A summary review is presented of the results of current and recently completed noise‐reduction programs for DC‐8 and DC‐9 airplanes. The relationship of these programs to other efforts by Douglas to develop quieter CTOL airplanes is briefly outlined. The engine/nacelle concepts studied were (1) application of nacelle acoustic treatment, (2) variable‐area exhaust nozzles, (3) jet exhaust noise suppressors, and (4) engines with larger diameter new front fans. Acoustic analyses leading to the selection of certain engine/nacelle configurations are described, as well as design constraints which limit the noise‐reduction capability of certain designs. Acoustic design features of the modified engine/nacelles are discussed along with estimates of the changes in aircraft community noise levels, as well as estimates of the changes in aircraft weight, performance, cost, and operational restrictions. The noise‐reduction estimates for the DC‐8 and DC‐9 are based on ground runup and flyover noise tests. Comments are presented on the potential noise‐reduction benefits, and associated aircraft operational problems, resulting from noise‐abatement operating procedures.
Subject Classification: 50.40, 50.50, 50.30, 50.55, 50.75.