Pratt and Whitney J57 with a Greatex Nozzle in the Altitude Wind Tunnel

A Pratt and Whitney J57 engine is tested with a Greatex No.1 nozzle in the Altitude Wind Tunnel at the National Advisory Committee for Aeronautics (NACA) Lewis Flight Propulsion Laboratory. At the time the aircraft industry was preparing to introduce jet airliners to the nation’s airways. The noise produced by the large jet engines, however, posed a considerable problem for communities near airports. The NACA had formed a Special Subcommittee on Aircraft Noise to coordinate research on the issue.

Preliminary tests showed that the source of the loudest noise was not the engine itself, but the mixing of the engine’s exhaust with the surrounding air in the atmosphere. The pressures resulting from this turbulence produced sound waves. Lewis researchers undertook a variety of noise-reduction studies involving engine design, throttling procedures, and noise suppressors. One of their first efforts focused on new types of nozzles to mix the exhaust with the surrounding air. The nozzles had a variety of shapes designed to slow down exhaust velocity before it combined with the air and thus decrease the noise.

From January to May 1957 a Pratt and Whitney J57 engine was equipped with various shaped nozzles, as seen in this photograph, and run in simulated flight conditions in the Altitude Wind Tunnel. A number of nozzle configurations, including several multi-exit “organ pipe” designs, were created. It was found that the various nozzle types did reduce the noise levels, but they also reduced the aircraft’s thrust.