A makeover for transport aircraft?

Stephen J. Mraz
Senior Editor

From the earliest days of aviation, transport aircraft, along with most others, have relied on a single type of design, tube (or fuselage), and wing. It has served well and engineers at Airbus and Boeing are still wringing more efficiency and performance from it. But about 20 years ago, NASA engineers, worried about crowded airports and fuel efficiency, asked airframe companies to redesign the large transport plane (more than 150 passengers), starting with a blank sheet of paper and no bias towards well-established approaches.

One concept that came out of the request was the blended-wing body (BWB) from Robert Liebeck at McDonnell Douglas Corp. (now part of Boeing Co.). It features a wide curved fuselage and a thick delta wing, both of which generate lift and carry cargo. Putting loads closer to the lift means less structure is needed. There is also less total surface or skin. Therefore, the overall plane is lighter with a higher lift-to-drag ratio (20 compared to the 747’s 17). These factors let it carry more cargo or fuel than conventional aircraft, and be less expensive to build in terms of materials.

About 10 years ago, NASA decided to put the BWB design to the test. It looked good on paper, but its flying characteristics and exactly how to control such an aircraft were purely theoretical. So NASA formed a team, including Boeing and the Air Force, to construct a BWB prototype, the X-48A, for wind-tunnel and feasibility testing. Then about seven years ago, Boeing and NASA decided to build a flying prototype, the X-48B

A FLYING BWB
Within six years, two X-48Bs were designed and put together by Cranfield Aerospace, a U.K. firm with experience building includsmall planes and drones from scratch. Boeing and NASA gave Cranfield the shape of the fuselage, a center of gravity, and approximate weight and thrust targets. Cranfield then designed and built the shell, airframe, avionics, and controls for the 20 flight surfaces on the trailing edge of the wing (10/side), and a ground control station from which the plane can be flown. “Instead of going to several vendors, then having to coordinate between them, we picked one subcontractor who could handle the entire job,” says Norm Princen chief engineer for the X-48B project at Boeing’s Phantom Works.

In a classic engineering compromise, Boeing decided on an8.5% scale factor based on available engines. At 8.5%, they could use three P200 turbines from JetCat, each with 50 lb of thrust. “That’s about the largest engine in the model or hobbyists category. There’s a large gap between them and the smallest ‘real’ jet engines, such as those used for drones,” says Princen. “So we went with 8.5%.”

(The initial X-48A was scaled to 14% based on NASA’s budget for the project and the size of the doors in the shop they were building it, according to Dan Vicroy, a NASA flight dynamics engineer on the X-48B.)

The team made similar compromises on the actuators for those 20 control surfaces. “To save money, we looked at actuators for models, but they didn’t have the speed or torque we needed. So we went with K-2000s from Kearfott Guidance and Navigation Systems. They are purpose-built aerospace designs and are used in the Army’s Shadow UAV,” says Princen. “They are almost too large physically, but they have the torque and rate requirements we need.”

The X-48B has a 21-ft wingspan, weighs about 400 lb, and flies at up to 130 kt at 10,000 ft. Three JetCat engines mounted above the wing each burn 24 oz/hr of kerosene, giving the plane 30 to 45 min of flight time on its 13-gallon fuel load. It uses a carbon- fiber airframe and carbon-composite skin. But with weight an issue, the skin covering the outer wingtips consists of a single ply of carbon fiber, about 0.001-in., and some epoxy.

Engines sit on pylons above the wing, a change from the original Mc- Donnell Douglas design, which had engine inlets flush with aircraft skin, letting them pull in boundarylayer air. Using air from this layer means airflow sucked into the engines doesn’t add drag. But Boeing wanted to get something flying and didn’t want to add the complexity of burying the engines in the fuselage nor lose cargo space in the fuselage.

“So the Boeing team decided to go with an approach they know, pylon-mounted engines,” says Vicroy. “And putting them atop rather than slung beneath the wings eliminates problems with landing-gear height and cuts back on FOD, or foreign object damage, a major source of engine damage caused by debris, pebbles, and other objects sucked into the intakes. Another benefit is that the body of the aircraft shields engine noise from the ground, making it quieter to operate. And putting the engines above the wing takes the engines out of the equation when it comes to exploring BWB control, the goal of the this project.” Because there is no tail on the

X-48B, the 20 movable surfaces on the trailing edge of the wing are responsible for all aircraft attitude control. Most of those surfaces are elevons, sort of a cross between elevators and ailerons. The outermost elevons split open like air brakes, so drag can be suddenly increased or decreased. And both wingtips, which are about 2-ft tall, have a rudder on them.

