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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.[6] 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.[7] The Airbus A380 is a double-deck, wide-body, four-engine jet airliner manufactured by the European corporation Airbus, a subsidiary of EADS. It is the world's largest passenger airliner and, due to its size, many airports have had to upgrade their facilities to properly accommodate it. Initially named Airbus A3XX, the aircraft was designed to challenge Boeing's monopoly in the large-aircraft market; the A380 made its first flight on 27 April 2005 and began commercial service in October 2007 with Singapore Airlines. The A380's upper deck extends along the entire length of the fuselage, with a width equivalent to a wide-body air

raft. This allows for an A380-800's cabin with 478 square metres (5,145.1 sq ft) of floor space; 49% more floor space than the next-largest airliner, the Boeing 747-400 with 321 square metres (3,455.2 sq ft), and provides seating for 525 people in a typical three-class configuration or up to 853 people in all-economy class configurations. The A380-800 has a design range of 15,400 kilometres (8,300 nmi; 9,600 mi), sufficient to fly from New York to Hong Kong, and a cruising speed of Mach 0.85 (about 900 km/h or 560 mph at cruising altitude). As of November 2012 there had been 262 firm orders for the A380, of which 92 have been delivered.[1] The largest order, for 90 aircraft, was from Emirates. A bending moment exists in a structural element when a moment is applied to the element so that the element bends. Moments and torques are measured as a force multiplied by a distance so they have as unit newton-metres (Nm), or pound-foot or foot-pound (ftlb). The concept of bending moment is very important in engineering (particularly in civil and mechanical engineering) and physics. [edit]Discussion Tensile stresses and compressive stresses increase proportionally with bending moment, but are also dependent on the second moment of area of the cross-section of the structural element. Failure in bending will occur when the bending moment is sufficient to induce tensile stresses greater than the yield stress of the material throughout the entire cross-section. It is possible that failure of a structural element in shear may occur before failure in bending, however the mechanics of failure in shear and in bending are different. The bending moment at a section through a structural element may be defined as "the sum of the moments about that section of all external forces acting to one side of that section". The forces and moments on either side of the section must be equal in order to counteract each other and maintain a state of equilibrium so the same bending moment will result from summing the moments, regardless of which side of the section is selected.