The future development of Boeing’s blended-wing-body (BWB) configuration depends entirely on its suitability for carrying cargo, and in the case of a military tanker-transport in particular, its ability to perform airdrop missions.
Key to this capability is an innovative cargo door that Boeing has designed to open like a clamshell in the trailing edge. However, it was unclear what effect such a radical door design opening in midair might have on the airflow around the trailing edge—the engines located on the upper wing surface and the area around the cargo ramp. Now, results of newly completed water-tunnel tests indicate that such a cargo door-configured BWB design is viable, providing a vital boost to the project and spurring plans for a series of follow-on wind tunnel evaluations of the same model.
The tests, undertaken by Boeing Research & Technology engineers at the company’s Huntington Beach, California, facility, showed that the air flow immediately aft of the door was sufficiently benign for airdropping parachutists and payloads with the clamshell-shaped door opening symmetrically. Testing indicated “we didn’t get a pitch change, and trim demands didn’t change,” says Norm Princen, BWB chief engineer. “You would expect more pitch sensitivity, but we also know the BWB has a lot of pitch control authority, so that makes up for it,” he adds.
Based on initial findings and earlier computational fluid dynamics assessments, Princen says, “we contend that this is in many ways a superior configuration for airdrop.” All other modern contemporary  airdrop transport aircraft have upswept T-tails, which “come with a lot of aerodynamic issues. Strakes have to be added to help manage the flow to make it a good environment, and we simply don’t have those issues because it is symmetrical,” he says.
The flow visualization water tunnel tank used for the test is a double return facility with a 3-ft.-deep test section. The 2.8% scale model was 24 in. wide by 31 in. long, with the wingspan truncated to enable it to fit in the test section. Water passed over the model at 10 in. per sec. to represent typical Mach 0.2-0.3 subsonic cargo drop airspeeds. Dye ports in the model were connected to dye streams and, in the circulating water, generated visible streams of colored dye from the model to simulate air flow.
The clamshell doors were tested fully open symmetrically in the water tunnel. Note deflected elevon on the upper door trailing edge. Credit: Boeing

“The Reynolds number [a scaling metric for the viscosity of the flow] was fairly low, but it does give a good approximation of the flow. It’s also the first step in a continuing development. The next step might be a low-speed wind tunnel using the same model at Huntington Beach,” says Princen. The force and moment effects on the model with the doors fully and partially open symmetrically and asymmetrically will be measured. The data will be used to determine the control effectiveness of the elevon mounted on the trailing edge of the upper clamshell door, as well as the likely ability of the BWB’s flight control system to handle center-of-gravity changes during cargo drop missions.
The clamshell doors are mounted on hinges located behind the aircraft’s rear spars and actuated by units attached to fixed ribs on either side of the doors themselves. “We have done the weight estimates and believe it is a pretty efficient design,” says Princen. The design also shows that the closure angle of the symmetrical door can be achieved in a shorter distance than under a conventional upswept tail. “The whole configuration works well in all these different areas, and it is a real enabler for the BWB,” he says.
Prior to the recent water tunnel tests, Boeing also successfully completed evaluations of control surfaces for a short-takeoff version of the BWB at NASA Langley Research Center as part of work to calibrate the facility’s refurbished 14 X-22-ft. subsonic wind tunnel. The test work also compared data obtained in 2015 using a 13-ft. span model in the 40 X-80-ft. National Full-Scale Aerodynamics Facility wind tunnel at NASA Ames.