Edgewise rotors operating at very high cruise speeds are required to reduce their RPM (as is done on lift-offset coaxial rotor helicopters and slowed-rotor compound helicopters) to avoid compressibility effects on the advancing blade tip. The very high advance ratios (resulting from the combination of high cruise speed and reduced RPM) put large portions of the retreating side of the rotor disk in reverse flow conditions where the air flows from the sharp trailing-edge of the blade toward the rounded leading edge. The blade sections operating in reverse flow suffer from extremely poor aerodynamics, generating negative lift, large drag and pitching moments resulting in poor performance (reduced efficiency) and high vibratory blade loads, pitch link loads and rotor hub loads. This project focuses on the use of camber deformation over the inboard sections of the blade to improve reverse flow aerodynamics. Wind tunnel tests as well as 2D computational fluid dynamics simulations to-date have already shown that use of reflex camber can reduce reverse flow drag by up to 40%. Ongoing studies are focusing on implementation of camber deformation on the blade, 3D CFD simulations and an assessment of operational methods (camber implementation schedules) to minimize the detrimental effects of reverse flow.
