The U.S. Air Force used to think every pilot needs to experience supersonic flight so we all got trained in it. They no longer do that so the training was either too expensive or unnecessary. In any case, most of us are constrained with a VMO that begins with a zero so this section of Basic Aerodynamics has been pared down a bit, but it hasn't gone away.
You might be thinking "High Speed Flight" doesn't apply to you and that might be true. But chances are it does apply to your wings. Compressibility not only impacts the way your aircraft flies, but the way its instruments relate your airspeed and altitude measurements. Critical Mach affects most jet aircraft that fly in the transonic range and that impacts your Stability and Control, so please read on...
Everything here is from the references shown below, with a few comments in an alternate color.
Photo: T-38, supersonic, from NASA.
We amateur aerodynamacists like to blame compressibility for all sorts of ills. I hear it most often when talking about the changeover from indicated or calibrated to Mach number in a climb. I don't think that is true, but I can't prove it. So I'll just discuss what I know to be true.
[Hurt, pp. 201-205]
Figure: Comparison of subsonic and supersonic flow patterns, (Hurt, figure 3.1.)
[Hurt, pp. 11-12]
We see that compressibility does have an impact going from CAS to EAS. But what about TAS?
[Dole, pp. 24-25]
It is true that compressibility will have an impact on TAS, but that is because it impacted EAS first. The reason TAS varies so greatly has more to do with dynamic pressure.
Figure: Critical Mach, from Eddie's notebook.
[Dole, pg. 217]
If the speed of the air foil is such that all of the local air flow is subsonic, the air foil is said to be below its critical Mach number. The lift and drag characteristics of the airfoil are, for the most part, conventional.
At the point where the first hint of supersonic air flow occurs above the wing, the air foil is said to be at its critical Mach number. This is the last point at which air can be considered incompressible and there is no shock wave to disturb the local air flow.
As critical Mach number is exceeded a normal shockwave forms between the boundary of supersonic and subsonic airflow. The area in front of this shockwave tends to be smooth, as the airflow has a gradual transition over the leading edge of the airfoil.
Behind the shockwave is a greatly increased static pressure, fighting to pull up the boundary layer. There is a tug of war between the kinetic energy of the air holding it to the airfoil, and the static pressure pulling it away. As the speed of the airfoil is further increased, the static pressure begins to win out and airflow separation occurs.
Figure: Wing sweepback effects, from Hurt, figure 3.14.
[Hurt, pg. 223]
Figure: Effect of Wing Sweep on Airflow Normal to the Leading Edge, from [Davies, pg. 97]
[[Davies, pg. 96-98]
See Stability and Control for more about wing sweep.
[Davies, pg. 24] With the advent of the big jet three things occurred to make life even more difficult. The size of the aeroplane increased enormously and, with increase altitude capability, the speed increase difficulty was compounded by Mach number effects which caused pitch changes on the aeroplane, upset the pressure distribution over the control surfaces and brought about unwanted changes in hinge moments. It was about this time that the design of the pure aerodynamic control was seen to be, perhaps not impossible, but certainly very difficult, costly and time consuming. Something was needed to remove the problems — and the answer lay in operating the control surface by pure power.
As we've seen, the aerodynamic forces on a wing change at critical Mach. Using powered flight controls and increasing the wing sweep help, but these fixes may not be enough for some wings.
14 CFR 25, Title 14: Aeronautics and Space, Federal Aviation Administration, Department of Transportation
Air Training Command Manual 51-3, Aerodynamics for Pilots, 15 November 1963
Connolly, Thomas F., Dommasch, Daniel 0., and Sheryby, Sydney S., Airplane Aerodynamics, Pitman Publishing Corporation, New York, NY, 1951.
Davies, D. P., Handling the Big Jets, Civil Aviation Authority, Kingsway, London, 1985.
Dole, Charles E., Flight Theory and Aerodynamics, 1981, John Wiley & Sons, Inc, New York, NY, 1981.
FAA-H-8083-15, Instrument Flying Handbook, U.S. Department of Transportation, Flight Standards Service, 2001.
Gulfstream G450 Airplane Flight Manual, Revision 35, April 18, 2013.
Gulfstream G450 Aircraft Operating Manual, Revision 35, April 30, 2013.
Hage, Robert E. and Perkins, Courtland D., Airplane Performance Stability and Control, John Wiley & Sons, Inc., 1949.
Hurt, H. H., Jr., Aerodynamics for Naval Aviators, Skyhorse Publishing, Inc., New York NY, 2012.
Technical Order 1T-38A-1, T-38A/B Flight Manual, USAF Series, 1 July 1978.
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