CYHeli:
A helicopter in a stable hover is producing the right amount of power (lift) to overcome the weight of the helicopter. The engine and blades are doing work because gravity wants to accelerate the aircraft down. Gravity is a constant, so there is always a downwards force, so therefore even if the helicopter is in a stable hover, (vertical - upwards) force is still being applied.
I completely agree. It might be good to specify that the "power (lift)" you mention is BHP(SHP) as we've been talking about THP as well.
blackhand:
Is that relatively speaking?
You're going to have to read the Wikipedia article on work. I'm not going to regurgitate what it says.