Many aircraft need to operate efficiently at more than one flight regime; for example, certain airframes are expected to perform in relatively high-speed cruise modes, as well as slower loitering scenarios. Unfortunately, an engine operates most efficiently when the 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 and off, depending on the required operating requirements. While this method can produce acceptable results, current efforts are geometrically constrained by internal nacelle size. Such a constrained variable cycle engine can only vary its bypass ratio relatively slightly from a low-to mid-bypass ratio. A less constrained geometry would allow for large variations in bypass ratio, allowing for an efficient high-bypass turbofan to switch modes to a low-bypass turbofan, or even turbojet configuration.
Current designs do not allow for a user to select between a high-bypass turbofan mode in order to operate efficiently at slow Mach numbers, and the option to turn off the high-bypass portion of the engine in order to convert the engine into a low-bypass turbofan (or even turbojet) to operate efficiently at higher Mach regimes.
This invention is a variable bypass turbofan engine that includes a bypass fan with multiple bypass fan blades mated to a low-pressure shaft segment. A second low-pressure shaft segment includes a low-pressure compressor and a low-pressure turbine to which it is mated. The engine also includes a clutch coupled between the two low-pressure shaft segments, and is configured to selectively couple and decouple the two low-pressure shaft segments. A brake is configured to selectively halt or oppose rotation of the first low-pressure shaft segment or the bypass fan.