Pelican's Perch #65
Where Should I Run My Engine?
(Part 3 -- Cruise)

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Cruise -- Time to sit back and enjoy the flight. But wait ... did you leave the mixture set where it was during the climb? Or do you just set it where it

Pelican's Perch

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News Flash for Carbureted Engines

As with my other columns in this "Where Should I Run My Engine?" series, this one is for owners of airplanes with fuel-injected engines. However, we have some recent information for those with carbureted engines -- see the sidebar below on the right side.

OK, Back to Our Flight

Where were we? Oh, yes, I left you hanging at the top of climb, wondering how to set up for cruise, in an airplane with a normally-aspirated, fuel-injected engine.

We discussed ground leaning, runups, takeoffs (always with full power), and climbs rich of peak (ROP), noting the EGT after takeoff (with proper fuel flow), and leaning to maintain that. We also discussed how to determine the proper fuel flow with full power at sea level, and how to detect too little fuel flow (VERY common problem!) or too much fuel flow (just lean a little). Full throttle throughout, and full RPM too, UNLESS there is a LIMITATION, or you like some slightly reduced RPM for noise and/or smoothness.

We talked about the leaning process in the climb. If you climb a little, the EGT (and CHT) drops a bit from the enriching effect of higher altitude (same fuel, less air). So we suggest you lean a little, bring the EGT (and CHT) back up to the target, and repeat that cycle. (Remember, when ROP, "leaner is hotter.") At some point in the climb, probably around 8,000' MSL, leaning the mixture will NOT bring the EGT up, it will drop it, showing you've gone lean of peak (LOP).

To emphasize the point, please remember, "Leaner is hotter" ONLY when ROP. "Leaner is ALWAYS COOLER" when LOP. (It's cooler for EGT, CHT, valves, and valve seats.)

Cruise

Ok, so you've reached cruise altitude. What to do, what DO you do?

Well, the first thing you do is leave alone whatever climb power you had set, and accelerate to your approximate cruise speed, while closing cowl flaps, if any. No, this has nothing to do with getting "on the step," for there is no such thing, Ernie Gann notwithstanding. It's just a good trick to get up to cruise speed sooner.

There is an OWT ("Old Wives' Tale") out there that the engine should be left to "stabilize" at cruise for some minutes before closing the cowl flaps. But think about this. If you quit climbing and accelerate without changing anything else, the CHT will drop, due to the increased cooling flow. So what you're really doing is running the CHTs down, only to turn around and run them back up again later, when you set up for cruise! That's harmless, but why wait? In fact, in some airplanes, you may benefit by climbing with the cowl flaps partly (or fully) closed. The CHTs tell the tale, on this.

On the other hand, CHT indication *on the ground* may NOT tell the tale, as there is not enough airflow for reasonably uniform cooling, or for uniform temperature measurement. Your CHTs may all be fine, but there probably will be "hot spots" elsewhere that may cook hoses, wiring, and other things in the engine compartment. So, follow the old military and airline manuals, keep your cowl flaps fully open on the ground, ALL the time, even in arctic conditions, please. Let the engine breathe.

At this point, the question usually comes up about "minimum CHT." We know of no data to support a minimum CHT. The only evidence I can find is that many of the manuals for the old radials had a note (not a limit) that said, "Do not take off with CHT less than 100C," which is about 212F. Modern liquid-cooled TCM engines have the thermostat set for 240F. That seems pretty reasonable to me.

First, Speed and Strategy

The first consideration for cruise has nothing to do with engine management at all, but "airplane management." Specifically, the speed you use to get the job done. This is, by far, the most dominant effect for range and efficiency. Everything else pales by comparison.

So many pilots always seem to use 65% or 75% power, because that may be the only power setting shown in the POH, or because "everyone uses 65%." Some actually believe that since the factory only shows certain power settings, those are the only "approved" settings! This is reinforced to some degree by the old procedures, where airline and military managers insisted on very specific HP settings for cruise: MP was set to the tenth of an inch, RPM to the nearest 10, and so on. It was enough to make everyone end up believing that running 1850 HP when the only approved settings were 1800 and 2000 would cause the engine to blow up!

The real reason for that was standardization across all crews, and data logging for a primitive version of "trend monitoring." Airlines carried data logging to extremes, and it simplified data analysis if everyone did the same thing.

Towards the end of the prop era, many were doing the long-range flights with constantly decreasing power, very low RPM compared to manifold pressure, and decreasing airspeeds for range as weight dropped from fuel burnoff. This works in our smaller steeds, but results in very low speeds, and few of us have the patience.

It seems pretty obvious to me that if you keep your temperatures under control, and if you keep the internal combustion pressures under control, what else is there, on an engine that is (almost always) good for full power, for the life of the engine?

Some say, "Just set 23 inches and 2300, and forget the mixture."

