Forward Swept Wings
Summary
Properties of the forward swept wing at low speeds have been known for some time (Weissinger, 1947; Multhopp, 1950). They include an uneven spanwise distribution of lift and excessive root bending moment.
The largest loads occur at the root, while an aft swept wing has a more gradual loading with a maximum lift around mid-span. Fast calculations can be performed with a linearized lifting surface theory.
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Figure 1: foward and backward swept wings
Features of Forward Swept Wings
At transonic speeds the sweep is needed to reduce and post-pone the drag rise. Research performed in recent years (Wachli, 1993), showed that for at constant shock location, shock wave sweep, taper ratio, aspect-ratio and area, a forward swept wing has a lower leading-edge sweep that an aft swept wing. This produces a lower profile drag and a lower root bending moment.
At constant root bending moment, the wing with forward sweep has a slightly higher
aspect-ratio, which leads to a further reduction of the profile drag.
Wing stall starts at the root and proceeds outwards, while on a wing with aft sweep
stall unusually starts at the tip and proceeds inwards. Root stall provides better
control capabilities at high speed, although lifting and stability capabilities may
be enhanced by appropriate canards.
Another problem of the wing is the critical wing divergence (e.g. the operation
point at which irreversible aeroelastic effects take place, with catastrophic consequences).
This difficulty would require a much heavier wing than the corresponding backward
swept wing. The problem could be partially solved with the use of advanced composite
materials.
The use of this wing is mostly confined to experimental fighter aircraft
(Grumman X-29A, Sukhoi S 37). These research airplanes feature tapered wings with
leading edge root extension (LERX), foreplanes, slightly canted fins, and extreme
agility at angles of attack above 90 degs !
One production aircraft with swept back wings, the German business
jet HFB 320 Hansa, was moderately successful in the 1960s.
Wing Stall
Wing Divergence
Current Applications
Related material
Selected References