you have to split two things here : propulsive efficiency/ power / useful work of the pure engine vs the aircraft as a whole package ( or a car or whatever elese) .
since all we know that any useful work needs a motion by physics a tied up plane or a car on a dyno does not any useful work since it does not move- efficency zero.
BUT
the engine of the car or the plane moves ( spins) , produces a given torque at a given rpm, the prop translates this torque to thrust etc. so the engine by itself of course makes useful work , produces power and has a given efficiency.
the useful work of the pure engine is used to run the dyno,to stress the brakes when they hold the aircraft, to keep our helicopter in hover against gravity.
long story short conclusion : the aircraft as the whole package does not provide any useful work, its engines do with the moment as they start to spin.
cheers !