Can you recapture kinetic energy of an electric aeroplane, via it's propellor, and return it to a battery. Yes of course.
Should you recapture the KE of an electric aeroplane? No.
Let me explain:
People are familiar with "regenerative" braking in an electric car, where when you brake, instead of using friction brakes to slow the car down, it runs the traction motor as a generator, and re-captures the energy stored as KE in the vehicles mass. On a typical passenger car, this has some advantages, but is by no means 100% useful.
But the Important point is:
Passenger cars have a useage profile that includes very variable speeds. Due to things like traffic lights, bends in the road, other cars, junctions, etc etc generally,soon after accelerating to some speed, the car will need to decelerate, often back to zero. As such, regen braking, can recapture approx 40-50% of the vehicles KE and hence extended range and reduce overall energy consumption.
Now, for a fixed or rotary wing aircraft, in order to remain flying, that wing must keep moving, and in fact, it generally only works across a fairly narrow range of dynamic pressures. As such, an aeroplane (or the rotor on a helicopter) flys at a fairly fixed speed, and without things like traffic lights, or junctions, doesn't have to "stop" (which is of course a good thing, as it's can't stop!! So, what exactly is the regenerative drive system recapturing in this case? The aircraft's speed isn't changing, and so the KE stored in it's mass is constant.
Of course, someone who doesn't actually understand how an aircraft flies, might say "AH HA, but what about the potential energy stored in the planes height!!". Well, there is of course PE, but as anyone with more than 3secs of flight experience will tell you, this is pretty much exchangeable for KE, infact, the phrase "trade height for speed and vise versa" is the first thing you will hear when learning to fly. The ONLY factor that matters for an aeroplane (fixed or rotary wing) is the ratio of Lift to Drag! The L/D ratio sets the efficiency of the craft, nothing else. In effect "Lift" is free, as long as we keep the wing planform moving, we go up, simple as that, so what matters then is the Drag of that planform, ie, how much power does it take to keep it moving through the air. Something like a glider, with an L/D of up to 60:1, doesn't even NEED an engine to stay flying, once the initial KE has been stored (ie the aircraft has been accelerated).
So, the only time recapturing KE in an aircraft would help is for the landing (the only time you descend without then ascending again) and unlike a car, you only every land once per journey. If you've got some spare time, you might like to consider how you would calculate the ratio of energies (total cruise drag, vs PE) for some typical aircraft (hows about a 747 on a 10hr transatlantic flight, or a light aircraft on a 45min sightseeing flight)
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