Characterisation of the Fluctuating Surface Pressure Field on sUAS Rotor Blades

This paper presents the design of a new, novel experimental rotor rig, called the Blade Fluctuating Surface Pressure (BFSP) rig. The BFSP rig is designed to measure the fluctuating surface pressure (FSP) using the remote microphone method, at various locations on the surface of a small diameter, two-bladed rotor, representative of rotors found on small unmanned aerial systems (sUAS). FSP data have been obtained across a range of blade collective pitch angles of 0◦, 6◦ and 12◦, rotational rates of 2000, 3000, 4000 and 5000 RPM, and varying edgewise freestream velocities from static to 10 and 15 m/s. The pressure taps on the blade surface have been arranged so that the pressure jump spectra across the rotor blade at three chord-normalised positions of x/c = 1/16, x/c = 1/8 and x/c = 1/4 are measured. These spectra show the presence of shaft rate and blade passage frequency (BPF) integer and half-integer harmonic tones under the static and edgewise freestream conditions tested. These tones of the BPF are moderately coherent and nearly out-of-phase at static freestream conditions and increase in coherence further with the introduction of edgewise freestream flow. These highly coherent, nearly out-of-phase tones as well as coherent out-of-phase broadband content are observed at a chord-normalised position of x/c = 1/16 between normalised frequencies of 0.5-6 f/BPF, with a double peaked shape in phase difference plots across all freestreams. At a flow speed of U∞ = 10m/s the coherence strengthens and expands to 0.5-6 f/BPF whilst at U∞ = 15m/s it reaches toward 9 f/BPF. Under these edgewise freestream conditions, these coherent tones are close to being exactly out-of-phase between 0.5-3 f/BPF at x/c = 1/8 and x/c = 1/4. By calculating the pressure jump spectra, the extraneous noise is assumed to be mostly cancelled, leaving only the aerodynamic loading sources of FSP. This assumption means that the BPF tones measured under static freestream conditions are most likely due to aeroelastic blade vibration.