Boeing has been mostly quiet on its future narrowbody thinking, though a recently revealed patent, filed in November 2009 and made public last month, may illuminate the airframer's thinking about how to replace the 737 with a composite aircraft and give its most important narrowbody customer, Southwest Airlines, the plane it's been begging for.
Titled Weight-optimizing internally pressurized composite-body aircraft fueselages having near-elliptical cross sections, or WIPCAFHNECS as I prefer to call it, is the work of Boeing engineers Mithra Sankrithi and Kevin Retz. Retz and Sankrithi have come up with several methods of developing a composite fuselage design to accommodate a seven-abreast 2-3-2 twin-aisle configuration ideal for quick loading of passengers and cargo.
The T-tail configuration and guppy-like aircraft shape is not what's important in this patent. What is important, however, is the shape and construction of the fuselage:
This twin-aisle fuselage cross-sectional shape has also been shown to provide a perimeter-per-seat ratio comparable to that of a corresponding single-aisle, six-abreast, conventional aircraft fuselage having a circular or "blended circular arc" cross-section, and consequently, can also provide a cross-section-parasite-drag-per-seat ratio and an empty-weight-per-seat ratio that, in a first-order analysis, are comparable to those of the corresponding single-aisle fuselage cross-section, while offering better passenger comfort and owner revenue options.
Though the patent spells out the challenges of such an arrangement including "the structural and weight penalties involved in moving from a fuselage design having a conventional circular cross-section to a fuselage design having a non-circular cross-section, especially those associated with the internal pressurization effects inherent in the design of high-altitude jet airliners."
The "weight-optimizing" explored in the patent doesn't define a specific method for construction but evaluates several methods of tailoring the fuselage skin for the appropriate loading. One example looks at how "composite-body aircraft fuselages can also advantageously incorporate shells comprising composite "sandwiches," i.e., stiff, lightweight "core" structures comprising either a continuous foam or a honeycomb of cells laminated between two circumferential skins, or face sheets."
Thus, for example, the core material can be tailored throughout the design process by varying, for example, core material, type and density.
Industry sources have indicated that Boeing's 737 Advanced Product Development Team is leaning toward an elliptical composite fuselage, coupled with an aluminum wing. That combination would represent a reversal in the strategies employed by Bombardier's CSeries and the Airbus A400M, which have metallic fuselages and composite wings.
Boeing Commercial Airplanes CEO Jim Albaugh told the Puget Sound Business Journal in August that the company had made significant advancements in optimizing a composite fuselage for a smaller aircraft. In March, Albaugh had conceded that hail and bird strike requirements dictated the thickness of the material and reduced the benefit of composite as the airplane became smaller.
"If you'd asked me six months ago, I would have said aluminum" for the 737 successor, Albaugh said. But he added that the potential attributes of the newer materials are changing the equation.
"We had limited our thinking to first-generation composites," he said. "As we look at things we can do with second- or third-generation composites, maybe we were a little ahead of ourselves saying it doesn't scale down as well... Clearly, second- or third-generation composites would lend themselves to more efficiency."
Albaugh said a Boeing team is now investigating how the Renton plant could be converted from making metal 737s to a new composite successor, without interrupting production. "It's not a trivial matter," he said.
"It's not like you turn the spigot off Friday, and turn on the new line on Monday. We're looking at how we might do that."
That transition is the central question to any narrowbody replacement. Using the current production infrastructure saves significant development time and cost, by leveraging the existing industrial footprint, as well as the already skilled workforce in Renton. By leaving the wing aluminum, Boeing avoids the 787's static load distribution challenges (read: side of body) and negates any non-optimized composite skin thickness for a 737-sized aircraft.
Twin-aisle medium to short-haul aircraft configurations are not new to Boeing, having explored a six-abreast 2-2-2 arrangement for the open-rotor design of the 7J7 of the late 1980s. The twin-aisle arrangement offers expedited boarding and deplaning, as well as containerized cargo that can be loaded more quickly than bulk luggage.
For an airline like Southwest that lives and dies by its 20 minute 737 turn times, a composite twin-aisle 737 replacement could give the airline a boost in its daily utilization. In essence, more flights, more revenue. The question remains, is this patent the special sauce that makes it happen?