Quiet Clean Short Haul Experimental Engine

A second wave of engine-improvement programs was initiated in 1969 and continued throughout the 1970s, as the noise around airports con­tinued to be a social and political issue and the FAA tightened its environ­mental regulations. Moreover, with the oil crisis and energy shortage later in the decade adding to the forces requiring change, the airline indus­try once again turned to NASA for help in identifying new technology.

At the same time, the airline industry was studying the feasibility of introducing a new generation of commuter airliners to fly between cities along the Northeast corridor of the United States. To make these routes attractive to potential passengers, new airports would have to be built as close to the center of cities such as Boston, New York, and Philadelphia. For aircraft to fly into such airports, which would have shorter runways and strict noise requirements, the airliners would have to be capable of making steep climbs after takeoff, quick turns without losing control, and steep descents on approach to landing, accommodat­ing short runways and meeting the standards for Stage 2 noise levels.[1301]

In terms of advancing propulsion technology, NASA’s answer to all of these requirements was the Quiet Clean Short Haul Experimental Engine. Contracts were awarded to GE to design, build, and test two types of high-bypass fanjet engines: an over-the-wing engine and an under-the-wing engine. Self-descriptive as to their place on the airplane, both turbofans were based on the same engine core used in the military F-101 fighter jet. Improvements to the design included noise-reduction features evolved from the Quiet Engine program; a drive-reduction gear to make the fan spin slower than the central shaft; a low-pressure tur­bine; advanced composite construction for the inlet, fan frame, and fan exhaust duct; and a new digital control system that allowed flight com­puters to monitor and control the jet engine’s operation with more pre­cision and quicker response than a pilot could.[1302]

In addition to those "standard” features on each engine, the under – the-wing engine tried out a variable pitch composite low-pressure fan with a 12 to 1 ratio—both features were thought to be valuable in reduc­ing noise, although the variable pitch proved challenging for the GE

team leading the research. Two pitch change mechanisms were tested, one by GE and the other by Hamilton Standard. Both worked well in controlled test conditions but would need a lot of work before they could go into production.[1303]

The over-the-wing engine incorporated a higher fan pressure and a 10 to 1 bypass ratio, a fixed pitch fan, a variable area D-shaped fan exhaust nozzle, and low tip speeds on the fans. Both engines directed their exhaust along the surface of the wing, which required modifica­tions to handle the hot gas and increase lift performance.[1304]

The under-the-wing engine was test-fired for 153 hours before it was delivered to NASA in August of 1978, while the over-the-wing engine received 58 hours of testing and was received by NASA during July of 1977. Results of the tests proved that the technology was sound and, when configured to generate 40,000 pounds of thrust, showed a reduction in
noise of 8 to 12 decibels, or about 60- to 75-percent quieter than the quietest engines flying on commercial airliners at that time. The new technologies also resulted in sharp reductions in emissions of carbon monoxide and unburned hydrocarbons.[1305]

Unfortunately, the new generation of Short Take-Off and Landing (STOL) commuter airliners and small airports near city centers never materialized, so the new engine technology research managed and paid for by NASA but conducted mostly by its industry partners never found a direct commercial application. But there were many valuable lessons learned about the large-diameter turbofans and their nacelles, informa­tion that was put to good use by GE years later in the design and fabri­cation of the GE90 engine that powers the Boeing 777 aircraft.[1306]