Noise Generated by Aircraft in Flight Free

The generation of noise from turbulent boundary layers has been investigated both theoretically and by model experiments in recent years. However, until the present, no flight data have been available with the exception of high speed flybys. The paper presents results of several inflight measurements of aircraft noise including jet noise, wake noise, and a radiated boundary layer noise, obtained by flush microphones mounted in laminarized pods external to the aircraft. The data indicate that the radiated boundary layer noise is primarily of quadrapole origin at flight Mach numbers above 0.5, and further, that the inflight measurements are essentially in accordance with the ground flyby measurements when the effect of motion on the transmission of sound is considered.