The trans-sonic region of flow

It is hard now to comprehend the excitement that accompanied the attempts to fly at speeds greater than Mach 1. It all happened soon after World War 2 when everyone was looking for pointers towards a new life free from the misery of war. It generated intense interest.

 

We have seen that the physics for supersonic flow had been established long before it was needed for flight but that same physics did not cover the region where shock waves just begin to form. I suppose that the attainment of supersonic flight might have been easier had the engines been more powerful but it was the fact of engine development that took us gradually and not suddenly into the supersonic era. Of course rockets were available but the popular interest was in aeroplanes with jet engines that they might be able to see at air shows. Supersonic flight was achieved relatively quickly but at the expense of a few deaths.

 

The problem was that if you try to fly with subsonic technology just into the transition from subsonic to supersonic flow you are inevitably faced with unpredictable behaviour. We can only use our imagination when we try to think about this transition phase. I have included a picture 14-24 of water flowing at its critical depth on the sloping apron of a weir. I think that no-one would believe this unless it is actually seen and recorded. How can the water create these little pillars? The fact that it is creating these little pillars all over the apron means that there is some predictability in its behaviour. It seems to me to be reasonable to expect some unimaginable behaviour in the air flowing past the many parts of an aeroplane attempting to fly in this transition region and also to expect some repeatability.

 

The real problem turned out to be with the elevators and ailerons. These were made like those for a subsonic aeroplane with inset control surfaces and operation of these controls led to rapid and unpredictable response by the aeroplane.  Redesign of these components to use all moving stabilisers made supersonic fight possible but the real answer was a big increase in thrust from the engines and to go through the trans-sonic region quickly.

 

Study of trans-sonic flow is difficult and outside my competence so I will stop.