Why does the mach limit lower the higher you travel?
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I looked up the coffin corner for the U-2, and the mach limit as it reaches 70,000 is 110 knots indicated airspeed. but isn’t the speed of sound above 600 knots? How does the u-2 reach the mach limit?
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The key phrase here is “110 knots *indicated airspeed*”. Indicated airspeed is not the same as true airspeed. Indicated airspeed is measured by the pressure buildup caused by the plane moving through the air, which means if the pressure outside is lowered, the indicated airspeed will drop even though true airspeed remains constant. It’s only accurate to true airspeed at sea-level atmospheric pressure, and for higher altitudes pilots must perform a calculation to convert between the two. Indicated airspeed is nevertheless useful to pilots because things like the lift generated by the wings depend on air pressure and are proportional to indicated airspeed, not to true airspeed.
The U-2 sees a low indicated airspeed because of the low pressures at extreme altitudes, not because it’s actually traveling slowly.
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