A side-question on the perspective of battery-driven electric-flights: when electric cars can recuperate power instead of breaking, why can't airplanes do the same on descent? Couldn't they build a propeller at the rear, which does propulsion and recuperation? Energy-wise, that would certainly be attractive...
My gut feeling is that it's probably not substantial amounts of energy to find there. Compared to a car in city traffic which might break to a standstill every other minute, an aircraft generally only "brakes" once when going down for landing.
Further, since air resistance scales with the square of the velocity, the drag forces on an aircraft is so high that you typically never reduce thrust to zero until to just before touching down. An aircraft never idle-glide any substantial part of the flight. I.e, it's like trying to recuperate energy while driving slowly in a car up a very steep hill. Just releasing the throttle will make you stop quite rapidly...
I'm no expert, so this could be wrong but I have a gut feeling that during descent it could probably idle-glide by using the high-lift device sooner. If anybody was serious about flying with less emissions that could probably save lots of energy already today - it would just take longer to reach the target.
It would be interesting to see numbers how the gravitational potential energy compares to the drag forces.
