Flyover Noise Prediction from Static Data

Abstract

The use of static ground test data to describe the acoustic characteristics of an aircraft flyover can result in a significant savings both in time and money, providing good correlation with actual flight test measurements is assured. This paper addresses itself to the elements associated with the calculation of EPNL values from static ground test data. Farfield noise level measurements were made over a range of engine thrust settings from idle to takeoff power of a JT3D-3B engine mounted on a test stand. Two key engine operation parameters were used to correlate the SPL values: low-pressure compressor rotor speed, which controls fan noise; and net thrust, which controls jet noise. The SPL spectrum, used to simulate a flyover, is generated by interpolating the measured data at the appropriate in-flight values of rotor speed and net thrust. The extrapolation to inflight condition also considers airplane parameters such as altitude, climb gradient, and number of engines, as well as Doppler shift effects, spherical divergence, atmospheric and extra ground attenuation, and ground reflection. Comparisons are made between EPNL values predicted from static ground test data and values obtained from flight tests. A tape will be played which demonstrates the difference between flyovers of a production B-707 airplane and a B-707 airplane equipped with quiet nacelles.