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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Air density is lower at higher altitudes. Air density is a big factor for the speed of sound in air, so it changes too.
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.
The key word here is “indicated”. The way that an aircraft speedometer works is by measuring the effect of the incoming air. But at higher altitudes there is less air so there is less effect to measure. This means that an aircraft speedometer is indicating too low speed at high altitudes. You would think this is something that could be fixed, and in fact modern airplanes have indicators for not only indicated airspeed but also true airspeed and ground speed. But the most important for avionics is still the indicated airspeed. This is because the same effects that you measure with the speedometer is the same effects that generates lift on the wings, authority on the control surfaces, drag, engine air supply, etc.
This means that the manual for the U-2 can specify a minimum airspeed of 110 knots indicated, no matter the altitude. It can be at landing or at 70,000 feet. As long as the needle on the speedometer stays above 110 knots the airplane is flying, no matter what the true airspeed is. The speed of sound however is based on the true airspeed, although it does vary a bit with altitude. So the U-2 needs to fly faster then 110 knots indicated but slower then 600 knots true. And this is where the altitude limit comes from. At 70,000 feet the air is so thin that the U-2 would have to fly above the speed of sound in order to generate enough lift for level flight. And the U-2 was not capable of safely flying above the speed of sound.
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