Factors that must be overcome include the 'drag' of the blades and airframe, retreating blade
stall, air flow reversal, and air compressibility.
Apparently, on the second pass, he banked the helicopter to the left, and the retreating blade
- the back end of the blade - struck the outcropping on the west side,'' Llorente said.
At high speed the main rotor is slowed to delay compressibility effects on the advancing tip, and unloaded by the wings to avoid retreating blade
The OSR reduces the rotational speed of the blades in forward flight, to delay compressibility effects on the advancing tip and inboard stalling on the retreating blade