Vortex surfing: Scott AFB scientist develops money-saving flight maneuver
An Air Mobility Command project called Surfing Aircraft Vortices for Energy (SAVE) showed a fuel savings of up to 10 percent in trailing aircraft.
Belleville (Ill.) News-Democrat
SCOTT AIR FORCE BASE -- Each year, the Air Mobility Command burns up $2.5 billion worth of aviation fuel -- more than 60 percent of the Air Force's annual fuel bill.
Donald Erbschloe, AMC's chief scientist, has found a way to slash the command's fuel costs after years of computer modeling and trials with C-17 Globemaster III air cargo planes -- and it's a method that could one day revolutionize civilian aviation as well.
It's called vortex surfing.
The concept behind it explains why geese and ducks fly in V formations. For aircraft, the concept is illustrated by the fact each wingtip creates a rotating pattern of air behind it as it generates lift.
Wingtip vortex is generated because an aircraft wing has an edge, Erbschloe said during an interview in his office at AMC headquarters at Scott.
"As it pushes its way through the air, it churns up the air, and it manifests itself into a swirling configuration, which we call the wingtip vortex," he said. "That exists for miles behind a large aircraft at altitude. When you look up in the sky and see contrails, those are the wingtip vortices."
A series of aircraft trials -- including one that involved two C-17 cargo planes that flew round-trip between California and Hawaii in early July -- showed that if one plane flew about half a mile behind the first, and flew positioned to the right, in the so-called "sweet spot," then the second plane's fuel burn rate was cut by as much as 10 percent.
"You're taking advantage because you're flying in the updrafts," Erbschloe said. "You're getting free lift. That means your engines don't have to work as hard to generate lift. And that fuel savings can be real."
Erbshloe's research team is trying to mimic what birds have known instinctively for millions of years.
According to researchers at Cornell University, the V formation greatly boosts the efficiency and range of flying birds, particularly over long migratory routes. All the birds except the first fly in the upwash from the wingtip vortices of the bird ahead.
In a V formation of 25 members, researchers said each bird can achieve a reduction of induced drag by up to 65 percent, increasing their range by 71 percent. Still, flight fatigue is a problem for the birds. To deal with it, the birds flying at the tips and front of the formation rotate in a timely, cyclical manner, sharing fatigue equally among flock members.
In this context, Erbschloe recalled a short ferry ride he took during a visit last December to Washington State. Almost as soon as the ferry left port, a sea gull flew to the ship's bow.
"And it just glided," Erbschloe said of the gull. "It didn't flap its wing once the entire trip over. It was riding the bow wave that was coming off the ferry. It was just hovering. Talk about the ultimate free ride."
That's what Erbschloe's research is trying to do for Air Force jets.
"Can you position your aircraft in places where they're getting an energy advantage?" he said. "Nature does this all the time."
Birds do it, planes do it
Erbschloe's research team consists of members from such disparate agencies as the Defense Advanced Research Projects Agency, NASA, and the Boeing Co., the C-17's manufacturer. They were brought together under an Air Force program called Surfing Aircraft Vortices for Energy, or S.A.V.E.
Aviation scientists for decades have been trying to find ways to cut fuel costs by flying in tight formation. But that goal has proven elusive because of the difficulty of finding the "sweet spot" of a wingtip vortex and sticking with it.
"In regular formation flying, you're flying and maintaining a position off an aircraft," said Erbschloe, 59, who retired from the Air Force in 2008 as a lieutenant colonel and has 4,000 pilot hours under his belt. "Here the trailer is flying a position off a vortex."
Although stunt pilots with the Thunderbirds and Blue Angels are trained to fly in tight formations, it has not been possible for larger aircraft.
A decade ago, NASA conducted trials at an air base in Southern California of fighter aircraft flying in "finger tip formation," which proved stressful for the pilots involved, Erbschloe said.
"I call it 'white knuckle flying', because talking with the pilots, it really was quite intense," he said. "The auto pilot didn't fly it. They were very, close. You had to make split-second decisions. Very turbulent air. Even so, the aircrews involved with those trial were seeing fuel reductions of up to 20 percent.
" So, it was significant, but it was ultimately decided it was too much, too hard on the aircraft, too hard on the aircrew, the pilots involved," he said.
Innovations in computer software, as well as computer modeling, however, have enabled flight crews in the trailing airplane to use their autopilot features to fly up to a mile behind the lead airplane, and to do so via autopilot, which greatly reduces aircrew stress, he said.
The high point in the project so far came in early July, when a pair of C-17s flew from Edwards Air Force, Calif., to Hickam Air Force Base, Hawaii -- a round trip of almost 5,000 miles -- in the first true test of vortex surfing. Fuel savings on sections of the trip were measured at up to 10 percent, generating test results that will be the centerpiece of a January meeting of the American Institute of Aeronautics and Astronautics.
But that test flight generated findings that raised questions that need to be investigated further.
It showed that "the closer you get to the vortex, you get more benefit, but the ride gets rougher," Erbschloe said. "And in fact, when you're right at the sweet spot, when you're getting the maximum benefit, it felt to me like I was in moderate turbulence. The plane is being shaken up quite a bit. And it's not something I would want to fly for extended periods of time."
A commercial benefit?
The potential applications of this research to civilian aviation, especially civilian air cargo, could be huge, Erbschloe said.
For instance, at night, when air cargo giant FedEx's planes leave their hub in Memphis, "It's kind of like ants coming out of an anthill," he said. "And many of them are going off in the exact same direction, either for extended periods of time or going to the exact same place. They're taking off within minutes of each other. For them to be able to join up and fly together makes an awful lot of sense."
The same logic could apply to civilian passenger airlines: American Airlines and Southwest Airlines flights, scheduled to take off within minutes of each other and going in the same direction, could benefit from the pattern.
"Could you ever reach a point where you have cooperation among the airlines, where they say, 'OK you get the benefit this way, and we'll get the benefit coming back,'" he said. "The numbers of fuel savings we're talking about are compelling enough that they ought to at least look at it."
John Stillion, a senior fellow at the Center for Strategic and Budgetary Assessment, in Washington, D.C., said he saw a lot of potential behind Air Force research into vortex surfing, but it might prove difficult to translate this technique to civilian applications.
If the aircraft must fly within 3,000 feet of each other, "that is well within the minimum spacing that the Federal Aviation Administration requires between airlines," he said.
To make vortex surfing work in civil aviation, he said, "There'd have to be a lot of rule changes, so it could take a considerable period of time for this to happen."
If everything falls into place, vortex surfing could be a standard operating procedure for military aircrews, but Erbschloe thinks that won't happen for at least another three years.
In the meantime, some important questions need to be answered about the effect of vortex surfing on human aircrews.
For instance, the plane trailing behind the lead plane must make small adjustments to align with the vortex generated by the lead aircraft.
"And as you move out further, those movements get exaggerated," he said. "So the one at the very end could be making very large gross movements. That's inefficient, and we call that a bullwhip instability."
Integrating vortex surfing into AMC procedure also will require a shift in the command's culture, he said.
"We are extraordinarily good at moving things from point A to point B, and doing things in a very systematic way, doing things in a quick way," he said. "The way we designed our system is we want to de-conflict aircraft. You don't want aircraft taking off at the same time or landing at the same base."
For vortex surfing to be adopted, "We're going to have to change our way of thinking," Erbschloe said, "and allow occasionally that sometimes we do want to take off from the same base, or perhaps from different bases that are nearby so that they can meet up and not lose too much fuel in the interim."