Hybrid cars work mainly by recovering energy from braking or going downhill to charge the batteries. Planes don't do that. Where do they recover energy?
While hybrid cars do those things, they aren't the main source of the efficiency of parallel hybrids, parallel hybrids like the Prius work by keeping the engine at the most efficient throttle setting (which is much higher than what would normally be used for cruising) whenever it is running, and using excess power from the engine to recharge the batteries.
The article describes this aircraft as doing the same thing.
It's possible that these requirements lead to engines which have a optimal power output above what would be required in straight & level flight.
This is a reason mathematicians loathe the overuse of "optimal" by engineers: in the real world there are often too much variables.
1. http://en.m.wikipedia.org/wiki/Atkinson_cycle 2. http://www.curbsideclassic.com/blog/the-atkinson-and-miller-...
Parallel hybrids like the Prius, Fusion, etc... do benefit from having something like a CVT, but the biggest gain comes from recovering kinetic energy during braking. Notice that they all get better city mileage than highway! That's because cycles of stop/accelerate have a very low penalty when most of the energy is recovered, so the difference in fuel economy is dominated by drag which is worse at higher speeds.
Hybrid planes and boats make little sense. This one wins by using a smaller engine and supplementing it with an electric motor for takeoff.
Now give me a series hybrid twin-engine aircraft and you've got some possible redundancy/safety improvements, but not much in the way of fuel savings.
I always thought that hybrids were more efficient because you can size the engine to the average power requirement, and not the peak requirement, thereby operating in a more efficient region most of the time, using the battery reserves for when you need peak power. And for some reason I was thinking that operating an engine at a fixed speed allowed for better engineering trade-offs.
"The petrol engine is optimally sized to provide the cruise power at its most efficient operating point, resulting in an improved fuel efficiency overall."
The fact that an airplane's speed and power delivery is much more steady state than a car's is the other big reason this has not been attempted until now.