Fundamental Aspects of Noise Reduction from Powered-Lift Devices
Proposed powered-lift STOL and RTOL (reduced takeoff and landing) aircraft are expected to require noise reductions on the order of 20 dB to meet community noise goals, according to a number of model and full-scale studies. Propulsive-lift STOL introduces to the designer a new class of noise problems in that the noise sources are exposed and not amenable to conventional muffling. Thus, the noise must be reduced at the source, a task for which thorough understanding of source mechanisms is required. A review of source mechanisms associated with various propulsive lift concepts is presented and current state-of-the-art of prediction technology is assessed for each important source. Sources are rank ordered for the various classes of lift-augmenting concepts. Parametric dependence of source levels on design and operating parameters is discussed. Fundamental concepts for noise reduction are developed and current progress toward implementation of these noise reduction concepts is reviewed.