Measurements Comparing Hover Performance of Single, Coaxial, Tandem, and Tilt-Rotor Configurations

Hover performance characteristics of torque-balanced coaxial, tandem, and tilt-rotor systems were compared against single-rotor systems with equivalent solidity. Towards this, a series of experiments were conducted on two identical, but independent, rotor test rigs that were configured to easily change from one system to another (i.e., coaxial to tandem etc.). The influence of various parameters, such as the axial separation distance for coaxial rotors or the shaft-to-shaft distance for tandem rotors, on the overall system performance was measured using untwisted (NACA 0012 airfoil) and highly-twisted (xv-15 airfoil) blades. Additionally, performance characteristics of the two individual rotors (upper/lower or front/aft) that comprised the coaxial and tandem rotor systems were measured independently while operating in a torque-balanced condition. The results, when compared against single-rotors, allowed several aerodynamic interference loss factors to be determined. Thrust sharing ratio, hovering efficiency of the rotor system, efficiency of the individual rotors, and influence of one rotor on the other were all measured at various rotor separation distances for both coaxial and tandem rotor systems. Momentum theory was used to compare and contrast the measurements against predicted values.