Stall Warner
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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!
- none of the four PA28 161's I've flown will!!!
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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)
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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
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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.
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.
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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 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
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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.
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"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.
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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 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
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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.
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.
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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.
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.
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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!
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!
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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.
(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.
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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...
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...
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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:
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:
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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
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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.
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.
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rob you of airspeed
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)