Stall Warner
 

Hi all ... a PA 28 charges down the runway 1 POB and climbs out at 85 knots with the stall warner, incorrectly set, buzzing away, 1100 feet per minute climb rate indicated and the houses getting smaller .... at exactly the same time a PA28 charges down the runway 4 POB and climbs out at 85 knots (uses more power etc ) with the stall warner, incorrectly set, buzzing away, 1100 feet per minute same conditions (except for power) .. does the stall warner (not the stall characteristics) know the amount of peope on board? .. do you really know the answer? Its caused a riot at the local club ! Discuss.. no fighting!

I'd love to see a PA28 climb at 1100 fpm :confused: - none of the four PA28 161's I've flown will!!!

I thought the take off and climb was always on full power and speed control was with attitude ?

Not sure what you are getting at with this scenario but thoughts are that stall speed increases with weight so heavier PA28 would stall at higher speed (assuming no extra wing loading due aeros, gusts etc)

Please keep your mind on "the stall warner" a pa 28 will climb out at that speed because its a Dakota! (still a PA28 but 235hp) variable pitch but thats not the issue ... ah well back to the G-BRNZ

Most stall warning systems rely on direct or indirect sensing of angle of attack, not speed. So they 'know' the weight of the aircraft because they are using AoA, and a heavier aircraft at the same speed will have a higher AoA.

Similarly, most systems will therefore 'know' what 'g' you are at. An aircraft at 1.4Vs at 1'g' flight will have no warnings under normal conditions. Put that same aircraft in a 2'g' turn (or try to - it's going to stall at 1.96 'g' (1.4*1.4)) amnd there should be a bunch of noises or whatever trying to attract your attention.

Not sure of your point there. Strictly an aircraft stalls at an Angle of Attack (AOA) not a 'g' loading. The AoA can be equated to a basic stall speed at 1g which is normally quoted in the manual for straight and level flight (and will depend upon aircraft all up weight).

That basic stall speed increases with the square root of the 'g' not 1.4x1.4Vs.

Therefore, at 2g the aircraft will stall at (squareroot of 2) 1.4142 x basic stall speed.

at 4g the aircraft will stall at (squareroot of 4) 2 x basic stall speed. etc

This equation of IAS and g loading will induce the critcial AoA which will cause the wing to stall. If it could be measured the absolute AoA required to stall the wing is the same whether straight and level or pulling 2g, 3g, 4g etc etc

That is true.


So do you say that a lighter aircraft can't fly at the same AoA as the heavier one? The stall warner (the small flap at the leading adge) does not know how heavy the aircraft is, all its know is AoA hence a relative airflow. The warner doesn't care if you fly at 200kts or 20kts. If the AoA is to high the realtive airflow will push the warner's flap up which will close a switch that activates a buzzer or a warning light.

The stall warner does not know the difference. It is sounding in both take offs. Stall warners are binary. On or off (based upon angle of attack). The stall warner can not differentiate anything else.
"does the stall warner (not the stall characteristics) know the amount of peope on board?"
The question does not make that much sense. You might need to ask it in a different way.

Er, you simply restated what I said - that an aircraft at 1.4Vs will have no stall warning/etc at 1'g' but will at 2'g'.

ACDC - the MfS is right I think...

What he's saying is that in the original post, the aircraft is a different weight in both scenarios, but the same IAS. Because of this, it must be true that the AoA is different in both scenarios.

Hence, the warner will operate at a different IAS dependent upon weight, even though it is at the same AoA when it operates. OK, no it doesn't 'know' the weight of the aircraft, but you could surmise whether the aircraft is heavier or lighter based upon the IAS at which the warner operates.

Somebody correct me if that's rot, long day.

No. But at a fixed speed - which was the original question - the heavier aircraft will be at a higher AoA and therefore a stall warner may go off for the heavier aircraft but not for the lighter, if the setting is between the two actual AoAs.

It's true that the stall warning system has no direct weight input, but because it looks at AoA, and AoA is a function of both weight and speed, at a given speed one can say that the stall warning system does take some account of weight - hence I said "know" in quotes.

Isn't it a drag curve question, the 1pob has a much higher nose attitude?
Though I've only ever seen 1100fpm in a zoom climb.

You see, what you need is an AoA indicator; perhaps there should be a thread about that. D'oh!

You've got two:

(1) The stall warner ... except that it's a bit binary (or maybe it has several different sounding squeaks so it's better than binary, but only a bit (or two)).

(2) The stick position ... except that there are so many ifs and buts that it's at best only a very vague indication.

Okay, let's try this way.

The stall warner works by sensing the stagnation point of the oncoming airflow -- where the oncoming air gets brought to a stop by the front of the wing. As the angle of attack increases, this stagnation point moves down and back along the bottom surface -- the flow either side of it (above and below) being away from the stagnation point. When the stagnation point passes over the little metal flap/small hole that makes up the stall warner, the local airflow changes (usually reverses direction) and the stall warner goes off.

