Analytical and Experimental Power Minimization for Fixed-Pitch Coaxial Rotors in Hover

Fixed-pitch coaxial, contrarotating rotors are often used in multirotor aircraft in applications ranging from payload delivery to urban air mobility. This paper examines minimizing net power at a given thrust for the coaxial pair using both an analytical approach based on blade element momentum theory and experimental results conducted in a hover test stand, as well as experimental results in hover for an X8 configuration multirotor. Appropriate distribution of thrust (realized via differential rotor speeds) to the upper and lower rotors results in approximately 5% savings in total power. Under this condition the thrust distribution is approximately 60% by the upper rotor and 40% by the lower rotor (which occurs when the lower rotor shaft speed is roughly 10% higher than the upper rotor shaft speed). Similar ratios were obtained in both the analytical solution and in experiment, at various total thrust levels. Hover flight tests of the X8 multirotor also showed approximately 5% reduction in hover power at this RPM setting.