DLR-Involvement in European Helicopter Rotor Noise Research Activities

Kurzfassung

Several major dedicated European/International research programs lead to significant advances in understanding and controlling helicopter rotor noise. The availability of appropriate wind tunnel facilities (the German Dutch Wind Tunnel, DNW, in particular) in conjunction with the application of ever more sophisticated testing techniques beyond acoustics, has helped the stated objectives. Among the noteworthy novelties are The visualization of the highly complex aerodynamic interaction processes between flow and rotor blades using the DNW-developed Lase Light Sheet (LLS) techniques (to determine e.g. vortex trajectories and blade/vortex miss distances), The quantitative assessment of the rotational flow phenomena near rotor blades (e.g. vortex velocity distribution, core size and circulation strength) by means of Laser-Doppler-Velocimetry (LDV), The determination of blade deflection and blade attitudes during rotation by means of the DNW-developed Moiré-technique, a photometric "projected grid" method. The quantitative resolution in 2-D (soon 3-D) of instantaneous velocity distribution overcompared to the LDV-method - much larger regions in a very time-efficient manner by means of the DLR-Göttingen-developed, highly promising, Particle Image Velocimetry (PIV). Moreover, progress in acquiring - directly on the rotating blade- unsteady blade surface pressure data (being strongly related to the important "loading noise" source mechanism) in conjunction with information on blade-vibration and radiated acoustics has furthered the knowlegde on rotor aeroacoustics to allow the design of new operationally "quiet" rotors and to devise technically feasible noise reduction methods through, for example, sophisticated global-harmonic or individual blade control mechanisms. - The application of such new, often highly complex and man-power-intensive testing techniques in wind tunnel allows the acquisition of the necessary detailed insight into the physics of rotor aeroacoustics, before applying and testing appropriate and promising techniques on real ("full-size") helicopters in the framework of flight testing, where conditions are inherently less ideal and controllable and where, foremost, the influence of rotor/rotor-fuselage-interactions must be accounted for, let alone trying to validate wind tunnel experiments with real flight test results. The ever better understanding of the intricate aeroacoustic mechanisms of rotor noise generation and radiation on the basis of such wind tunnel and flight experiments is instrumental in furthering the computational capabilities where substantial effort is presently spent to develop accurate prediction schemes for helicopter flyover noise, none in the least for purposes of land use planning and noise certification. This survey paper will discuss recent progress made in the above areas.