Low speed buffet

[TotalBeginner]

Can someone explain why the indicated airspeed at which an aircraft stall remains fairly constant with altitude, however the low speed buffet increases significantly?

[Goldenrivett]

Hi TotalBeginner,

Mach Number. See:
Aero 12 - Angle of Attack (http://www.boeing.com/commercial/aeromagazine/aero_12/attack_story.html) about half way down,

"Because of the effect of Mach number on stall AOA, the stall warning AOA typically was set at a conservative level to accommodate gross weight and altitude variations expected in the terminal area.

The early stall warning system thresholds were not set to be effective at cruise altitudes and speeds because they did not correct for Mach number (fig. 10). This kept the system simple. The stick shaker was set at an AOA effective for low altitudes but at too high a value for cruise. Natural stall buffet was found to give satisfactory warning at higher Mach numbers."

[TotalBeginner]

Sorry, I still don't understand. What is it about high altitude flight that increases the LOW speed buffet margin?

If IAS is a measure of the actual airflow over the wing, then why would buffet occur at a higher speed just because the aircraft is higher? If the Mach number plays a part, what's the scientific reason?

[Chesty Morgan]

Compressibility.

[Sidestick_n_Rudder]

At high Mach number, increase of the AoA intensifies the shock wave in the wing, which in turn causes flow separation and buffet. This occurs at significantly lower AoA, than classic low speed stall - think ~5 deg AoA instead of 15...

P.S. What airplanes are we talking about? Cessna 150, or jet transport. The former rather doesn't suffer too much from compressibility effects... :)

[physicus]

Compressibility and viscosity of air always play a role but are lesser factors at low altitude (high density). At high altitudes however (30kft+), compressibility of air becomes a dominant factor. Under low pressure conditions, shock waves form in the turbulent flow, i.e. in the portion of the wing where the flow stops being laminar. This leads to a reduction of CLmax, onset of buffet at higher IAS, and less AoA wiggle room to maintain unstalled lift conditions.

[Vessbot]

To put what others here have said into some reconcilable context, note that a wing does not always stall at the same angle of attack. That it does, is a good enough approximation to learn basic flight dynamics with, but strictly speaking it's false. Mach number and Reynolds number change it, and when you get into high altitude aerodynamics, the difference is enough to matter.

[misd-agin]

Some 737's and 777's have AOA gauges. At low altitude the stall margin indicator is at approx 1:30 on the AOA gauge. At high altitudes it is at approx 2:45.

As you climb, and Mach compensation comes into play, the AOA limit moves from 1:30 towards 2:45. Under 10,000 at 250 kts it's at 1:30 clock position. Above 10,000 and increasing climb speed, Mach exceeds .4(roughly minimum Mach displayed on the airspeed indicator), at the limit starts changing.

Clock positions equate to roughly 6 units (2:45 position) and 12.5 units(1:30 position). No guidance if 'units' is exactly the same as actual degrees of AOA.