Wikipedia:
"Mach number: the ratio of the airspeed to the speed of sound should be identical for the scaled model and the actual object (having identical Mach number in a wind tunnel and around the actual object is -not- equal to having identical airspeeds)" source
Why identical Mach number in a wind tunnel and around the actual object is -not- equal to having identical airspeeds?
If plane fly at 0.5 Mach(617km/h) that mean airflow speed for test model must also be 617km/h ,to has same compresibility effects? Isnt it?