Air Force

Variable bypass turbofan engine

Selectively switches between high-bypass and low-bypass turbofan configurations for optimal efficiency at multiple speeds


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When the engine is underway, the bypass fan blades (17) are angled to the same degree as found in fixed bypass fans (16). To convert the engine from a bypass turbofan to a turbojet configuration, the clutch is disengaged, and the brake is applied. The bypass fan is therefore decoupled from the low-pressure shaft and rotation is halted. At that point, the bypass fan blades are rotated about their respective pivot points (54) to be substantially parallel with the incoming ram air. In this configuration, drag from the halted bypass fan is substantially reduced.

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 improving 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.

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