What is it about winglets?

Business jets have them, more airliners are getting them, now the US National Research Council says the US Air Force’s KC-135 and KC-10 tankers would benefit from them. Developed by NASA in the 1970s, winglets are becoming a hot item in the hunt for fuel savings.

An NRC committee was asked by the USAF to look at its inventory and identify which aircraft might be good candidates for drag-reducing winglets. The committee evaluated the fleets that burned the most fuel – in decreasing order the C-17, KC-135, C-5, KC-10 and C-130 – and ranked them in terms of priority and potential benefit from wingtip modifications. The KC-10 and KC-135 were ranked “high”, the C-5 “medium” and the C-130 “low”.

Surprisingly the C-17, which already has winglets, gets a relatively positive “medium/low” ranking – because the committee believes it should be possible to design a better wingtip these days.

The USAF’s next tanker? (NASA photo)
The debate over whether fashion or function dictates their use has raged for years, but it is important to distinguish between those aircraft designed from outset with winglets and those that are retrofitted with them.

Winglets reduce lift-induced drag - the component of drag caused by the wing’s lift – but so does increasing the span, so they work best when span is constrained. This was the case with the C-17, and is when retrofitting existing aircraft.

Most aerodynamicists will tell you a properly designed wing doesn’t need winglets, but they have become de rigeur for business jets because the customers expect to see them. Bombardier and Gulfstream have used them for years and now the last hold-outs – Cessna, Dassault and Hawker – have joined the fold.

Retrofitting winglets to wings that lack them can provide useful fuel savings and Aviation Partners Boeing is doing brisk business selling kits for the 737, 757 and 767. There are designs for the 747 and 777 and Aviation Partners (minus Boeing) is working with Airbus on an A320 winglet.

As a daily consumer of over 8 million gallons of fuel it is not surprising the USAF should be interested in the things. The airlift, tanker and ISR aircraft fleets evaluated by the NRC account for over half that consumption and among these conspicuous consumers the KC-135 and KC-10 would be the easiest to modify – a KC-135 and a DC-10 having flown with winglets in the late 1970s and early 1980s.

But it is difficult for the US Air Force to justify spending money to save money, and how it would pay to retrofit winglets to its tankers and transports is far from clear. An innovative proposal to re-engine B-52 bombers with fuel-efficient turbofans on long-term leases paid for out of the operating cost savings went nowhere in the early 2000s.

For now, the USAF is focused on buying a new tanker, the KC-X, and is likely to resist anything that lessens the justification for a new aircraft. For winglets to stand a chance the Pentagon’s Assured Fuels Initiative will need a more aggressive mandate to improve the military’s fuel efficiency as it seeks to reduce US dependence on foreign oil.

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