Mach No. vs the stall vs V alpha prot
 

Hi all,

Would anyone like to offer an explanation of how increasing Mach no affects the other 2 parameters?

In subsonic flight, VS is constant with EAS. However, airliners fly at transonic regimes, in which compressibility has a double effect:

on the one hand, VS IAS increases with mach number, even if VS EAS remains constant.

Secondly, Lift and Drag Coefficient curves change with mach number, and VS EAS increase (separation occurs at an earlier AoA due to compressibility effects, shockwaves and such).

The overall result is a very noticeably increase in VS indicated.

Both VS and VaPROT are AoA based speeds, so when Mach number increases, both speeds increase.

Thanks, but I'm no wiser.

Let me try to rephrase my question.

When Mach no increases how does that affect the speed and angle at which the wing will stall? And why.

The problem comes down to the usual KISS approach's leading to misunderstandings associated with well-intended simplification.

You don't appear to tell us much about your knowledge level so I am presuming you are a student pilot or thereabouts without an engineering or similar technical background ?

Lift coefficient depends on angle of attack, Mach Number (ie compressibility effects), and Reynolds Number (ie viscosity effects). The last two (M and Re) generally are ignored for most aircraft which operate at well below transonic speeds and in a narrow band of the atmosphere. Hence the low speed pilot rarely gets to talk about their effects - in any case they are conveniently hidden away in the CL data.

Many pilots have a basic understanding of Mach Number and, generally, none in respect of Reynolds Number.

Any number of net references, including these -

(a) Mach Number

(b) Reynolds Number

Most pilots tend to think of the simplistic CL versus alpha curve as being the whole story ... when it isn't.

The effect of M on CL is to reduce max CL as M increases. Some data relating to the DC9 is shown at Fig 11 in this paper. A useful Boeing article on angle of attack is worth a read as well - fig 4 shows some Mach effects.

The effect of Re on max CL can be seen at p60 in Aerodynamics for Naval Aviators.

Increasing flight Mach number into the transonic range eventually causes small shockwaves to form on the wing, which grow in size as the flight Mach number and/or AOA increase. Google "critical Mach number" for more info on this. These shockwaves disturb the airflow behind them, reducing the lift of the wing compared to subsonic flight conditions for the same AOA. Aerodynamicists deal with this by adjusting the relationship between CL and AOA to compensate. The effect is the stall EAS increases because the shockwaves cause CLmax to decrease. Another effect is that above sbout Mach 0.6, CAS and EAS diverge due to compressibility effects, with CAS over-reading compared to EAS. Most aircraft PFDs show CAS on the airspeed tape, so at higher Mach numbers the stall speeds shown in CAS must increase to compensate for the growing difference between EAS and CAS.

Finally, the shockwaves can also cause the wing to stall at a lower AOA, but this is all dealt with through reduced CLMax and thus higher stall speed.

Regards, WF

vikena, putting others' contributions together with this Boeing article may help understanding.