Effects of two/rev higher harmonic control on rotor performance

The paper examines the effects of a 2/rev pitch input on rotor performance, for an articulated rotor helicopter. The underlying physical mechanisms are explored by comparing the distributions over the rotor disk of several aerodynamic quantities, and the variations of drag coefficient as a function of angle of attack and Mach number. The results show that properly phased 2/rev inputs can reduce the required rotor power by up to 3.8%. The improvements are very small. The primary physical mechanism is the change in distribution of the profile drag coefficient C-d over the rotor disk. The nonlinearities of the C-d variations with respect to both angle of attack a and Mach number M play a key role. The effects of the 2/rev input cannot be predicted with constant or linear C-d(alpha) approximations, and without including Mach number effects. Because changes in rotor power with a 2/rev input are strongly influenced by the presence or absence of profile drag peaks at the advancing tip, it is very important to properly model the dynamics and the aerodynamics of the tip region. For the 16,000 lb (C-r/sigma = 0.073) case, the minimum power is obtained for a phase angle, phi(2) = 210degrees which increases the angle of attack alpha on the advancing, thus reduces the profile drag peak at the advancing blade tip. The same phase angle also increase the angle of attack alpha on the retreating blade, where the quasi-steady values of a can go beyond the stall limit. The minimum power for the 22,000 lb (C-r /sigma = 0.099) case is obtained for a phase angle, phi(2) = 60degrees, which decreases the angle of attack alpha on retreating blade thus reducing the blade stall area. Therefore, depending on the blade loading and the specific configuration, power reduction and stall alleviation can be conflicting objectives.