Effect on critical alpha with increased altitude

[sudden twang]

Is the stalling angle of attack the same at sea level as it is at FL400?
if not why not?
There are conflicting statements on the internet.
I admit to not being very bright.

[Vessbot]

It's not the same, which causes 1 G stall speed to be higher at high altitude even in IAS (in addition to being way higher in TAS due to the much more familiar density error).

We say that the wing always stalls at the same AOA when teaching/learning basic flying, but that's a simplifying approximation to make the basics easier, or a "lie told to children" if you will. It's true enough in the envelope of a piston trainer. But compressibility (Mach) and viscosity (Reynolds) effects change the air flow behavior even at the same AOA, and cause an earlier (i.e. lower AOA, higher airspeed) stall at jet altitudes.

https://cimg8.ibsrv.net/gimg/pprune.org-vbulletin/609x460/yezhd_2ac6607bc5c448c779a6236725caa7cb39982a7e.gif

[vilas]

https://youtu.be/4WuPoVjOXLY
All you want to know about stall.

[pineteam]

Hi Vessbot,
When you say: higher IAS at higher altitude. From my undestanding it’s higher cause of compressibility effect? That’s my understandings of why the VLS will increase by 1kt every 1000 feet above FL200 on A320. FCOM says “ Above 20 000 ft, VLS is corrected for Mach effect to maintain a buffet margin of 0.2 g.” But I assume they want to keep it simple. Please correct me if I’m wrong.

Nice video about stall by Airbus: https://vimeopro.com/easyjetpeople/crew-training/video/95385218

Edit: Oups, Vilas beats me on the video. :8

[RBF]

The best explanation I have found so far comes from the "principles of flight" book from oxford (studying material for the EASA ATPL)

Basically as you encounter high mach numbers, air molecules ahead of the wing are given less notice about the in incoming airfoil. At low speeds there is plenty of time for the molecules ahead to upwash ahead of the leading edge. At high speeds this up wash occrus much closer to the wing and the upwash motion is "compressed". End result is that the air meets the wing at a steeper angle. This is cited as the main reason for the change in CL max with mach.

Compressibility effects start to become evident above 0.4 M.

L/D max also decreases at HIGH Mach

[RBF]

Aparently I am not allowed to add images until I have 10 posts....