Stall Warning

[skipper91]

Hi all,

Have recently been asked a question that I can't find the answer to anywhere. It is:

Why do we only have one stall warning sensor when we have 2 wings?:confused: I thought it could be to do with one wing stalling first but have been told that is not the case.

If anyone knows the real reason it would be awesome.

[Ultralights]

if one wing stalls first, you will know about it quite quickly.

[Nomde plume]

Stall warning is not designed to sound at the stall AoA, rather in most cases 5-10 knots above the stall speed (actually whatever AoA that corresponds to).
So a stall warning only gives you a 'ballpark' warning that you're approaching the critical AoA, So in theory neither wing should be stalled at the point it sounds.
It's purely a warning device.

[TOUCH-AND-GO]

Ask yourself why is the stall warning located on the left wing?

T&G :ok:

[Ultralights]

and when its designed to sound.....:E

[avconnection]

Some aircraft have a "stall angle" on both wings... but thats because of certification issues.

[Slatye]

On light aircraft, I can see a few reasons:

(1) Cost - installation and maintenance. You'd want them to be calibrated so that they go off at exactly the same angle of attack; otherwise the left one might go off first even though the right wing is actually closer to a stall.

(2) Weight. It isn't much weight, but every kilogram counts.

(3) If one wing is in a stall, the other wing is probably very close to a stall. Not much point warning about a stall on the left wing and then warning about the right wing half a second later.

(4) How to indicate it? The planes I've been in use a light and/or a buzzer. If there are two different warning tones (one for each wing) then that's just going to confuse people. If there are lights, by the time you've actually identified which wing is close to stalling, the other one is probably stalling too.



In bigger aircraft, where the computers might be able to do something sensible even if the pilots can't, there might be a good reason for it - but if the computers are in control then the plane will never get anywhere near a stall on either wing.

[dubbleyew eight]

statye .....nonsense.

in a lightie it is usually on the left wing because we do left turn circuits and the left wing is occasionally slower.

your point 3 WTF!! you would want the stall warning to come on if either wing was about to stall. think about it!

avconnection. the stall angles as you call them are 'trips' to induce stalling on the inboard end of the wing while keeping the ailerons flying.

[Ultralights]

think prop wash, forget about speed, think AOA, get a decent instructor.

[CHAIRMAN]

My 1930's designed Tiger has stall warning indicators on BOTH sides..........they're called slats:D

[Homesick-Angel]

Go out to your local training area, and do some stalls. Really take notice of whats happening (if you have some SCT cloud above even better for vis ref)

Power to idle
Maintain height with steady app of BP

eventually you will have a..

High nose attitude and You will start to gently yaw left (usually stopped with rudder-clouds as a backdrop help to see this)
Low and reducing airspeed
Reduced control effectiveness
5-10kts above the stall-Stall warning
Control buffett around the stall(high wing or low wing will determine exactly when this happens)

CoP moves rearward, nose drops and if you do absolutely nothing the aircraft will try to recover.

In the app config the stall occurs at a lower nose attitude and tends to be stronger stall with the possibility of a WD, and the vast majority of wing drops are to the left.(why?higher power setting, lower airspeed)

I cant find any written evidence as to why the stall is designed on the left wing, but I doubt it is due to left CCTS.

Id say it is more to do with aerodynamic issues approaching and during the stall as most SE AC have a tendency to yaw left at low speed due slipstream/torque (yes it still has an effect at idle particularly at low speed).

A number of AC if they have electric stall warning will share same circuit as pitot heat and or lights(usually on the left as well) Although this seems less of a deciding factor, but then again a lot of twins are the same with the vanes on the left?....

[dubbleyew eight]

I cant find any solid evidence as to why the stall is designed on the left wing, but I doubt it is due to left CCTS.

if you have no evidence why do you doubt it?

[Homesick-Angel]

if you have no evidence why do you doubt it?

Because.. Just as a stab in the dark, I think most airfoil designers would be thinking about Aerodynamics and not circuit direction.

And..Im reading this on PPrune

Im happy to be proven wrong..

Id like to know definitively as well.

[gassed budgie]

Last time I looked the Aerostar that I use didn't have any stall warning devices of any description installed on the airframe. Go figure!
I suppose Mr.Smith (and the FAA) was of the opinion that the airframe itself gave off enough hints that it was about to stop aviating.

