Originally Posted by
Uplinker
Not being an aerodynamicist, my untrained mind would imagine there to be less drag if air was allowed to go through a rotating fan, than if the fan was held fixed in the airflow - presenting a huge “airbrake”. As several have said, turning the fan and engine and gearbox will take power to do, but would that be more power than the equivalent “airbrake” would absorb though?
Think of the blades like a clutch that engages the airsteam to the rest of the engine, and unstalling them (by letting it windmill) as the engagement of that clutch. With them unstalled and windmilling (clutch engaged) now it's like you're engine braking downhill in a car. Locking the engine from turning stalls the blades and disengages the clutch, allowing the car to roll downhill freely. (Yes they still make more drag than if they weren't there at all, but less than they would if they're transferring energy to the engine.)
Talking about prop engines, some people think of the feathering function as a way to reduce drag by reducing the frontal area, but that's only a fraction of their benefit. By far most of the benefit is in preventing windmilling and transferring energy from the airsteam to the engine. If the engine seized and the blades failed to feather (i.e., stuck flat) the drag would be slightly higher than if they were feathered, but still far less than if it was windmilling.