane with a drag area of 386/111 = 3.5, which is about the drag of a Columbia 400.

So we have a Columbia 400 with two of these engines doing 255 mph at 12,000. This beats what a Columbia 400 can actually do at that altitude.

Let's see how much fuel we are burning. Total thrust of 386 lb * SFC of 0.745 = 288 lb/hr = 41 gph.

The 400 POH says that at 12,000 and ISA the plane will do 207 ktas on max cruise power (85%), burning 24 gph.

That is about 144 lb/hr. Our 310 hp engine at 85 perccent power is making 264 hp. So BSFC = 144/264 = 0.55.

Now that's what the POH says. I'm sure some guys are reducing power and flying LOP.


Posted by: Gordon Arnaut | October 8, 2009 7:04 AM    Report this comment

Now the same plane with the two small fans. Total thrust is 386 * velocity of 374 fps / 550 = THP of 263. Assuming prop efficiency of 0.85 we get shaft hp of 262/0.85 = 309 shp.

That's the power those two fans are making if we were to measure it somehow. BSFC is therefore 288/309 = BSFC of 0.93.

Not quite double the Columbia, but still pretty high. That is an apples to apples comparison.

The fan plane is flying about 15 kt faster than Columbia but it is burning 70 percent more fuel.

It will need a lot more fuel capacity to equal the range of the piston plane.

Still, if they can achieve even these numbers with their flying prototype, it will be pretty good. Burning 40 gph is not much more than some of the jetprop conversions, and that's if you fly them in the flight levels.

If you fly them at 12,000 ft, they will probably be more than 40 gph.

I would say this might just work.


Posted by: Gordon Arnaut | October 8, 2009 7:13 AM    Report this comment

Now if we had a high-compression engine let's see what cruise SFC we would need to shoot for if we wanted to get down to 30 gph at the same speed and altitude (255 mph, 12,000 ft). IN the same airplane with the same amount of thrust.

We take our 30 gph and multiply by fuel weight of 7 lb/ga and get 210. Divide that by our thrust of 386 and we get 0.54 cruise TFSC.

Is this doable? That is the question. It is a jump in efficiency of about 37 perccent.

If we go back to our previous calculation of thermal efficiency, we see that going from CR of 5 to CR of 15 gave us a jump of about 46 percent.

So yeah I would say it is doable. I would add that a turbofan Columbia burning 30 gph at 255 mph and 12,000 ft would be revolutionary.

Posted by: Gordon Arnaut | October 8, 2009 7:20 AM    Report this comment

A bit of a correction on those numbers. We had assumed propulsive efficiency of 0.85 for the fan airplane, which we might see at higher Mach numbers, but not at M 0.35.

Here we are only going to see about 0.75, as discussed earlier. It means that the fan engines are making more hp than we figured so SFC will be lower.

Let's try this again. 386 lb thrust * 374 fps airspeed / 550 ft-lb/s = 262 thrust horsepower. Our previous calculation was okay up to this point.

But now we divide that THP by 0.75 and get engine power of 350 hp. Fuel burn is the same at 288 lb/hr, so SFC = 288 / 350 = 0.82.

That is much closer to the actual number we would get if we could measure this engine for power output. That is not terrible if we compare it to the piston engine's 0.55. Even improving that by 10 or 20 percent would make this airplane more viable.

Posted by: Gordon Arnaut | October 8, 2009 10:04 AM    Report this comment

Gordon, I've been in touch with Price Induction and they've sent me a 13 page attachment from the Paris Air Show this year on their DGEN engines, including two new engines that are in the pipeline. One will be tested with a counter-rotating ducted fan. If you're interested post an email address and I'll send it along.

Posted by: Will Alibrandi | October 8, 2009 1:43 PM    Report this comment

Sounds good. goarnaut(at)

Posted by: Gordon Arnaut | October 8, 2009 2:42 PM    Report this comment

I always knew turbines can't so much idle as just back off slightly from full fuel flow, but those are some astonishing ground-idle fuel flow figures given above. Of course these are much more powerful engines than car engines, but they're not Boeing 777 class, either. My car burns about a gallon an hour on the highway, so even the TSIO-550's 2.5 is a whole lot for idle, though, yes, it is a big monster compared to a car engine. Stopped, the car burns nothing at all, no need for the engine to run if the car is not moving, it can always be restarted in a moment when needed. Starting a turbine is going to take longer than starting up a Prius's engine, and the thermal cycle is stressful, but, wow, that's a lot of fuel for just sitting still. Burn a week or two worth of auto fuel use just starting up and taxing toward the runway.

