Where Hybrid Aircraft Fit

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I was looking through the archives the other day to see when we did our first serious coverage of electric aircraft. It was 2009; five years ago this summer. We imagined then that production versions would be trickling to market in a few years. They arenít. But thatís not to say thereís not interesting progress being made in electrics and especially hybrid drive for aircraft.

I wasnít particularly surprised when Airbus announced last week that its skunk works had flown a ducted fan electric that will serve as a gateway into research on what Airbus really has in mind: a hybrid-drive regional airliner. Itís not surprising that theyíre going in that direction, but the technologies they hope to useówhich barely exist never mind being matureóare right out there on the edge. More on that later.

Counting Airbus, I know of five hybrid-drive projects, including versions from Flight Design, Pipistrel, Diamond and another company set to announce a new project of its own that I canít mention yet. You donít even have to dust off your calculator to conclude that these hybrid ideas are simply daft economically. Diamond, in partnership with Siemens, claims it has the weight under control, but as with so many of these projects,†the return on the required investment makes three-acre lots in the Everglades sound like a good retirement play. But then general aviation has always had its share of dingbat ideas that everyone knows will never work. Flying cars anyone?

With commercial success unlikely for the moment, why are these companies chasing these moonbeams? Itís simple and actually quite sensible. Theyíre the test-bed projects that will lead to the next generation of electric airplanes and the ones after that, where commercial viability will be in reach, if not assured. Hybrid-drive research aircraft will allow sustained electric flight in the thousands of hours and eventually prove technology that might finally circle back to hybrids that do work economically. Theyíre experience generators and could provide some eye-opening performance points.

When I was in Slovenia last month, I got a look at Pipistrelís Panthera Hybrid, which is just in the concept stage. It will be a serial hybrid, meaning the gasoline engine drives a generator, but has no direct motive connection to the prop. Thrust comes from a seriously powerful brushless DC motor capable of as much horsepower as you have electricity to pour into it. In fact, Pipistrel envisions a five-blade prop to absorb the torque of what could be a very sporting takeoff indeed.

This design is sort of a booster concept. The batteries, which live in the wings, will sustain full power for only six minutes. Then the airplane transitions to hybrid flight on a turbocharged, four-cylinder engine vaguely based on a Rotax 914 driving a generator, which in turn drives the motor at lower power levels suitable for cruise. The plan is to have sufficient surplus power to recharge the batteries after takeoff. Why go to all this trouble? Pipistrel sees five reasons: less noise, high-performance regardless of density altitude, multi-fuel capability and dual power sourcingóa twin in a single, if you will. But the overarching reason is that this is a research project; the first step in whatís next.

The same ethos is driving Flight Design, which is pursuing a parallel hybrid concept (PDF). Same booster idea, but the electric motor works in parallel with the gasoline engine to provide more power for short bursts, when itís needed. So youíd use the electrics for takeoff or perhaps climb, but throttle†back during cruise, as with Pipistrelís serial design. The efficiency gain comes in having a small combustion engine punch above its weight with the addition of 40 more horsepower from the electric motor. It wouldnít require the same battery capacity as a pure electric airplane, so the overall weight penalty isnít as great. The electric motor provides some redundancy if the combustion engine fails, but just glide stretching, since it lacks the power for sustained level flight. To make these things work, both Flight Design and Pipistrel will have to work out challenging details related to battery technology, charging and electronic control logic. Pipistrel, for instance, is using distributed computing to manage the hybrid power train. Theyíll have this technology available when battery capacity catches up and makes electrics practical. That could be awhile, but catch up it will.

Then thereís Airbus, which announced the E-fan last week.†Again, thereís no practical airplane lurking in this prototype, just the beginning of a developmental thread that the company hopes will lead to something breathtakingly ambitious: an electric airliner. When I first heard this concept, I thought it was even nuttier than flying cars, but I didnít have the imagination to consider it as a hybrid. Further, when ducted fans were mentioned, I envisioned a DC-9 type airplane with big fans on the back. Another failure of imagination, for electric airplanes will require far lighter and more efficient airframes than are being built today. A battery company has been testing an electric Cessna 172, but I can't imagine there will ever by a commerically viable electric Cessna 172. If GA remains so hidebound to still support Skyhawk production by the time batteries are capable of flying it, we're all screwed.

You can get a feel of the airframe requirements from this prospectus (PDF) describing the so-called E-Thrust concept which would conceivably be employed to power the electric airliner Airbus has its eye on. EADs and Rolls-Royce are throwing serious money at this idea. It has a timeline of about 2050, which seems realistic to me. One driving goal is to reduce carbon emissions by 75 percent. While the world debates carbon control, companies designing aircraft for the mid-term future clearly realize aviation is vulnerable to high carbon emissions and they know that to stay in the game, they canít rely on turbine engines forever.

