Loads Transfer Principles of Coaxial Helicopter Rotors

The load transfer principles of coaxial rotors are analyzed from a theoretical point of view. The theoretical expressions of hub forces and moments are derived by summing the Fourier series of the root forces and moments from all the blades of a coaxial rotor system with the assumption that the rotor loads are identical between the two rotors. The analytical relations indicate that changing the rotor-to-rotor phase angle may be used to reduce (and even eliminate) the magnitudes of specific harmonics of rotor thrust, drag, and pitching moment, but the reduction is accompanied by an increase in the magnitudes of rotor side force, rolling moment, and torque for the same harmonic and vice versa.