Figure of merit definition for coaxial rotors

The development of appropriate figure of merit expressions for a coaxial rotor system is discussed. To this end, momentum theory solutions for a hovering contrarotating coaxial rotor system have been formally derived for equal thrust and torque-balanced conditions. The solutions were validated against coaxial rotor system performance measurements using average values of induced power interference factors, with good agreement. A new figure of merit expression for a coaxial rotor was established using the momentum theory solutions for an ideal coaxial rotor as a basis, and also accounting for the effect of thrust sharing between the upper and lower rotors. This metric properly reflects the fact that each rotor of a coaxial system operates at different disk loadings and, therefore, at different effective aerodynamic efficiencies. The basis for comparison with single rotor systems is also discussed. It is shown that the maximum efficiency of a coaxial rotor system, however defined, occurs when the rotors are separated such that the lower rotor operates in the fully developed slipstream of the upper rotor.