In a remarkable coincidence, the split-tip winglet that went out of fashion with the demise of the MD-12 concept in the early 1990s has suddenly re-appeared in in two applications -- Boeing's straight-edged Advanced Technology (AT) winglet for the 737 Max (above) and Aviation Partners' scimitar-edged split-tip blended winglet on the 737 Next Generation series.
Aviation Partners is jealous about guarding what it considers proprietary information (ask Airbus), but founder Joe Clark says he has no suspicions that Boeing copied the Aviation Partners design. Robb Gregg, Boeing's chief aerodynamicist on the 737 Max, agrees that both companies came to a similar conclusion from different directions. But the joint venture partners remain divided on the potential benefits of applying a more efficient split-tip design on the 737NG as a retrofit. We explore the history and opinions on both designs in next week's issue of Flight International, but here is a sneak-peek.
Aviation Partners poised to offer split-tip winglet for 737NG, but Boeing not convinced
Aviation Partners has started showing airlines a split-tip winglet with blended, "scimitar"-edged feathers as a retrofit option that the joint venture estimates can reduce fuel consumption by 2.5 to 3% on next-generation 737s.
The move precedes a launch decision by the board of directors of the joint venture, but that approval should come "shortly", says Joe Clark, founder of Aviation Partners, the Seattle-based firm that designed the standard blended winglet ordered on more than 4,600 737NGs.
Aviation Partners unveiled the scimitar-edged winglet last October and launched flight tests on a 737 Boeing Business Jet in April, which confirmed the estimates of computational fluid dynamics models to within one-tenth of a percentage point, Clark says.
"We are very pleased with what we've achieved," he adds. While Aviation Partners prepares to offer a scimitar-edged split-tip winglet on the 737NG, Boeing is readying a straight-edged split-tip winglet on the 737 Max.
Both companies claim to have arrived on the split-tip configuration for the 737 at nearly the same time by coincidence. Aviation Partners had no prior knowledge of Boeing's "dual-feather" split-tip winglet for the 737 Max, and has received no information on the design from its joint venture partner, Clark says.
For its part, Boeing also was unaware of the Aviation Partners design when it began working on the Advanced Technology (AT) winglet around June 2011, says Robb Gregg, a chief aerodynamicist for the 737 Max. "As I was looking at the configuration, we needed to get more performance out of it and really the only place we hadn't spent a lot of time was looking at the [wing]-tip," Gregg says.
Boeing completed trade studies between August and September last year, he says, then fabricated a set of optimal shapes for testing in a wind tunnel. Although the split-tip design appears to be a new innovation, it traces back to Robb's previous work as a chief aerodynamicist at McDonnell Douglas.
The airframer that merged with Boeing in 1997 had pioneered the installation of winglets on airliners in the mid-1980s. The MD-11 entered service with an up/down winglet, with a shortened lower surface forward of the upper surface. The lower surface was shaped to improve stall characteristics at low-speed, Gregg says. McDonnell Douglas also proposed a split-tip winglet for the short-lived MD-12, a late-1980s concept for a four-engined double-decker. As the chief aerodynamicist of the MD-12 concept, Gregg says, he proposed the split-tip to optimize lift of a wingspan artificially constrained to a length of 64.9m (213ft) to fit into existing airport gates.
Likewise, the 737 Max also demanded more performance than a blended winglet could produce. "Because we needed more performance to satisfy the customers we felt we needed to push the technology a bit further," Gregg says.
A split-tip winglet has never been tested in flight test, and Boeing currently has no plans to test the 737 Max AT Winglet on a surrogate platform. Boeing is confident that computational fluid dynamics models have predicted drag characteristics accurately, Gregg says.
At the same time, Boeing is not convinced a split-tip winglet will produce performance improvements as a retrofit option on the 737NG, although it has not conducted an analysis yet.
Holding Boeing back is the knowledge that the AT Winglet increases the aerodynamic loads on the outboard wing section. "The better the winglet the more load it's going to drive outboard. Otherwise it didn't do anything for you," Michael Teal, chief project engineer on the 737 Max, said in a July interview. "The question is how difficult it would be to retrofit," he added. "You're getting out there on the end of a wing; it's not that thick. It's not something that's easy to take apart and add gauge to."
Despite being joint venture partners, Boeing and Aviation Partners also have different views on the margin of benefit provided by a split-tip winglet. Boeing predicts the straight-edged split-tip on the 737 Max will contribute 1.5% to fuel burn reduction. Aviation Partners, on the other hand, is proposing a 2.5% to 3% benefit from installing the scimitar-edged winglet on the 737NG, which shares the same airfoil as the 737 Max. Even so, Aviation Partners is optimistic that scimitar-edged split wing-tips will be retrofitted on as much as 60% of the 737NG fleet, Clark says.