[The Green Goblin]

In bigger aircraft, where the computers might be able to do something sensible even if the pilots can't, there might be a good reason for it - but if the computers are in control then the plane will never get anywhere near a stall on either wing.

Probably the dumbest comment on here.

P.S The computers are never in control. They are merely doing what they are told to do by the Pilot.

[dubbleyew eight]

ok
in part of a circuit you are doing a descending turn to the left.
since this is not far from the ground it is important that you dont stall there.
in the descending turn the left wing is flying slower and has a higher angle of attack than the right side wing. thus the left wing is closer to the stall.
so it make sense to place the stall warning on the left wing.

next time you turn finals just remember that the designer did his best to help you avoid stalling in that turn. the rest is up to you.

[fujii]

My stall warning switch is on the right.

[allthecoolnamesarego]

Fujii,

Only RH CCTS for you then!:p:rolleyes:

[Mach E Avelli]

I suspect that in most bugsmashers, the location of the stall sensor is more to do with ease of design, i.e. the wiring and plumbing arrangements for that particular airframe. Simplicity and cheapness are more likely to have been the drivers.

In heavier iron there will be duplicate stall warning systems - one per side. - and all sorts of clever comparitors to prevent false warnings. Many of these sensors are not even on the wing, but are angle of attack 'vanes' situated on each side of the nose.

Which way you normally turn would have little to do with the designer's thought process because he at least knows that the relative speed of the wings in a turn is so small as not to matter. Do the math. On a typical bugsmasher it is so little that it can't be measured with conventional instruments.

As airframes age, stall characteristics can change. I have flown old beat-up aircraft of exactly the same type where in a full-on stall one will drop the left wing first while the other will drop the right wing. One will flick into an incipient spin without notice, the other will stay wings level and buffet and mush in a benign manner.

[Old Akro]

Last time I looked the Aerostar that I use didn't have any stall warning devices of any description installed on the airframe.

The Seneca has two on the left wing. Maybe Piper used them all up by the time they got to the Aerostar?

[Wally Mk2]

As they say there are no dumb questions when it involves aviation especially if yr airborne!:-):)
Part of the original Q says 'we have two wings'......well technically that's correct for say a modern day A/C but essentially there is only one wing just interrupted by the body of the machine. What about a parasol wing? That's just one wing from tip to tip uninterrupted so it could be said for aerodynamic purposes there's only one wing not withstanding the other associated issues with this common arrangement such as parasite drag or interference drag.

Typically having the only stall warning device on the L/H side as for say Cessna means the PIC whom usually sits in the L/H seat can see the probe for a variety of reasons. Obviously a low wing machine doesn't have this feature.

Both wings (if you want to call them that for clarity) in a perfect world would stall at the same time as they are considered as one being equal lift either side of the body but obviously one side of the wing will stall first due all sorts of reasons such as bank angle etc but for simplicity of design in light A/C there's only one stall warning device.
Lots of good answers here.

'GG' the original poster re computer controlled planes is quite correct & that's exactly why modern day planes like the Airbus won't allow under normal law a pilot to exceed the planes set parameters,keeping it from stalling being just one of them, hence that comment computers are in control is essentially correct & not the dumbest Q:).


Wmk2

[john_tullamarine]

FAR 23.207 (http://www.ecfr.gov/cgi-bin/text-idx?c=ecfr&SID=3d7d7315d1083334500c4aefda8c1ec2&rgn=div8&view=text&node=14:1.0.1.3.10.2.65.41&idno=14) provides some useful background.

As always, one needs to read the relevant AC to find out what the rule might actually mean in practical FAA-speak. AC 23-8C indicates -


4. § 23.207 Stall Warning.

a. Explanation.

(1) Purpose. The purpose of this requirement is to ensure an effective warning in sufficient time to allow a pilot to recover from an approach to a stall without reaching the stall.

(2) Types of Warning. The effective warning may be from either aerodynamic disturbances or from a reliable artificial stall-warning device such as a horn or a stick shaker. The aerodynamic warning is usually manifested by a buffet, which vibrates or shakes the airplane. The type of warning should be the same for all configurations.