Posted by: Michael Pereckas | October 8, 2009 9:23 PM    Report this comment

Yes, POH says 65 percent power at 12k ISA, 2500 rpm will give 186 ktas at 17 gph.

But now the fan airplane is flying 35 knots faster. Another comparison is to look at gas mileage. The fan plane burning 41 gph at 255 mph is getting 6.3 mpg. The piston plane is getting 12.5 mpg, nearly double.

So the question for the Dgen engine is whether people are going to trade away half their gas mileage for the privilege of a turbofan?

I would bet there are enough people that would, especially if cost of entry is in the same ballpark as the piston plane.

Posted by: Gordon Arnaut | October 9, 2009 4:33 AM    Report this comment

Also I have some doubts about these Dgen cruise numbers. They ran the engine on the test stand and got a static TSFC of 0.4 which is very good. However, this does not jibe with their numbers at 12,000 ft.

That is a very small fan, looks like no more than 15 inch. It is going to take something like 600 hp to get 550 lb of static thrust with a fan that small. Yet at 12k ft. This engine is loafing at maybe 200 hp? And fuel specifics have doubled? This does not add up.

It doesn't help matters that they don't want to give you the pertinent numbers that would allow a real analysis. The big engine makers are doing this too with their small engines. They are giving practically zero information. You can bet if their numbers were something to brag about they would be bragging, like they do with the big engines, where they immediately post their CR, cruise SFC and lots more.

Bottom line is neither the dgen nor the small engines from the big guys are the engine that we need. It does not exist. But it could.

Posted by: Gordon Arnaut | October 9, 2009 4:35 AM    Report this comment

Gordon: *Legal* highway speeds. But, yes, actually closer to 1.1 gal/hour at 55 and if you're doing 65MPH more like 1.4 for the Prius.

I put a Hobbs hour meter in my last car (a Subaru station wagon) and the grand average speed traveled over a few years was 33.5MPH. Drone along on the highway all day and I could get over 3 gal/hour for the tank-full, but it took 1400 hours on the Hobbs to burn about 1860 gallons, an average of 1.33 gal/hour. Now I'm well off-topic, but it's a comparison to something we're all familiar with but never think about in gal/hour terms.

Posted by: Michael Pereckas | October 9, 2009 7:26 AM    Report this comment

Greg provided some impressively high fuel flow numbers for a PT6A at ground idle. I asked my contact at P&WC about this and was told at idle the engine is turning around 48% Ng and the idle speed is set above minimum flow to sustain combustion. Now I'm curious if a higher CR would reduce idle fuel flow, or is that just the nature of the engine?

Posted by: Will Alibrandi | October 9, 2009 8:18 AM    Report this comment

I just had a look at that news Release from Price Induction that Will sent me.

Well the good news is they are going in the right direction. They are planning on increasing fan size to about 20 inch on the next engine. Bypass rastio will be over 15. This is more like it.

I did the numbers for duct fan rotary engine of 300 hp and the minimum size that will give decent takeoff thrust was 20 inches.

Also planned is a contra-rotating fan engine with an even bigger fan diameter and a bypass ratio of 35. Now this is getting serious.

Now it is important to point out that the core engine is still the same low-compression engine. However, with the bigger fans they are addressing the propulsive efficiency issue.

With the contra-fan they claim a propulusive efficiency competitive with turbofans and that is believeable with such a large fan diameter and the efficiency of contra-rotating fans.

They straighten out the swirl you get with a single fan or prop and recover the energy lost in the tangential direction. This also means less asymmetric thrust issues, since the flow is going straight back and not helically.


Posted by: Gordon Arnaut | October 9, 2009 10:34 AM    Report this comment

The goal is cruise SFC of 0.55 with the contrafan, which is almost exactly what I said earlier would be an achievalbe number.

