With every point of drag reduction critical to Boeing's ambitious performance targets for the 737 MAX, the company is celebrating two significant milestones concerning its novel-shaped split winglet: verification of better-than-expected aerodynamic benefits and confirmation that it can be mass-produced at rates approaching 100 winglets per month.
First unveiled publicly in May 2012, the advanced winglet—which Boeing initially described as a “dual feather”—was designed to provide up to an additional 1.5% fuel-burn improvement on long flights, with the promise of even more if a proposed laminar flow surface treatment worked as planned. Unlike the current upward-tilted blended-wingtip design developed in association with Aviation Partners, the MAX winglet is an all-Boeing design that extends divergently both above and below the wingtip.
Detailed performance analysis, revealed by Boeing for the first time to Aviation Week, indicates that for long-range-cruise missions up to 3,000 nm, the new winglet on the 737-8 carrying 162 passengers in a dual class configuration will have up to a 1.8% better block fuel burn than a blended-winglet-equipped aircraft. The advantage increases with flight length, but even on shorter ranges of 500 nm the new winglet shows a relative benefit of about 1% at Mach 0.79 and slightly more than 1% at long-range cruise speeds.
The configuration is designed to maximize lift for a wingspan restricted to the same Category C-sized gates as current 737s. The design traces its origins to the split wingtip adopted for similar gate restraint reasons for the proposed, but never built, MD-12 twin-deck concept devised by McDonnell Douglas in the 1990s before it merged with Boeing. “The span considerations of the MD-12 determined that it had to live within the 213-foot span of the 747-400,” says Boeing Flight Sciences' chief aerodynamicist and former Douglas designer, Robert Gregg. Inboard of the winglet and outboard of the aileron, the MAX will also incorporate a divergent trailing edge. “This puts more load into winglet and makes it work harder,” he adds.
“At the time, we thought for MAX this design was the better course, and NLF [natural laminar flow, a method of increasing efficiency by reducing skin friction] was one of the approaches that we could also take. We couldn't do that with the original winglet both because of issues with the integration of design and because at the time the technology was new. So we thought we might be able to mature NLF into it, and we wanted to make sure we could understand the build requirements,” Gregg says.
To test if a smooth, applique-based skin treatment suitable for the operational winglet could produce sufficient laminar flow, Boeing flew a modified version of the standard upper winglet on the company's 737 EcoDemonstrator testbed in late 2012. “The decision to go for the MAX winglet was taken in the January-February 2012 period, and we had the option to do NLF or not,” Gregg says. “But we always had it planned, and it was not until the EcoDemonstrator that we could prove we could build a production configuration with the required aerodynamic performance.”
“The EcoDemonstrator flights proved we were able to meet the profile and assembly tolerances,” says Boeing MAX winglet design lead Philipp Witte. Working with Boeing Research and Technology's Advanced Composite Center, the MAX winglet team perfected an all-composite design made from a small number of parts. Although the thin cross-section of the winglet made it a challenge to attach to the wing tip, Witte says the finalized design provides added weight margin for repairs. “There's a 'do-not-exceed' weight limit on the current winglet, and we're sizing this for more margin, so repairs will be easier.”