April 3, 2018, © Leeham News: New airplanes for the foreseeable future are unlikely to look radically different than the tube-and-wing configuration that’s been around since the dawn of manned flight.
Yes, there are unmanned aerial vehicles (UAVs) that look very different. Yes, there are the B-2 and B-21 bombers that are flying wings.
And, yes, there is the Blended Wing Body concept that was created by McDonnell Douglas and tested, in scaled-size models, by Boeing, which acquired MDC in 1997.
But don’t expect to see a BWB either as a freighter or as a passenger airplane any time soon, says Boeing’s VP of Product Development and Future Airplane Development.
Mike Sinnett says the configuration works. It’s economic. But its very configuration works against the airplane for commercial application, he says.
“The characteristics of a freighter, in general, are you want to be able to use the airplane at airports that aren’t typically as improved as airports are that we use passenger airplanes at,” he told an evening meeting March 28 at the University of Washington in Seattle. The event was organized by the American Institute of Aeronautics and Astronautics.
“Airport compatibility is a really important part of that,” he said.
“A BWB works great because it’s got [wing] span. It’s got more empty weight than a tube-and-wing configuration for a revenue ton of cargo. It’s going to carry more operational empty weight. It’s got span and it’s got a fair amount of wetted area.
Sinnett said designers will make the weight trade for span when there is a lot of range needed. For short missions, for example, four hours, the weight trade isn’t worth it.
“A four-hour mission isn’t going to work because you’ve got more weight and you’re not taking advantage of that span,” he said. Long cruise segments are required to make the aerodynamics work to their greatest benefit.
Large wing span may also conflict with airport compatibility, he said.
Cargo aircraft also need to be “loadable” quickly, he said. Existing freight containers and cargo loading systems need to work with the BWB.
“And, in general, unless it’s more than a 20% improvement over what you have today,” a new airplane isn’t justified. “For more than a 20% improvement, people will make the change,” he said.
Sinnett that that Boeing’s studies have only shown a few cases in which the economics are better than 20%.
Military cargo operations aren’t tied to commercial requirements.
“You can imagine someday where military cargo operations, long-range, where you’re going from hub-to-hub,” he said. But so far, there hasn’t been a customer requirement for which a BWB makes sense.
Sinnett said passenger use presents different challenges.
In a BWB, the center wing box is also where the passenger cabin is. The wing box needs to be tall enough to accommodate 95% of the world’s men to be able to stand upright.
Space is required for the carry-on bags. There has to be room above and below the floor for ventilation and systems.
“That means you have to have a certain height. Once you have a certain height, you’ve got to have a certain width for the box to work,” he said. “That translates into a certain span. You’ve got a really big airplane at that point.”
This results in the multiple hundreds of passengers, he said.
“It’s hard to do with a small airplane to get enough height in the center box.”
The design creates another challenge: developing a family of airplanes.
Boeing, as with all manufacturers, like several models of a basic design.
This enables the OEM to create, relatively inexpensively, several sub-types to cover the massive R&D costs and post high profit margins (at least in theory) the longer the program is in production.
With tube-and-wing design, it’s a relatively simple task to stretch or shrink the base design to create a three-member family.
The 737 MAX has four sizes, the MAX 7, 8, 9 and 10. Although there were special challenges to overcome for the MAX 10, the stretch was nevertheless accomplished for comparatively little money.
A BWB, with its unique shape, doesn’t lend itself to a stretch or a shrink, Sinnett said. “You take the most expensive part of the airplane, the non-constant section, and growing it in a non-constant way, or shrinking it in a non-constant way. It’s really hard.”