Boeing has unveiled a refined design for its Transonic Truss-Braced Wing (TTBW) ultra-efficient airliner concept. The 737-class aircraft, with a 170-ft.-span low-drag wing, is designed to fly at Mach 0.8, a typical airliner cruise speed and faster than previous versions of the design.

The TTBW concept was originally developed in 2010 under the Boeing and NASA Subsonic Ultra Green Aircraft Research (SUGAR) program to study new configurations for ultra-efficient airliners that could enter service in the 2035 time frame.

The model of the revised TTBW configuration was unveiled Jan. 8 at the American Institute of Aeronautics and Astronautics SciTech conference in San Diego. Boeing estimates the TTBW has an 8% fuel-burn advantage over a conventional configuration with cantilevered wing.

The TTBW configuration has a high-aspect-ratio wing to minimize induced drag in the cruise. The long, thin wing is braced by trusses—similar to the Hurel-Dubois designs of the 1950s—that minimize the weight penalty of the longer span.

In the SUGAR study, the aircraft was designed for a reduced cruise speed on Mach 0.75 to minimize fuel consumption. Under NASA contract, Boeing has optimized the truss design and increased wing sweep to enable the aircraft to fly at Mach 0.8, the speed at which today’s airliners typically cruise.

Artist’s concepts of the refined TTBW design released by Boeing show the increased sweep of the high-set wing and the revised design of the truss. The main member has increased chord at the fuselage, forward sweep and tapers toward the junction with the wing.

The junction with the fuselage has been moved back and the main truss now angles up to meet the wing at some distance from the fuselage, presumably to reduce aerodynamic interference. There is a small jury member connecting the truss to the wing close to its junction with the wing.

The long-span wing folds just outboard of the truss junctions to enable the TTBW to use the same airport gates as a 737, which has a wing span of almost 118 ft.

The aircraft pictured appears to be conventionally powered, with turbofans mounted under the wing, but NASA is studying versions of the TTBW concept with hybrid-electric propulsion. These have electric motors integrated into the turbine engines. An electric-powered ducted thruster on the tail ingests the fuselage boundary layer and re-energizes the wake, reducing drag and energy consumption.

Graham Warwick