Updated by Bill Lofton 12/21/16
If automobile EVs can regen while braking, then e-aircraft can regen while aero-braking. They can and they do.
Flight Profile and Regen
Aero-Braking Regen Requires a Negative Angle of Attack (AoA) of Propeller Blades
Maximizing Regen Power With Your Propeller
Propeller Design for Maximum Regen