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UH-60A Airloads Wind Tunnel Test Summary

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Flight testing of the UH-60A airloads rotor in 1993 resulted in a unique and extensive database for many level flight and transient maneuver conditions. The key data from this test were the rotor airloads (section normal force, pitching moment, and chord force) integrated from chordwise pressure arrays located at nine radial stations. These data, in combination with other measured parameters (structural loads, control positions, and rotor shaft moments), have helped to provide physical insights into specific flight regimes, including high speed flight and dynamic stall, as well as providing critical data for validating new and emerging predictive tools.

To provide an expanded database for validation of these predictive tools, NASA and the U.S. Army have recently completed (May 2010) a full-scale wind tunnel test of the UH-60A airloads rotor, including the pressure-instrumented blade. This test, conducted in the USAF National Full-Scale Aerodynamics Complex (NFAC) 40- by 80-Foot Wind Tunnel, was designed to produce unique data not available from the flight test. This included data from a number of new measurements, such as rotor balance forces and moments, oscillatory hub loads, blade displacements and deformations, and rotor wake measurements using large field Particle Image Velocimetry (PIV) and Retro-reflective Background Oriented Schlieren (RBOS). This also included data acquired at conditions outside the conventional flight envelope, such as at high speed, high thrust, and slowed rotor conditions.

The primary wind tunnel test data were acquired during speed sweeps at 1-g simulated flight conditions, up to advance ratios of μ=0.4. For each wind tunnel test condition, the rotor RPM and tunnel speed were set to match the target Mtip and μ, and the shaft angle set to match the predicted value. The trim controller then targeted the appropriate lift and hub moments and estimated the shaft angle change necessary to match propulsive force. The lift and propulsive force used as targets included corrections for wind tunnel wall effects. The shaft angle was then manually adjusted until all trim targets were met. The 1-g level flight sweeps were performed at three lift levels, CL/sigma=0.08, 0.09, and 0.10.

The parametric sweeps were conducted at three different tip Mach numbers, with the majority of data acquired at the baseline Mtip=0.650. A limited number of sweeps at Mtip=0.625 were conducted to attain higher non-dimensional thrusts and advance ratios without reaching load limits. A limited number of sweeps at Mtip=0.675 were also conducted, with the objective of exploring higher advancing blade Mach numbers.

Data were also acquired at matching conditions from previous full-scale flight test and small-scale wind tunnel test to assess rotor and wind tunnel scaling issues. For each simulated flight condition, the rotor RPM and tunnel speed were set to match the target Mtip and μ, and the shaft angle was set so the wind tunnel wall corrected angle matched the flight test. The trim controller then targeted the appropriate rotor thrust (CT/sigma), and fixed system hub moments (derived from the same shaft bending gage used in flight). If necessary, the shaft angle was further adjusted to match corrected shaft angle exactly. A total of three Airloads flight test points were simulated: C8525, C8424, and C9020.

Finally, data were acquired while performing unique slowed-rotor simulations at reduced RPM (40% and 65%), up to advance ratios of μ=1.0. The measurements reveal new and rich aeromechanical phenomena that are special to this exotic regime. These include reverse chord dynamic stall, retreating side impulse in pitch-link load, large inboard-outboard elastic twist differential, supersonic flow at low subsonic advancing tip Mach numbers, diminishing rotor forces yet dramatic build up of blade loads, and dramatic blade loads yet benign levels of vibratory hub loads. The objective of this research is the fundamental understanding of these unique aeromechanical phenomena. The intent is to provide useful knowledge for the design of high speed, high efficiency, slowed RPM rotors of the future.

Recent Publications

Norman, T.R., Shinoda, P., Peterson, R.L., and Datta, A., “Full-Scale Wind Tunnel Test of the UH-60A Airloads Rotor”, 67th American Helicopter Society 67th Annual Forum, Virginia Beach, VA, May 2011

Romander, E. Norman, T.R., and Chang, I-C., “Correlating CFD Simulation with WindTunnel Test for the Full-Scale UH-60A Airloads Rotor”, 67th American Helicopter Society 67th Annual Forum, Virginia Beach, VA, May 2011

Datta, A., Yeo, H., and Norman, T.R., “Experimental Investigation and Fundamental Understanding of a Slowed UH-60A Rotor at High Advance Ratios”, 67th American Helicopter Society 67th Annual Forum, Virginia Beach, VA, May 2011

Wadcock, A.J., Yamauchi, G.K., Solis, E., and Pete, A.E., “PIV Measurements in the Wake of a Full- Scale Rotor in Forward Flight”, 29th AIAA Applied Aerodynamics Conference, Honolulu, HI, June 2011

Barrows, D.S., Burner, A.W., Abrego, A.I., and Olson, L.E., “Blade Displacement Measurements of the Full-Scale UH-60A Airloads Rotor”, 29th AIAA Applied Aerodynamics Conference, Honolulu, HI, June 2011