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Aviation History
1984
1984 - 1014.PDF
Forward sweep Several advantages are claimed for forward sweep, including: 9 higher ut t lower su] e drag • better kv, handling # reduced wing bending 0 better area ;ind volume distribution. Of primary interest to the X-29's designers was the improvement in transonic I -1 by- forward swi " .is is the region in which shock waves start to form and drag begins to rise, and is of vital interest to fighter designers wfco seek to delay the drag rise in an effort to increase c ity at Mach numbers approaching unity. The X-29 wing uses a thin super critical aerofoil section. This aft loaded aerofoil helps delay shock-wave formation to higher su numbers. The magnitude of benefit, however, depends on maintaining supercritical characteristics over the entire span. This requires a twist -. wing which minimises spanwise flow. On a swept wing the component of forward velocity parallel to the leading edge causes the airflow to drift, in that direction. '* ap the more prone On a forward-swept wing the lead ing-edge sweep is substantially less than that on an aft-swept wing, requiring ie ~.et the drift. In addition, the drift is towards the root, not towards the tip as in an aft-swept wing. The reduction in !wist produces a wing with better transonic/supersonic compatibility, as twist generates drag above Mach 1. The terminating shock on a super critical-section wing rests at 70 per cent i! en. On an aft-swept wing the trailing edge is less highly swept than on an equiv alent forward-swept wing. As a result the terminating shuck, which is near normal to the flow on an ASW, is swept on an FSW. The swept shock can support a higher pressure jump before precipitating separation. This increases the drag-rise Mach number. A second advantage is that the reduced leading-edge sweep supports a higher suction peak and the laminar leading-edge boundary layer is less A forward-swept wing generates less negative pressure on its upper surface, reducing (he strength of the terminating shock Shock location -Aft swept wing -Forward swept wing Forward sweep could result in an aircraft between 5 and 25 ;x?r rent smaller than an equivalent aft-swept design, estimates Grumman, reducing production and operating costs prone to separation, reducing induced drag and delaying leading-edge vortex formation to higher angles of attack. The result, according to Grumman, is that the forward-swept wing, at high transonic lift coefficients, generates more positive pressure on its lower surface and less negative pressure on its upper surface, and therefore causes a weaker shock, c? less drag. The wing has several other proper ties. Because the airflow tends to drift inboard, the root stalls first, leaving the tips clear. As the lift loss is close 3 still unseparated. lateral and directional control is maintained to high angles of attack.: Before the root stalls, however, a spanwise lift distribution with loading concentrated towards the root is produced, unlike ASW, where the tips are the first to become highly loaded. This moves the c.p. inboard, reducing bending moments at the root. As a result, a lighter wing can be built, or aspect ratio can be increased to reduce mrnxmi &ag.. Root stall, however, is the principal disadvantage of forward sweep, and uniess some way is found to delay airflow separation on the inboard section a forward-swept wing wili .-swept wing. Sweeping the inboard section aft by using a fillet is one way. " Another is to place a canard just ahead of the wins, so that downwash off the foreplane reduces the effective angle of attack on the wing behind it. This delays the root stall and enables the wing to achieve its lift potential. According to Grumman, the effect of the X-29's close coupled canard is that separation begins on the wing :ne tip and spreads inboard and outboard simultaneously. The stall is therefore well behaved and. with the tips -nuiou'ij' -ftV;;v-? tr -rgles »f at+% k, beyond 80°, the X-29 is virtually spin ; I. There is evidence to suggest that the a. es of a close-coupled canard to pitch response and high ly are best -! forward sweep, which allows an extremely compact layout. The short moment arm produced by sweeping the wing trailing-edge forward avoids any limit on the use of high-lift devices being imposed by the canard power available. Assf Vie benefits of forward over aft sweep, Grumman estimates a drag reduction due to reduced leading- f 13 per cent for wings of identical aspect ratio. A further reduction of 8 per cent is available in the form of increased aspect ratio for the same bending moment. This only applies at higher lift coefficients, however. Grji'i'iia* estimates that a forward- swept-wing design could generate between 10 and 20 per cent less drag for a given mission. Less drag means a smaller engine, requiring less fuel, and leads to a smaller aircraft Drag polars shou the expected improvement in lift:drag ratio for a forward-swept wins Forward swept wing , Aft swept wing CD 1568 FLIGHT International, 16 June 1984
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