(3) Artificial Stall Warning. Stall warning devices may be used in cases where there is inadequate aerodynamic warning. The warning signal from the devices should be clear and distinctive and not require the pilot's attention to be directed inside the airplane. A stall warning light by itself is not acceptable. If a stick shaker is installed, the warning should be unmistakable even if flying hands off.

(4) Margin. The stall warning margin, as defined in the rule, is applicable with speed reduced at a rate of one knot per second for §§ 23.201 and 23.203(a)(1).

b. Procedures.

The stall warning tests should be conducted in conjunction with the stall tests required by §§ 23.201 and 23.203.


I'll leave it to those interested to research older versions for any differences applicable to a specific Type/Model Certification.

[Ultralights]

Low and reducing airspeed

5-10kts above the stall-Stall warning


Stall is irrelevant to speed.

[dubbleyew eight]

while the stall is not related to speed, merely angle of attack, the angle of attack that leads to a stall is most often encountered in slow flight.

[john_tullamarine]

Stall is irrelevant to speed

For quasi-steady flight conditions, on the other hand, we all use speed as a useful guide to what's what in the aeroplaning world. Different matter when it starts to get more dynamic ...

[Wally Mk2]

Stall is very relevant to speed seeing as it's the only indicator a pilot has (apart from any aural or buffeting effect) That's why they have a white arc on the speedo, the lower end of which represents the stall speed in a certain flap config for Eg.
AoA is the dynamics of the actual stall (as has been mentioned)with regards to wing shape & any delaying devices used, the leading up to it is only shown to the pilot by the speedo:-)

It is my opinion that the speedo is the single most important inst in a plane, that plus the other important commodity, height of course:)

Good to see healthy discussion here about a killer disease & good ref by 'JT':-0)



Wmk2

[43Inches]

The Pitot and Stall sensors are positioned to allow minimum disturbance from things such as prop wash or other localised airflow.

On large aircraft this is on the forward fuselage.

For aircraft with nose-mounted engines this is not as easy so outboard on a wing is where they go.

Piper must have found that the left wing is less prone to disturbance so it was all packed in that area. Since most of the small Piper twins have build commonality with the single version the sensors were kept in a similar place.

The Navajo has the stall warning fitted to the right wing for something different.

[avconnection]

Stall is irrelevant to speed.

The stall warning can still come on 5-10kts before the stall. The speed at which the aircraft stalls in its current configuration or load is irrelevant but the corresponding angle for the warning activation will still be relative to the current dynamic stall speed. So the statement above is correct and true, the stall warning activates 5-10kts before the stall, for the current flight condition.

[Ultralights]

why can i activate the stall warning at cruise speed while still straight and level, or even in a dive, just by pulling sharply on the stick?

[dubbleyew eight]

you can if the attitude is correct.

I've seen a stall in a cessna 150 with warning blaring at something like 100 knots. ....vertically down after a flick stall.
it was done to demonstrate and prove just that.

[Capt Fathom]

Ultralights, you've got to stop pulling on that stick!

[Ultralights]

what about when inverted over the top of a loop?

[Capt Fathom]

Maybe a gentle push at the top to round it out......:=

[avconnection]

why can i activate the stall warning at cruise speed while still straight and level, or even in a dive, just by pulling sharply on the stick?

I'm really not sure if you're serious or not, but for your benefit: Regardless of the aircrafts speed or configuration, there is still 5-10kts between stall warning and aerodynamic stall. The usage of knots as a unit of measure is to offer something tangible to the person at the controls. To make the statement "the stall warning activates 2 degrees before the stall" is useless in practical applications.

Does this clear the previous statement up?

The fact that reference to speed above aerodynamic stall for pusher activation is mentioned in AFM's of slightly more sophisticated aircraft than a 172 would indicate that it's a reasonable datum measure.

[tecman]

Fuji, my stall warning is also on the right (low) wing. Being a tab/microswitch type, it's vulnerable to all sorts of 'precision' adjustments by well-meaning folk. Currently set very conservatively, but it barely matters: to get into trouble in most flight regimes you'd have to work hard to induce a stall, and then consciously ignore the aerodynamic signs. But I guess it's possible. Stats for RH/LH wing drop (if it happens) on power off stalls are about equal (checked after trim tab adjustments).

