Speed of the air of a hovering helicopter

[nowayn]

Homework Statement

A helicopter is hovering 10m above the ground, weighs 6230kg and the rotors are 14,6m in diameter. Which speed does the air have that the rotors are pushing down?

The Attempt at a Solution

i really have no idea where to begin but the chapter has to do with impulse so i assume its solved using that in some way.

 

[willem2]

If the speed is v, how much air is going through the rotor every second? What is the momentum of this air?