High speed stall

[JABBARA]

Hi, all

As an ex-airforce pilot I can explain what high g stall is. There are two other names for this kind of stall: Accelerated Stall or High Speed Stall.

D= Density
V= Speed
Vs Stall speed
Cl= Lift Coefficient
Clmax = Max Lift Coefficient (where the stall occurs)
SQR = Square (2nd exponent)
SQROOT= Square root
Vs = 1 g stall speed
Vsn = Stall speed when n times of g is applied

In fact it is very simple:

L=1/2 x D x SQR (V) x S x Cl

If airplane is flying level unaccelerated (1g flight), when the speed is low enough so that when Cl=Clmax, that is the time of stall and that speed is the stall speed (Vs), so we can write:

L= 1/2 x D x SQR (Vs) x S x Clmax (1)

Now if airplane is maneuvering with a higher g, (lets say ng, n= number of g), practically L should also increase "n" times (= n x L). So we can write the above equation(1) like this:

n x L = 1/2 x D x SQR (Vsn) x S x Clmax (2)

Here Vsn is the stall speed when the airplane is maneuvering with n times of g (e.g 3 g).

Now if we divide equation (1) and (2) together, we will get the following:

Vsn= Vs x SQROOT (n)

A solid example, if 1 g stall is 100 kts, then 2 g stall is,

Vs2g = 100 x SQROOT (2) = 141 kts.

So what happens with high g stall: As soon as you relieve g, stall recovers.

For spin there are two requirements :
- Stalled airplane
- Sideslip (whatsoever the reason, either created by misuse of rudder or naturally happens)

Unless these two are not there, spin is impossible. Simply overbanking does not cause stall. Stall only occurs when level flight is tried to be maintained with that bank, as applying more "g". Inanother word the main reason for stall is "g". I.e zero g equals to zero Lift. If Lift is zero stall speed is zero as well. If somehow airplane is stalled, now at least we should avoid from sideslip to avoid from spin. That is why when trying to recover airplane upset, it is always recommended to be very careful while using rudder.
But I believe, today`s design technology and computer systems are good enough to prevent an airliner from spin, both aerodynamically and electronically.