1985 Small Propeller-Driven Aircraft Noise Test Program

The international aviation community is currently reviewing noise certification procedure for small propeller-driven aircraft. Under discussion is a proposal to substitute a ground-plane microphone for the currently specified elevated microphone as a means of suppressing spectral irregularities. Given the strong low and mid-range tonal frequencies attendent to propeller-driven aircraft, the constructive/destructive interference pattern in an aircraft frequency spectrum can result in inconsistent certification test results for aircraft with different blade passage frequencies. The Federal Aviation Administration conducted several flight tests during the summer of 1985 in-order-to compare noise levels measured at four feet to ground-plane levels as a function of the blade passage frequency. The purpose of this report is to present a preliminary assessment of the data acquired during the flight tests. A more comprehensive analysis of the test results will be documented in the final report. Three flight tests were performed using a Cessna 210, Cessna Caravan I, and a (Beechcraft) U.S. NAVY T-34C. A vertical array (3.75 to 7.0 ft.) of microphones and a range of test RPMs were used in-order-to test the theoretical prediction of the ground reinforcement effect. As a secondary objective, the microphones were redeployed in horizontal arrays to test for a difference in variability between elevated and ground-plane microphones. The primary installation for the ground-plane microphone was over a 0.4 meter circular metal plate. A comparison was also made between 0.4 meter and 36 inch diameter ground plates.