What justifies replacing a direct-drive IC engine with a system requiring both an engine and a motor, both batteries and fuel tanks, and complex control systems to keep the whole thing transparent to the pilot? It has to be a significant gain in efficiency.
This is where the calculations get tricky. The Volt, which like all hybrids bombards its driver with displays intended to let him know how green he is being, would end each fuel-free outing with the announcement that we had achieved 250-plus miles per gallon. Actually, this was too modest; we had used no fuel at all and had therefore achieved infinite miles per gallon. The EPA had quite a struggle to come up with a miles per gallon equivalent for partially electric cars. But it would have been a different story if we had been going 300 miles at 100 mph. It is claimed that a hybrid aircraft power plant would be 25 percent more efficient than a simple recip, but in the absence of practical experience I don’t think such estimates mean much.
Perhaps practical experience will soon be available. AeroVironment has built a high-altitude, long-duration unmanned aircraft using multiple electric motors and a Toyota engine, and at the Paris Air Show this year Siemens, EADS (the European aerospace conglomerate that includes Airbus) and Diamond Aircraft unveiled an experimental hybrid Dimona with a 100 hp electric motor, batteries and a rotary-driven genset. It’s always a bit mystifying to see an outfit the size of EADS dabbling with two-seat motorgliders, and one suspects some ulterior motive, like a hybrid UAV that travels to its target under IC engine power and then switches to a silent electric motor to perform surveillance. You can dream up all sorts of uses for hybrid power plants, so long as you don’t compel them to meet the current standard of practical personal transportation.
The standards of performance we have become accustomed to are very hard for a battery-based system to meet. Current electric airplanes are either small, or slow, or limited in range and duration, or cumbersome because of gliderlike proportions, or all of the above. There are no battery-powered, 900-mile-range, 170-knot four-seaters, and there will be none for the foreseeable future. Hybrid systems escape the constraints of batteries; they can do anything a recip can do and anything a pure-electric airplane can do. No doubt they can deliver improvements in efficiency, but whether those improvements warrant the costs of the extra complexity is doubtful. Look at the Volt. It’s a sweet piece of engineering, no doubt, but it’s a $41,000 car. How many trips to Provincetown is that?