Sorry if I offend, but that's unspeakable, a complete cop-out, and a lousy way to operate these engines.

"When all the fools in town are on your side, that's majority enough." - Anon

Folks, there's NO reason (except laziness) to fix on just one power setting, and no reason at all to even think in terms of "percent of power." Especially that 65%, that was a CRUTCH, invented by marketing departments and magazines, who wanted a common number for easy airplane comparison. It can be handy to use percentage as a reference tool when talking about engine management, as we do here.

Now, all that said, thousands of these engines used to run to TBO with only minor jug work, using the procedures I deplore so much. But those were mostly "conforming" engines, with good quality control during assembly. We don't enjoy that luxury today, and many of these engines will begin having problems at around the 500-hour mark. Make no mistake, this is NOT due to LOP operation, or anything else under the heading of "pilot technique." These engines will begin failing (valves and cylinders, mostly) REGARDLESS of your methods of operation.

Truly, your first job is to determine what your mission calls for. Most of the time, I just want to get there as fast as I can (oh, for a BD-10), regardless of using a few extra gallons of fuel. That's easy, I just set the most power I can get without hurting or overheating the engine, and go. I call this "Go Fast Mode." As an old friend once growled at me when I asked why he used nothing but max power, "I didn't buy a fast airplane to go slow." He was ahead of his time.

Go Fast, or Go Far

Surprisingly often, I am faced with a flight that is near the maximum range of my "tiptankless" airplane, and on those flights, I need to pay a bit more attention. With the accuracy of modern fuel flow instrumentation and GPS all connected together, it's very easy to just set up any old guess at a power setting, and see what the system shows for fuel at the destination. My personal absolute minimum fuel remaining with excellent weather, lots of airports very close to my destination, and when I'm feeling frisky, is 10 gallons, in my own airplane. I know how much fuel each tank holds to the tenth of a gallon, exactly where it is during flight, and how much I'm burning, with several cross references to back that up. That meets the regulatory requirement, and it meets mine, but you can bet I'm paying attention during that last hour or two!

This method works surprisingly well, even with wind changes on your route. You must wait until fully established in cruise, power (and mixture) set, speed stable, and very close to "on track" to your destination, because GPS calculates motion relative to your next waypoint ("closure speed"), and NOT "current groundspeed" for time and fuel remaining. Relative motion and current groundspeed coincide only when tracking directly to the next waypoint. (This is VERY different from INS, for those accustomed to that system, which uses current groundspeed regardless of the direction of flight.)

Once you are stabilized, with accurate data showing, keep in mind that shows data for your current weight. As you burn off several hundred pounds of fuel, those numbers will improve, offsetting any reasonably normal adverse wind changes.

For those who cannot just "go direct" to the destination, punch it in every once in awhile, and take a hard look at the numbers. Keep in mind you may have to fudge those numbers a bit for the actual "dog-leg mileage." Some of the high-end systems compute fuel time and distance for multi-leg routes, but most of us have to suffer, and do a little addition and subtraction.

(How spoiled I've become! Give me full-time, precise position and accurate, real-time data, and I want more!)

Many is the time I've leveled off, gotten stabilized, and noted "5.0" (gallons) in the "reserve fuel" display. As the trip goes on and I start thinking about alternatives, that will slowly count up to about "10.0" or a bit more at the end, with no wind changes. On those rare trips with a major wind change, this may not work, of course. Bear in mind you'll probably pick up a bit on the descent, as your airspeed builds and your fuel flow drops.

If I'm at a high cruise setting ("Go Fast mode"), and I don't like the range (or reserve fuel) numbers I see, I reduce the airspeed a bit by leaning more for optimum brake specific fuel consumption (BSFC), then pulling the RPM back as needed. I'll let that stabilize for a minute or so, then I'll take another peek at the numbers. It doesn't take a lot of speed reduction to have a major impact on "Predicted Fuel Remaining." (Under NO circumstances will I reduce throttle: That remains full open on all normally aspirated engines, and on turbonormalized engines. We can talk later about turbocharged engines that pull 35" MP and more.)

Why the sequence of leaning first, then RPM? For simplicity. The idea is to attain optimum BSFC first, then set the RPM as needed for the mission.

As a general rule in normally aspirated engines, you won't go too far wrong by leaning until you have a MIXTURE setting of about 15 to 25F LOP (at lower manifold pressure settings) or 25 to 50F LOP at higher manifold pressure settings in order to obtain the most favorable (minimum) BSFC. This is not really critical, anywhere around 50 LOP is fine, if the engine runs smoothly. Thereafter, reduce RPM as needed to get the reduced AIRSPEED you want or need to get the range you require to complete the trip with adequate reserves. On most airplanes, mixture and RPM give ample control to do anything needed.

