Hello Mayday. I've found compressibility blamed for all sorts of problems/analomies. The following is how I've come to appreciate it's effects.
Compressibility has 3 main influences that I see on aircraft and systems. Incompressible flow, ie without compressibility, occurs when density of the air can be considered a constant as it interacts with the aircraft. Generally less than .3 mach meets this criteria. Greater than .3 mach, and the variation can become important, particularly for aircraft with the performance envelope of today’s high subsonic airliners.
1. Airspeed measurement. Effects of compressibility on airspeed measurement can be seen form 7p3i7lot’s post above. Typically they are small, the effect on the cas is only typically up to 30 kts.
2. Lift. Effect of compressibility on the wing are much more difficult to understand and predict, and most texts do a poor job of explaining it. Experimental data shows that for .6 mach and less, for a given aoa, the value of Cl **can** actually be greater than without compressibility. That is, as you speed up, the Cl value for a fixed aoa can increase. The slope of the lift curve, delta CL divided by delta aoa, is greater with slight compressibility than no compressibility. Further, the max Cl value can be greater at a fixed mach, although as mach increases, a slight rise in max Cl seems to be followed by a drop to below the non compressible value of Cl max. And while von Mises “theory of flight” shows stall aoa to be greater with slight compressibility, I’ve not seen that in other texts. In fact, others show the stall aoa to be less with compressibility. Note that all of this is for an airfoil with flow entirely subsonic.
3. Drag. Although one may argue that it is not compressibiltity that affects drag in the transonic range, I generally consider it to be included. The coefficient of drag Cd is constant at low machs for a constant Cl. However as mach increases, there is a sudden and rapid increase in Cd as the effects of shock wave formation along the wing surfaces and flow separation come into play. Once complete supersonic flow envelopes the wing, a regular pattern of shock waves are formed, and an overall reduction of Cd occurs.
As KW typically says, “I could be all wrong”. I have no degree in an aerodyamic field, so this is all information I’ve gathered on the side.
Perhaps others have more pertinent, or correct information to add.
Make sense?
Hawk