Tom Neuman designed a practical electric plane for NASA’s design contest, and he thought about retrofitting a Cirrus SR22 with fuel cells:
The SR22’s normal power plant is a Continental IO-550-N, a six-cylinder, horizontally opposed, air-cooled engine that weighs 187 kg and provides 310 horsepower (231 kilowatts). By removing the engine and fuel and replacing them with a fuel cell of the same weight, I could possibly produce a similar amount of power. But to do that, the fuel cell would have to provide 500 W/kg of specific power. And at that level of specific power, the specific energy of the fuel cell would be about 400 Wh/ kg, roughly as good as the best batteries I could expect to use, which I already knew wouldn’t let my plane fly very far. To provide a specific energy of 800 Wh/kg, the fuel cell’s specific power drops to 200 W/ kg, well below the power needed to fly at 200 mph.
Unlike combustion engines, electric motors are compact and efficient. These small, light motors can be placed in many more locations on the aircraft than would be practical for a combustion engine. If applied strategically, this tactic can distribute the power production across more or larger propellers. And the greater the area swept by propellers, the more efficient and quieter they become.
I ran yet another analysis and found a sweet spot in efficiency using two rather large propellers attached to a pair of motors. Instead of mounting them conventionally, on the wing or fuselage, I put them in my design atop the plane’s V-shaped tail, where the airflow is cleaner.
This simple strategy not only improved propulsive efficiency (from 85 to 92 percent), it also benefited the plane’s aerodynamics. Now air could flow more cleanly over both fuselage and wing. And although the propellers were large, putting them on the tail meant that I didn’t have to increase the height (and therefore, weight) of the landing gear. Having short gear made choosing retractable wheels much more palatable, and this reduced drag even further.
When I ran the next analysis, I found that this change, combined with some more optimization, decreased the plane’s energy consumption by another 27 percent. Indeed, this design change had lowered the power demand to the point that it became feasible to fly the plane on hydrogen-powered fuel cells. That’s when I dubbed my V-tailed, hydrogen-powered design “Vapor.”
I see he even mentioned Pipistrel, the Slovenian light-aircraft manufacturer in the article. You can see a nice video of their flagship Panthera plane here. It’s super-sleek, carbon-fiber, and currently the most efficient certified four-seater, which will have hybrid and electric variants. Enjoy the landscape!