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Aviation History
1982
1982 - 0009.PDF
[pyipxnr 1 XL n. Composites key to Grumman forward-swept-wing Bethpage—Grumman has received a $71 million Defence Advanced Re search Projects Agency contract to build the X-29A forward-swept-wing (FSW) technology demonstrator. The aircraft is expected to fly in October-December 1983. The contract keeps open the option for a second demonstrator, providing funds for certain long-lead-time items. The X-29A is a 16,0001b aircraft powered by a single General Electric F404 turbofan of around 16,0001b thrust. General performance is ex pected to be similar to that of the F-16. To reduce costs, an F-5 forward fuselage, supplied by the US Air Force, will be married to the new rear fuselage, wing, tail, and canard. FSW is expected to provide a num ber of benefits, including improved low-speed handling and an overall in crease in aerodynamic efficiency. The former is achieved by reversing usual swept-wing behaviour at the stall— with FSW the root stalls first, leaving the wingtips and ailerons in clear air, maintaining controllability. Improved aerodynamic efficiency is a combination of better wing-body shaping, the aft location of the wing root improving area ruling, and the optimisation of wing/canard pressure distribution, including interference be tween canard and wing, to bring it closer to the ideal elliptical spanwise loading for lowest lift-induced drag. The problem with FSW has always been to build a wing strong enough to withstand the divergent aero- elastic twisting force from the wing- tips, yet light enough to be feasible. Previous forwardnswept-wing applica tions, all subsonic, have used metal structures considerably heavier than the equivalent aft-swept wing. The X-29A wing has a carbonfibre composite wing torsion box and carry- FLIGHT International, 2 January 1982 through structure. The secret of re sisting the divergent twisting force is in the layup of carbonfibre laminates in the wingbox. Laminates are orientated 0° (spanwise), 45°, and 90°. Overall proportions are: 20 per cent 0°, 60 per cent 45°, and 20 per cent 90°. To control twisting, outboard 0° laminates are rotated forwards by 9°, and, because of the increasing bending load towards the tips, there is a slightly higher proportion of 0° laminates outboard. Tunnel tests Windtunnel tests show that the wing leading-edge re-entrant, where forward sweep changes to aft sweep, has the biggest static load. This is caused by the centre of pressure mov ing forward relative to the rest of the wing, putting a disproportionate load on the front spar. This area is beefed up, with 156 laminates in the region around the V-corner beam. Wing weight is around 1,6001b, or some 10 per cent of the X-29A's take off weight. Wing-section is an aft- loaded, supercritical aerofoil, with discrete variable camber. Strakes ex tending aft from the wing-root trailing-edge terminate in flaps, which give a clean stall break, and which help unstick the nosewheel. The X-29A is highly unstable and has a Honeywell triplex digital fly-by- wire flight control system. The basic wing/body combination has neutral longitudinal static stability. Addition of the all-moving canard shifts the overall centre of pressure forward of the centre of gravity by 35-40 per cent of the mean aerodynamic chord (area/span). The combination of neutrally stable wing/body and destabilising canard provides the lowest "tail-off" control power requirements. A 35-40 per cent statically unstable tailed aircraft, equivalent to the X-29A, would be 60- 70 per cent unstable tail-off. Brute force actuators would be required to balance the aircraft. There are no plans to carry stores/ pylons on the first aircraft, although the wing will be stressed for pylons. The second aircraft, if it is built, may incorporate additional technology, in cluding a two-dimensional exhaust nozzle, radar absorbing materials, and light-level matching "active" camouflage. Initial shake-down flight-tests will be carried out by Grumman before the demonstrator is transferred to Nasa Dry den, Edwards AFB, Cali fornia, for the majority of tests. Grumman expects the aircraft to be spin-resistant because of its low-speed controllability. The company is look ing forward to the first flight: "It should be interesting, but, we hope, not too interesting."
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