Hi V1,
Local Speed of Sound (LSS) is directly proportional to the square root of the absolute temperature. Using the relationship between pressure and density you can get a constant to represent them in a formula.
LSS (kts) = 39 x sqrt(Temperature in Kelvin)
For Kelvin just add 273 to Celcius.
Ex. LSS = 39 x sqrt (15 + 273) = 662 kts at ISA sea level.
and LSS = 39 x sqrt (-56.5 + 273) = 574 kts at ISA above 29 000 feet.
Mcrit is the speed at which the first part of airflow (boundary layer) goes supersonic. This usually happens on the wings and be dependent on camber (and obviously various other factors to do with the shape of the airfoil section). Another name for Mcrit is Mcdr (Mach critical drag rise) and this occurs with the formation of the first shock wave. I can't immediately think why Mcrit should rise with altitude. I always thought it to be pretty constant, but I can be wrong.
The danger of subsonic wings at Mcrit is that the center of pressure moves rapidly with the formation of shockwaves and this can cause tuckunder.
I'll pull out my books when I next go home and phone you with some more info.
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