Thrust and Acoustic Performance of Small-Scale, Coaxial, Corotating Rotors in Hover

The findings from a test campaign aimed at understanding the thrust and acoustic performance of a small-scale, coaxial, corotating rotor in hover are discussed. Measurements of axial thrust are acquired alongside surveys of the near-field acoustics at eight observer positions above and below the tip path plane of the upper rotor. The variables of interest are the index angle and stacking distance between the upper and lower rotors, as well as rotor speed. Standard metrics like thrust coefficient, sound pressure spectrum level, and the sound pressure level of the various blade harmonics and subharmonics are studied. Human ear effects are also considered using the A-weighting standard, which is shown to have a profound influence on the perceived noise levels. The analysis demonstrates how rotor thrust and sound pressure levels are more dependent on rotor index angle than stacking distance. However, the index angle where the peak thrust coefficient resides is more sensitive to stacking distance at low rotation speeds than it is at high rotation speeds. Various rotor speed and index angle combinations of the stacked rotor are also shown to produce the same rotor thrust, all the while resulting in uniquely different sound levels of the various rotor harmonics.