Performance and Acoustics of Small Rotors with Non-Uniform Blade Spacing

Thrust, torque, and radiated noise are investigated for small rotors typical of unmanned air systems concepts with non-uniform blade spacing. Blade spacing of 5-bladed rotors is arranged for five unique angles and four levels of non-uniformity. Two configurations are considered, based on a 40-inch rotor with rectangular, untwisted blades, and a 7.5-inch rotor from a commercial quadcopter drone. The performance of the larger rotor was found to be insensitive to blade number and non-uniform spacing while the smaller rotor showed a degradation between two blades and five of 5% in hover and 9% in forward flight, but was also insensitive to non-uniform spacing. As the blade spacing was varied from uniform to the most non-uniform, the acoustic spectra gradually changed from a primarily tonal noise at the blade passage frequency to a broadband noise with multiple harmonics. Overall noise level increased as non-uniformity increased, but was significantly lower than a 2-bladed rotor with the same solidity.