Objetivo
To bring about substantial reduction of the noise emission of helicopters and future tilt-rotor aircraft, the corresponding noise prediction capabilities must be considerably improved to provide the European helicopter manufacturers with the necessary competitive edge.
Towards this objective a joint European effort will be conducted to investigate the aeroacoustic mechanisms of rotor noise generation by means of a comprehensive wind tunnel test programme. The unsteady pressure distribution on rotor blades is determined through a large number of pressure sensors embedded in the blade contour. By simultaneously measuring the radiated acoustic signals for a large variety of "flight-conditions" ( take-off, high speed, horizontal flight, landing approach, high-g turns), a comprehensive database will be obtained which allows the new advanced prediction codes and improvements to existing codes for helicopter external noise.
Theoretical results
The methods applied differ mainly with respect to the necessary computation effort and the physical effects addressed. Different levels of code complexity correspond to the different stages of the helicopter design process. For the first estimation, a rough and relative simple computation is needed, whereas for the final lay out, sophisticated theories are mandatory. So, the work of Bristol University leads to a saving of computer time by partial use of prescribed wake configuration. The approach of ECD and ALFAPI concentrates on blade vortex interaction under neglection of compressibility effects. The latter aspect is addressed by the work of Morino.
The partners AGUSTA, CIRA, ALFAPI, IST and Bristol University established codes for the prediction of rotor noise created by different sources, viz thickness, loading, quadrupole and broadband noise. The theoretical approach was performed in the frequency and time domain. The procedures differed between the Farassat and the Lowson solution of the Ffowes Williams Hawkins (FWH) equation. A work share existed in the addressing of different noise sources. In the areas, where the same problems were tackled by different partners, future comparisons between the theoretical and experimental HELINOISE results will help to identify the superior theoretical approach.
Experimental Results
The test results will serve as data banc for the verification of aerodynamic and aeroacoustic prediction codes and as a tool to explain the complex physical background of rotor noise generation and radiation. The data comprise in detail:
- radiated acoustics
- sound pressure time histories
- narrowband spectra
- mid-frequency summary level contour plots
- low-frequency summary level contour plots
- blade surface pressure
- time histories (in the azimuth),
- chordwise distribution
- (per one revolution) azimuthal distribution
- vortex trajectories
- blade vortex miss distance
- blade deflection
The tests will be conducted in the best aeroacoustic wind tunnel in existence, the DNW, using a high quality modular rotor test stand. The model rotor will be a large scale (40%), to avoid scaling towards full size problems.
The helicopter exterior noise prediction methods will be developed for high speed and blade/vortex interaction, impulsive noise and broadband noise. They will be based on the pressure distribution of the rotor blade surface, which is the source of the rotor noise radiation. The pressure distribution will be evaluated by advanced aerodynamic codes taking into account 3-dimensional, unsteady and compressibility effects.
The wind tunnel tests will provide a validation of both the aerodynamic and the acoustic prediction results and thereby control and improve the codes. The prediction codes will permit consideration of noise constraints in the early design phase of a helicopter rotor system.
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