Airbus and Boeing may happily be churning out tube-and-wing, engine-under-wing airliners by the dozen, but they are not stopping their designers having the odd out-of-the-box thought, as recent US patents show.
Airbus has been awarded a patent (8,157,204, filed in 2008) for a "double fuselage aircraft" - a high-capacity airliner that combines two passenger-carrying fuselages with twin forward-swept wings and turboprop propulsion. (German WWII twin-fuselage aircraft were called Zwilling, for "twin".)
All graphics: USPTO
The main advantage of a twin-fuselage design is that it reduces wing bending moment and therefore minimizes structural weight for a very large aircraft. Turboprop propulsion - and I think here Airbus is broadening the term to include open rotors - reduces fuel consumption significantly compared with conventional turbofans.
According to the Airbus patent, the configuration is designed to overcome some of the limitations of an earlier twin-fuselage design patented by Boeing in 1979 (below, US patent number 4,165,058) and described as a "tandem wing airplane".
Boeing's design is described as having "two independent spaced apart fuselages and two wings of similar area", the lower forward wing attached to the forward fuselages and the upper aft wing mounted on struts so the tips of the wings are separated vertically by at least 25% of the span.
The wings have to be spaced apart to avoid the wake from the forward wing having an adverse aerodynamic effect on the aft wing. The Airbus configuration mitigates this by carrying less lift on the forward wing than the aft wing, and using forward sweep to minimize the vortices shed by the forward wing. Together, these allow the aft wing to be attached directly to the rear fuselages, saving weight. The engines can then be mounted on the aft wing, on the centerline, with adequate ground clearance.
The second Airbus patent (8,152,095, applied for in 2008) is for an "aircraft having a reduced acoustic signature", particularly for operations from runways in noise-sensitive urban environments.
The configuration is designed to maximize airframe shielding of propulsion noise. The engines are mounted together atop the aft fuselage in an extended nacelle so the shared inlet is above the wing, forward of the trailing edge, shielding fan noise, while the exhaust is between the twin vertical tails and forward of the aft end of the fuselage, shielding jet noise.
To improve aerodynamic integration and noise shielding, the engines are sunk into the fuselage by about 10-30% of their diameter. In this and other respects, Airbus' configuration resembles MIT's D Series "double-bubble" airliner concept with its boundary-layer-ingesting, wake-re-energizing integrated engines and Pi-tail.
Not to be beaten on the quiet airliner front, Boeing has refreshed its Sonic Cruiser design with a new patent (8,016,233, filed in 2006) that focuses on the canard double-delta configuration's ability to shield the noise from overwing-mounted engines.
The patent also notes that the configuration's broad inboard wing section (and twin outboard vertical tails) could provide shielding of infrared radiation in a military application and shockwaves from the propulsion system in a supersonic configuration.