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Compared with the low subsonic regime, in the high subsonic regime the streamlines ahead of the wing change direction...
• A
further from the leading edge, resulting in a reduced Mach number for the high-speed stall.
• B
at the same distance from the leading edge but are closer together above the wing due to compressibility.
• C
closer to the leading edge, resulting in an increased indicated stall speed.
• D
further from the leading edge, resulting in an increased Mach number for the low-speed stall.

Refer to figures.
As an aircraft flies faster, the streamline pattern around the wing changes. Faster than about four tenths the speed of sound (M 0.4) these changes start to become significant. This phenomenon is known as compressibility.

Figure 1 shows that at low speed, the streamline pattern is affected far ahead of the wing and the air has a certain distance in which to upwash. As speed increases, the wing gets closer to its leading pressure wave, and the streamline pattern is affected a shorter distance ahead so must approach the wing at a steeper angle.

This change in the streamline pattern accentuates the adverse pressure gradient near the leading edge and flow separation occurs at a reduced angle of attack. Above M0.4 CLMAX decreases as seen in figure 2.

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 1/2ρV2 . 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 3 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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