Most planes look the same: a tube with some wings and a tail. But for nearly a century now, since Vincent Burnelli built the UB-14 in the 1930s, engineers have been plotting the end of traditional aircraft design. Instead of just relying on wings, they thought, why couldn't an aircraft's body—its fuselage—generate enough lift to keep the aircraft aloft?
The idea tantalizes, and recent advances in flight control systems and materials science have pushed this blended wing concept from pipe dream to reality. Elements of the concept can be seen in airplanes like the B-2 Spirit bomber and the Navy's X-47B experimental carrier drone. Yet the enduring triumph of tubular airplanes is on obvious display at every airport and airfield in the world, while the blended wing airplane remains on aviation's periphery.
Boeing's Norm Princen is a believer. He has spent two decades pursuing the blended wing dream and sees concept coming of age for 21st century airplanes. "We get smarter all the time, and so we keep evolving the concept and improving it," he says. "There are still benefits left to find. We've only scratched the surface."
Princen has the enviable position of working within Boeing Research & Technology, the arm of the company tasked with developing aerospace breakthroughs that other units can use to build airplanes. In other words, they toil in labs to make up the real mind-bending stuff. "We are working on technology now so that we are ready for any future application that comes along," he says. "In 10 to 15 years we'll be ready with subsonic blended wing transports."
Today is a big day for the engineer, as Princen reveals new blended wing research that solves a thorny problem for such airplanes: the loss of control they suffer when the cargo door is open during flight. This may seem like a strange problem for civilians, but military cargo aircraft drop combat-ready paratroopers, pallets of GPS-guided humanitarian aid or military vehicles, and the occasional fuel air explosive from their rear doors.
The problem, as you could imagine, is that this plays hell on the aerodynamics of an airplane. The disrupted air from an open door throws off the airplane's balance, which makes for very uncomfortable or even dangerous airdrops. Modern cargo haulers, like the C-17, have distinctly upturned aft sections, an unnatural kink in the airplane's profile that minimizes the impact of the open door. Aircraft also install strakes and other devices that help smooth the airflow and help pilots keep the transport steady. None of this is ideal for the weight and fuel efficiency, though.
Princen wanted to solve this problem as it applies to blended wing airplanes. The video shows a new clamshell design: Two independent doors open, one going up and the other down. "Because the doors are almost symmetrical, it doesn't change the pitch characteristics of the airplane," he says.
During an airdrop, a crew would eject the drogue chute into the stable air behind the cargo plane, which pulls the pallet and ensure a smooth exit. Paratroopers would be even more appreciative while leaping from the rear door and facing less jarring violence during first few seconds of their drops.
The video shows a 3D-printed model of a blended wing aircraft inside a water tunnel. The model lacks full wing, and is about 24 inches wide by 31 inches long. That's 2.8 percent the full size aircraft, but big enough to prove the concept.
Dye in the water, moving at 10 inches per second, charts how the airflow moves across the airplane's surface, called the "wetted areas" by engineering types. Like smoke in a wind tunnel, turbulence in the dye show where the airflow is disrupted. Information from the tests can validate computer models, which can be used to design future tweaks in the design, which can then be created by a 3D printer and put back into the water tunnel.
The results suggest that the clamshell doors not only work but are less disruptive than current methods, encouraging enough for Boeing to release test footage. For Princen, it's another step toward ultimate adoption of flying wings in future military aircraft. "We turned a problem into an advantage," he says.
Boeing is hardly alone in pursuing blended wing aircraft. Lockheed Martin, for example, in 2016 proposed a wingless aircraft as a competitor for the U.S. Air Force Air Mobility Command's next-generation tanker aircraft program. Lockheed tested a heavy metal half-span model of the aircraft designed to withstand transonic speeds at NASA Langley Research Center, Aviation Week reported a year ago. Still, there are no programs of record with the Department of Defense. But when the next cargo airplane or refueling tanker program is announced, blended wings will be on offer—if the big aerospace companies can prove they are airworthy.
The benefits of the blended wing design are traditionally tied to efficiency from weight loss. An airplane that can generate as much lift without wings can carry more cargo. As the old adage goes, you can make any shape fly if you give it big enough engines. But those engines add a lot of weight as well, and devour fuel. There goes your efficiency and cargo capacity.
Princen says emerging technologies have increased the chances of the blended wing coming to fruition. One is the vastly improving flight control computers that can make intricate calibrations to flight control surfaces, enabling odd shapes to fly. As chief engineer of Boeing's X-48 program, he was instrumental in proving the airworthiness of blended wing airplanes. Making sure they flew like other airplanes, what they call "flight control laws," was critical in those days and remains so today. The X-48C flew in August 2012.
Engineers also had to prove they could beat the benefits of the conventional airplane fuselage. A cylinder is the best shape for a pressurized vessel because the curved structure distributes the force. This is why every pressurized tank looks the way they do. A flat surface is an initiation for the material to bulge. Alas, flat surfaces also generate a good amount of lift, which is why blended wing aircraft designers want to abandon cylinder-shaped fuselages. However, just putting a cylinder inside a boxy aeroshell doesn't work because it weighs too much.
Princen says the game-changer is modern composite materials, which confer metal-like strength to myriad shapes with no weight penalties. Boeing has a new style of composite that could be especially useful in blended wing designs, he says. Most composites use epoxy alone to keep layers of composite material together. Boeing is lining the layers with stitches that prevent the layers from peeling by acting like sutures.
With today's video release of a blended wing cargo plane, the dream of generations may be one step closer to reality. It's also a gauntlet that Boeing has thrown at other aerospace companies pursuing a fleet of blended wing airplanes that operate more cleanly, cheaply and (with engines mounted on top of a flat surface) quietly.
"We believe our body of work, both in design, test and engineering, is technically superior when compared to other designs in the marketplace," said Naveed Hussain, Boeing's vice president of aeromechanics technology said last year. "The bended wing body is showing great potential to offer structural, aerodynamic and operating efficiencies as well as the capability to be more fuel efficient and quieter over more traditional aircraft designs."
The race is on, against fuselages and other blended wing airplanes. To the victors go the contracts. The rest can only hope for a soft landing – at a museum.
Joe Pappalardo is a contributing writer at Popular Mechanics and author of the new book, Spaceport Earth: The Reinvention of Spaceflight.
