About M crit.
Early 'high speed' but subsonic jet aircraft such as the Meteor and Vampire had Mcrits of about 0.8 and 0.78. They tell me that the Spitfire and Mustang were in that range.
The enormous drag increase and the random movements of the shock waves as those mach numbers were reached created an almost impenetrable boundary principally as a result of the buffet and inadequate elevator power to overcome the resultant nose down pitch as the centre of pressure for the wings jumped around as it moved aft.
For the single seat Vampire Mcrit was reached as airflow over the fuselage reached mach 1.0. In a dive to reach Mcrit it felt as though the aircaft would break up at any moment with the stick kicking around randomly. Take it just a little faster and the nose would start to go down uncontrollably with buffet yet increasing. One rarely had a desire to do it twice.
Subsequent wing and tail designs resulted in lesser effects at M crit and caused the critical to become non critical.
The F86 Sabre and Hunter, much assisted by wing sweep, slipped through Mcrit fairly easily in a steep dive. Some noticeable buffet and unpredictable roll going through followed by smooth flight supersonic as the shock waves became established and consistent.. In some rare light conditions I have been able to see the shock waves dancing around on the wings as the light going through the shock waves became refracted through the changing air densities of the shock wave..
Meanwhile engine powers kept on increasing, substantially with afterburning/reheat together with ever improving wing /fuselage design to result in predictable and relatively smooth shock wave behaviour.
A sonic boom is one of those shock waves going past you at the speed of sound.
The Super Sabre, the F104 and the Lightning were then able to go supersonic almost without the pilot being aware. So over time M crit has become that Mach No at which there is a sharp increase in drag and a need to substantially increase thrust to go faster.
Most of today's aircraft have not been able to efficiently penetrate the drag rise so have to cruise at Mach Nos somewhere between 0.8 to 0.85. Try to go a tab faster and
and your fuel flow will be a big cause for concern.
The Vulcan delta had twinges of buffet as it went above 0.9 IMN and an automatic Mach trimmer in the elevator controls smoothly applied increasing up elevator from 0.86 through to 0.96 which became the limiting Mach No for aircraft operation as there was then not much elevator movement remaining. One got away from me once and it went to Mach 1.04 pitching nose down with the stick hard back. Seriously considered letting it go under inverted to roll out coming up the other side. However with power off the Mach No dropped off with rapidly decreasing altitude and increasing air density and the fully up elevators gradually took effect.
Now there are a few fighter designs that have enough engine power and low enough drag to be able to cruise supersonic without resorting to ineficient afterburning..They have a 'supercruise' capabiliy. The F22 can supercruise at about Mach 1.6 which will alow it to run rings around F15s and the like.
Others such as the Concord and Blackbird managed to have engines and engine intakes which gave increasing thrust as speed increased at about the same rate as the increasing drag. I guess that if a Concord becomes short on fuel the pilot will resort to subsonic flight at about Mach 0.9. Any ex Concord pilots out there?
What next with Scramjets? The Heat Barrier.