wangus
From a book in my library:
There are more exciting things to read about than the theory and practice of what speed should be flown on the climb. However one question I have been asked more than once is why Concorde had a climbing speed that started as a constant indicated airspeed (IAS) but later became a constant indicated mach number (IMN). This changeover is not unique to Concorde. It is something that affects all aircraft capable of flying high as well as those with a supersonic capability in level flight. As ever the explanation of this phenomenon comes in two sizes - short and long.
The short explanation is that, as you climb at a constant IAS, regardless of your aircraft type, your mach number is actually steadily increasing thanks to the reducing air temperature and density. If your aircraft has sufficient power to climb high enough, the mach number will eventually reach that at which a pretty steep drag rise starts to happen, even if your climbing speed is only 80 kt IAS. This mach-related drag rise would probably start in the range of say 0.60 to 0.90 IMN depending on the shape of the aircraft and, in particular, the thickness chord ratio of the wing. Naturally such a drag increase will quickly put a stop to any climb rate. It is therefore necessary to hold an IMN a little below the one where the steep drag rise happens in order to keep the best climb going. Eventually of course full power will be needed to hold height at that speed. Once at this point, you have reached the absolute ceiling of the aircraft. To summarise, all aircraft climb initially at constant IAS and increasing IMN and then change over to a constant IMN and a reducing IAS.
The longer explanation has to do with how climbing speeds are chosen and the theory that lies behind them. Here some differences start to emerge both between the theoretical optimum and what it is sensible to expect pilots to fly and also between piston-powered and jet aircraft. Any reasonable text book on the subject is likely to cover the topic of climbing in a few equations but I have always found it more useful to understand the words behind the equations.
There are three things going on in any climb at a constant IAS. First the engine is having to overcome the drag that would be present at the same speed in level flight, second it has to provide the energy necessary to ‘winch’ the weight up vertically at a speed equal to the rate of climb and finally it has to accelerate the aircraft because at constant IAS the true airspeed (TAS) is steadily increasing as height is increased. I do accept that the latter term is small enough to be ignored in low performance aircraft but it is not insignificant with jets that have high rates of climb.
There follows several pages of detail but you probably are bored by now........
Once the climb is done and you are in the high speed level (or diving) business then ALL aircraft have an IAS limit for structural reasons as you clearly understand. However not all aircraft have a Mach limit. If a type has one it is probably related to an undesirable trim change or other control issue. Only those types which can sustain a very high TAS are likely to have a structural temperature problem which may be reflected to the pilot as a sustained Mach limit or an actual structural temperature indication limit.