In seeking a compromise between helicopters and fixed-wing aircraft, engineers in recent years have opted for tilt rotors, but NASA has dusted off and improved on a tilt wing aircraft design that takes off and lands like a helicopter and flies like an airplane. Called the Greased Lightning, or GL-10, the unmanned prototype made a successful vertical takeoff and transition to horizontal flight at Fort A.P. Hill, not far from NASA's Langley Research Center in Hampton, Virginia.

One of the major challenges for aviation engineers is combining the vertical takeoff and landing (VTOL) and hover capabilities of a helicopter with the speed and range of a fixed-wing aircraft. The V-22 Osprey and similar aircraft achieve this by rotating their engine nacelles while keeping their wings fixed because it's mechanically simpler and reduces crosswind drag. However, the GL-10 is an unmanned hybrid-electric aircraft that uses 10 electric motors for propulsion and NASA believes that a tilt wing is the better option for handling so many nacelles at once.

Currently in the design and testing phase, the composite construction GL-10 has four motors on either wing and two on the tail section. These motors are powered by two 8 bhp (6 kW) diesel engines, which charge the craft's lithium-ion batteries. The Unmanned Air Vehicle (UAV) has a wingspan of 20 ft (6.1 m), a dry weight of 55 lb (24.9 kg), and a takeoff weight of 62 lb (28.1 kg).

"All four engines on the left wing are given the same command," said Zack Johns, the GL-10's primary pilot. "The four engines on the right wing also work in concert. Then the two on the tail receive the same command."

The GL-10 was developed using a rapid prototyping process, with NASA saying that a series of 12 prototypes were made. It started with modified foam before working up to hobby planes weighing 5 to 25 lb (2.3 to 11.3 kg) that suffered a number of "hard landings" as the engineers worked to improve the flight control system.

According to the space agency, the GL-10 can loiter for 24 hours in horizontal flight and is described as being as quiet as a petrol lawn mower. The UAV is expected to eventually find applications in small package delivery and long endurance aerial surveillance for agriculture, mapping, and other survey tasks. A much larger version could one day carry one to four passengers.

"During the flight tests we successfully transitioned from hover to wing-borne flight like a conventional airplane then back to hover again," says Bill Fredericks, aerospace engineer. "So far we have done this on five flights. We were ecstatic. Now we're working on our second goal – to demonstrate that this concept is four times more aerodynamically efficient in cruise than a helicopter."

The GL-10 will be exhibited at the Association for Unmanned Vehicles Systems International 2015 conference in Atlanta from May 4 to 7, then will go on to aerodynamic testing.

The video below shows the flight test of the GL-10.

Source: NASA

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