Critical angle of attack and stall

[dimsp81]

I am confused.Again.

According to theory,the critical angle of attack it the angle of attack:
1)where the CL (coefficient of lift) reaches its maximum value,and beyond which it will decrease
2)beyond which the separation point will syddenly move well forward
3)beyond which the center of pressure will suddenly move rearward
4)where the aircraft stalls.

We know that the aircraft stalls when the boundary layer has mostly separated from the wing (breakdown of streamline flow above wing) and there is no sufficient lift.So,due to decreased lift,it will start to sink.

But,the theory says that stall happens at the critical or stalling angle of attack where (as I wrote above) the CL is at its maximum value,and the lift produced at that moment can counteract weight.

So,stall happens at the critical angle of attack of shortly after?

Can anyone help me?

Cheers!

[Radix]

............

[dimsp81]

@Radix
Thanks for your answer!

As far as 3) is concerned: you are right,I didn't explain it correctly.I meant that when we have CLmax,we can produce as much lift to counteract weight,and I refer only to 1G straight and level flight.

Thanks again!

[gums]

@dims

I am not sure where you are getting this "critical angle of attack" term from.

I highly recommend a visit to the library and see some books. Of course, Wiki is likely to have good stuff on aero or some links.

You can "stall" at damned near any speed, gross weight, gee, bank angle - there must be "fifty ways". It all comes down to one thing - AoA. The classic graphic of AoA and Cl seems out dated to this old fart. The older, straight wing and large camber wings behave like that. The swept wings and airfoils since the 60's do not. You can fly well beyond the peak of the Cl point on the graph and not "stall". On some planes ( like deltas - think Concorde), you never lose a lotta roll authority or directional authority, but develop a high sink rate. Call it stall, but simply lowering the nose will regain "normal" flying characteristics real quick.

Go to the library and then come back.

[laz219]

Critical Angle of Attack is referenced in nearly all my textbooks (usually as approx 16 degrees) and described as the point Coefficient of Lift is at it's maximum- Exceed the 'critical angle' and the lift begins to reduce.

Have a look at the lift graphs in the book, you'll see the graph progressively rising, peaking at the critical angle and then falling off sharply from there.

[gums]

@ Laz

O.K., "critical angle of attack" seems to be the max AoA before lift begins to reduce according to you and Dims and whatever reference you guys are using.. Oooops, not lift, but Cl.

The newer wings have a lot more gentle curve up near max Cl. The old graphics with the sharp break are not there except for very old airfolil designs, most likely used by general aviation planes. For example, some newer planes reach the max Cl at "x" degrees, but do not get a lot lower Cl for 5 or 10 degrees more AoA. They just have more drag and sink.

The Viper I flew could get up to 25 degrees AoA with great performance and not a lot of induced drag. Leading edge flaps and the "flaperons" on the trailing edge changed the camber of the wing. So I would not place a firm value on AoA for the "critical AoA'", like the 16 degree number. And as I asserted, the deltas had a peak Cl on the graph, but simply started to sink when you got a lot higher AoA. Some planes can not even fly with AoA above 10 degrees or less without stalling. All depends on the airfoil used.

[hawk37]

A bit off topic but I was wondering, when an airfoil is at the aoa for max CL (critical aoa term used here in this thread), can we expect to already see some flow separation on the upper surface of the wing? Or is it generally the case that the flow is still completely attached, in order to get this maximum CL value.

anybody?

[BOAC]

I'm pretty sure that the flow is NEVER 'completely attached', so we are talking levels of 'separation'.'.

[laz219]

Gums- That is straight out of my BAK and CPL aerodynamics book, so I guess that is what they are teaching these days.

[henra]

I'm pretty sure that the flow is NEVER 'completely attached', so we are talking levels of 'separation'.'.
Exactly.
Except maybe for an ideal laminar flow wing. This exists only on paper though.
In reality it is pretty safe to assume you are correct.

The polars @dimsp81 is probably referring to are usually for an infinite and ideal wing without twist and taper, sweep, slats and other geometrical properties or also physical properties like rivets or dents or other imperfections of a real wing which are significantly influencing separation behaviour.
So this critical AoA thing is nice but to which section in which condition of a real wing does it refer?
Due to these mentioned geometrical features the separation will be a progressive process with increasing AoA but starting at very low AoA.

[hawk37]

OK, flow never completely attached, I get that. any guesses on how far back it may separate for a generic type airliner wing at max CL aoa?

I'm just thinking, if it was say at approximately the 3/4 chord point, ie 3/4 the way back approximately, that would mean the wing was getting it's max CL value with the air only attached for 3/4 of the area or so. That would be an interesting thought to me.

Maybe that 3/4 is too far forward. But is separation at 99% chord too far back for the max CL point?

