But are anybody knowing of an attempt to develop electrical/hybrid "regenerative braking" ?
I mean propellers capable to change pitch to run as RATs along with reversible electric motors switched to generating regime ? I think that on a long descent there are a lot of energy which could be "harvested"
I suspect the most efficient way to recoup the energy used to get to cruising altitude is a power-off descent in the lowest drag configuration possible (i.e exactly as now).
Regenerative props will, by definition, create much drag and therefore only be effective as speed brakes. However, as such they will recover at least some energy from an otherwise non-optimum managed descent profile.