DYNAMIC MODELING
For scaled-down wind-tunnel prototypes, shape is the most important factor. Density, overall weight, and inertias are not part of the mix. But dynamic models must mimic actual flight motion of full-sized versions, and that brings into play inertias, weight, and other factors.

“The model has to respond to inputs the same way a larger one would,” says Princen. “But because our plane is smaller and less massive, it actually responds faster, by a factor of about three, than a full-sized plane. But otherwise, responses are the same.”

“Hitting those weight and inertia targets was tough and we went through five design iterations always trying to drive out weight,” says Princen. “That’s because the density of our engines and actuators, for example, don’t scale with size. For example, if you just scaled up the 50-lb thrust engine to the point it delivered 45,000 lb of thrust, it would be much heavier than an advanced turbofan jet with the same power. So on our scaled plane, these parts are effectively too heavy and we have to make the structure lighter to account for that. And it was a challenge making an aircraft that has lower density than a full-sized military transport, which are efficiently designed to begin with.”

“We built a 5% dynamically scaled X-48 to fly in a wind tunnel,” recalls Vicroy. “It was so sensitive in roll (motion around its longitudinal axis), that having to add 1 oz at the wingtip forced us to spread an additional pound around the rest of the aircraft to get all the inertias balanced.”

Another attribute that doesn’t scale is Mach number, a function of airspeed and altitude, “For example, a fullsized plane might be flying at 400 knots and Mach 0.7,” says Princen. “But scaling that down, our plane would only be going 150 knots or Mach 0.2. So the X-48B cannot be used for testing at transonic speeds, making this a low-speed test vehicle. We will use it to explore flight controls and strategies for terminal area operations, or takeoffs, climbing to cruise altitude, and landings. And we wanted to tackle those issues first. After all, this is not supposed to be a faster transport, and this way we can do testing with a relatively low-cost vehicle.” (Costs of the X-48B is proprietary and Boeing isn’t saying.)

The goals of the project, one the team is well on its way towards hitting, is writing the software code that translates pilot commands into aircraft actions. The code has to do this predictably, reliably, and in accord with what generations of pilots have learned. So, for example, even though the X-48B lacks a conventional rudder, the ground station cockpit where a pilot remotely flies the craft has rudder pedals. “And pushing them elicits the same response as if the aircraft had a rudder,” says Princen.

The software also had to let pilots react conventionally to ground effect. Ground effect is most noticeable when flying less than one wingspan above the ground. It adds a cushioning effect. When pilots land, they expect to encounter ground effect and know they have to pull up lightly on the stick at the right time to flare, or bring the nose up, for landing. BWB designs react differently. “In conventional aircraft, the nose would pitch up if the pilot did nothing upon going into ground effect,” says Princen. “BWBs do just the opposite; they pitch down. We adjusted the algorithms to make the plane react like a traditional plane. So when the pilot comes in to land, he pulls back on the stick, the software activates the right sequence of elevon movements, and the nose gently pitches up, flaring for landing.”

“We don’t want to retrain pilots to fly this particular plane,” emphasizes Princen. “They already endure years of training, learning to fly a certain way, and we want this plane to respond the way pilots expect. So our control algorithms should let pilots think they are flying any other Boeing airplane.”

With algorithms controlling 20 control surfaces, not to mention three engines, it needs a capable computer. So Boeing used a dual DSP-chip setup.

One area that still needs more research is tumbling. “The X-48 lacks a tail, so it should be much more susceptible to tumbling,” says Vicroy. “We want to learn what it takes to set the BWB tumbling, whether it even has the power to instigate it, and whether it has enough power to get out of it.”

Another key area that needs more R&D before BWB airliners grace the skies is in structures and pressurization. A cylinder, like the fuselage on most planes, is relatively easy to turn into a pressure vessel. But how about a BWB? “Researchers have come up with candidate designs for a pressurized BWB,” says Vicroy. “And none of them present any substantial weight penalty. Most are variations of a weblike structure of an interconnected series of tubes, more organic than current designs.”

With the first phase of flight testing complete, the Boeing team is already upgrading the control software and planning further testing this year. They want to complete basic flight testing around the middle of next year. If funding is available, they would then like to do some low-noise testing for NASA.