Box wing jets are also sometimes called loop wing jets.
A box wing aircraft uses two sets of wings, one forward pair configured normally, and a second set positioned where the tail would normally be and sweeping forward to merge with the first set of wings, forming a complete 'loop' of lifting surfaces. Jets using this configuration are undergoing serious study and development by a number of government and private interests, such as the US Air Force, aircraft manufacturer Lockheed Martin, and the German non-profit Bauhaus Luftfahrt.
In the early years of aviation, biplanes dominated the skies, and with good reason. The two layers of wings created a stronger airframe, and the increased wing surface area provided more lift, which well suited the modestly-powered engines of the time.
The concept has been updated for the twenty first century, with the aim of increasing fuel efficiency, reducing pollution and noise, and ultimately, saving money. The vertical spars or plates connecting the two sets of wings help to stabilize the craft.
The biggest advantage of this configuration is the increased lift created by using two sets of wings. The design being pursued by Lockheed Martin would increase the lift to drag ratio by 16%, allowing the craft to use less fuel and therefore could fly the same routes as conventional aircraft for less money. The savings on fuel could also allow it to stay in the air longer or to fly farther for the same size fuel tank.
The increased wing area would also allow it to make steeper descents and ascents safely than current conventional jet liners. The swept-back design of the double looped wing would also allow the aircraft to readily handle transonic and supersonic speeds.
Lockheed Martin's design also incorporates ultrahigh-bypass turbofan engines and advanced lightweight composite materials, which combined with its box wing configuration, promises to make the aircraft up to 50% more fuel efficient than contemporary jetliners. Similar cost and fuel-saving innovations are being combined with box wing designs by Bauhaus Luftfahrt, with the ultimate goal of creating aircraft that not only operate more cheaply, but are more environment-friendly.
The US Air Force has also looked into box wing designs. Though details of their actual progress are sketchy online, its likely the box wing's greater lift-to-drag ratio, fuel efficiency, and greater endurance in the air is what is attracting the armed forces to such aircraft, especially for the large airborne tanker craft used to refuel military jets in flight.
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