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How does stall speed (IAS) vary with altitude?

• A

It remains constant.

• B

It increases with increasing altitude, because the density decreases.

• C

It remains constant at lower altitudes but increases at higher altitudes due to compressibility effects.

• D

It remains constant at lower altitudes but decreases at higher altitudes due to compressibility effects.

Refer to figure.
The stall speed is defined by:

VS=√((2∗W)/(ρ∗S∗CLMAX))

At the stalling angle CLMAX and S are constant, and the indicated airspeed is given by the dynamic pressure 1 / 2 ρV2. Therefore, for the same airplane mass and configuration in straight and level flight, if the slight difference between EAS and IAS caused by compressibility is ignored, an airplane will stall at the same IAS at all altitudes.
If, however, compressibility is not ignored then the stalling IAS value slowly increases with increased altitude, but the change only becomes significant at very high altitudes.At a constant EAS, Mach number will increase as altitude increases.

Figure 1 shows the variation of stalling speed with altitude at constant load factor (n). Such a curve is called the stalling boundary for the given load factor, in which altitude is plotted against equivalent airspeed.At this load factor (1g), the aircraft cannot fly at speeds to the left of this boundary. Over the lower range of altitude, stall speed does not vary with altitude. This is because at these low altitudes, the Mach number at VS is too low for compressibility effects to be present. Eventually (approximately 30000 ft), Mach number at VS has increased with altitude to such an extent that these effects are important, and the rise in stalling speed with altitude is apparent.
As altitude increases, stall speed is initially constant then increases, due to compressibility.

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