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Aviation History
1980
1980 - 1644.PDF
36 FLIGHT International, S ]uly 1980 modulation; winglets; cambered fuse lage; self-trimming wing; underfus- lage intake; integrated propulsion and flight controls; digital fly-by-wire; active controls and relaxed static stability; composite structure; aero elastic tailoring; stress-limiting con trols; direct lift and sideforce; and high aspect ratio, two-dimensional (2D) nozzles. Those which did not make it onto HiMAT include the blended wing/ body, vortex strakes, cambered fuse lage and the 2D nozzle. The blended wing/body and vortex strakes are already familiar features on the F-16. The highly swept, sharp^edged strakes generate vortices which sweep back over the wing, energising the air and maintaining lift to greater angles of attack. Blending the wing and body reduces interference drag at their junction and creates a forebody strake. Windtunnel tests on the original HiMAT configuration—with strakes and blending—showed that the vor tex lift created nonlinear pitching characteristics which could not be trimmed out without the jet flap created by the 2D nozzle. There was not enough time to incorporate the advanced nozzle in HiMAT so the blended wing/body and canard strakes were deleted. Canard sweep was increased to 63° to generate the vortices required to maintain wing lift. Nasa research has shown that a suitably located canard can interact beneficially with the wing. The wing behind the canard is subjected to downwash from that surface while the wing outboard of the canard is in upwash. This increases the load on the outboard panels, which are aeroelastically tailored to twist nose down, tilting the lift vector forward to overcome induced drag. This in crease in loading on the outboard panel compensates for the reduction of lift on the wing directly behind and below the canard. At high - lift coefficients, when separation has occurred, the vortex created by the canard induces lift on the wing. The forward surface also HiMAT general arrangement HiMATs structure is 26 per cent carbon-fibre; 26 per cent aluminium; 18 percent titanium; 9 per cent steel; 3 per cent glassfibre and 18 per cent miscellaneous materials stabilises flow over the fuselage, in creasing directional stability. Several changes were made to the HiMAT wing-canard combination to achieve the desired induced drag. More verti cal separation was required but was limited by the depth of fuselage. Instead, canard dihedral was in creased, which also improved the direct sideforce capability of canard flaps and rudders. Increased dihedral meant more side area forward of the centre of gravity. To compensate, winglet chord was increased and the surfaces ex tended below the wing. This was a more efficient way of regaining direc tional stability, as winglets reduce in duced drag by taking some of the sting out of vortices cast off by the wing. The winglets also increase the loading at the wingtips, increasing the aeroelastic nose-down twisting of the wing. Winglets ensure excellent directional stability at all angles of attack, as at least one surface is always in free air. Shock waves created by winglet-wing interference can reduce their advantages in the transonic regime, however. To meet the demanding transonic design point, precise control of local angle of attack is required. To pull 8g at Mach 0 • 9, the HiMAT wing re quires 9-5° washout from root to tip. An aeroelastic wing deforms under load and can be made to twist nose down. HiMATs modular design means that only the wing sections outboard of the undercarriage fair ings can deform. This short span limits the twist under load to 5-5°. The rest is built into the wing. The canard has 3° washout built in and another 4° supplied by aeroelastic deformation at 8g. Minimum induced drag can be achieved over a wide range of lift coefficients if the camber is control led to vary the design lift. Camber reduces the leading-edge suction peaks created at high lift but means a thick aerofoil section which generates supersonic drag. Variable camber allows the wing to be matched to pre vailing conditions. Mechanical cam ber changing was selected for the fighter design but economics dictated that HiMAT use fixed, interchange able leading edges matched to 8g at Mach 0-9, 30,000ft and to lg at the same conditions. HiMAT uses a supercritical aerofoil developed by Rockwell. The section is not constant over the span of either the wing or canard but varies to optimise the surfaces for 10° to 11° angle of attack at Mach 0-9. Rock well made the first practical applica tion of advanced techniques for cal culating three-dimensional transonic flows. In HiMAT, basic airframe stability is replaced by stability augmentation through an active control system.
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