You will rarely catch me slowing down a whole lot, because it's very possible to end up so slow that it's quicker to make the quick fuel stop, and using "Go Fast Mode" for the flights.

For extreme long-range work, it becomes necessary for me to slow WAY down, with about 120 knots being the MAXIMUM indicated at gross weights, and perhaps as low as 105 knots when very light.

The math involved for true long-range flight is complex, and beyond the scope of this column. There are many variables. If interested, please see John Eckalbar's fine work, "Flying High Performance Singles and Twins," for all the details on this. My own personal planning revolves around checking the time to the destination in "Go Fast Mode," then if that doesn't leave enough fuel, I'll slow down (as above) and check again, and repeat that until it's going to take me about 30 minutes longer than the original plan. Once I get to that point without being able to do the flight non-stop, I'm looking for a fuel stop.

Whither the Wind?

There is a common misconception "out there" that you should go faster into a headwind, and slower in a tailwind. This time-honored technique doesn't work!

Now that I've got your full attention with that outrageous statement, let me modify it a bit. The way most people understand it, it doesn't work. The way I fly, it doesn't work. The fundamental point that almost everyone misses is that this works only if your reference point, your starting point, is "best range speed" in still air! Most Bonanza pilots are shocked to learn that "best miles per gallon" (MPG) speed is about 110 knots, indicated airspeed, with no wind (See Eckalbar's work for more on this).

Optimum BSFC

Once you have figured out the RANGE and SPEED issue, the rest is easy, if you use a little science.

You simply want the optimum (i.e., minimum) BSFC, at the power setting it takes to get your desired performance. Here's a graphic we use in the seminars.

Landmarks for Mixture Settings

(Click on all charts to see larger versions.)

The EGT, CHT, HP, and BSFC curves are straight out of the TCM factory for an engine at 25" of MP, 2500 RPM, and have been included in various TCM publications for decades. We have verified those charts on the test stand, and in our airplanes. They are very close to reality.

Using actual data from the test stand, we have added "ICP" (Internal Combustion Pressure) to the curves. Note this tracks CHT almost perfectly, which makes sense. (So does valve temperature). We have also inverted the classic BSFC curve to be more intuitive for pilots (and more offensive for engineers). Instead of showing the usual "Pounds of fuel per HP per hour," we invert it and show "HP per pound of fuel per hour," on the theory that "more is better."

(Scientists insist this calculation must use a weight of fuel, and they are correct. For our purposes, converting to gallons is appropriate and useful. Aviation gasoline actually weighs about 5.82 pounds per gallon in shirt-sleeve weather. If you fly in much colder or hotter weather, move.)

As power settings change, the chart curves retain the same shape, and move the same relative to each other, except for the BSFC, curve, which does shift left and right just a little.

At lower power settings, "optimum" BSFC occurs at about 10-20F LOP, and at higher power settings, as much as 80 LOP. But the BSFC curve is so flat in that area, it really doesn't make that much difference in range. What may matter is that the engine may not run smoothly at lower MP if you get too far LOP.

Most folks are aware of the old story about reduced RPM reducing the "friction horsepower loss." For once, there is some reality here; the difference in my IO-550 between 2100 RPM and 2700 RPM is about 20 HP, by the chart produced at the factory.

Motored Friction HP vs RPM To develop this chart, the engine is driven on the test stand by a motor, with HP measured.

That's a "free" 20 HP, and well worth considering, if extreme range is your goal.

There is also an effect from "prop efficiency," which varies from prop to prop!

Wear and tear? Well, all the friction points should be well-lubricated, and that means no metal-to-metal contact anywhere. There should always be a microscopic film of oil holding the metal bearing surfaces apart.

Finally, while reducing RPM MAY improve your BSFC, there are many props that will lose enough efficiency at the lower RPM to offset most of this!

If you use the climb suggestions I gave last month, and you choose to level out and cruise at or below about 8,000' MSL (normally aspirated), you'll be pretty well set up for ROP cruise by simply closing the cowl flaps, and leaving the throttle, prop, and mixture alone. Simple. You probably don't want to lean any more, unless you go all the way to LOP. Leaning a bit more would put you in "The Danger Zone" (a.k.a "The Red Box," in our seminars).

If you prefer to use LOP (and I do, it's cleaner and cooler), do "The Big Mixture Pull" to get right through the "danger zone," (we call it "The Red Box" in our seminars), and get all cylinders over to the lean side. Let that stabilize, then check to determine where peak EGT occurs from the LOP side, on the RICHEST cylinder. Then lean again until that richest cylinder is out of "The Red Box."

(Note the "Lean Find" feature works just as well from the lean side as it does from the rich side. Once you're familiar with your numbers, and learn which is your richest cylinder, you'll never need the "Lean Find" feature again.)

