The wing in supersonic flight

We are accustomed to the idea of fighter aeroplanes being capable of supersonic flight. The only civil supersonic aeroplane was the Anglo-French Concorde. It lasted 30 years and was not replaced. There is an obvious advantage in flying at high speed and Concorde cruised at about 2,000 km/hr or 1300 mph. Nevertheless modern airliners fly much more slowly at about 1,000 km/hr or 650 mph or about Mach 0×85. There are good reasons for this.

 

There is the social problem of the sonic boom but that may be more in the mind than the fact.[1] Even so Concorde was not allowed to fly supersonically over land. There are real engineering and logistic problems. The main engineering problem is that the supersonic aeroplane must be able to fly at subsonic speeds in order to take off and land. Unfortunately the lift force generated by the wing is in a very different place for subsonic and supersonic flight. Some military aeroplanes, the Tomcat for example, have swing-wings and others have dodges to overcome this problem but Concorde moved its fuel about to move its centre of gravity to re-balance the aeroplane. When Concorde flew at low speed the angle of attack of the wings was very high and the engines were used to overcome the very large drag. Still the aeroplane took off and landed at high speed. Concorde also needed preferential landing slots to avoid excessive wastage of fuel in a landing “stack” when compared with a subsonic aeroplane.

 

It is evident from the foregoing that the shock waves and the attendant sonic booms that would be created by a large supersonic aeroplane would be a serious impediment to its acceptability. The shock waves present a serious problem to designers. This means that any design must aim to minimise the number and intensity of these waves. There are two unavoidable but very loud waves, one created by the nose, however sharp that may be, and the other at the tail. There will be other waves emanating from the engine intakes etc.. They all become more intense as the size of the aeroplane increases. Concorde carried about 100 passengers but a new supersonic civil aeroplane would have to carry many more in order to compete with modern subsonic aeroplanes carrying 500 passengers. It may well be that the supersonic civil aeroplane would be too wide, too heavy and too legally restricted to be viable.

 

Military aeroplanes are small and have their own landing facilities. They have their own design constraints not least that they are always built in an atmosphere of haste generated by political competition between countries.