Originally Posted by BOAC
Basically because it was correct!
No, it's not correct. The angle of attack at which a high-performance wing will stall is a function of the speed of the air flowing over it, exactly contrary to what DFC said in hisher first sentence.
These AoAs, for different airspeeds, are established during certification of the airplane. You put a wing in a wind tunnel, and look at its behavior at different airflow speeds. The AoA at which, say, C_L drops off sharply is a function of the airspeed.
I must say I am somewhat surprised at your response. I had awaited something different.
Your comment
[PBL: The angle of attack at which a modern commercial jet stalls at 0.8M is something around half of the angle of attack at which it stalls at Vs1g.]
is totally confusing.
I am sorry you are confused. I think it is a precise statement, and I don't know how to say it more precisely. Let me try.
I can put numbers on these, for a specific wing X:
"The AoA at which wing X stalls at 0.8M" = S;
"The AoA of wing X at Vs1g" = T
S is round about half of T for wing X.
This won't necessarily be true for all high-performance wings X, but it is certainly true for some of them.
Less confusing?
Originally Posted by BOAC
Secondly you are mixing a Mach effect at an unstated 'g' with a stall at '1g' at an unstated airspeed! Absolute confusion. It will still stall at 'Vs1G' at high level, will it not? Just a different Vs due to airflow Mach effects.
Vs1g is a specific airspeed. What do you mean, "unstated airspeed"?
The AoA at which a wing will stall, and how it will stall, is dependent on Mach effects. These must be determined when certifying aircraft.
I don't agree that this is "off topic". Some posters are talking about stall recovery at high altitude, and many of them seem to be confused about aerodynamics, even what "stall" may mean. The vocabulary I am using is one which I use on an almost daily basis in my work with an aerodynamicist. I am gradually learning through these exchanges on PPRuNe that many pilots seem to be unaware of the aerodynamic phenomena which govern the behavior of their airplane. I don't think this disconnect can be healthy.
As I explained earlier, the "stall" is generally a range of phenomena playing out over a range of airspeeds. The certification-defined "stall speed" is the one at which (crudely) handling becomes compromised due to buffet. This may be, often is, lower than the speed at which C_L falls off. I don't agree with Mansfield that this is not a "practical" distinction. It becomes very practical when considering gusts at altitude. For certification, one has to take the wing to defined-stall ("uncomfortable buffet") and then subject it to gusts of a defined magnitude, and make sure nothing untoward happens, such as suddenly losing all your lift.
PBL