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. NOMENCLATURE f frequency, in Hz. G(f) Spectral density, in dB. r Radial coordinate, with origin at the center of the rotor hub. R Rotor radius. z Axial coordinate, with origin at the center of the rotor hub, positive going downstream. Vz Inflow velocity in m/s, perpendicular to the tip path plane; positive going downstream.