Some comparisons of the flyover noise characteristics of DC-9 aircraft having refanned and hardwalled JT8D engines, with special reference to measurement and analysis

Abstract

FAR-36-type noise measurements and analyses of selected refan and hardwall DC-9 aircraft flyovers were made in parallel with the Douglas Aircraft Company (DACO) at the DACO Yuma, AZ test site in order to evaluate the refan modifications as well as data acquisition and analysis procedures. A limited comparison of results from NASA and DACO measurements and analyses for landing approach conditions indicated agreement within ±1 EPN dB. The worst repeatability results in the NASA analyses (±3 EPN dB), were obtained for the refan takeoff-with-cutback condition. It is believed that the difficulty is associated with the duration factor computation rather than being due to measurement systems performance or atmospheric effects. NASA analyses of the refan and hardwall data indicated that the refanned aircraft provided a centerline noise reduction of about 9.1 EPN dB for takeoff with cutback and about 10.0 EPN dB for 50° flap landing approach. This paper provides a general description of the measurement and analysis systems and procedures used by NASA in these tests.