Many aircraft, particularly in military and unmanned roles, need to operate efficiently at more than one flight regime. Certain planes are expected to perform in relatively high-speed cruise modes, as well as slower loitering or circling scenarios. Unfortunately, it is well established that an engine operates most efficiently when the engine exit velocity closely matches the speed of the airframe. As a result, designing an engine that is suitable for multiple roles, yields an engine that only performs moderately well in each of those operating conditions.
One approach to improve performance over a wider range of missions, is to add an additional flow path to the engine that can be turned on or off depending on the required operating requirements. While this method can produce acceptable results, current efforts are geometrically constrained by the internal nacelle size. Such a constrained variable cycle engine can only slightly alter its bypass ratio (the ratio of the flow rate of the bypass stream to the flow rate entering the core). A less restricted geometry would mean larger variations in the bypass ratio allowing for an efficient high-bypass turbofan to switch modes to a low-bypass turbofan or even turbojet configuration (zero bypass).
Air Force scientists and engineers have addressed this gap in technology by inventing a variable bypass turbofan engine. The engine includes a bypass fan with blades mated to a first low-pressure shaft segment. A second low-pressure shaft incorporates a low-pressure compressor and a connected low-pressure turbine. The engine has a clutch between the first and second low-pressure shaft segments and a brake to selectively halt or oppose rotation of the first low-pressure shaft segment or the bypass fan. Fan blades are rotated to increase flow.
- The engine allows the pilot to select the configuration best suited to a given flight segment
- When the bypass fan is activated, additional cool dense air is propelled around the core housing and this augmented propulsive air allows for reduced emissions from the core of the engine
- The bypass air also cools the exhaust, thus reducing the thermal signature of the engine
- US patent 9,605,557 available for license
- Potential for collaboration with Air Force reseaarchers