Incidentally, I notice the LSA version of the aircraft doesn't have a stall warning - and they don't seem to be dropping from the sky.

At least the tab type switch is checked without the suck test. I see a lot of people sucking vigorously on C172 etc reeds without the obligatory barrier hanky. Obviously don't have spiders like the ones in our hangar!

[training wheels]

In bigger aircraft, where the computers might be able to do something sensible even if the pilots can't, there might be a good reason for it - but if the computers are in control then the plane will never get anywhere near a stall on either wing.

I think that's what the crew of AF447 were thinking as well...

[training wheels]

Stall is irrelevant to speed.

A case of theory vs practice, I believe. Aircraft manufacturer's POH/FCOM all have references to stall speeds, eg, Vso, Vs1, approach speed Vref = 1.3Vs .. etc.

But yet, PPL training manuals will teach that a stall can occur at any speed when the critical angle of attack is exceeded.

[john_tullamarine]

The discussion is going around in aimless circles.

(a) POH information on stall is for quasi steady state - slow reduction of speed (< 1kt/sec) to stall. Whether that be reasonable or not is not for me to say but, at least, it gives repeatable and quantifiable data of use to the certification process. If the aerodynamic warning is inadequate, one needs some artificial warning device.

(b) If you are yanking and banking the devices will respond to inertial and fluctuating airflow effects according to the design and installation. Why is anyone surprised in the slightest ? I'd be more concerned to keep a sensible lid on the yanking bit rather than worrying about spurious stall warning indications.

One can get an accelerated stall (increase/decrease) under g-loading (positive/negative). This should be covered in the principles of flight courses with the usual equation discussions.

.. as well as an unusual animal (usually described as a dynamic stall) where, under quite high pitch rates, the stall generates a short lived vortex which pushes the effective stall angle quite a bit higher than the usual item. Not a well scanned paper - the joys of 1-bit photos - but this (http://www.google.com.au/url?sa=t&rct=j&q=&esrc=s&frm=1&source=web&cd=6&ved=0CEoQFjAF&url=http%3A%2F%2Fwww.dtic.mil%2Fcgi-bin%2FGetTRDoc%3FAD%3DADA234788&ei=ZqxdUtDhIInziAeWi4C4DQ&usg=AFQjCNEHk_xEMnmtZZORVscuCfwP9JLf9w) gives a bit of information for those interested.

[43Inches]

I think that's what the crew of AF447 were thinking as well...

From reading the report it looks as though the flying pilot tried to execute a go-round manoeuvre at one point not understanding that the aircraft has insufficient performance at altitude to power out of a stall.

The reason behind focusing on angle of stall in the early theory is to strengthen the mind in relation as to how to recover from a stall and that is to reduce the angle of attack.

You can't really fly an aircraft through all stages of flight with reference to an angle of attack. It does have its place in efficiencies but as soon as you change configuration, gather ice, dirt, dent a wing etc the angles change, including stall angle.

Using speed to reference the basic max weight level stall in ISA gives a speed datum which a well trained pilot can manipulate to find where the minimum place you want to be is. You can then maintain a speed well above that and be safe.

The stall warning is just that and should go off a few knots at least before you stall. Who cares what angle you are at, reduce the attitude and power up to regain some speed ensures you move away from the stall point.

One point to make is that if you get iced up or other form of wing degredation you may stall at an angle and speed prior to stall warning activation.

Another posible reason for the placement of piper switches and pitot is on the opposite side to the cabin doors. Small Pipers are right side doors, left side sensors, PA31 left side doors right side stall sensor. It may have nothing to do with it but this may stop random bending of vanes etc. Also may offset drag caused by handles and probes, switches etc.

[Wally Mk2]

'JT' it may seem that this subject is going round in circles but I think it's healthy that there are a steady flow of posts on the subject 'cause at the very least it gets some thinking about a subject that has taken many an aviator prematurely possibly thru a little misunderstanding in the first place.
Yr posts are excellent more based on the technical side of things going beyond the original posters query but hey we can never have enuf knowledge about flying as it is a dangerous environment we play in at times:)


Wmk2

[john_tullamarine]

it's healthy that there are a steady flow of posts

.. absolutely, old son. Always a good thing to get folks thinking ..