The E-Thrust idea envisions a gas-turbine serial hybrid distributing electrical power to clusters of motors driving high efficiency fans. As with the Pipistrel hybrid, batteries--or energy reservoirs of some kind-- would provide power for high-demand takeoff and landing go-arounds and would provide sufficient redundancy for landing in case of a generator failure. Conceptual review suggests that one large single gas turbine is more efficient that several smaller ones so, yup, this is a single-engine airliner. Will it spawn the inevitable acronym, ESEOPS?†(If you think certification in 2014 is a challenge, imagine driving something like this through the regulatory hoops. It gives me a headache.)

But thatís the nature of progress. The engineers who will sit across the table from EASA and FAA to complete the short strokes on this project probably havenít entered undergrad school yet and I envy them the interesting future ahead of them. As for light GA electrics, Iím bullish, but patient. These hybrid projects will jolly things along, but battery capacity gains are steady, not rapid. Within a year or two, I think a couple of companies will work around that limitation with quick-change packs and get minimal capability electrics into the market. And theyíll find buyers, too, albeit not a frenzy. Thatís okay with me. Iím just anxious to get my mitts on one.

The hybrid idea for light aircraft seems inevitable if distant, benefitting from both battery progress and another trend most of us tend to forget: internal combustion engines are improving just as rapidly as batteries are. Toyota has been using Atkinson-cycle engines in its hybrids for several years and next year plans to introduce in the U.S. an Atkinson-based model called the Aygo, which it claims will yield 78 mpg fuel economy. The Aygo is already marketed in Europe. My guess is an Atkinson-cycle engine would adapt well as an aircraft hybrid powerplant, since it doesn't need a gearbox or any kind of thrust bearing arrangement to run a generator. I wouldn't be the least bit surprised if Toyota has already done this, since the company has shown a predilication for dabbling in aviation.

Interesting times.

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Comments (13)

Are these hybrid systems even close to certifiable? I know one of the intractable issues with the FAA with things like GAMI's PRISM system and similar was that they could result in an unpredictable, if still minor, power loss. I'm thinking a case where a takeoff is made and an approach needs to be made back into the departure airport immediately. Would go-around power be available?

Posted by: Joshua Levinson | May 9, 2014 7:27 AM    Report this comment

Airbus is building a new 300 worker factory just outside Bordeaux to build two seater trainers based on the E Fan prototype. Target market is training and flying clubs, and the promise is an hour's flying from 3 hours of quick charging using 380 volt tri-phase plugs. It could even happen, especially as both Renault and Bollore (Bluecar) are saying newer and better batteries for their electric cars are two years away.... As for hybrids, the real advance will be in the fossil fuel part. Anyone who has driven or heard a diesel-electric locomotive at work will know the diesel part is not a fixed speed generator, but a motor which reacts to differing loads on it, albeit in a limited rev range. The demands are different from car or existing aircraft engines, so when an efficient, light weight one is invented and built, the way will be open for it.

Posted by: John Patson | May 9, 2014 9:15 AM    Report this comment

When it comes to hybrid designs for aircraft, I have wondered why the drive train concept used by a diesel locomotive wouldn't work. I found my answer. The reason is that solutions are for completely different problems. In a locomotive, engine powering the axles directly would require some type of clutch device. The extreme mis-match in engine RPMs as opposed to the axle RPMs while starting to move a train, would soon disintegrate any clutch. Also, using electric motors at the axles provides more power than just the diesel engine is able to provide by itself. In an aircraft there is some mis-match between prop RPM and engine RPM but not any were near as extreme. Of course weight is not a major consideration for a locomotive. For power to weight ratio, is they anything better than a 2-stoke? Regarding flying cars... there may be one that will actually work http://www.samsonmotorworks.com/

Posted by: Dana Nickerson | May 9, 2014 9:33 AM    Report this comment

Actually, Dana, a diesel-electric locomotive is a serial hybrid and apart from the fact the diesel operates across a narrow band of RPM in response to load, its duty cycle is similar to an aircraft.

It requires high breakaway thrust, then less power once speed is reached. Unlike an airplane, it doesn't have a big aerodynamic load to overcome, just rolling resistance of all those wheels. In the early days of diesel and throughout the period of steam, railroads commonly used helper engines to start a heavy train. The helpers would push while the main unit pulled and once it was rolling, they would back off. I wonder if they still do that for those big Powder River coal trains.

