Sweepback
Raising the critical Mach Number – sweepback
- The second main way of raising the critical Mach Number (and this applies only to the wings, tail, fin, and control surfaces) is sweepback – not just a few degrees of sweepback that was sometimes used, rather apologetically and for various and sometimes rather doubtful reasons, on subsonic aircraft, but 40°, 50°, 70° or more.
- Sweepback of this magnitude not only delays the shock stall, but reduces its severity when it does occur.
- The theory behind this is that it is only the component of the velocity across the chord of the wing (V cos a) which is responsible for the pressure distribution and so for causing the shock wave.
- The dotted line in how a wing swept back at 45° has a higher critical Mach Number than a straight wing at all values of t/c ratio, the advantage being greater for the wings with the higher values of t/c.
- sweepback has very little advantage – but that is another story and, in any case, airplanes cannot fly at M 2 without first going through the transonic range.
- Of course, as always, there are snags, and the heavily swept-back wing is no exception. There is tip stalling – an old problem, but a very important one; in the crescent-shaped wing an attempt has been made – with some success – to alleviate this by gradually reducing the sweepback from root to tip.
- There are also control problems of various kinds, and the designer doesn’t like the extra bending and twisting stresses that are inherent in the heavily swept-back wing design.
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