Abstract
Noise measurements were made on two jet flap systems proposed for STOL aircraft. In one case a 6-in.-diam fan was mounted under the wind such that the exhaust impinged on downwardly deflected flap segments. In the other case, the fan was located on the upper surface of the flap such that fan and flap moved as a unit, with no exhaust impingement. Results for take-off and approach fan speeds and flap deflections were used to estimate STOL airplane perceived noise levels. Internally generated noise directivity corresponded with fan axis orientation for the fan-on-flap, but no consistent redirection of internal noise was observed with changes in flap angle for the fan-underwing. With the fan-on-flap arrangement, the wing shielded some fan inlet noise from the ground. Since no impingement of the exhaust on solid surface occurred, the external noise was jet noise alone. In contrast for the fan-under-wing, the jet/flap interaction noise dominated the external noise even with no flap deflection. If internal noise is reduced by fan design and acoustic treatment, the jet/flap interaction noise will dominate. Additional work is needed to establish a spectral correlation of interaction noise.
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