Raising the critical Mach Number – slimness

When increase in engine thrust – due to the rapid development of jet engines – first made transonic flight possible, research was concentrated on the problem of raising the critical Mach Number, of postponing the shock stall, of getting as near to the barrier as possible without getting into it – in short, of keeping out of trouble rather than facing it.

There are two main ways of raising the critical Mach Number. The first is slimness. The need for slimness will be abundantly clear from all that has been said about shock waves and their effects – and the slimness applies to all parts, the aerofoil section, the body, the engine nacelles, the fin, tail plane and control surfaces, and perhaps most of all to small excrescences (if there must be such things) on the aircraft. The aerofoil section must be of the low-drag laminar – flow type already referred to, and must have a very low ratio of thickness to chord. The Spitfire of the Second World War has already been mentioned as an example of slimness, and of a high critical Mach Number – all the more remarkable in that it was not designed for transonic speeds.

The full line in Fig. 11.15 shows very clearly the effect of thickness/chord ratio on the critical Mach Number for a straight wing (the dotted line will be referred to in the next paragraph); at a tic ratio of 10 per cent this wing has a critical Mach Number of only just over 0.8, at a tic ratio of 8 per cent it is raised to 0.85, and at 4 per cent it is over 0.9. Not long ago the tic ratios of wings for fighter aircraft were from 9 to 12 per cent, but they have now been reduced to 7 or 8 per cent, and may yet be still further reduced to a figure as low as 3 per cent, though there are of course very great design and manufac­turing difficulties in producing such thin wings. In thus speaking of thin wings it is important to keep in mind that what really matters is not the actual thick­ness, but the ratio of thickness to chord.