Now, the angle of attack is directly proportional to the Coefficient of Lift (generally, within the attached region -- i.e.: pre-stall), which in turn is directly proportional to the Lift the wing's generating. Therefore, using a small amount of maths (and assuming no component of the aircraft's thrust acting in the vertical direction: true for small climb angles which will probably hold for a knackered PA28):

LIFT = WEIGHT
therefore WEIGHT = some constant x (angle of attack) x (Velocity)^2
where some constant ~ rho.(wing area)

therefore: for a lower weight and the same speed, the angle of attack required for steady, unaccelerated* flight is lower, therefore 1 up the stall warner should go off at a lower airspeed than fully loaded...

*and by unaccelerated, I'm including gz-loadings higher than 1...

And yes, stick position would make a nice indicator of alpha and I might be bothered and do the maths tomorrow morning...

I think we all fell for this one hook, line and sinker.
It was even pointed out that the second aircraft was using more power than the first (to keep the other parameters constant).

THE STALL WARNER IS INCORRECTLY SET:

I agree well done ... the 1100 was not relevant or the speed .. its simply airflow not speed and its the angle of attack that will make the stall warner sound, it wont know what weight is in the plane, I did say we were not talking about stalling ....simply the warner.... well done again AC and Miserlou

The PA28 can also suffer transient stall warner activations due to gusts.

I am not sure I understand the question, but the stall warner should work equally accurately (to the extent of detecting the onset of stall) regardless of aircraft loading, airspeed, etc.

The PA28 can also suffer transient stall warner activations due to gusts

Any plane can, and AFAIK it's because the gusts (or more likely wind shear) rob you of airspeed and thus can bring the wing close to stall, hopefully only momentarily. The stall warner itself knows nothing about wind, etc. I've done departures where the ASI is showing 100kt and the next moment the stall warner is going off, with the ASI showing 70kt; 30kt of wind shear.

Although I appreciate what you're saying with regards wind shear.....isn't the real reason due to the AoA of the wing? In other words if you suddelnly meet a strong upward airflow, this brings the relative wind flow over the wing to greater than the max AoA and hence the wing stalls?

An aeroplane can stall at any airspeed, any altitude and any attitude. If you fly along at 100 kts in a 172 and yank the yoke back, the plane will instantly stall. I wouldn't recommend trying this in a 30 year old aeroplane though ;) (accellerated stall)

In other words if you suddelnly meet a strong upward airflow, this brings the relative wind flow over the wing to greater than the max AoA and hence the wing stalls?

I think more likely the other way round.

Let's say you are flying along at 100kt, and you meet a 100kt (+10,000fpm approx) vertical airflow. (Let's ignore for now the fact it will rip your wings off). Your effective airspeed has now risen from 100kt to the vector sum of the two i.e. 141kt. I don't think the wing will stall! If it did, then the wing loading (due to this airflow) would be very small and one could fly through CBs, at any speed, without concern.

A stalled wing has a low wing loading, which is where Va (basically) comes from - at Va, the wing will stall (and unload) at the design limit of 3.8g, thus (supposedly) protecting itself from failure.

I think the thing which might cause the stall warner to go off is downward airflow. But I think this is unlikely in reality; air doesn't usually flow vertically that fast. The momentary stall warner activations we all get come from plain simple gusts, or from wind shear i.e. variations in horizontal wind speed.

englishal

No, if you yank the stick back on a 172 that is in unaccelerated straight and level flight at 100kt, it will instantly climb! (due to inertia and the increased angle of attack of the wing chord to the relative airflow). Depending upon the angle of climb induced, it may subsequently stall or continue to climb at a lower speed. Sorry to be picky...

Actually it will stall. I did it for my FAA CPL training, where we explored the envelope and did all sorts of stalls. The FAA requires all sorts of fun stuff to be demonstrated.....

If you are say at 100kts, or Va for your weight for safety sake, and suddenly yank back on the yoke, the wings stall and the stall warner will go off. This is because you increase AoA to beyond critical, no matter what your airspeed, and the wing stalls. A common misconception is that the aeroplane will only stall at slow speed, however one very valuable piece of info I was taught and had demonstrated is:

An aeroplane can stall at any airspeed, any altitude and any attitude

Back to IO's 100kt vertical airflow :) Wouldn't the vector sum of the two airflows put the relative airflow at 45° (+ or -) to the wing (tan-1 100/100)? Which if so, and the wings remained attached, would put it above the critical AoA of the wing (and hence stall it?)

cheers

It seems strange that nobody has mentioned the possibility of the pitot cover being left on or a bockage of the pitot head and drain hole by ice/insect eggs.
That being the case, the ASI would act in the form of an altimeter and over-read in the climb.
Obviously, in visual conditions the increased nose attitiude would ring alarm bells but this could be extremely dangerous in IMC without the usual visual clues. I would be very wary of assuming that a stall warner is incorrectly set just because the ASI reads what is expected; it is also worth remembering that the stall warner is set to sound well before the stall, not at the point of stall.

englishal

We are splitting hairs here. Of course what you say can be true - it depends how hard you yank and what you mean by "instantly".

