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Subsonic and supersonic leading edges
The requirement that the velocity component normal to the leading edge should be subsonic implies that the degree of sweep must be greater than the local Mach angle (Chapter 5). If this is so, we can see from Fig. 8.9 that the section AA’ on the wing lies within the Mach cone emanating from a point B. Point B will influence not only section AA’ but also the flow approaching the section AA’. The distance ahead of A where the approaching flow is first influenced will increase as the distance between A and B increases and, for a wing of infinite span, the approaching flow and the flow over section AA’ are (ignoring boundary layer effects) precisely equivalent to the flow over AA’ being at the subsonic velocity Vn.
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Fig. 8.9 Swept wing with subsonic leading edge Air flow approaching section AA’ is influenced by point B, but A cannot affect B |
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Fig. 8.10 Swept wing – supersonic leading edge Section AA’ cannot be influenced by B. Sweep angle is less than Mach angle |
This may, perhaps, seem like a rather tortuous way of repeating what we know already. However, the value of looking at things from this point of view will be apparent when we come to examine the tip and centre section flows shortly.
A wing whose leading edge is swept at an angle greater than the Mach angle is said to have a subsonic leading edge. If the sweep angle is less than this value then the leading edge is said to be supersonic (Fig. 8.10). In this case the flow over the section will be supersonic in nature, albeit at an apparently reduced Mach number as a result of the sweep.
