Originally Posted by
RandomPerson8008
I know this isn't the perfect scientifically accurate explanation, but I always though of it this way:
The wing needs a certain minimum number of air molecules flowing over it every second to generate sufficient lift for flight, or the wing stalls. The pitot tube measures how many molecules are flowing over the wing (well, really through the pitot tube but it's proportional) and thus your IAS is an indirect measure of airflow over the wing.
At high altitude, the air is less dense, there is more "space" between air molecules. For the pitot tube to gather the the same number of air molecules (IAS) the airplane has to be moving faster over the ground at high altitude (in TAS) to scoop up the same number air molecules. Similarly, the wing will have roughly the same number of air molecules flowing over it at 200 KIAS at sea level as it will at 200 KIAS at 10,000 feet, although the TAS will be about 20% higher at 10,000 ft because the airplane has to cover more distance to pass through the same number of molecules at 10,000 ft.
This is a folksy yet valid explanation, but it only explains the density difference that increases TAS stall speed by the same amount that the pitot underreads, thus keeping EAS/IAS stall speed constant (assuming the density difference is the only difference)
But it doesn't explain the OP's question, which is a different one: why does EAS/IAS stall speed increase with altitude? And the answer is that there are differences besides density, which are compressibility and viscosity. And they cause the wing to stall at a lower AOA, thus decreasing the CLmax.
For many commercial airplanes, the IAS barber pole actually doesn't increase with altitude. In the B-747 for instance, Vmo is about 365 KIAS from sea level all the way until 365 KIAS becomes equivalent to Mach 0.92 at some point in the Mid 20,000's. Above that altitude, the "barber pole" slowly decreases in terms of IAS, bur remains constant at Mach 0.92. Of course 365 KIAS at 0 MSL is a lot slower in terms of TAS than 365 KIAS at 25,000....but in terms of air molecule flow over the wings and into the pitot tubes, it is roughly the same.
What's being asked about is the minimum speed barber pole, not the max. Ambiguous term, gotta be careful with them.