Over the years, theories of the forces on bodies moving with purely subsonic or purely supersonic velocities through gases have been evolved as required by current aeronautical practice; the development of such theories has not proved unduly difficult. Moreover, the theories have been monitored and checked by corresponding experimental investigations. In view of the complexity of some of the subsonic work, it might have been expected that supersonic theory would prove intractable; in the event, it has proved that, in some respects at least, supersonic theory is simpler than subsonic. By contrast with these fields, studies of transonic phenomena have lagged badly. For one thing, experimental work at transonic speeds is comparatively recent: until the development of slotted or perforated walls for transonic tunnels, it used to be said that one could only test at M=l a model of infinitesimal size—anything finite choked the tunnel. On the theoretical side the investigator is faced with the study of a region involving mixed flows, different parts of the field obeying different laws, with an unspecified sonic line at the boundary. Clearly, even in steady conditions this is a problem of great complexity.
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