What Are Hybrid Electric Airplanes Good For?
When I drive around town, I never fail to see some kind of hybrid car, say a Toyota Prius or maybe a Ford Fusion and the occasional Nissan Leaf. This tells me that hybrids and electric vehicles are now dominating the automotive market and airplanes are soon to follow. And here, let me welcome you to Paulie’s Moment of Magical Thinking. The reality is electric vehicles—both hybrids and pure electrics—command less than 2 percent of total market share in the U.S., although it’s greater in other countries. Even in the U.S., electric vehicle sales are on the upswing. Still, tap the brakes here and pitch the nose up for application to airplanes.
Some perspective is needed, especially following the announcement this week that three companies, Tecnam, BRP Rotax and the giant German electrical concern, Siemens, are developing a hybrid drive for aircraft. But read carefully between the lines and you sense what they said—“the project will allow broadening the horizons of knowledge in the field of parallel hybrid propulsion systems”—is describing not a market entry, but a technology demonstrator. I think these companies are smart enough to know that market potential for an aircraft piston hybrid drive is thin at best, non-existent at worst.
Consider the technical points. In a car, a hybrid drive like the Toyota Prius has marginally better economy than the equivalent best-case internal combustion model, but its life cycle costs may actually be more than the equivalent ICE, depending on what you pay for gas and how you drive. If CO2 emissions are a thing for you, hybrids are marginally greener.
What about airplanes? The only hybrid close to fruition is the Euro-sponsored HYPSTAIR, a video of which you can see here. This is a serial hybrid design that has a Rotax 912 or 914 engine powering a generator to charge batteries carried in the wings. The prime power is a 200-kw (270-HP) brushless DC electric motor. The typical operating cycle is to take off on batteries, cruise with ICE while charging the batteries, then complete the trip with theoretically charged batteries. It hasn’t flown yet, so it’s yet to be proven if the claims will pan out There’s also a weight penalty. The hybrid drive system is comparable in weight to a diesel power plant of similar output so useful load will take a hit. Range might or might not be better.
There are some of realistic plusses. One is that the battery-driven motor is unaffected by density altitude, so the full 270 HP is available anywhere, even if max prop efficiency is not. Second, reliability. Although the electric motor is a low probability single-point failure, the ICE is at least there to keep the batteries alive. I would call this a sort of redundancy. Overall efficiency is simply an unknown. While it uses a smaller engine to charge the batteries, there's a conversion penalty over simply throttling back a larger gasoline engine for similar performance sans the hybrid drive. Don’t forget noise. That‘s huge in Europe, a little less so in the U.S.
The Tecnam/Rotax/Siemens project is a parallel hybrid, not HYPSTAIR’s series design. They haven’t revealed details, but this means it uses an electric motor as a kind of helper or torque booster for the gasoline engine. FlightDesign fooled with this idea a few years ago by adding a parallel motor to a Rotax engine. It was sort of a giant starter motor on top of the engine and could be engaged for takeoff—relying on battery power—and in standby mode for emergency power. Of course, to be an effective standby, the electric motor has to have a mechanical path to the prop that won’t be tanked if the crank breaks. Again, there’s a weight penalty here in motor and batteries against the benefit of added takeoff performance.
That’s the market entry challenge. Try this thought test: Does a hybrid drive solve a significant shortcoming of gasoline engines, such as poor takeoff performance, low efficiency and lack of reliability? And if the hybrid does these things, will it attract buyers in sufficient volume to make the juice worth the squeezing? For now, I doubt it. I’m not even seeing a future where it makes much market sense at all except one: where a hybrid is used as a range extender in an electric aircraft that uses distributed power to leverage the advantages of electric motors, say the Volocopter, for example.
Pure electrics are different. Entirely. As Pipistrel is showing, there may be just enough early adopter interest to make airplane BEVs practical. As battery capacity inches upward, the curve will eventually advance to the point that schools can look at the numbers and make a business case for a mix of electrics and gasoline airplanes. Some are almost there now, but we don‘t have enough operational data to verify cost and performance claims for electrics. In the U.S., there are approval details to sort out, but this is just bureaucratic noise. It’ll eventually get done and electrics will carve a share.
At AVweb, we do a lot of coverage in the electric, alternative power and autonomous flight fields. Casual readers may assume that such technologies are gaining increasing momentum and practical market entries are just around the corner. “Much sooner than you think,” as I’ve been told by more than one electric airplane enthusiast. Depends I guess I what they think I’m thinking, which is usually a decade or two.
Still, Tecnam et. al. are pushing this technology hard because they have no choice. If they don’t, someone else will and even in the stodgy backwater of piston general aviation, things don’t stand perfectly still. Only the companies pursuing blue sky initiatives that may go nowhere in the short term will be competitive in that decade or two I’m thinking about. We should all be happy to see such things.