In order to demonstrate the potential benefits of variable-turbine geometry in a turbofan engine, the U.S. Air Force Aero Propulsion Laboratory funded Garrett-AiResearch to build and test a variable-cycle turbofan engine. The engine selected for the demonstration is the Garrett-AiResearch Model TFE731-2 two-spool, geared-turbofan engine modified to accept variable geometry in the LP turbine, LP compressor, and exhaust nozzles.
Throughout approximately 72 h of engine testing at sea level, static conditions, the variable-cycle engine has demonstrated that these variable-geometry components provide an effective means of rematching the engine components to obtain improved performance characteristics. For example, the concept of maintaining constant inlet total airflow and low-pressure-compressor surge margin while modulating engine thrust was demonstrated during this testing. This concept, in turn, relates directly to the potential of improving installed engine performance over that of a fixed engine by reducing inlet spillage and aft-end external drag.
The variable-cycle TFE731 engine program is continuing and will include simulated, high-altitude, high-Mach-number testing. In addition, analytical studies will further explore the benefits of the current variable geometry, as well as those afforded by additional variable components as related to overall system performance.