[MX Trainer]

Ultralights and Spam Cans - as above!!!

Modern designed high speed aircraft - not so much.

Angle of attack indicators, airspeed, and outside air temp fed to computer.

Approaching stall = stick shaker - this is the warning system.

Approaching stall by a lesser margin = stick pusher - this is stall prevention system.

Have tested many of them as the aircraft is trimmed nose up - power back - and watch the airplane do it all by itself. Nice gentle oscillations.

On the other end of the spectrum is the mach trim system. When aircraft speed build up to sensed critical point - before mach tuck will occur - the stick pull will gently cause the aircraft to climb and bleed off speed. Again all by itself. Nice gentle oscillations as the speed bleeds down and the stick pull releases - then aircraft speeds up etc.

So a correctly functioning computer system can actually prevent stalls from taking place if the crew lets it.

Source of information - Bombardier Lear 31 maintenance training manual that I am currently revising my training notes for. One of many aircraft with the above 2 systems.


Mx

[The Green Goblin]

Ultralights,

It's called a dynamic stall. You can stall at any speed if you wish to test Darwin's theory of natural selection.

Wally,

You are completely incorrect re the airbus and normal law. You can easily stall the aircraft in normal law. Alpha floor isn't going to save you.

The inflight protections are there to protect the average aviator having a bad day. There will always be a situation that the aeronautical engineer couldn't fathom.

Next time you're in the sim and have a spare 15 mins reposition to 34 plus at a high gross weight for the level. Introduce an overspeed close to coffin corner. Follow the QRH procedure including speed brake full.

Always an interesting conversation waiting for them to stand down 25 seconds after you watched the gator coming to bite you from below. :)

Airbus protections are not infallible.

[garrya100]

All you need to remember is when you push the stick forward the houses get bigger, when you pull the stick back half way the houses get smaller, and when you pull the stick all the way back the houses get bigger again.. :E

[Ralis]

maybe (at least in the case of Cessna 100 series(and no not all!!!!)) it is placed on the left wing because

1 In testing this was the most reliable place to give an indication. (outboard of the slipstream and prior to the ailerons or outboard sections due to washout.)

It was probably also found that in varying a/c configurations and operating conditions (flap,slat,gear,speed,loading and bank) that the pressure envelope over that portion of the wing stayed adequately consistent with regards to its migration at varying angles of attack thus providing the most reliable information.

2 its a reed type warning and placed such that a pilot flying without a headset (as some used to and still do) has the best chance of hearing the warning.
vane types don't have this concern

3 the weight and plumbing of the warning, pitot tube and fuel vent are on the left to offset the weight of the flap motor on the right. (lets not chicken or egg that one)

4 cause that's where they effing stuck it...... You know how, why, where and when it works.........put some time and thought into preventing it being correct.

Its already been said though COST, WEIGHT, FUNCTIONALITY what more do you need to know for crying out loud.:ugh:

PS number one is the most likely reason for the position of stall warnings

[Ultralights]

You can stall at any speed if you wish to test Darwin's theory of natural selection.

get an aeros endorsement, and learn to not fear the stall, at any speed, and any attitude.

actually, an asymmetric stall while rolling off the top can actually help complete the manoeuvre and score you full points!
interesting stalling while inverted and ending up upright! the hard bit is recovering from the stall when upright and not lose height (actually, recovering from stall and not losing height is the easy bit)

[The Green Goblin]

I do not fear the stall mr ultralight after years of aerobatic flight in my earlier career.

I was merely pointing out you can stall at any speed, if you want to test natural selection. I.e excess g loads, wings breaking off etc etc.

At least you won't be able to report your own crash on pprune!

[Ultralights]

ill be happy if XXX investigates! :E

[Homesick-Angel]

Post number 50 in this thread. Who woulda thought.

Stall is irrelevant to speed.

Yep it is, but show me an AoA indicator in a 152 or pretty much any light trainer?

The stall speed is generally tested and decided for a particular AC at MTOW, most forward CoG,ISA, Power at idle etc and as someone else mentioned, the aircraft is usually brand new.So older aircraft are generallt less predictable. That fella had obviously flown some of the heaps of sh1t I have.

The stall can occur at any speed, if approx 16º AoA is exceeded (you could be pointing at the sky or the ground and exceed this critical angle). Thats why Ultralights can get the stall warning to go off at high speed (remind me not to fly with you after a spag bol.)


