Analysis and Testing of Index Angle-Controlled Stacked Rotors in Hover

This article discusses a novel thrust control method involving a coaxial co-rotating, or stacked, rotor system. The thrust of the stacked rotor system is controlled by electronically varying the index angle, that is, the azimuthal angle between the upper and lower rotors. A 2-m diameter stacked rotor system was tested at 950 RPM in hover to measure steady state as well as transient thrust and torque during an index angle change. A +10° change in stacked rotor index angle at a rate of 25 °/s resulted in a thrust change of 15.4% in 0.4 s with a 30% increase in total torque. An electromechanical model coupling the powertrain dynamics with aerodynamic loading captured the measured trends in rotor thrust and torque but underpredicted rotor torque by as much as 43%. The electromechanical model was used to compare the index angle control with collective pitch control and speed control. For the same thrust change of +15.4% in 0.4 s, a speed-controlled rotor required 50% additional torque, and a collective pitch-controlled rotor required 20% additional torque assuming a pitch rate of 60°/s. Therefore, index angle control could potentially enable higher bandwidth changes in thrust than are possible for fixed pitch, speed-controlled rotor systems.