NASA research shows Hybrid-Electric Propulsion can Achieve 4 times Increase in Cruise Efficiency for a VTOL Aircraft

Engineers at NASA’s Langley Research Center in Hampton, Va., are studying the concept of vertical takeoff and landing electric engine drones with models such as the unmanned aerial system GL-10 Greased Lightning. The GL-10, which has a 10-foot wingspan, recently flew successfully while tethered. Free-flight tests are planned in the fall of 2014.

This research has helped lead to NASA Aeronautics Research Mission Directorate efforts to better understand the potential of electric propulsion across all types, sizes and missions for aviation.

NASA says that a hybrid-electric design is “scale free” — meaning the same principles could be used to revolutionize everything from helicopters, to military UAVs, to massive jetliners.

NASA has a 21 page study – Benefits of Hybrid-Electric Propulsion to Achieve 4x
Increase in Cruise Efficiency for a VTOL Aircraft

Electric propulsion enables radical new vehicle concepts, particularly for Vertical Takeoff and Landing (VTOL) aircraft because of their significant mismatch between takeoff and cruise power conditions. However, electric propulsion does not merely provide the ability to normalize the power required across the phases of flight, in the way that automobiles also use hybrid electric technologies. The ability to distribute the thrust across the airframe, without mechanical complexity and with a scale-free propulsion system, is a new degree of freedom for aircraft designers. Electric propulsion is scale-free in terms of being able to achieve highly similar levels of motor power to weight and efficiency across a dramatic scaling range. Applying these combined principles of electric propulsion across a VTOL aircraft permits an improvement in aerodynamic efficiency that is approximately four times the state of the art of conventional helicopter configurations. Helicopters typically achieve a lift to drag ratio (L/D) of between 4 and 5, while the VTOL aircraft designed and developed in this research were designed to achieve an L/D of approximately 20. Fundamentally, the ability to eliminate the problem of advancing and retreating rotor blades is shown, without resorting to unacceptable prior solutions such as tail-sitters. This combination of concept and technology also enables a four times increase in range and endurance while maintaining the full VTOL and hover capability provided by a helicopter. Also important is the ability to achieve low disc-loading for low ground impingement velocities, low noise and hover power minimization (thus reducing energy consumption in VTOL phases). This combination of low noise and electric propulsion (i.e. zero emissions) will produce a much more community-friendly class of vehicles. This research provides a review of the concept brainstorming, configuration aerodynamic and mission analysis, as well as subscale prototype construction and flight testing that verifies transition flight control. A final down-selected vehicle is also presented.

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