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

Flyover noise measurements were made (using Federal Aviation Regulations, part 36 procedures) of two DC-9 aircraft, one equipped with refanned JT8D-109 engines and the other equipped with hardwalled JT8D-9 engines. NASA analyses show a refan centerline noise reduction of about 9.1 EPNdB and 10.0 EPNdB for takeoff with cutback and 50 deg. flap landing approach, respectively. A comparison of refan and hardwall PNLTM spectra shows that the refan noise reduction may be attributed to lower jet noise levels on takeoff and reduced high-frequency tonal content on landing approach. A general description of the test procedures and results are included along with detailed descriptions of the measurement and analysis systems.