Flight at transonic speeds

Introduction

The earlier chapters of this book have dealt with flight at speeds well below that at which sound travels in air – in short, at subsonic speeds. It is true that frequent references have been made to the problems of high-speed flight, but detailed consideration of these problems has been deferred until now.

From what has already been said, it is clear that the speed of sound has an important influence on the flow, and this is what we examine first.

The speed of sound

When a body moves through the air at speeds well below that at which sound travels in air, there is, as it were, a message sent ahead of the body to say that it is coming. When this message is received, the air streams begin to divide to make way for the body, and there is very little, if any, change in the density of the air as it flows past the body. The way in which air is thus ‘warned’ of the presence of the body which is approaching, or of changes in the shape or atti­tude of that body, can be clearly illustrated in a smoke tunnel in which the air flows over an aerofoil fitted with a flap (Fig. 11.1, overleaf). If the smoke streams are allowed to settle when the flap is in the closed position, and the flap is then lowered, all the streams of smoke many chords length in front of the aerofoil immediately change course, and streams which previously flowed below the aerofoil, now flow above it. Once again, if we could ‘see the air’ in front of an approaching aeroplane, this fact would be immediately obvious, and the air would begin to be disturbed perhaps 100 metres or more in front of the aeroplane of the approach of which it must have had some warning.

What we have called a ‘message’ or ‘warning’ is really due to a wave – motion in the air set up by the areas of increased and decreased pressures

Airflow —— ::

Fig 11.1 Effect of lowering flap on airflow in front of aerofoil

around the body. These pressures are communicated in all directions to the surrounding air by means of ‘waves’. These waves are similar to sound waves, and they travel at the speed of sound, which is about 340 m/s (or 1224 km/h) in air at sea-level conditions. There is no mystery in this relationship between pressure waves and sound waves because sound is a pressure wave set up by some local compression of the air, and the speed of sound is simply the speed of propagation of rarefactions and compressions of small amplitude in the air.

So if a body travels through the air at the speed of sound, there will be no time for the message as given by the wave to get ahead and the air will come up against the body with a ‘shock’.