D.R. Hoad, D.A. Conner, C.K. Rutledge

NASA Langley Research Center, in cooperation with NASA Ames, has conducted acoustic hover tests with the XV-15 Tiltrotor research aircraft at Moffett Field in California. The vehicle rotor system had been modified to include the Advanced Technology Blades that were designed to provide for higher operating weights and improved maneuver load factor in helicopter and transition modes of operation. The acoustic test program was designed to provide a comprehensive data base of the hover acoustic characteristics of the XV-15/ATB, particularly to provide a data set for validation of current and developing analytical methods in tiltrotor acoustics. In addition, some operating conditions were chosen that matched those obtained in previous tests with the original (metal) blade set in order to address evaluation of the acoustic impact of the advanced blade design. An acoustic model that predicts the discrete frequency noise characteristics of helicopter rotors has been used in the analysis to help identify source characteristics. This method incorporated a NASA Langley developed acoustic prediction method (WOPWOP), and a simplified flow field model to account for rotor wake re-ingestion in hover - identified as the "Fountain Effect". The "Fountain Effect" model was developed by Charles Coffen and Albert George of Cornell University. The impulsive character of the acoustic time domain information was used in a "source localization" technique to identify the relative position of the source mechanisms on the vehicle, and to further improve the confidence in the analytical procedure for modeling the fountain effect. Comparison of the data with the original Metal Blade (MB) and the Advanced Technology Blade (ATB) was conducted by sorting and correcting the ATB test data into an array of 96 microphone locations and the MB test data into an array of 20 microphone locations. These locations were chosen to represent the distribution of acoustic radiation at ground level for each vehicle hovering at an altitude of 100 ft. The results of the comparisons of Overall Sound Pressure Levels (OASPL), fundamental blade passage frequency Sound Pressure Levels, and OASPLs above 130 Hz have indicated that the ATB rotor system, with the tip shape tested (operating at the same tip speed and gross weight) produced comparable, or less, levels of noise as the MB rotor system, except for the higher frequency characteristics behind the vehicle where the "fountain effect" was dominant. The results al the higher frequencies showed that the ATB levels in certain regions behind the vehicle were 9 dB higher than the MB levels at distances up to 1000 ft from the vehicle. Strong directional radiation characteristics observed in the A TB test low-frequency results related to blade passage frequency phasing effects were not evident in the MB test results due to limited spatial resolution in the measurement process. In-plane acoustic radiation characteristics were not evaluated in this paper.

DGLR/AIAA 14th Aeroacoustics Conference, 1992

Conference Paper

englisch

21,0 x 29,7 cm, 19 Seiten

DGLR-Bericht, 1992, 1992-03, DGLR/AIAA 14th Aeroacoustics Conference - Proceedings; S.134-152; 1992; Deutsche Gesellschaft für Luft- und Raumfahrt - Lilienthal-Oberth e.V., Bonn

NA

aeroacoustics, tilt rotor aircraft

Bibliothek

1992