NASA-Boeing X-66A
For almost 80 years the U.S. experimental X-plane series has relentlessly pushed the boundaries of speed, altitude, power and aerospace technology.
Now the Boeing-NASA X-66A—the latest in this pioneering series, will take the X-plane program into yet another new realm —sustainable aviation.
Based on a heavily modified ex-Delta Air Lines MD-90, the X-66A is focused on proving the aerodynamic benefits of its unique design feature—a high aspect ratio, 145-ft. span transonic truss-braced wing (TTBW). The size of the wing alone—which is about 50% longer than the cantilever wing on an equivalent conventional single-aisle airliner—will make it the largest X-plane yet.
Structurally braced by trusses that also generate lift, the increased span of the TTBW configuration reduces lift-induced drag. Through the X-66A Sustainable Flight Demonstrator (SFD) project, Boeing and NASA aim to prove, at aircraft level, the TTBW concept has the potential to cut fuel burn by around 10%.
As the wing is mounted above the fuselage, it can also accommodate higher bypass ratio engines, including open fans, which together with lightweight composite construction can collectively contribute towards a potential 30% fuel saving compared to current single-aisle airliners.
But aside from the experimental nature of the TTBW concept, the X-66A also differs from all previous X-planes by deliberately creating a production-scale platform for a next-generation commercial airliner family. The program is heralded in some NASA circles as the 21st century equivalent of Boeing’s 367-80—better known as the Dash 80 progenitor of the 707 and KC-135—and the company has reinforced this view by announcing the SFD could lay foundations for “vision system” airliners covering the 737-7, -8, and -9 replacement market.
In July, Boeing further differentiated the TTBW from earlier X-planes by announcing the formation of a sustainability coalition of leading U.S. airlines to advise the design and testing of the X-66A. The group includes Alaska Airlines, American Airlines, Delta, Southwest Airlines, and United Airlines—all of which Boeing says will provide input on operational efficiencies, maintenance, handling characteristics, and airport compatibility.
The airline participants “will share feedback on sustainable operations and airport compatibility,” adds Boeing, which goes on to explain, “the TTBW design could be used by airplanes of different sizes and missions and may benefit from folding wing tips to accommodate existing airport infrastructure.”
As well as providing guidance on airport operations, Boeing says, “airline pilots will have a chance to experience the X-66A through a flight simulator and assess the vehicle’s handling characteristics.” During flight tests, which are slated for 2028 and 2029 out of NASA’s Armstrong Flight Research Center at Edwards Air Force Base, the aircraft manufacturer says airline operations and maintenance teams will assess the X-66A as modifications are made to the aircraft.
In another sign of divergence from the space and defense-rich heritage of X-plane history, Boeing is making no secret of its high commercial expectations for the TTBW. At the company’s second quarter earnings call on July 26, Boeing CEO and President Dave Calhoun said, “We are intent on proving this technology and we are hopeful. If it matures the way we think it will, and NASA frankly thinks it will as well, I do think it will see service.”
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