Vortex-surfing

Vortex Surfing: Formation Flying Could Save the Air Force Millions on Fuel

Researchers are flying specially outfitted C-17s on missions to find out exactly what formations the Air Force should use to cut the amount of fuel its planes burn.

On July 9 two specially instrumented C-17s took off from Edwards Air Force Base in California, heading to Hawaii. The pair of cargo airplanes carefully took their positions: one following the other just a few thousand feet behind, slightly off to the port side. The reason for the careful placement: The pair of C-17s was generating data for an experiment intended to figure out how to maximize the fuel savings that planes get by flying in formation.

The wingtips of every plane generate swirling coils of air called vortices. If an airplane is positioned in the right spot, the updraft from the vortex will help keep the airplane aloft. “It’s like free lift,” says Donald Erbschloe, the chief scientist of the USAF Air Mobility Command (AMC). It’s called vortex surfing, and it’s the reason geese fly long distances in those V-shaped wedges.

Erbschloe, who was on the trailing airplane during the test, says the trip to Hawaii reduced fuel by 6 percent. During the trip home on July 11, this formation flying saved 10 percent.

The key to airplane drafting is not to be as close as possible—what Erbschloe calls “fingertip formations … white-knuckle, hard flying.” Instead, the trailing C-17 was spaced in sweet spots 2000 to 6000 feet behind the lead airplane, places where the vortices are fully formed and pushing air upward. (If a plane flies in a spot where the vortex is pushing down, it uses more fuel.) This updraft provides lift with no extra fuel. Compounded over many flights—and Air Force Mobility Command averages more than 80,000 flights a year—even single-digit percentages of fuel conservation would be counted in the millions of dollars.

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http://www.popularmechanics.com/military/a9257/vortex-surfing-formation-flying-could-save-the-air-force-millions-on-fuel-15703217/