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First of all, we all know what IAS is and if not for some people you can follow this link to read the basics about stall and the differents speeds. So we also know that our TAS/ Mach number will increase with increasing altitude.

Let's assume that our IAS stall speed is 100kts at sea level, what will be our IAS stall speed at 30 000 feet with exactly the same aircraft ? (Assume ISA atmosphere)

Everyone would be tempted to answer that it doesn't change, right? but what about the low speed buffet in this picture ?low and high speed buffet

We agree in saying that the low speed buffet here is not the same as the one at sea level but look, our IAS/CAS stall speed has a higher value now. Maybe that it's a representative value of IAS/CAS calculated by the Air Data Computer from an EAS but still, our IAS stall speed has rised.

We can also keep it simple and think about it for a non pressurized single engine aircraft. I don't need an exact value but I would love to have a logic and smart answer.

It seems to be very basic but when we correctly think about it we can find ourself a little bit confused.

For me, the answer is : The IAS stall speed will increases

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Thomas
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    "IAS stall speed is 100kts at sea level". What do you mean? An aircraft can stall at any speed. – Simon May 21 '16 at 21:24
  • I know that, I know that an aircraft stall at the same angle of attack regardless of its speed. I just took an example – Thomas May 21 '16 at 21:31
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    @Simon I think it's safe to assume he is referring to the straight and level flight stalling speed. This type of question is asked in various theory publications and I'm not sure why it has a close vote – Ben May 21 '16 at 22:06
  • That's visible on the coffin corner diagram. Also on Wikipedia (U2 diagram). – mins May 21 '16 at 22:17

2 Answers2

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Airplanes do not stall at the same indicated speed or even at the same angle of attack - it all depends on circumstances.

The angle of attack dependency is discussed here. An increased pitch rate can push the stall angle of attack 50% higher than what the stall angle of attack is in stationary conditions.

The next big factor is the Mach number. When increasing the angle of attack, the flow around the airfoil's nose will develop a suction peak. This suction is equivalent to higher local speed, and if the critical speed (when local flow speed equals the local speed of sound) is exceeded, the flow past the suction peak will no longer behave similarly to the flow at the same angle of attack but a lower flight Mach number. Let's just say that the local Mach number in the suction peak has a strong influence on the stall angle of attack, and flying at a higher Mach number lowers the stall angle of attack, sometimes dramatically.

Increasing altitude will raise the flight Mach number in two ways:

  1. The reducing density means you need to speed up to fly at the same dynamic pressure, and
  2. the atmospheric lapse rate decreases the speed of sound in air.

Both effects conspire to reduce the stall angle of attack at 30.000 ft to a value quite a bit below that at sea level. Details depend on the airfoil and specifically on its nose radius and wing loading.

Only very light aircraft will not be affected by the change in Mach number, but even here the stall angle of attack at altitude is lower than at sea level due to the reduction of the flow's Reynolds number with increasing altitude.

In short, the indicated stall speed goes up with increasing altitude for a variety of reasons and does so nonlinearly. The magnitude of the change depends on a multitude of details.

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Peter Kämpf
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  • Thank you for this precise answer! I waited for this type of answer from you specially! Can you just tell me what would you answer to my question ? – Thomas May 21 '16 at 22:23
  • @Thomas: To answer the question I need much more information. Airfoil and wing loading would be the most important details, but maybe not sufficient. I tried to explain that the change in stall angle of attack depends on many things, and your question only gives a sea level stall speed. – Peter Kämpf May 21 '16 at 22:26
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Vstall is measured as a true airspeed, it increases approximately 1% for every thousand feet increase in altitude. 100KTS its sea level - 110 KTS at 10000 feet TAS increases 2% per thousand feet for any given IAS. 110 KTS TAS at 10000 feet = 92 kts Thus, Vstall TAS increases, IAS decreases

PerformanceAviator
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    Are you sure? As far as I can tell, all $V$-speeds are always given as indicated speeds. The only where the actual limit is TAS is $V_{NE}$, where the limit is flutter (which depends on TAS) or Mach number (which depends on TAS and temperature only). All the other, including $V_S$, depend on dynamic pressure, so IAS (EAS). – Jan Hudec May 23 '16 at 09:56
  • Vs is not measured in TAS. Vs is given in CAS (or EAS when aircraft is likely to exceed 0.4M). If Vs was given as a TAS, it would be unusable to pilot the aircraft. Because Vs would be always changing. http://aviation.stackexchange.com/questions/7500/why-are-planes-slower-at-higher-altitudes/7502#7502 – Fox May 26 '16 at 11:39
  • Both parts of this answer are incorrect. TAS does not increase by 2%, it is a nonlinear increase related to the decrease in density. That rule of thumb is only a very rough estimate usable at lower altitudes. – John Oct 15 '19 at 06:49
  • I'm pretty late to the game, but TAS as a function of CAS so long as the atmospheric lapse rate follows ISA. At any temperature, TAS goes up by 1.015x (in other words, 1.5%) per thousand feet, accurate to well within 1 knot. If your TAS is 100 Kts at 5,000 feet, your TAS will be 101.5 Kts at 6,000, 103 at 7,000, and 116 at 10,000. – Max R Aug 06 '22 at 05:13