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Modern airliners are usually low-wing designs with two engines mounted in underwing pods. The Boeing 747 and Airbus A380 are the only airliners in production which are too heavy (more than 400 tons maximum takeoff weight) for just two engines. Smaller airliners sometimes have their engines mounted on either side of the rear fuselage. There are numerous advantages and disadvantages to this arrangement. Perhaps the most important advantage to mounting the engines under the wings is that the total aircraft weight is more evenly distributed across the wingspan, which imposes less bending moment on the wings and allows for a lighter wing structure. This factor becomes more important as aircraft weight increases, and there are no in-production airliners with both a maximum takeoff weight of more than 50 tons and engines mounted on the fuselage. The Antonov An-148 is the only in-production airliner with high-mounted wings (usually seen in military transport aircraft), which reduces the risk of damage from unpaved runways.
Except for a few experimental or military designs, all aircraft built to date have had all of their weight lifted off the ground by airflow across the wings. In terms of aerodynamics, the fuselage has been a mere burden. NASA and Boeing are currently developing a blended wing body design in which the entire airframe, from wingtip to wingtip, contributes lift. This promises a significant gain in fuel efficiency.
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