C-17s Go Surfing, to Save Fuel

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Taking a leaf out of the goose's flying manual, a pair of US Air Force C-17s have demonstrated that flying in formation can save fuel. The flight tests were conducted in September and October at Edwards AFB and showed that the trailing aircraft could reduce its fuel consumption up to 10% by flying in the wingtip vortex of the lead aircraft.

Photo: AFRL

Flights were conducted under the Air Force Research Laboratory's Surfing Aircraft Vortices for Energy ($AVE) project, which aimed to demonstrate modifications to the C-17's formation-flight system software enabling the aircraft to fly inside the lead-aircraft's vortex for long distances using the autopilot and autothrottle.

Photo: NASA

$AVE is a follow-on to NASA's Autonomous Formation Flight program in the early 2000s, which used a pair of test F/A-18s (above) and indicated the potential for fuel savings of 10-15%. This was followed in 2010 by DARPA's Formation Flight for Aerodynamic Benefit program, which used a pair of production C-17s and used the formation-flight system to provide control inputs to the autopilot.

The concept works because upwash inside the lead-aircraft's vortex increases the angle of attack on the trail-aircraft's wing, rotating the lift vector. Because lift is an order of magnitude greater that drag, this small upwash angle produces a significant reduction in drag for an insignificant increase in lift. Geese use this phenomenon, flying in a V formation so that each bird flies in the upwash of the one ahead.

Previous flights have shown that precise formation flying can be performed manually, but requires concentration equivalent to aerial refueling and over extended periods is fatiguing for the pilot. So DARPA and now AFRL have been focusing on automating the task so the trail aircraft will maintain position without active assistance from the pilots.

Graphics: AFRL

The Edwards flights, which were flown with the trail aircraft 4,000ft or more behind the lead aircraft, also collected engine and aircraft structural data to confirm earlier conclusions that $AVE operations would not impact the airframe or engine life cycle through flying in increased turbulence inside the vortex.

The USAF quotes Air Mobility Command chief scientist Donald Erbschloe as saying the Edwards tests show the $AVE concept meets AMC's safety, crew workload and viability criteria. The next step for AFRL is to analyze the data and investigate implementation for other aircraft types.

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