Abrego1_AHS02 - A Wind Tunnel Investigation of a...

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1 A Wind Tunnel Investigation of a Small-Scale Tiltrotor Model in Descending Flight Anita I. Abrego, Aerospace Engineer [email protected] NASA Ames Research Center Moffett Field, CA Kurtis R. Long, Aerospace Engineer [email protected] Naval Air Warfare Center Patuxent River, MD Abstract A small-scale tiltrotor model was tested in the 7- by 10-foot Wind Tunnel at NASA Ames Research Center, with the goal of better understanding Vortex Ring State (VRS) effects on tiltrotor aircraft. Test objectives were to obtain performance data of a tiltrotor model over a wide range of descent conditions, to explore the effects of sideslip at these descent conditions, and to investigate the validity of using a single-rotor with a physical image plane to simulate dual- rotor performance characteristics. The model consisted of a pair of 2-bladed teetering rotors with untwisted, 11.125-inch diameter, rectangular planform blades. Model configuration variations included a dual-rotor, an isolated-rotor, and a single-rotor with a physical image plane. Rotor performance data were obtained for the dual-rotor configuration operating over a wide range of descent and sideslip conditions. Isolated-rotor and single-rotor with image plane configurations were tested over an abbreviated range of descent conditions. Results of this investigation are presented and show mean thrust reductions in the region of VRS for each model configuration. In comparison with the dual-rotor configuration, the isolated-rotor and single-rotor with image plane configurations produced thrust results similar in trend but different in magnitude. Notation C T Thrust coefficient, T / ρ N R (Ω R ) 2 π R 2 N R Number of rotors R Rotor radius, ft T Total model thrust, lb V Air velocity, ft/sec V tip Rotor tip speed, R, ft/sec v h Equivalent hover induced velocity, (T/2N R ρπ R 2 ) 1/2 α Shaft angle, positive in descent , deg α G Turntable angle, positive clockwise, deg β Model sideslip angle, deg β G Sting pitch angle from horizontal, positive up, deg θ Rotor collective pitch angle, deg ρ Air density, slugs/ft 3 Rotor rotational speed, rad/sec Presented at the American Helicopter Society Aerodynamics, Acoustics, and Test Evaluation Technical Specialists Meeting, San Francisco, CA, January 23-25, 2002. Copyright © 2002 by the American Helicopter Society International, Inc. All rights reserved. Introduction Vortex Ring State (VRS) develops when a rotor’s descent velocity approaches the induced wake velocity, causing the formation of a doughnut shaped vortex ring around the circumference of the rotor disk. As VRS develops, the flow becomes very unsteady, causing a reduction in rotor thrust at constant collective blade pitch. The aerodynamic characteristics of helicopters operating in VRS have been studied for many years. In the 1960’s, two propellers were tested in vertical or near vertical descent, indicating a loss in thrust, as well as thrust oscillations, in the region of VRS (Ref. 1). Researchers have also completed studies on both single and tandem
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Abrego1_AHS02 - A Wind Tunnel Investigation of a...

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