Advanced control technology resurrects idea for lighter wings

Tom Shelley reports on how technology from the dawn of aviation history had just been given a new lease of life Active Aeroelastic Wings (AAW) represent a re-birth of the wing warping used by the Wright brothers to control the roll and bank of their first aeroplanes, but with the addition of advanced control. The original Wright machines were extremely difficult to control, and often fatal crashes were frequent, so wing warping went out when the Wright's great competitor, Glen Curtiss re-invented ailerons. (They were originally invented by New Zealander Richard Pearse in 1902) Flexible, warpable wings, are, however, potentially lighter than stiff wings with flaps and actuators to move them, which is one reason birds do it this way. The new version has been developed by NASA's Dryden Research Center in Edwards, California, using a modified F/A-18A obtained from the US Navy. The Boeing Company's Phantom Works division in St.Louis undertook the wing modifications, Lockheed-Martin and BAE Systems developed the research flight control computers, and Moog developed the actuators for the outboard leading edge flaps. Rather than mechanically twist the wings in the way the Wrights did, with the pilot moving a saddle connected to the wing tips by cables, the present development inclines the leading edge flaps, together with trailing flaps, so as to make the wings twist in such a way as to enhance roll performance. During early F-18 flight tests, the wings had been found to be too flexible at high speeds for the ailerons to produce the specified roll rates. This was because the high aerodynamic forces against a deflected aileron would make the wing deflect in the opposite direction. In the AAW development, several of the existing wing skin panels along the wing box section of the wing just ahead of the trailing edge flaps and ailerons have been replaced with thinner, more flexible skin panels and structures, similar to those of the original prototype F-18 wings. By using the outboard leading edge flap and the aileron to twist the wing, the aerodynamic forces on the twisted wing provide the required roll performance, so that with AAW control technology, the flexible wing now gives a positive control benefit instead of a negative one. The project began in 1996, but has only now just been completed. Flights have been undertaken at speeds of from Mach 0.85 to Mach 1.3 at altitudes from 5,000 to 25,000 feet. Roll rates adequate for lateral control, or within 15 to 20 per cent of those obtained by a production F/A-18 were obtained by active control of the wing flexibility alone, without the use of the differential rolling horizontal tail used by standard F/A-18s at transonic and supersonic speeds. Roll rates were highest at Mach 0.85 and Mach 1.2 and lowest at Mach 0.95, similar to conventional F/A-18s. NASA believes that the results of its work will allow future designers to reduce the weights of wings by 10 to 20 per cent. This will allow increased fuel efficiency or payload carrying capacity, along with potentially reduced radar signature. The technology has application not only to military aircraft but also high altitude, long endurance, unmanned aircraft, transports and airliners. NASA Dryden Flight Research Centre Moog Controls Pointers * By using advanced control, it is possible to make a light weight, flexible aircraft wing distort in such a way as to enhance roll control * Weight savings are expected to be between 10 and 20 per cent * This represents the successful re-introduction of a technology dating from the very beginnings of aviation, but long abandoned because of the difficulties caused by putting it under mere human control