They don't say what the diameter will be on the contrafan (probably cause they haven't got that far yet), but we can extrapolate it to be about 26 inches or so. That is still a manageable size

With the smaller 20 inch fan engine they are looking to get cruise SFC of 0.62. That's not bad either.

Again it is important to note that this is with a low-compression engine. With the contrafan, even if they miss theri goal and get to 0.6 cruise SFC they are still competitive with small turboprops like the pt6. I think this is more realistic.


Posted by: Gordon Arnaut | October 9, 2009 10:46 AM    Report this comment

Again, this is for unpressurized planes operating in the teens. This is what makes these engines different from the VLJ engines we have now.

The contrafan engine (called the TAOR 390) will be about 1000 lb thrust, while the 20-inch engine (DGEN 410) will be about 860 lb thrust.

They mention these for twin-engine 4 to 6 seaters weighing from 4,000 to 6,000 lb. I would add that one of these could be used on a single of about 2,500 to 3,000 lb.

Again, this is encouraging because it is moving in the right direction of large fans and very high bypass ratio, which is what is needed.

True, they have no roadmap for a high-compression engine, which would really be the piston killer that we all dream about. But this is a good start.

Posted by: Gordon Arnaut | October 9, 2009 10:56 AM    Report this comment

Correction: With the contrafan engine they are shooting to be competitive with turboprops, not turbofans.

Btw, bypass ratio is not usually a measure that is used on a turboprop, but if we were to measure it on a pt6, for example, it could be quite high. Easily over 50 to 1, maybe even 100.

The measure is simply the ratio of air mass flow going through the fan (or prop), versus the air mass flow going through the engine for combustion. That's why it is called bypass ratio, because the air bypasses the engine.

That engine with the big fan 26 inch (?) and 35 to 1 bypass ratio is therefore not that far off a turboprop of even 100 to 1.

I mentioned previously that a fan in a duct will be about twice as efficient (for its size) as an open prop. This is because the tip losses in a ducted fan are largely eliminated and so is the induced drag that goes with that.

That means that a fan of half the area of a prop will give the same thrust on the same power. When we add contrarotating blades, the efficiency of the fan is increased even more.

So yes, the contrafan and the large diameter (bypass ratio) means it could well be close to a turboprop in propulsive efficiency.

Posted by: Gordon Arnaut | October 9, 2009 11:13 AM    Report this comment

Bottom line is this: The contrafan engine with big diameter fan could in fact be as efficienct as small turboprops like pt6, even though it is a low-compression engine. Maybe even a little better at low altitudes because that is the design point of the engine.

The slightly smaller non-contrafan engine will burn a little more fuel but it will be a good improvement on that small 15 inch fan engine they have now.

Snecma and Turbomeca certainly know how to build engines. There are thousands of CFM (Snecma-GE) B-737s out there flying quite efficiently.

Posted by: Gordon Arnaut | October 9, 2009 11:24 AM    Report this comment

Just to give an example here. Cruise thrust they are shooting for with the contrafan engine is 370 lb thrust at M 0.38 (about 215 kt), 10k ft.

If we take that C400 we have been talking about and put one of these contrafan engines on it (we would have to give it a v-tail and mount the engine pod on top of the aft fuselage), we would be able to push that airplane to just over 200 knots, which is about M 0.32 at that altitude.

Let's say we don't quite make the SFC of 0.55 that they are shooting for and we see 0.6. Cruise thrust of 370 lb * 0.6 = 222 lb/hr, just over 30 gph.

This would be much more attractive to the single pilot than fuel flow of 40 gph. Might make all the difference.

Posted by: Gordon Arnaut | October 9, 2009 11:42 AM    Report this comment

Actually they do give the fan size of the contrafan (I missed it on first glance). It is 700 mm, which is 27.5 inches.

That is a good sized fan. With those contra-rotating blades, it will not be giving up much efficiency to a prop, if anything.

Again, we are talking just propulsive efficiency here. The core engine remains the same low-compression engine.

Of course all of the small turboprops we have now are low-compression engines too.

Posted by: Gordon Arnaut | October 9, 2009 12:18 PM    Report this comment

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