'Gravitational' power vs. engine power A glider gets the power it needs to fly from the decrease of gravitational potential energy associated to the descent. My question is: for the same weight $W$, airspeed $V$, and a prop efficiency of 100%, is the amount of ‘gravitational power’  $W · w$ that drives a plane in an unpowered glide exactly identical to the ‘engine power’ it needs for s/l, unaccelerated flight?
Short answer yes, a powered aircraft at any given airspeed in straight and level flight (note that a glide path is also "level", at constant airspeed) needs to use the same amount of energy to over come drag as the glider.

The potential energy for the glider is altitude. The potential energy for the powered aircraft is fuel.
The conversion of potential energy to kinetic energy gives the aircraft velocity and matches drag forces.

There for, gliding is powered flight! They might "fill up" by catching a thermal, or by having someone tow them up.