Subsonic – transonic – supersonic

We have already talked about flight at subsonic, transonic, and supersonic speeds, and it should now be clear that the problems of flight are quite dif­ferent in these three regions, but the dividing lines between the regions are of necessity somewhat vague. Figure 11.7 shows the subsonic region as being below a Mach Number of 0.8, the transonic region from M 0.8 to M 1.2, and the supersonic region above M 1.2. There are arguments in favour of consid­ering the transonic region as starting earlier, say at a Mach Number of about 0.7 or near the point marked in the figure as the critical Mach Number, and extending up to say a Mach Number of 1.6 or even 2.0. In terms of sea-level speeds this would mean defining subsonic speeds as being below 450 knots, transonic speeds as 450 up to 1000 or even 1200 knots, and supersonic speeds above that.

Perhaps the best definition of the three regions is to say that the subsonic region is that in which all the airflow over all parts of the aeroplane is subsonic, the transonic region is that in which some of the airflow is subsonic and some supersonic, and the supersonic region is that in which all the airflow is super­sonic. Once again we are in trouble if we take our definition too literally. Even at very high speeds we may have local pockets of subsonic flow – just in front of a blunt nose for example. So the Space Shuttle would only be transonic even at the fastest point of re-entry! Also with this definition we are none the wiser as to the speeds or Mach Numbers at which each regions begins or ends; the beginnings and endings will of course be quite different for different aeroplanes.

In this chapter our main concern is with speeds in the transonic range, and particularly in the narrow range between Mach Numbers of 0.8 and 1.2. This

range, as is probably already evident, presents us with some of the most baf­fling but fascinating problems of flight; it is the range in which most of the change takes place, the change from apparent incompressibility to actual com­pressibility, the gradual substitution of supersonic flow for subsonic flow; it is the range about which we are even now most ignorant.