Measurement Around a Rotor Blade Excited in Pitch, Part 1: Dynamic Inflow

Abstract

This is the first of two papers describing experiments on a two-bladed rotor in hover, under controlled pitch excitation. Measurements of the inflow to the rotor are described, first under steady conditions, and then under prescribed 4-per-rev pitch excitation. The high data rates, data acquisition techniques and data quality required for this experiment are demonstrated, including 4-per-rev velocity variations due to 1-degree pitch oscillation, extracted over a single period of rotor revolution at 600rpm. The steady case results agree with lifting-line-based analytical predictions except in the tip region, as expected. The excited-blade case shows the 4-per rev velocity perturbation, and the dependence on the phase of the excitation. Spectral analysis of the LDV data shows multiple harmonics of the excitation frequency. The rate of pitch change is seen to be high enough to produce substantial hysteresis effects, whose variation with radial position is measured. The hysteresis is seen to arise from the inboard shed vorticity, and to decay towards the tip.

Experimental Characterization of a Rotor to Dynamic Collective Pitch Inputs in Hover

“…The existing body of experimental work is comprised primarily of studies utilizing single-point velocity measurement techniques in the wake of a single rotor. Liou [7] investigated the inflow response of a rotor to sinusoidal collective pitch inputs of 1 • amplitude at four times the rotor rotational frequency (4/rev) using laser doppler velocimetry (LDV). Measurements of axial velocity were taken along the 45 • radial line at axial locations of 4% and 10% radius above the rotor plane.…”

The dynamic induced velocity field of a model rotor in hover conditions

“…Work by Liou' 22 ' who examined the induced flow response over a radial range at just one excitation frequency, revealed a hysteresis effect when the induced flow at a point is plotted against the excitation root pitch. It was also found that this hysteresis effect diminished towards the tip and seemed to be predominantly affected by the inboard shed vorticity.…”