Jet Noise Reduction Technique for Military Reconnaissance/Surveillance Aircraft. Phase I. Bench/Wind Tunnel Tests

A novel method of jet noise reduction has been subjected to bench and wind tunnel tests. This method uses very small nozzles arranged on struts protruding from the upper surface of an aircraft wing. These nozzles are intended to replace the conventional single propulsive nozzle of the jet engine for the primary purpose of reducing the aural detectability of the jet noise by modifying its frequency spectrum. Tests of this concept were conducted during this program including acoustic comparison of a dozen different nozzle arrangements; tests with heated air; acoustic and propulsion tests of an array of several thousand nozzles; acoustic wind tunnel tests of struts and inlet configurations; and aerodynamic wind tunnel tests of an array of 6,000 nozzles on a wing section. Additional tasks included preliminary design of a quiet research test vehicle and prediction of its aural detectability. Tests of various nozzle arrangements resulted in selection of a single row of nozzles on each strut. Tests with heated air showed that jet noise could be predicted based on jet velocity, independent of jet temperature. (Modified Author abstract)