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Altitude doesn't affect Indicated Stall Speed

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Altitude doesn't affect Indicated Stall Speed

Old 6th May 2020, 12:51
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Altitude doesn't affect Indicated Stall Speed

Hi there,

I'm re-reading my ATPL and can't seem to wrap around the idea of IAS does not change with altitude. Given that for commercial airlines, the barber pole increases with altitude, how then can we say altitude does not change with indicated stall speed?

Thanks for the knowledge
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Old 6th May 2020, 17:06
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IAS is actually a measure of ram-air pressure, not speed directly. All it measures is the ram-air pressure captured in the pitot tube. Which is then displayed as an effective aerodynamic speed on the IAS gauge.

Since lift is related to the aerodynamic speed or IAS, it is roughly true that a wing that stalls at, for example, 175 kts indicated at sea level will also stall at about 175 kts indicated in the thinner air at higher altitudes. This is why IAS is still a useful measure of performance and for "situational awareness," even if it is grossly "wrong" for estimating time-of-arrival when flying at 35000 feet.

However, there are other effects, such as Mach effects (incipient shock wave formation, buffet) or compressibility/calibration effects (CAS = calibrated airspeed, EAS = equivalent airspeed). All of which can change with altitude and air density and TRUE airspeed.

And of course we know that what really causes a stall is angle-of-attack - a wing can stall at any IAS if the AoA is high enough.

The relatively high speed envelopes, Mach envelopes and altitude envelopes of jets thus complicate what is a simpler and more direct calculation with a C172.

An electronic "glass" speed tape/barber pole will (or should) include computed corrections for those effects. In much the same way that the Vmo barber pole at the top end of the speed range will slowly decrease with increased altitude.

See also "Coffin corner" - the place where the Mach limit and the stall limit (both barber-poles) are only a few knots apart. https://en.wikipedia.org/wiki/Coffin..._(aerodynamics)

Last edited by pattern_is_full; 6th May 2020 at 17:25.
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Old 6th May 2020, 23:27
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"The wing always stalls at the same AOA" is a simplified telling for student and private pilots. If it was strictly true, then for a given weight and 1G, it would always stall at the same EAS (roughly, IAS) regardless of altitude, and the (low end) barber pole would never move - so your discrepancy wouldn't exist.

But, in a more detailed and advanced look, compressibility (Mach) and viscosity (Reynolds) effects change that - and those values that correspond with higher altitude, reduce the stalling AOA, thus increasing the EAS/IAS stall speed. And that is an actual increase, not just a measurement error.

This is a separate effect from, and not to be confused with, the (much bigger) TAS stall speed increase due to density, that we're all thoroughly familiar with.

https://en.wikipedia.org/wiki/Coffin...(aerodynamics) here is the link fixed

Last edited by Vessbot; 7th May 2020 at 03:22.
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Old 7th May 2020, 20:44
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Originally Posted by RandomPerson8008 View Post
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.
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