If you are say at 100kts, or Va for your weight for safety sake, and suddenly yank back on the yoke

I hope you are doing this in something rented ;) If I have to demo this for my CPL in the TB I will just refuse. Chandelles and lazy eights are as far as I am going. This is despite knowing that a TB20 is much stronger than a PA28; no known in-flight structural failure ever.

You are right about the 45 deg airflow. I can't get my head around all this right now; you may well be right. I think there is a difference between transient and longer term conditions though. The stick, for example, has no effect on the wing AoA - it's only through the action of the elevator, and relatively much later on through a pitch change, that the wing AoA changes.

Yea, a 172. It worred me slightly as the instructor said "I hope the structural integrity of this aeroplane is good" before yanking on the yoke and stalling us. It was luckily

Yank ..beep... about that sort of time :)

Because the airflow has increased by 41%, the lift at constant AoA will double, but the AOA will move from say 3 to 46, way beyond stall. But interestingly developing say .4 lift Coef vs. the 0.2 at cruise (so lift goes up -Coef Lift is about 20% at Aoa = 3, peaks at say AoA =15 then drops to about 30% at AoA=20 and then goes back up at AoA=40 to about 40% - I had to look that one up and it surprised me). But you are now on an elevator adding power (Wind Vertical Velocity * G * M) so to reduce lift sufficiently to remain level the power output of the wing has to be decreased by a factor of 2.8 (in IO'scase) - which means pitching to an AoA of about 1.

IO's situation in English, the wing would be stalled, stall warner would be going off, but due to the increased airspeed and the fact that the coeficient of lift is high at such an extreme AoA, you are generating bucket loads of lift going up (until the wings fall off)

Indeed! Something I had rammed home during my CPL training as well but had also had experience of during my Aeros training.

The following is from this weeks AOPA newsletter and happens to be about ciritical AoA and stalls.

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CRITICAL ANGLE OF ATTACK

Pilots learn that a stall can occur "at any airspeed, in any attitude, with any power setting," as explained in Chapter 4 of the FAA's Airplane Flying Handbook. How well do you understand this concept?

The typical method of practicing and demonstrating straight-and-level stalls and recoveries is to slow the aircraft down in the takeoff or departure configuration until it reaches the minimum controllable airspeed (MCA) depicted on the airspeed indicator, inducing the stall, and recovering. (See the February 10, 2006, Training Tips article "Pre-solo Stalls.") This is a safe way to demonstrate the effects of exceeding the critical angle of attack. But letting the discussion end there carries the risk of fixing in a student's mind the inaccurate notion that exceeding that angle of attack can be prevented simply by flying above minimum controllable airspeed (MCA). That's not so.

Suppose you are on your final approach glide with flaps down, throttle at idle, and maintaining an indicated airspeed of 65 knots, well above MCA for your airplane. Another aircraft suddenly appears below and in front of you. Your first reaction is to haul back sharply on the yoke to avoid collision. Without decelerating to MCA, the aircraft wing exceeds its critical angle of attack and stalls—unexpectedly and at low altitude. Because this is a so-called accelerated stall, the added lift induces a load on the airframe. To understand maneuvering speed's importance to stall avoidance, see Rod Machado's "A New Look at Maneuvering Speed" in the March 1999 AOPA Flight Training.

A glance at your pilot's operating handbook reminds you that stall speeds increase with bank angle. Practicing a level-flight steep turn, or increasing bank to complete a turn in the pattern, you are surprised to hear stall-warning activation at what seems to be a healthy airspeed. Remember that you are flying a maneuver with a high-load factor, meaning that the aircraft wing is producing horizontally inclined lift to turn the aircraft plus the necessary vertical component of lift to regulate altitude. It is flying closer to its critical angle of attack than in unaccelerated level flight.

Managing angle of attack at any airspeed, attitude, or power setting is the key, as the opening words above remind us.

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J.

True or False?
 

We have lost the theme of the original text .. we were talking about the (incorrectly adjusted) stall WARNER. For those who fancy a question on stalling ....is the following statement true or false ? This is from the Jepperson manual for FAA I.R. preparation "The angle of attack at which a wing stalls varies depending on total aircraft weight, bank angle, load factor and airspeed"... answer next week .. now thats clear isn't it? This also answers a previous question by default .. are licence holders arrogant.. I should say so!

Answer - False

flyingphil1

The stalling angle of attack is the stalling angle of attack. The speed at which it is reached depends upon the weight/load factor; I would suggest that you have either mis-quoted the manual or that it is wrong/misleading.
I would also be wary of assuming that just because a stall warner has sounded at a higher IAS than you would expect that it is incorrectly set. Pitot-static errors are covered in most training manuals and may go some way to explaining your example.

How have I got the text answer wrong ? .. I havent told you what the answer is yet!!!.. read the text, dear person ....me arrogant ... mais non !!!