Airbus has been granted US patents for two configurations that tackle one of the fundamental limitations of most aircraft designs – the wing is fixed to the fuselage. Other than variable-geometry, or swing-wing, designs where the objective is to combine the lower high-speed drag of a swept wing with the higher low-speed lift of an unswept wing, there might not seem much advantage to a moveable wing. Not so, it seems.
Graphics: Airbus via USPTO
In the first configuration (above, US patent 8,360,357 B2, filed in 2010), the wing is movable longitudinally along the aircraft, allowing the center of gravity to be positioned at the center of lift throughout flight and enabling the balancing horizontal and vertical stabilizers to be eliminated, leaving only what Airbus calls “monobloc saber rudders”.
The patent notes that an aircraft design must respect certain constraints in the relative positions of the center of gravity (cg), center of lift and other “aerodynamic foci” – the points at which the aircraft’s weight, wing lift and balancing tail force are assumed to act. But cg position depends on loading and changes with fuel consumption, while aerodynamic forces move with Mach number.
To cope with this, designers define an allowable range of cg positions within which the aircraft has acceptable flying characteristics under all flight conditions – one that does not require unacceptably large and draggy horizontal stabilizers for trim in pitch.
Some aircraft transfer fuel between tanks in the wing and tail to shift the cg in flight and reduce drag from the trimming force required. In Airbus’ design, the wing is moved fore and aft, to keep the cg and center of lift together and reduce or eliminate the need for a balancing tail, saving weight and reducing drag.
In the second configuration (above, US patent 8,336,811 B2, filed in 2010), the wing, tail and engines are combined into something Airbus calls the “aero-propulsive unit”, which is mounted above the fuselage and attached via a system of rods. Actuators in the rods enable their length to be controlled, so that the position of the wing relative to the fuselage can be varied.
The patent notes that the setting angle of a wing, its fixed angle of incidence relative to the fuselage, is usually determined during design to ensure an essentially horizontal fuselage at average cruise weight, to minimize drag and provide a horizontal cabin floor. But, it notes , outside that one design point neither the fuselage nor the floor are horizontal.
In the patented configuration, the aero-propulsive unit can be moved relative to the fuselage - fore/aft, up/down, left/right, or rotated in pitch, roll or yaw – as a function of the aircraft’s center-of-gravity position to increase load-carrying capacity; of its flight phase to keep the fuselage and cabin floor horizontal; for flight control; or to alleviate turbulence.
In the Airbus concept, a twin-boom tail is integrated with wing to avoid interference with the fuselage and allow greater freedom in design of the fuselage and loading systems, the patent illustrating a version with a swing tail for roll-on/roll-off cargo loading.