In research I did on diesels, I came across a locomotive--there may have been more than one--that wasn't a hybrid but conventional transmission, with multiple gearing. It's called a diesel-mechnanical. The diesel-hydraulic used a giant torque converter--an automatic transmission. But it was too complex and maintenance intensive and by then, electric traction motors were efficient and powerful.

You can see the same thing going on in aircraft technology, albeit they need batteries for the breakaway part, otherwise there's no advantage. Might as well stick with an engine big enough for takeoff, then throttle it back for cruise. Interestingly, because of their weight, diesel engines aren't attractive for aircraft hybrids. Their fuel economy isn't enough help.

Twenty years from now, the landscape could be entirely different in GA. Or...my successor will be explaining why it isn't.

Posted by: Paul Bertorelli | May 9, 2014 10:50 AM    Report this comment

A separate, but somewhat related thought: General aviation engine manufacturers have always seemed to be behind the progress curve of automotive engines. For instance, exhaust headers have been a tried-and-true recipe for car and truck engines for 50+ years. Yet, GA engine manufacturers refuse to introduce new engines with this basic technology. Exhaust headers have been proven, time and again, to provide more power and better fuel economy with no increase in operating costs. In fact, exhaust headers REDUCE operating costs. The same is true about variable valve timing.

It is time for GA engine manufacturers to step up to the plate and bring their engines in line with the progress made by the automotive sector.

Posted by: Matt Williams | May 9, 2014 11:00 AM    Report this comment

"It is time for GA engine manufacturers to step up to the plate and bring their engines in line with the progress made by the automotive sector."

What incentive do they have to do that? The vast bulk of their market is re-manufacture of existing designs. Where would they sell new designs? You can't hang a new design on your existing aircraft without an STC. The new-vehicle OEM market is tiny. The FAA remains allergic to anything that's truly new.

So why should the engine manufacturers "step up to the plate?"

Posted by: Thomas Yarsley | May 9, 2014 3:33 PM    Report this comment

Matt, Thomas,

Most of the automotive technologies would honestly not do a whole heck of a lot of good in an aircraft engine. Variable spark timing is just useless complexity in an engine running at full throttle 95% of the time. Variable valve timing is useless for an engine running at a constant RPM 95% of the time. Automatic mixture control, sure, might be nice, but oftentimes pilots want to be able to choose between 150ROP go-fast and 75LOP for economy. For something you set once at top-of-climb and don't touch until before-landing, or somewhere in the decent, I'd say for the most part, FADEC is a useless bolt-on as well.

As for exhaust headers, fine, maybe a few percent improvement in fuel economy, at the cost of more metal piping under the cowl, getting in the way of airflow. In a liquid cooled car engine that's running at 15% power, who cares. In an air-cooled engine with rubber baffles, you really don't want more stuff in the way. Also, not trivially, more metal pipes means more weight.

And really, I don't know that an exhaust manifold really does much good in a low-RPM engine anyhow (I'd have to look into this more). The more time there is to scavenge exhaust, the less of an issue back-pressure is. In fact it might even hurt. A car engine might put out 200HP, but with a peak torque of 200 ft-lb. Our airplane engines put out 200HP at between 550 and 650 ft lb of torque. Back-pressure HELPS, not hurts, at low RPM, high torque.

I'm all for pointing at the lack of technological progress in aviation, but for the most part, our engines are not the issue. Running lower octane gas would be nice. But the fact is, these engines have pretty damn good BSFC numbers. It's tough to compare with a car given the different duty cycles, but I wouldn't be surprised at all if they were as good or better steady-state.

Posted by: Joshua Levinson | May 9, 2014 3:56 PM    Report this comment

Interesting article and comments. So we're looking at a system where something like an APU is actually the main power source. You can be sure that there'll be secondary power being generated using the waste heat, as with industrial fixed turbines.

Tesla seems to have the best gear at the moment. A Tesla 85kw.h battery is about 450kg all up. So that's about 110hp for an hour for 1000 pounds of weight. Maybe the aviation batteries are slightly lighter but it's easy to see why pure battery power isn't going to revolutionize aviation any time soon. An exception might be the super ultra-light stuff with short range and/or powered glider flight profiles. This hybrid thing excites me because of the potential you mention Paul for some battery capacity for the quick squirt of power and a hybrid system for cruise. That Tesla battery could provide 1100hp for 6 minutes.

