Republic F-84 Thunderjet with Slotted Nozzle

A Republic F-84 Thunderjet dramatically modified at the NASA Lewis Research Center to investigate the use of slotted nozzles to reduce exhaust noise. The F-84 was a single-seat fighter-bomber powered by an Allison J35 turbojet. It was the Air Force’s first post-World War II tactical aircraft and was used extensively in the Korean War. The laboratory had acquired the aircraft in 1954 and modified it in order to demonstrate the reverse thruster. The tail end of the aircraft was then removed for a series of large nozzle investigations. Lewis researchers launched an extensive program in the mid-1950s to develop methods of reducing engine noise as the airline industry was preparing to introduce the first turbojet-powered passenger aircraft. The early NACA investigations determined that the primary source of noise was the mixing of the engine’s hot exhaust with the cool surrounding air. Lewis researchers studied many different nozzles designed to facilitate this mixing. Nozzles with elongated exit sections, as seen in this photograph, produced lower noise levels. These long slot nozzles were also considered for Short Take-off and Landing aircraft because their long flat surfaces provided lift. In 1958 Lewis tested several full-scale slot nozzles on the F-84. The researchers, led by Willard Cole, sought to determine the noise-generation characteristics for nozzles having large a width-to-height ratio. The nozzle in this photograph has a 100 to 1 width-to-height ratio. Cole determined that the experimental nozzles produced the same levels of sound as the standard nozzle, but the changes in the directional noise were substantial.