Heres another coupla simple questions for the gallery:
1-Why does CoG effect stall speed.
2-Why does the stall speed increase so much in a steep turn?
3 Why is there a difference in the buffet (when it happens) between high and low wing.

[FokkerInYour12]

Angle of attack

Angle of attack is only measurable during refuelling operations. A slightly high angle results in a dribble. There is no need to report this to either your colleagues or on your Maintenance Release. However, too much angle results in significant spillage. CASA Penalty for large angle of attack: One round of refuelling for all pilots within your direct and peripheral vision.

The term can also refer to operations after refuelling. After significant refuelling operations you may involuntarily experience psychsomatic symptoms. To counteract this, most pilots involuntarily deploy their an otherwise ineffective protective device called beer goggles. Such pilots may think that their angle of attack is impressive but they are typically subject to visual illusions, especially if operating night VFR (some even attempt this IFR). From time-to-time, after-refuelling angle of attack can, surprisingly, operate within the designated operational envelope. A recently published CAAP recommends that you if you are in this operational envelope you should open it, especially if there is a French communication within.

Stall warning

Related to the refuelling operations above, it can either mean: a) A failure in the use of radio communication to attract someone previously positively radar identified through the "beer goggles"; or b) An imminent pump failure. You may need to initiate the boost pump and/or add additional type certified lubricant to rectify this situation.

A CASA-approved alternative means of compliance is also available in this situation. You should disregard the angle (of attack) of the dangle and ensure that the motion of ocean has been calibrated through non-destructive testing.

[Check_Thrust]

Heres another coupla simple questions for the gallery:
1-Why does CoG effect stall speed.
2-Why does the stall speed increase so much in a steep turn?
3 Why is there a difference in the buffet (when it happens) between high and low wing.

1. The position of CoG effects the amount of lift required to be generated for flight, this lift has to overcome both the weight of the aircraft and the downward force generated by the tail plane (to prevent the nose from dropping in normal flight). The further forward the CoG a greater amount of downward force generated by the tail plane therefore more lift is required at the same speed resulting in a higher AoA at that speed and vice versa for a more reward CoG.

2. Stall speed increases in a steep turn due to an increase in load factor, this increased load factor is caused by the need to generate more lift due to the lift vector not acting in the directly opposite direction to the weight vector (therefore increasing the aircraft's effective weight). The formula for calculating the effect of load factor on stall speed is "New Vs = Old Vs x Square Root of the Load Factor". A 60°AoB turn results in a 2g load factor which translates to a 41.4% increase in the straight and level flight stall speed.

3. Buffet occurs due to the disturbed/turbulent airflow seperating from the main wing and traveling over the tail plane whilst approaching and during the stall. Buffet tends to occur earlier and be more pronunced on low wing aircraft than high wing aircraft because this airflow seperation tends to miss the tail section on high wing aircraft.

[T28D]

Fokker in your 12 Thank you, 1st rational post I have seen on this waste of time thread.

[Mach E Avelli]

T28 it is only a waste of time for those smart enough to already know everything about stalling (and that's not me - I did get some new info out of it).
The OP is clearly a newbie who was asked a question but could not find an answer. Now, if he has been paying attention, he has quite a few credible answers to offer to his inquisitor. And hopefully enough knowledge to set his inquisitor right if he is full of sh!t. As are so many instructors, it seems.
Unfortunately, some wise-guys here proffered light-hearted (at least I hope it was) comment about the left wing being closer to the ground in a stall and how that must be avoided at all costs, how the down-going wing always stalls first (really?), cows getting bigger then smaller then bigger etc.
When justified, I am all for taking the p!ss, but spare genuine newbies with legitimate questions, please.

[Jack Ranga]

I've seen lots of vids of doods who don't fear the stall plowing in........from a stall. I did a loop in an aircraft, 280knots over the back and felt the stick shake, we were doing more than twice the 'stall speed'

[T28D]

Surely if a newbie was under instruction the person to answer the question would be his/her instructor.

That way the nonsense about wing position would be avoided and the whole issue around A of A would be answered properly not nonsense about left wings and left hand circuits, spare me !!!!!!!!!