Stalling without stall warner going off

[vancouv]

I did ask this question in another thread, but I thought I would put it in its own place.

Are there circumstances (and I'm talking about your average trainer here, C152 PA28 etc.) when the plane can stall without the stall warner being activated?

For example, a skidding turn where airflow over the wing with the stall warner may be disrupted?

I've often wondered whether you could get into a situation where the stall happens without the warning going off - and I am just talking about the stall warner here, no other signs such as buffet, high nose etc. etc.

Any thoughts gratefully received.

[Flik Roll]

Yes, if you are in favourable conditions for icing and the stall warner doesn't move due to being iced up.

[foxmoth]

Or if the CB pops (or is pulled by the instructor;) )

[Circuit Basher]

<continuing frivolous banter from Spitfire thread!! ;)>

I'll have you know that I generally give warning when about to pop!! (and the AB thread about her Remove B4 Flight T Shirt / 2 Lovely Bristols got a happy man feeling very old!) ;)

Also, I've never been pulled by my instructor (not that I'd have really wanted to be - they were mostly old, fat & hairy - and male!!)!!

CB

[mazzy1026]

The wing without the stall warner attached to it could stall first - wing drop?

[Paris Dakar]

Not the same thing but....................I was checked out on a PA28 that had a 'light' as a stall warner and not a 'buzzer' (something I totally forgot about when carrying out some stalls - it made the instructor smile if nothing else!).

PD

[FullyFlapped]

The stall warner could be knackered (I've had this happen) ....

FF :ok:

[Mike Cross]

Mine can stall without the stall warner being activated - it hasn't got one!

It's not activated by the wing stalling so stall warner is a slight misnomer. It activates when the wing reaches a preset angle of attack. As AoA increases the point at which the airflow divides moves down the leading edge. Once this point goes below the vane it is lifted by the upward airflow and the warning sounds. It's usually adjustable and set to go off prior to the stall so you have enough time to react and do something before the stall actually occurs. Providing the thing is in good working order and not iced up it should consistently go off at the same AoA, which if it is in adjustment should be below the stalling AoA.

However if you were to suddenly pull a lot of G at low speed you could easily leave yourself no time to react.

(Above refers to the vane type, haven't thought about the suction version.)

[High Wing Drifter]

The only aerodynamic situation I can think of is: If you have power (preventing root stall) on are near stall speed and then you use the aileron, the down going aileron can take the wing tip AoA past critical and stall that tip resulting in a sudden wing drop in the opposite direction to the stick.


>>For example, a skidding turn where
>>airflow over the wing with the stall
>>warner may be disrupted?
I don't believe this to be possible as the stall is by definition the stagnation point (point of stationary air that neither goes over the top nor underneath the wing) moving down the leading edge of the wing and pushing the stall flap up. If that is prevented then at least that part of the wing is below critical AoA.

[Miserlou]

I've always maitained that the first aerodynamic sign of the approach to a stall is an increase in back pressure on the controls.

If you haven't changed the centre of gravity or configuration and the aircraft was trimmed to a certain speed then any increase in back force must be because you are slowing down (increasing pitch attitude).

The next symptom is a reduced effectiveness of the controls.

Then you usually get some kind of airframe buffeting.

How many stall warnings do you need?

[NinjaBill]

Slightly off topic,

Can I legally pull the CB on the stall warner before take off? I find the stall warner on the firefly very irritating when I'm doing aerobatics, and pulling the CB would sort that problem.

NB

[foxmoth]

Rather than pull it before TO why not put it in as part of your before and after Aeros checks, then you have it for departure and arrival but not when it is just going to be a nuisance.:ok:
(The people who might get funny about this would be the insurance co. if you crashed on departure or arrival, they would probably not consider this a factor so much during aeros)

[djpil]

For info, from FAR 23 para 207 at Amdt 23-50:
(f) For acrobatic category airplanes, an artificial stall warning may be mutable, provided that it is armed automatically during takeoff and rearmed automatically in the approach configuration.]

[foxmoth]

Not sure how much that applies in the UK

[Miserlou]

Ninja,
Can anybody prove you pulled it? CB's do pop by themselves and sometimes for no apparent reason.

We used to tape over them for aeros.

[stiknruda]

With you Foxmoth, don't believe that it does apply.

Many of the aerobatic machines that I know have the stall warner C/B out and disabled (taped over) so that it can't inadvertently be selected on and therefore give the poor stick-Ninja the fright of his life!

On my own mount, the audio feed is insufficiently loud to be heard above cruise power, so for take-off it is pretty useless and for landing - it scares me to bits! So the non-cycle type C/B has been changed to the selectable one. With pax, I always have it ON as their weight does make quite a difference!

One young lady recently asked after we'd landed what that siren noise was? I told it her that it was the aeroplane's lie detector. She wanted to know; what was lying? You were, it doesn't believe that you are only eight and a half stone!

Stik

[Miserlou]

And you wonder why you never saw her again!

[stiknruda]

Miserlou old chap,

Had I wanted to see her again, I'd have taken her to lunch then found somewhere far more comfortable than the front hole in the Pitts to take her upside down!

Stik:E

[GuinnessQueen]

Stik,

Some of us really go for that sort of thing!

GQ:O

[mad_jock]

The stall warner on the C150 I learn't in never went off for the whole of my PPL.

Was first shown to suck on it during my walk round ( the stall warner). And quickly thought that was bloody stupid idea so didn't bother.

Did the course and the three instructors never made a comment it not going off during stalls.

On the final practise I was given a new Riddle instructor to break in and show JAR methods to.

He watched me do the walk round and gave me a bollocking for not checking the stall warner. I lied and said i had never been shown how. At which point he gave it a good suck and ended up with 2 months of compacted Love bugs and other beasties in his mouth. At which point he barfed. But give him credit the stall warner did work in the next flight. And to this day i have never felt the urge to find out if the constipated duck works on a pre flight check.

MJ

And to answer the question.

1. High load stall in the cessna after say a spin recovery ie the pilot trys to pull the nose up to hard to stop the plane going through Vne.

2. When the plane stall warner is a wee bit to liable to go off. It has been know for instructors to stick a bit of chewing gum on it after the student has done there walk round to stop it going off on every gust.

[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 :O

[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).:sad:

[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 AoAI 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