BOEING HAS SELECTED a new wingtip design for the stretched 767-400 which saves more than 1,000kg over the winglets previously planned. The raked tips reduce wingspan from just over 55m (180ft) to around 52m, yet reduce drag sufficiently to enable the -400 to achieve its 10,500km (5,700nm) range.
The company has completed low-speed and high-speed windtunnel testing of the 767-400 with the raked tips, and has applied for a patent. Each tip is some 2.4m long and has greater leading-edge sweep than the wing itself. Boeing describes the design as "counter-intuitive", but says that the raked tips provide "the best overall balance for the aircraft", by reducing weight while still improving cruise efficiency. The design with raked tips "-achieved nearly the same efficiency as the bigger-span wing with winglets", the company says.
The 51.9m-span wing makes the 767-400 more compatible with the Lockheed L-1011 and McDonnell Douglas DC-10 which it is designed to replace. Another advantage is that the shorter-span wing does not need the extra set of outboard leading-edge slats Boeing had planned to add when increasing span from the 47.2m of the basic 767, hence reducing complexity. The tips will have aluminium leading edges and carbonfibre skins over metal spars, although the company is studying composite spars.
Boeing plans to freeze the 767-400 design in September, leading to a first flight in late 1999. Other recent design changes include increasing the maximum take-off weight, from just under 200,000kg to slightly over 204,000kg.
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Source: Flight International
