In theory, this additional thrust should have an upper advance-ratio limit due to a growing region where the retreating blade
Factors that must be overcome include the 'drag' of the blades and airframe, retreating blade
stall, air flow reversal, and air compressibility.
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 flow field is unsteady and three-dimensional, with transonic flow on the advancing blade, dynamic stall on the retreating blade
, blade-vortex interactions (BVI), and vortical wakes.
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
. A piston-engined A160 flew in 2002, having been developed by Frontier Systems, which was taken over by Boeing in 2004.
Patrick Agyemang sprinted down the right with a man advantage over the retreating Blades