Blended Wing UAV
Unique unmanned craft’s robust composite design a plus for rough duty.
By Sara Black, Technical Editor | May 2007
Design for rough duty
For the Killer Bee, the Swift team had to meet several key project objectives: a manageable system for launch and recovery in the field; a lightweight airframe to maximize the aircraft’s long-term loiter capabilities in day or night operations; and structural durability to withstand extreme environments. McCue and Page used a suite of software to optimize the KB-UA’s shape, aerodynamics and composite structure. SolidWorks 3-D CAD software (SolidWorks, Concord, Mass.) helped define the craft’s initial shape, while NEiNastran, supplied by Noran Engineering Inc. (Westminster, Calif.), was used to size and develop the airframe structure and laminate schedule to meet anticipated loads.
Source: NASA
NASA’s blended wing body (BWB) for large civil and military cargo craft has been under consideration since 2000. A 1/10th scale remotely controlled concept built by The Boeing Co., dubbed the X-48B, was scheduled for a test flight in April at Edwards Air Force Base (NASA is partnered with The Boeing Co. and the Air Force Research Laboratory). The concept continues to generate interest because its shape would mean 20 percent less fuel consumption than today’s tube-and-wing aircraft. The wingspan would be comparable to a Boeing 747 and could operate at existing aircraft terminals. It would also weigh less and generate less noise. The NASA study will determine flight and handling characteristics and how propulsion systems would be integrated.
In contrast to other tactical UAVs, Swift’s operates without a runway. The KB-UA is launched via a trailer-mounted, compressed air-powered catapult and is retrieved with a net deployed from the trailer, enabling operation anywhere that a truck and trailer can travel, an advantage the military calls “organic” capability. The aircraft’s small, two-cylinder, 100-cc gasoline-powered engine sits at the rear of the craft with a pusher propeller, which simplifies retrieval with the net. The craft is fitted with primary net hooks on its nose and secondary hooks on the ends of its winglets that provide for “three-point recovery” to ensure that the craft does not tumble out of the net and incur damage. Its autonomous control system doesn’t require a ground pilot, and the craft can be maneuvered easily via a joystick. (Multiple aircraft can be controlled using a single controller.)
For this launch/recovery scenario, the team considered flight loads (launch and propulsion thrust, lift, drag, wind gusts, net-recovery loads), mass inertial forces (produced by payload equipment and the fuel load) and ground forces — the inevitable abuse incurred during handling. Reding says that between 15 and 20 “strenuous” load cases were developed, incorporating launch and recovery accelerations/decelerations of 15 Gs and safety factors ranging from 1.3 to 3. To facilitate handling and shipping, the downturned outer wings are removable via reusable screw-type metallic fasteners. “We adopted a spiral design approach — when the model showed that stresses were too high on areas of the airframe, like the outer wing joints, we adjusted the layup and added plies, then reran the FEA analysis.”
To minimize weight yet obtain the highest strength and stiffness for sustained operations, the airframe was designed with carbon/epoxy composites, using a combination of cored and solid laminates made from unidirectional and plain-weave prepregs. Total laminate thickness varies from as little as 0.030 inch to 0.100 inch (0.75 mm to 2.5 mm) depending on location. Ply buildups are added along the leading edges of the wings and fuselage nose, for example, for added strength.
The optimized airframe permits the KB-UA to loiter over a target for up to 24 hours at low altitudes of around 3,000 ft/914m or as many as 15 hours at higher elevations, says the company. When its fuel is spent, the KB-UA automatically flies back to its launch site for capture in the net.
A triangular spar, resembling a billiards rack about 3 ft/1m on a side, provides the primary structure for the main fuselage/wing body. Made from phenolic honeycomb-cored sandwich panels with unidirectional carbon/epoxy prepreg skins, the spar creates a large interior area for a payload, which can include cameras, radar equipment, ordnance or other cargo. Fuselage and wing skins, leading edges, trailing edges and the outer wings are solid laminates. A large access panel on the craft’s upper fuselage skin allows easy loading and unloading of cargo and equipment. Like the outer wings, the access panel is attached with removable metallic fasteners.
