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Winglets could mean fuel savings, study finds

The Air Force could reap some fuel-cost savings by adding winglets — vertical wing-tip additions — to mobility and tanker aircraft, according to a recent study.

Rising fuel prices, coupled with the high operations tempo for Air Force airlifters, have made Congress eager to find ways for the Air Force to save on fuel. The 2007 Defense Authorization Act directed the Air Force to study the fuel savings that winglets might produce. The service tasked the National Research Council’s Air Force Studies Board to conduct the research, and the group filed its report earlier this year.

Michael A. Clarke, NRC director, summarized the findings this way: Winglets are good, but the Air Force needs to study the costs and benefits of modifying each individual airframe before making any moves.

Winglets reduce drag on an aircraft by altering the airflow off the plane’s wingtips, which can improve fuel efficiency during takeoff and cruising. Almost ubiquitous on large commercial aircraft, winglets could produce fuel savings of 3 percent to 5 percent on Air Force tankers and airlifters, the study said.

Tankers could benefit the most from adding winglets — reducing drag as much as 8 percent — because the KC-10 and KC-135 are based on commercial airframes that already feature winglets, the study said. The C-5 fleet could see modest improvement, while C-130s would gain little from such modifications. The C-17 Globemaster III already features the devices.

But Air Force engineers say retrofitting C-5s with winglets would, if anything, hurt efficiency.

Joanne Rumple, spokeswoman for Air Force Materiel Command, said engineers at Robins Air Logistics Center, Ga., found “an add-on winglet is a poor performance idea for the C-5.”

“If the winglets are designed into the wing at the outset ... then there is a small gain to be made in cruise efficiency as a tradeoff to a longer wing,” Rumple wrote in an e-mail. “If they are just added on, then they will typically improve low-speed climb by reducing induced drag, but will hurt cruise by increasing profile drag.”

Moreover, retrofitting an aircraft with winglets adds weight to wings, and that could increase wear and tear. It can also be difficult to match the special materials used in the original aircraft, said Dr. Don Erbschloe, a scientist with Air Mobility Command.

Engine upgrades or winglets?

History suggests the tankers aren’t likely to get winglets now. An Air Force and NASA study in the late ’70s strongly recommended winglets for KC-10s and KC-135s, but as the House Armed Services Committee noted in its 2006 winglet study request, the Air Force opted instead for engine upgrades for the tankers.

“Engines tend to wear out a lot more quickly than airframes do,” Erbschloe said. “Particularly during that era, jet engines were improving very, very quickly in performance and efficiency.”

Then as now, Erbschloe said, Air Force officials might have preferred the long-term benefits of new engines over the modest fuel-efficiency gains offered by winglets. The fuel crisis of the late ’70s was relatively brief, so after studying the issue and implementing a winglet program, it might have taken longer to recoup the investment.

It’s difficult to say whether today’s high oil prices represent a temporary spike or a more long-lasting trend.

That could have been part of the thinking behind choosing re-engining for the C-5s over adding winglets. However, that program has already exceeded costs by 15 percent, a level that could lead Congress or the Air Force to terminate the upgrade.

As for future aircraft, Pentagon officials overseeing the KC-X acquisition aren’t demanding winglets on the new tanker.

“Fuel efficiency is a performance factor, however, specific design solutions are not being directed,” said Jennifer Bentley, an Air Force spokeswoman.