Abstract:
The effect of the ground on a rotor wake in
forward flight has been investigated experimentally in the
working section of an 8ft x Oft straight-through wind
tunnel. A three bladed fully articulated rotor with a
solidity ratio of 0.07 and diameter of 1.06m, powered by
a hydraulic motor, has been tested at a height of 0.47 rotor
diameter above a solid ground board which has an elliptical
leading edge.
Tests have been run at various low advance ratios
(<0.1) with two collective pitch settings.
A three-element hot wire anemometer probe has
been used to measure the average value of the three
components of velocity simultaneously in the forward half
(advancing side) of the rotor wake and in the main stream
surrounding it.
The rotor wake and the ground vortices have been
visualized by smoke. Surface flow patterns on the ground
board have located the interaction region between the rotor
wake and the oncoming flow on the ground board.
Theoretical estimates of the flowfield based on
Heyson's vortex cylinder model (Ref. 2) are compared with the
experimental results.
Both experimental results and theoretical estimates
show that the ground-induced interference is an upwash and
a decrease in forward velocity. The upwash interference'
opposes the vertical flow through the rotor, and have large
effects on the rotor performance in producing thrust. The
streamwise interference decelerates the mainstream and
becomes more noticeable as the wake boundary is approached.