IN-FLIGHT ROTORCRAFT ACOUSTICS PROGRAM


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Introduction

A key part of NASA's aeronautics research is reducing noise to make helicopters and tiltrotors more acceptable to the public. The objective of the In-Flight Rotorcraft Acoustics Program (IRAP) is to acquire rotorcraft noise data in flight for comparison to wind tunnel data. The type of noise of concern is "blade-vortex-interaction," or BVI, noise. Microphones on the wing tips and tail fin of the quiet NASA YO-3A Acoustics Research Aircraft measure BVI noise while the YO-3A descends in close formation with the helicopter or tiltrotor emitting the noise. The data acquired through IRAP is needed to validate wind-tunnel test results, or, where the results cannot be validated, to provide researchers with clues as to how to improve testing methods.

Links to related pages:

BVI noise. During certain rotorcraft flight operations, particularly forward descents such as landing approaches, each rotor blade can run into the tip vortex shed by a preceding blade. The resulting "blade-vortex interaction," or BVI, causes a distinctive type of noise: the annoying "blade-slap" of helicopters with slow-turning rotors, or a sharp fluttering noise for rotorcraft with fast-turning rotors. A comprehensive review of rotorcraft noise is given in Schmitz's 1991 summary, listed on the references page. In-flight measurement of BVI noise is the primary effort of IRAP.

History. In-flight measurements of BVI noise were undertaken in the mid-1970s with a Bell UH-1H "Huey" as the test helicopter and an OV-1 Mohawk as the microphone platform (Schmitz, 1991). By 1979, the OV-1 had been replaced by the much quieter YO-3A, which was used to measure BVI from a UH-1H and two models -- AH-1S and AH-1G -- of Bell Cobra helicopters (Boxwell and Schmitz, 1980; Cross and Watts, 1984). The Army further used the YO-3A to test a variety of helicopters, including the UH-60, YUH-61, YAH-63, and AH-64 (Cross and Watts, 1984). There were also flight tests with a McDonnell Douglas MD 500D.

Drawing on this experience with in-flight acoustics testing, the In-Flight Rotorcraft Acoustics Program (IRAP) was established in 1991 for the specific purpose of acquiring measurements of rotor noise in flight for comparison to wind-tunnel data. IRAP is managed by the Rotorcraft Aeromechanics Branch at NASA Ames Research Center. The YO-3A has been used for all IRAP tests, four of which have been carried out to date:

Follow the links for details of each flight test. Each discussion of a flight test program includes links to hypertext descriptions of associated wind tunnel tests.

Related Publications:

Boxwell, D. A., and Schmitz, F. H. "Full-Scale Measurements of Blade-Vortex Interaction Noise." 36th Annual Forum of the American Helicopter Society, Washington D.C., May 1980.

Cross, J. L. and Watts, M. E. "In-Flight Acoustic Testing Techniques Using the YO-3A Acoustic Research Aircraft." NASA TM-85895, February 1984.

McCluer, M. and Dearing, M. "Measuring Blade-Vortex Interaction Noise Using the YO-3A Acoustics Research Aircraft." 22nd European Rotorcraft Forum, Brighton, UK, September 1996.

Schmitz, F. H. "Rotor Noise." Acoustics of Flight Vehicles: Theory and Practice. Vol. 1: Noise Sources. H. H. Hubbard, ed. NASA RP-1258, WRDC TR-90-3052, August 1991.

See also the references page.

Point of Contact:

C. W. Acree
NASA Ames Research Center
Moffett Field CA 94035-1000

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Last Modified: Thursday, 27-Dec-2007 14:37:25 PST