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Delta wings
The amount of sweep required to maintain low speed flow patterns on a wing depends on the maximum flight Mach number required (the ratio of the maximum speed of the aircraft to the speed of sound). Subsonic airliners which are designed to cruise at Mach numbers less than 0.9 (90 per cent of the speed of sound) require a sweep angle of around 25 to 30 degrees measured along the – chord line (a line drawn along the span – of a chord back from the leading edge). An aircraft designed to travel at twice the speed of sound would require a sweep angle in excess of 60 degrees. The BAe Lightning shown in Fig. 8.1 is an early example of a swept-wing aircraft designed to fly at twice the speed of sound (Mach 2).
With such large angles of sweep, it becomes difficult to build a wing with sufficient bending stiffness or strength. The wing is long in relation to the overall span, and for reasons given in Chapter 9, it may also need to be very thin.
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Fig. 2.21 Swept and delta wings compared Relatively short straight spars can be used in the delta wing. The delta wing is also thicker at the root for a given thickness-to-chord ratio |
On the Lightning, the thickness to chord ratio was only around 6 per cent, and even this is quite thick by modern standards!
The delta planform allows the spars to run straight across, as illustrated in Fig. 2.21(b), instead of along the wing, as in 2.21(a). The delta wing was yet another feature developed by German engineers during the Second World War.
It should be noted that there are two types of delta wing. They are characterised by the type of flow regime employed rather than their shape, but for convenience we may classify them as broad and slender deltas. The older broad type was introduced first, and is typified by the Avro (BAe) Vulcan bomber seen in Fig. 2.22. This type of delta wing is essentially a form of swept wing with a large degree of taper. Wings of this form are designed to operate with attached flow for most flight conditions, but separated conical-vortex flow will occur at high angles of attack.
For low speed flight, the low aspect ratio, high taper, and sweepback of the delta planform result in a poor lift-to-drag ratio. This is offset by the structural advantages, and by the large wing volume that results; useful if a high fuel load is required. However, it is in high speed flight, where large amounts of sweep – back are required, that the delta shows its main advantages.
