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Highly swept and slender delta wings
Highly swept wings tend to produce the stable separated conical vortex type flow described in Chapter 1, at relatively low angles of attack. For aircraft
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Fig. 2.22 Broad delta wing The Vulcan bomber originally had a simple triangular delta wing. This was later modified by the addition of a leading edge extension which improved the stability of the leading edge flow |
designed to fly at twice the speed of sound or more, it becomes possible to use this type of flow for all flight conditions. On the slender-delta-winged Concorde, the leading edge was made very sharp to provoke separation even at the low angles of attack required at cruising speed. It was also warped along its length in such a way as to ensure that the vortices grew evenly along the leading edge. Figure 2.23 gives some idea of the complexity of this wing. In addition to the leading edge warp, the wing has spanwise variations in camber for reasons that will be explained later.
The conical leading edge vortices extend downstream, and the usual trailing vortices are formed, as illustrated in Fig. 1.21. One advantage of this type of flow is that tip stalling does not occur, since the flow is already separated and stable.
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Fig. 2.23 Complex leading edge shape of the slender-delta Concorde wing (Photo courtesy of British Aerospace (Bristol)) |
From the plan view of Fig. 2.24 it will be seen that the wings of Concorde were not a true delta, but had curved leading edges; a shape that is known as an ogive. The ogive shape has the effect of moving the position of centre of lift rearwards and also reduces the variation of the position of the centre of lift with angle of attack and speed.
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Fig. 2.24 Plan view of Concorde Although classified as a slender delta, this wing is known as an ogive |
Concorde was originally envisaged as flying with conventional attached flow in cruise, but it was found that the optimum cruise condition was obtained with a small amount of leading edge vortex flow.
Highly swept wing root strakes are used on some aircraft to provide a combination of separated conical vortex flow inboard, and conventional flow outboard. Wing root strakes may be seen on the F-18 in Fig. 2.25. With this arrangement, at high angles of attack, the loss of lift on the outboard wing sections may be more than compensated for by the extra conical vortex-lift generated by the strakes. The vortex produced by the strakes also helps maintain flow attachment on the wing, as described in Chapter 3.
