Abstract
O promising concept for powered-lift short takeoff and landing (STOL) aircraft is the externally blown flap (EBF), with conventional turbofan engines exhausting either above or below the wing. Several methods exist for predicting EBF noise at zero flight speed. However, takeoff and approach flight speeds of EBF aircraft will be roughly !4 the exhaust velocity. It is likely that such flight speeds would alter the exhaust jet flow properties and therefore the noise radiation. Measured noise beneath EBF models in external flows has been found to decrease at some but not all frequencies. These reductions generally are least at frequencies that dominate full-scale annoyance-weighted noise levels. If the effects of forward flight on EBF noise mechanisms were understood, it might be possible to choose configurations having less noise annoyance at takeoff and approach.
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