Jazzy....When flying at altitudes above 20k ft the indicated airspeed starts to decrease at a good pace..to make flying easier above 20,000 ft the mach number was employed because it is much better at being user friendly...like Drela's post mentions it is only dependant on temp......modern aircraft use ADC,s( air data computers) to compute Mach Number ...at say 20,000 ft your indicated airspeed is 350 knots now check in @ 37,000 ft it might now be 240 knots indicated where by using a mach number a pilot could see .80 all the time....also keeping in mind the air thins out the higher you go making "Buffet" margins easier to define...because there are low speed margins (encroaching stall area) and high speed margins(airframe limits) and these margin areas compress as the altitude increases....temperatures in the 35,000 to 40,000 ft altitudes can be 50* to 60* below zero.....If you compute your props tip speed , I know for full scale aircraft anything over roughly 650ft/sec props start making lots of noise...like a T6 prop...really wails @ t/o power........Bill....