The Dangerous Red Box

Just where is that "red box" I keep talking about? Some rough numbers, good (that is to say, BAD) for most of these engines -- these are "no fly zones," DO NOT set the mixture between them:


Red Box = No Fly Zone

  • At and below about 60% power, there is no red box. Put the mixture wherever you want it.
  • At about 65% power or so, 100F ROP to Peak.
  • At about 70%, 125F ROP to 25F LOP.
  • At about 75%, 180F ROP to 40F LOP.
  • At about 80%, 200F ROP to 60F LOP.


All those numbers are approximate! Please don't start splitting hairs, here!

You probably don't want to run your engine between those mixture settings. If you do, you are running very high peak pressures inside the combustion chambers, and that peak pressure is occurring too close to top dead center.

There's a chance you read too fast, and missed this very important point, so let me put it another way:


Outside the Box

  • At 65% power, use richer than 100 ROP, or leaner than peak EGT.
  • At 70%, use richer than 125F ROP, or leaner than 25F LOP.
  • At 75%, use richer than 180F ROP, or leaner than 40F LOP.
  • At 80%, use richer than 200F ROP, or leaner than 60F LOP.


(On most of these engines, with a properly set mixture at full rich, at sea level, full power, the EGT ends up at about 250F ROP, with some as high as 300F ROP.)

Cruise at Altitude

It's important to realize that if you choose to run at full throttle in a normally aspirated engine (and you almost always should), then your only control over manifold pressure is altitude. The higher you fly, the less the manifold pressure, and the smaller that "red box."

It's really simple if you climb above about 8,000 feet or so. The altitude will have reduced your power so that no matter what you do (normally aspirated), you cannot recover it -- you cannot get more than 60% or so -- and you can't hurt the engine with ANY mixture setting. If you want the most power you can get at the higher altitudes, then you must operate ROP, and 50 ROP is a pretty good place to be for that. As you climb higher and higher, you can experiment with the mixture control, and find out where peak EGT occurs, then just ballpark about 50 ROP.

Even with good, balanced fuel flows, you still have six (or four) engines flying along in loose formation, connected only by a common crankshaft. Some will be a bit leaner, some will be a bit richer. Which EGT to use? Think about this. If we are trying to avoid "the red box," then when ROP, we want to use the LEANEST cylinder, so that the others will be "less critical" out there even more ROP. If we want LOP operation, then use the RICHEST cylinder, leaving the others further from "the red box."

If you are running ROP, and decide you need more range, I think you'll do better to first drop the power by shifting towards LOP operation. Once you're properly LOP (as above), you're probably better off to start reducing RPM next.

Set RPM for Smoothness

I have one more consideration, smoothness. I'll go for a smooth engine every time, and some airplanes have specific RPM settings where they seem smoothest. You also don't need to be at some exact multiple of 50 or 100! 2534.202 RPM is a perfectly valid setting! It amazes me how some pilots will work so hard to get an RPM at exactly some "even" setting, especially those with digital tachs, accurate to 1 RPM. If it helps, put a piece of masking tape over that final digit! Heck, if you're LOP, cover up the whole tach!

A few engines just won't run smoothly LOP, no matter what you do. Bearing in mind that "smoothness" and "roughness" are VERY subjective terms; a little roughness from small cylinder-to-cylinder power variations won't hurt the engine at all. It MAY have a long-term effect on the airframe, accessories, instruments and hemorrhoids.

If you find that this roughness hurts your ears and your head (from your spouse beating you upside the head, or screeching in your nearest ear, "DON'T DO THAT!"), then perhaps you need your subjectiveness adjusted to more nearly match your spouse's.

While you're fiddling around, trying to find a power setting that will give you the range/speed you want, be aware that small changes in RPM do not affect the mixture ratio very much (fuel pump RPM changes with engine RPM), so you don't need to twiddle the mixture at every RPM change. Once you're where you want, if you want, do one final check for peak, and set the mixture to the appropriate setting.

Sound complicated? It's a lot harder to write and read about it, than to just do it! It's complicated here, because I'm trying to give you the reasons, the logic, and the science behind it, along with a few bad jokes. With a few practice runs, while thinking about it, these procedures become second nature, are very easy, and you're using science for power settings, rather than witchcraft, old wives' tales, and the "knowledge" of a 300-hour CFI. The single most common comment I get from folks who ride along with me is, "But you're not doing anything!" As in anything worthwhile, it takes a bit of practice to get used to it. Take along a safety pilot, the first few times, to watch for traffic, or even have him fly while you learn your engine.

Here's another way of looking at this problem. I am going to leave you with these charts, hoping you'll try to figure them out as an exercise between now and next month's column. I'll try and wrap up this series on flying normally aspirated engines next month.

Be careful, up there!

Mixture Example Chart Mixture Example Chart -- Annotated