Stalling without stall warner going off

[vancouv]

Thanks for the replies. I was talking about a plane with a working stall warner, and specifically a stall warner, not warnings of the stall approaching.

When I was learning, one of the things when doing stall practices was to set carb heat to cold when the warner sounded ready for the recovery. I think my instructor would have had words if it wasn't working and I hadn't spotted it during my walk around.

[Brooklands]

Point of Information: The stall warner in Cessna 152s and 172s (and probably 150s, but I've never flown one) doesn't need electric power, so has no circuit breaker, and so can't be disabled that way. I think its based on a 'reed' rather like you find in a mouth organ.

Brooklands

[BEagle]

One of the many dangerous results of severe leading edge icing is that the Cl v alfa curve of the wing may change and, as a result, the wing may have a lower stalling angle of attack. If the AoA probe is calibrated to provide a warning at the normal stalling AoA, it is indeed possible for an a/c to stall before the stall warning operates.

[Final 3 Greens]

Just speculating aloud, but what would be the effect of a particularly strong gust from behind the aircraft?

Could it cause a stall without triggering the warner - i.e. sudden loss of airspeed without significant AoA change?

Could someone with rather better aeordynamic knowledge comment on this hypothesis (and disprove if it is wrong.)

[High Wing Drifter]

I don't think so F3G.

Stall is only a function of AoA, so by definition if you loose IAS (or better put as loosing TAS), then to maintain height you must increase the AoA to increase the coefficient of lift (CL) to maintain the lift = 0.5 * rho * V^2 * CL * S equation (as all other things are assumed to remain constant).

V=TAS, CL is a function of AoA and wing characteristics, S=wing area and rho=air density.

[foxmoth]

I think what HWD means is that you will lose lift and hence the A/C will sink but you will not actually stall. If you then increase AoA to stop the sink you may then stall, but the warner will go off as well.

[High Wing Drifter]

Yes that is what I mean

[Final 3 Greens]

HWD/Foxmoth

Thanks for your input.

So what would be experienced by the unfortunate pilot of the gust affected aircraft would be a strong pitch down to regain trimmed IAS (and thus TAS), rather than a stall.

But it might feel like a stall.

The only instance of this type of incident that I have heard of (and it was second hand) was a DC8 hit by windshear from a CB, so it's no doubt pretty unlikely anyway.

[foxmoth]

No significant pitch down (could even be a slight pitch up as you have lost IAS), just a sink which you would feel, you would then need to pitch down and/or add power to regain IAS/height (IAS would recover after the gust but you would have lost height).

[Charlie Zulu]

The Beagle Pup has a stall warner circuit that is disabled when flaps are fully up.

Thus we can stall the pup without the stall warner system beeping at us...

[Circuit Basher]

HWD -

Stall is only a function of AoA

I beg to differ from that statement, without wishing to start an argument. IIRC, stall speed is a function of:

• Wing Loading (A/c Weight)
• Aerofoil Section (inluding whether additional lift devices, such as flaps, or if spoilers, are deployed)
• Any icing / surface degradation (such as flies / dents) on the mainplane
• A of A
• Manoeuvering Loads
• Air Temp / Density

Whilst a typical wing under typical conditions may stall at around 15 deg A of A, this is not an absolute figure.

[Mike Cross]

mmmm.......

Wing Loading (A/c Weight)
An increase in weight will require a higher AoA to provide the same lift.

Aerofoil Section (inluding whether additional lift devices, such as flaps, or if spoilers, are deployed)
Fairly obvious that changing the configuration of the wing is going to alter the stalling AoA.

Any icing / surface degradation (such as flies / dents) on the mainplane
Same comment as above. Anything that reduces the efficiency of the wing is going to require an increased AoA to produce more lift and stop the thing from sinking.

A of A
agreed

Manoeuvering Loads
How pray do you apply them without changing AoA? Increased wing loading=increased AoA, decreased wing loading=reduced AoA.

Air Temp / Density
As above, lower air density will require increased AoA to provide the same lift.

Mike

[High Wing Drifter]

Hi CB,

I beg to differ from that statement, without wishing to start an argument. IIRC, stall speed is a function of:

Agreed, but you have changed the context of the statement by trying to capture the variable of the stall speed. What I am saying is that the stall speed may change for environmental, weight, wing loading,what have you, reasons. But the stall still only starts at the critical alpha.

Basically, for any wing configuration (flap, slat, etc setting) there is only one cirical alpha. The aircraft will only be in a stalled condition if that AoA is exceeded regardless of the speed (EAS) it maybe travelling at.

Icing is unquantifiable. Yes, it will change the wing shape (configuration) and add weight so both the stall speed and the critical alpha will almost certainly change for the worse. For any given situation that is all we know. But, even under those circunstances, this wing will still only start the stall at the critical alpha (whatever it maybe) regardless of speed.

[Circuit Basher]

MC / HWD - yep, accepted all of that. Just felt that there are too many people (and I believe quite a few PPLs amongst these) who have a belief in the absolute fact that a wing will stall at x deg AoA (some who I have spoken to seem to even just think that AoA is just the angle between the wing and horizontal!).

I just felt it was worth pointing out that it isn't just a magical thing that you'll stall at x degrees (where x is a constant for a particular aircraft type) and that's all there is to it - a lot of things affect what your critical AoA is for any situation.

I teach Principles of Flight to Air Cadets (a lot!) and this is the one thing that they have a genuine difficulty remembering when they go aerobatting in Tutors!

[Shaggy Sheep Driver]

CB

I just felt it was worth pointing out that it isn't just a magical thing that you'll stall at x degrees (where x is a constant for a particular aircraft type) and that's all there is to it - a lot of things affect what your critical AoA is for any situation.

Actually, there's no magic involved - just simple aerodynamics. Any given wing will stall at 'X' degrees AoA and that is all there is to it. In the case of an iced-up wing, the AoA at which it will stall is different to that of the same wing with no ice - because the aerodynamic shape of the wing has been changed by the ice.

All the stuff you mention about manouvre loads and weight are irrelevant - they may well alter the aircraft's AoA for any given flight condition, but the wing will always stall at the same AoA. And the elevator is your AoA control.

If you fly aeros, you ignore IAS as far as departure is concerned, You fly AoA.

SSD