ORIGINAL: yallaair
This is more or less why we don't set up the heli rotor with a pitch more than 11 degree in any cases. Above 11 dregree, the efficiency goes down and you'll risk rotor stall.
Yallaair,
Helicpters fly in ground effect, otherwise most of the rotor will not be stalled at much greater AoA than 11ยบ.
If the rotor is stalled, its blades will not make enough lift to take-off.