Where Hybrid Aircraft Fit
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.
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