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enter image description here

Theorectically, the stall speed in IAS doesn't change with altitude, but this graph above shows the stall speed actually starts to increase a little from above a certain altitude. How can this be explained? Is it something to do with a compressibility effect?

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The U-2C (which the graph is for) had a stall speed of about 79 KEAS, that is 79 knots equivalent airspeed (EAS) at all altitudes. At 50000ft, 79 KEAS is about 80 KIAS (indicated airspeed), but at 75000ft, 79 KEAS is about 83 KIAS. This is simply a consequence of the way that indicated airspeed is measured by pitot-static systems - the measured speed is not quite a true representation of the effect of the airflow upon the airframe. So, the apparent increase in stall speed is just an artefact of the way airspeed is measured, rather than the performance of the airframe itself.

You can confirm the EAS/IAS conversions for yourself using the Hochwarth Aviation Calculator.

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    $\begingroup$ Many thanks for the link to the Hochwarth Aviation Calculator! $\endgroup$
    – Terry
    Commented Aug 6, 2017 at 20:40

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