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## CL AOA stall angle

• Aj114
• Topic Author

### Aj114 created the topic: CL AOA stall angle

Hi All
Regarding to critical angle of attack and stall...
For a given aerofoil... does the stalling angle change (provided flaps are up) ofcourse...
I know the stallin speed changes and is affected by many things sucb as weight, load factor, etx etc etc. But does the stalling angle given for a specific aerofoil change? Or is it fixed and once exceeded it will stall? Is it true in regards to larger jet aircraft?
The graph CL over AOA is given to a specific aerofoil and thus aircraft type ... but does that mean it is at that one speed ? Is that graph true for cruising speed? Or thru out the whole speed for that aircraft and weight...
I was told because CL is affected by speed and Reynolds number the stalling angle cannot be fixed for a specific aerofoil thus it is variable....

• Posts: 168

### Carello replied the topic: CL AOA stall angle

I think that the graph below will answer some of your questions

For a given wing, the stall angle will depend on what is hanging off it - trailing edge flaps, leading edge flap, slats etc. In a nutshell, any device that changes the basic shape of the wing will change the stall angle of attack.

That being said, the graph is independent of speed as CL is the unit of lift per unit of dynamic pressure (speed) - so for a particular wing and configuration the stall angle is fixed.

In reference to the Reynolds Number this is of little interest to a pilot - perhaps someone versed in fluid dynamics can answer this part of the question.
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• John.Heddles
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• ATPL/consulting aero engineer
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### John.Heddles replied the topic: CL AOA stall angle

As with training environments generally, the story presented is simplified to an extent appropriate to need. Stalling is no different.

For a given aerofoil... does the stalling angle change (provided flaps are up) of course...

Well, yes and no.

The basic training story ignores both Re (Reynolds Number) and M (Mach Number), both of which have a significant effect on CL .. but not to any significant extent at low speed and altitudes where basic training occurs in the civil environment.

Similarly, one needs to be cautious in the interpretation of the usual wing section CL properties typically seen in textbooks .. these are basic engineering starting points but relate to wind tunnel work with wing sections but not the whole aeroplane. Stall characteristics for the aeroplane will be influenced considerably by tail loads and fuselage effects.

Further, the typical POH stall speed relates to the relevant Design Standard certification stall .. an animal which doesn't get seen much in the real world albeit it is the starting point for a lot of speeds which we do use routinely in line operations. The real world stall is very much affected by pitch rate, yaw inputs, etc. The actual incidence angles achieved at stall can vary to some extent as a result. There is a particular phenomenon associated with very high pitch rates where the formation of a spanwise vortex sees the stall angle go to quite a high, if short-lived value ... not something you will see on the typical bugsmasher but, certainly, something of interest to the rotary wing folks.

Is it true in regards to larger jet aircraft?

Apart from Mach effects on CL, in particular, a stall is a stall and one sees a degree of consistency in the basic characteristics for a Type-Certificated aeroplane flown in a certification manner.

The graph CL over AOA is given to a specific aerofoil and thus aircraft type

The CL characteristic curve relates to the the wing section. A particular wing section may be seen on various aeroplanes. As indicated previously, once the wing section is incorporated into the whole aeroplane animal, things change a bit although the basics are still much the same, if not the specific numbers.

Is that graph true for cruising speed?

The typical section graph relates to very controlled, steady state, wind tunnel work. With that caveat, and sensible pitch rates, the main effect of speed will be to demand a different body angle to achieve the required CL for the particular stage of flight.

I was told because CL is affected by speed and Reynolds number the stalling angle cannot be fixed for a specific aerofoil thus it is variable...

Perhaps your story teller referred to Mach Number, rather than speed ? Two different things. If you are interested in Re and M effects, there is plenty of useful information on the net via the services of Mr Google et al. As indicated previously, for low altitude, low speed flight, Re and M have very little effect and can be ignored, as they are for light aircraft.

There is some more of the story in this thread - www.bobtait.com.au/forum/rpl-ppl/5022-different-types-of-stalls

Engineering specialist in aircraft performance and weight control.
The following user(s) said Thank You: Aj114

• Aj114
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### Aj114 replied the topic: CL AOA stall angle

So, it is incorrect to say that a specific airfoil will stall at that particular aoa. Because there are many factors involved. Yet text books keep it simple for us and expand on stalling speed rather than stalling angle... pretty much that cl, aoa curve is pretty much useless for the pilot. But great to show us what happens when flaps are applied-the stalling angle reduces so does the stalling speed ... and the guage- aoa indicator today they are installing on our aircrafts and already installed on certain jet aircrafts is not an absolute reading rather just an idea or a precaution not to get you into a stall.. ?
Does this all relate to the thin airfoil theory?
I am still confused as to why in textbooks they speak alot about stalling speed and not stalling angle.
I was suprised when i was giving theory to a class whom are all engineers learning to fly, until one said how can it stall at a specific aoa and not be affected by other factors..
Ive been taught that for a specific aerofoil there is a stalling angle and once exceeded the wing will decline in cl lift. And ive taken it for granted... expaning on it saying if we were to fly s+l, and slow down the aircraft... i will to a point where i reach my stall speed after than the plane will no longer support the weight and thus stall . Without now going into the details of the wing and boundary layer with seperation point and all that, ive always though that the angle was constant for stalling... once i reached lets say 16degrees aoa it will stall. The reason that doesnt happen when im flying fast lets say cruising at 200 kts i lower the aoa to maintain s+l really im just moving the airfoil where the relative airflow is hitting upon by minimising its frontal exposure to that airstream ofcourse any sudden manoeuvre such as pulling back hard on the elevator will make me exceed that angle because inertia will tend to keep me going on that path until the aircraft reacts and takes me to a climb. So pulling back hard without giving time for inertia will just make me exceed that stalling angle and thus i go down even though my nose is up.
That was where my confusion was thinking that stalling aoa is fixed to that specified angle given to that particular airfoil.

