Gents, I am but an ordinary (now retired) ex military airline TC.
In writing an article about 'speeds' for my last Airline Training Department, one of my very clever ex-military TP (Edwards trained) penned the following summary for me which you may or may not agree with - I found it helpful:
“We can’t see AOA, compressibility or boundary layer characteristics in the cockpit, but the wing can feel them. So the designers give us a pressure gauge and decide what banana units it should read. By clever maths….they make it read CAS (kts) because that’s what gives us the most useful info at slow speeds and near the ground. By fluke, it also works quite well throughout most of the envelope – keeping us from stalling and overspeeding. But it doesn’t show Reynolds and compressibility….so they decided they should electronically add warnings of stall and overspeeds usually by lookup tables cross referenced with air data inputs and fed into the cockpit speed tape, audio etc……The SR71 uses KEAS because the CAS indicator becomes virtually useless at high Mach numbers.”
Cheers,
mcdhu