Posted by: john hogan | May 11, 2014 11:10 AM    Report this comment

Joshua, Evidence from the experimental world shows a big improvement from the installation of electronic ignition with variable timing. When you're cruising up high, you aren't operating at full power even with the throttle wide open, and variable timing gives you smoother operation at those lower power settings. That's the key; it's not raw BSFC at full power, but rather being able to smoothly and reliably operate at leaner mixture settings and lower power levels. Matched with balanced fuel injection, you can see a significant fuel burn savings over a traditional magnetos-and-carb engine where leaning is often limited by one particular cylinder that runs leaner than the rest. Variable ignition timing might not benefit the flight school C150 that buzzes around down low at full rental power all the time, but most other airplanes would see a nice benefit if the FAA and most pilots would simply accept that aviation technology didn't actually peak in the early 70s.

Posted by: Bob Martin | May 12, 2014 7:57 AM    Report this comment

So far, most electric airplane development has focused on plug-ins. I've always felt that was mostly applicable to very light sport airplanes. I have long believed that the hybrid concept holds more promise for general aviation than all-electric.

I'm not out to "save the planet" by reducing emissions--or even looking for a cheaper way to get around--though efficiency does count. Instead, I'm looking for multi-engine redundancy in an aircraft--and the hybrid holds promise.

The battery system in a Prius weighs 134 pounds. I don't have the exact weight for the engine and electric motor, but don't believe that the entire drive system would be much different than a pair of IO-520s. It should be feasible to adapt the proven drive system to a Bonanza-type airplane. The combination of the powerful electric motor and the automotive engine would provide sparkling takeoff performance. In the event of an engine failure, it takes very little power to keep an aircraft aloft--the battery would allow getting back to an airport--perhaps not allowing a missed approach, but certainly better than the alternative.

Certifying the STC for electric power alone, and letting the auto engine provide the extended range should be do-able--certainly easier than certifying an entirely new engine. The use of an auto engine as a GENERATOR would solve the fuel availability issue, would allow us to take advantage of the improvements made to auto engines, and wouldn't cost as much as a large aircraft engine.

"Standing by to receive incoming fire as to 'why it won't work'.

Posted by: jim hanson | May 12, 2014 11:38 AM    Report this comment


I don't think there's a conceptual reason why it won't work. But I think you're vastly underestimating the weights involved. The newest Prius puts out peak power of 134 HP, gasoline and electric combined, and no automotive system is rated for high continuous power output the way an aircraft powerplant is. So, that's probably not good for any more than 40 or 50 HP max continuous power. How many of them are you going to need to replace that pair of IO-520s?

Posted by: Joshua Levinson | May 12, 2014 11:05 PM    Report this comment

With the new fuel injected Rotax 912iS Sport, imagine -- if you will -- what sort of single place speedster could be built if Rotax applied that technology to just a slightly larger engine AND the speed restriction on light sport was eliminated in favor of a horsepower rating ... say 125HP. At some point, someone in the FAA has to be FORCED to recognize that it is they who are holding things back in aviation. A two place version could be produced for training and a single place for recreational use.

Everyone is betting on the come for higher capacity and safe batteries and higher efficiency electric motors ... sorry, but I won't be climing into either a pure electric OR a hybrid any time soon.

Posted by: Larry Stencel | May 13, 2014 8:53 AM    Report this comment

The latest version of the Prius battery weighs 99 pounds.

The weight given for the motor(s) is 315 pounds.

Given the fuel efficiency, not as much fuel need be carried.

They (and other manufacturers) do make much higher horsepower systems--look at the Tesla high-performance car--but I couldn't readily find the weight of other systems.

As to continuous power output, The hybrid car has no transmission--the electric motor simply provides the power required. It takes relatively little power to drive a propeller in cruise--we typically use slightly more than half of the rated power in cruise, or 180 hp on a 300-horse engine. Once aloft, most "clean" GA airplanes can maintain altitude on 1/3 of the installed hp--or in the case of a Bonanza-type airplane, 100 hp. In the case of a similar installation in a Skyhawk, that requirement is only 50-60 horsepower.

As we have seen on diesels--there is a difference in horsepower rating due to torque capability--few diesels have the horsepower of the gasoline engines they replace, yet they give equal performance. Electric engines are capable of tremendous amounts of torque (think railroad engines), and an aircraft propeller could easily be constructed to absorb lots of that torque from an electric engine--further reducing aircraft noise.

Again--I'm not out to "save the planet"--I came at this looking for a power reserve so that we wouldn't be dependent on a single aircraft engine--I'd like a backup

Posted by: jim hanson | May 13, 2014 9:06 AM    Report this comment

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