Since 2005 UAV builder Aurora Flight Sciences has been working on an innovative hybrid turbine-electric propulsion for VTOL flight. The Excalibur demonstrator features a Williams F145 turbofan engine (the same used on the Tomahawk cruise missile) in a mid-fuselage tilting nacelle combined with battery-powered electric lift fans in the wingtips and nose. In VTOL flight, the lift fans augment the vertical lift provided by the jet engine and in conventional flight, the engine also recharges the batteries for the lift fans.
The challenge for VTOL flight is that the high power needed for vertical flight means that the engines are larger than necessary for cruise flight. By using electric lift fans, the engine can be sized for cruise flight, thereby lighter, and can more easily use off-the-shelf engines than one designed for vertical flight. In VTOL flight, the Excalibur's jet engine provides 70% of the lift thrust with the fans providing the balance.
The Excalibur is a small proof-of-principle aircraft that is already flying and plans are for a second Excalibur with the necessary changes to make a full transition to high speed level flight.
Source: Aviation Week & Space Technology, July 20, 2009. "Hovering Hybrid: Aurora flies turbine-electric UAV designed to combine high speed and vertical flight" by Graham Warwick, p38.