Jet-Engine Exhaust Noise—A Measure of Thrust

Abstract

The thrust of the jet engine is theoretically shown to be related to the over-all sound pressure of the jet exhaust regardless of environmental conditions. Within the range of tolerances required to measure thrust accurately on a competitive basis with existing systems, it is found that over-all sound-pressure-level measurements are not satisfactory. Numerous experiments have indicated that specific frequency regions of sound can be utilized to determine a correlation between sound and thrust. An instrument, the Jet Thrust Indicator (JTD), is described as having special filters such that the sound-pressure level of the frequency ranges correlates with thrust. The JTD provides direct readout in pounds of thrust. Actual field-test data are provided for aircraft and test stand-mounted jet engines. Thrust-measurement comparisons are made of the JTD with the conventional thrust-measuring techniques presently employed. Data obtained from field tests conducted over a wide range of atmospheric conditions and with various engines are reviewed. Results of the test program indicate an average accuracy of 2%. Applications including measurement of jet-engine thrust for ground check out on test stands and aircraft, go/no-go indications, and other uses are discussed.