• John.Heddles
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### John.Heddles replied the topic: CL AOA stall angle

So, it is incorrect to say that a specific airfoil will stall at that particular aoa.

Not quite, you need to distinguish between the (wing) section properties and the real-world situation with an aeroplane. However, with a few caveats, it is fine to teach the idea that stalling angle is constant.

The section graphs you are looking at will be very repeatable test values in a wind tunnel environment. In the real-world, providing the pitch rate is low and yaw is well-controlled, the aeroplane will provide a similar sort of stalling result. More particularly, you should see a repeatable stall situation if the conditions for any test are sensibly repeatable. The extent to which the numbers reflect the section data is not all that important to the pilot. Rather, you need to appreciate that the aeroplane will produce something similar in shape to the section properties graph.

The main problem in the real world situation is the extent to which the pilot is able to establish repeatable conditions for any stall comparison with the main problem being pitch rate (ie g loading)

text books keep it simple for us and expand on stalling speed rather than stalling angle

That’s a fair comment.

The consideration with speed rather than angle is that, traditionally, the pilot has not had an accurate angle measure presentation in the cockpit as the airflow direction is neither observable nor presented on a gauge.

A different matter for the military folk, especially Naval operators, where the aeroplane is operated much closer to the stall environment than is the case in civil operations.

For the more sophisticated civil aeroplanes, AoA certainly is measured in the aeroplane systems .. whether or not that information is presented to the pilot, and in what form, is an OEM matter as there is no specific regulatory requirement to do so.

What we do have, though, is the good old ASI. Providing conditions are controlled, the ASI is a pretty good indicator of the proximity to the stall. Add buffet and so on to this and the pilot has a reasonable means of appreciating where the aeroplane is in respect of the stall condition.

cl, aoa curve is pretty much useless for the pilot.

Not at all the case. Just don’t try to read across from the section properties to the real-world aeroplane too literally. Certainly, the aeroplane displays characteristics similar to the section properties data and that is the value for the pilot’s understanding.

Does this all relate to the thin airfoil theory?

I suggest that engineering theory considerations probably are not all that necessary for the pilot to have detailed knowledge.

I am still confused as to why in textbooks they speak alot about stalling speed and not stalling angle.

The pilot texts for typical aeroplanes talk speed as that is what the pilot has to work with .. not angles. If the cockpit presentation includes explicit AoA information, then one certainly can work with that in lieu of the ASI although, as I understand things, the better result is to use both.

I was suprised … not be affected by other factors..

If you are restricting the discussion to section properties, that is a reasonable position.

Ive been taught that for a specific aerofoil there is a stalling angle and once exceeded the wing will decline in cl lift.

Correct for the section properties and fair enough for the aeroplane providing that the conditions are kept repeatable .. in the real-world that remains the difficulty.

if we were to fly s+l, and slow down the aircraft... i will to a point where i reach my stall speed after than the plane will no longer support the weight and thus stall

That’s correct.

The reason that doesnt happen when im flying fast lets say cruising at 200 kts i lower the aoa to maintain s+l really im just moving the airfoil where the relative airflow is hitting upon by minimising its frontal exposure to that airstream

Not really. Things to keep in mind –

(a) look at the lift equation and you see that, as speed increases, CL has to decrease to keep the lift constant. The section properties graph (and the similar graph for the aeroplane) tells you that you need to lower the nose to lower the angle of attack to get to a lower CL.

(b) Keep in mind that the basic low level, low speed CL equation ignores Re and M, both of which, especially M, have to be included for lift calculations involving jets at height.

pulling back hard without giving time for inertia will just make me exceed that stalling angle and thus i go down even though my nose is up.

More importantly, that higher pitch rate approach to the stall will provide you with quite different stall handling characteristics .. one needs to be very wary of pitch rate control and stalling.

That was where my confusion was thinking that stalling aoa is fixed to that specified angle given to that particular airfoil.

Only a matter of needing to keep the whole picture in mind rather than just the very controlled section properties data. Certainly, for routine, gentle manoeuvring, there is nothing wrong with the idea that the aeroplane will stall at the same AoA.

Engineering specialist in aircraft performance and weight control.
The following user(s) said Thank You: Aj114

• Aj114
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### Aj114 replied the topic: CL AOA stall angle

Thankyou for your thorough explanation i really appreciated your response, now... 1 last thing...
The angle for Best L/D ratio is given to us as pilots as a speed.... does that angle also differ? This is important because i intend to have my glide speed set to that specific angle? Glide speed changes with weight for example but does my glide angle change ? Is my glide angle fixed for a specified aerofoil? Best aoa for l/d ratio

• John.Heddles
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### John.Heddles replied the topic: CL AOA stall angle

The angle for Best L/D ratio is given to us as pilots as a speed....

Again, the pilot has a speed indication (ASI), but no angle indication. Speed is far more useful as a result.

does that angle also differ?

A similar argument follows to the previous post. The best glide angle will be at maximum L/D (or CL/CD, if you prefer)

This is important because i intend to have my glide speed set to that specific angle?

For practical purposes, with reasonably steady motion, you can use speed in lieu of a specific knowledge of AoA.

Glide speed changes with weight for example but does my glide angle change ?

Speed changes to keep the sums in order. Some analysis here if you wish .. www.dept.aoe.vt.edu/~lutze/AOE3104/glidingflight.pdf

Is my glide angle fixed for a specified aerofoil? Best aoa for l/d ratio

As a general observation, yes, for the best glide performance.

Engineering specialist in aircraft performance and weight control.