Winglets help refine flow and increase efficiency
Conventional winglets have come to be widely used on airliners, whether in the form of the Airbus A320’s wingtip fence or the up-swept devices characteristic of the A330/340 and Boeing 747-400. And if winglets are good, shouldn’t bigger ones be even better? Wingmaker Airbus UK has been investigating their potential.
Michael Smith, Airbus UK’s engineering product manager, research and technology, told the Royal Aeronautical Society’s recent annual conference in London that winglets affect aerodynamic benefits at both high and low speeds. The mod adds aerodynamic efficiency in cruise and at low speeds, resulting in improved mission performance, notwithstanding the increase in overall weight and wing loads.
Adding a winglet introduces a bending moment at the wing root, which a strengthened, hence, heavier structure must counter. A relatively small increase in the wing root bending moment can produce a large reduction in drag, and increasing the size of a winglet can produce a big improvement in aerodynamic drag, but with more profile drag as well as an increase in the bending moment. In fact, increasing wing twist as weight increases can easily result in disbenefit, Smith said.
Increased wing loads, meanwhile, accompany reduction in vortex drag, so designers must control the direct force induced by the winglet, for example by varying the angle. Also, at higher speeds vortex drag remains lower but profile drag and wave drag increase.
As part of the project called Aircraft Wing with Advanced Technology Wing Operation (Awiator), Airbus fitted an A340-300 with large winglets–each 13 feet long and 6.5 feet wide–designed and built by GKN Aerospace Services (GKNAS). The project involves 23 European industry and academic partners and 50 percent funding from the European Commission.
Testing included 40 hours of flying with the large winglets and some tests with no winglets, Smith said. And while designers seriously considered large winglets for the A380, they opted not to use them because they found the classic Airbus wingtip fence most effective.
Large winglets constitute just one of an array of tools for cutting drag, he added. Others include an increase in span, conventional winglets, wingtip fences and innovative devices; the selection of the most appropriate depends on the balance between aerodynamic performance and increased weight.
The €80 million ($98 million) Awiator program held two objectives: increase European competitiveness in the aerospace sector and address environmental issues. According to GKNAS, a 1-percent reduction in drag on an ultra-high capacity aircraft would result in an estimated 65-nm increase in range or a 6,200-pound reduction in the fuel requirement.