Hover Performance Measurements Toward Understanding Aerodynamic Interference in Coaxial, Tandem, and Tilt Rotors

The aerodynamic interference between rotors in a multirotor system in hover was analyzed using a series of experiments. First, single-rotor measurements were acquired over a wide range of test conditions by varying thrust, tip speed, and number of blades (two to six). Next, parametric studies were conducted methodically on torque-balanced coaxial-, tandem-, and tilt rotors. For coaxial rotors, the effects of axial separation distance, blade twist distribution, and rotor rotation direction on the system performance were studied. For the tandem rotors, the effect of overlap between rotors on the system performance was measured using untwisted and twisted blades. A unique aspect of the experiment was the ability to measure the performance of the individual rotors even when they were operated as part of a torque-balanced multirotor system. The multirotor measurements, when compared with isolated single-rotor measurements, revealed the influence of one rotor on the other, thereby enabling various interference loss factors to be quantified. Momentum theory and blade-element momentum theory were used to understand and explain the measurements.