[FE Hoppy]

Air Flow and Flow Separation - YouTube

[dimsp81]

I am referring to a tupical finite wing,only for educational ppl purposes.I am not interested for a special case of wing.

Sorry If I confused you with the critical angle term.It is a term that trevor thom uses in his books.Just call it stalling or stall angle of attack,and it is usually 16 degrees.

[john_tullamarine]

A problem here is the tendency to want to make things black and white when the real world is many shades of grey.

A few thoughts ..

When we refer to a stalling angle of around 16deg, that is appropriate for an older, traditional aerofoil profile and is the typical dumbed down story for new pilots.

As with any form of training or information transfer, the detail level needs to be appropriate to the needs and comprehension of the trainee. For the new chum, the story needs to be simplistic until the basic ideas are comprehended and then the student can be introduced to more complex and detailed stories about what might be happening.

Doesn't matter whether we are talking about Scouts or similar learning the absolute basics for a badge of some sort through to trainee pilots then progressing to airline folk or, for that matter, first aiders progressing over a period of time to become brain surgeons .. same principle applies.

Keep in mind that any of the nice pictures you see will be generated from (typically tunnel) data for quasi-steady flow states. Once you get out into the real world it starts to get a little muddied.

It is a term that trevor thom uses in his books

Critical angle is fine to think about .. whatever it might be for a particular aerofoil section .. somewhat below that and things are comfortable .. progressing to more interesting things as you increase to higher angles. Please don't suggest that 16 deg is the number for all wings .. that's nonsense. The basic textbook Cl by alpha characteristic is sort of typical but varies a LOT according to the profile and, also, pitch rate (especially for helicopters).

I haven't spoken to Trev for many years now .. he did a lot of good work in the original development of his books (with a bit of help from a few of us along the way). Flew with him a few times in years long gone by ... thoroughly nice chap. I would imagine that the present business, run by Dave Robson, still is a purveyor of fine books albeit, I imagine, with a bit more of an emphasis on technical rigour. Flew with Dave only the once and that, many, many years ago.

[Wizofoz]

Also just to point out that this-

As far as 3) is concerned: you are right,I didn't explain it correctly.I meant that when we have CLmax,we can produce as much lift to counteract weight,and I refer only to 1G straight and level flight.


Isn't necessarily so. Post stall, you still have a Cl, so you can still generate Lift that equals weight, if you have sufficient speed.

[henra]

A problem here is the tendency to want to make things black and white when the real world is many shades of grey.

A few thoughts ..

When we refer to a stalling angle of around 16deg, that is appropriate for an older, traditional aerofoil profile and is the typical dumbed down story for new pilots.


Exactly.

Knowldege of the different AoA at which Clmax occurs depending on the exact profile chosen is even quite old for that matter. The good old NACA Report 460 dating back to the thirties shows nicely how different the chrarcteristic of Cl can be around clmax and at what a wide range of AoA this Clmax can occur.
And those are all only old NACA 4 digit profiles.
http://naca.central.cranfield.ac.uk/reports/1933/naca-report-460.pdf

And it doesn't include Leading Edge devices or other geometrical properties of a real world wing which can enourmously change Alpha(Clmax).
So even for a theoretical infinite wing in a wind tunnel a statement that a wing stalls at 16° AoA is nonsense.
It might be helpful as a rule of thumb but in reality even that is not worth much.
If the Buffeting starts, it might be wise to stop pulling, even if your AoA gauge only shows 14°.

[gums]

I am glad to see JT and Henra post here.

I apologize to Dims for suggesting he visit the library. It was crass.

The best thing I saw was Henra's warning about "feeling" the buffet. And then JT's observation that a specific AoA may or may not define max Cl or a stall.

I was personally blessed to learn in Aeronica's, Luscombs and Taylorcraft. Then to USAF, and Tweets and the T-33, and then a mix of bent wings and straight wings..... The big thing in all of them was "touch" or "feel". The plane talks to you, and those who do not listen or cannot listen will pay the price, regardless of what the indicators display or the control system is designed for. See the AF447 threads.

Good thread, IMHO.

[Jetblu]

+1

Brilliant thread. Learnt a lot, thanks. :ok:

[Love_joy]

Just chipping in with my 2c...

There is a lot of talk of AoA, specific angles, and initiation of stall recovery here. The question is, are we discussing stall of an aerofoil or an aircraft?

As above, a fairly standard aerofoil section will stall at or around 16 degrees AoA, this is repeatable and well understood.

Once the aerofoil is attached to an aircraft, and that aircraft is in motion all factors change. Its also of paramount importance to understand that the AoA of the wing, is completely different to the aircraft 'pitch', they are two mutually exclusive states.

Once you suss out the concept of the Relative Air Flow, you'll find all this to be much more clear. Best of luck!

[barit1]

gums:The big thing in all of them was "touch" or "feel". The plane talks to you, and those who do not listen or cannot listen will pay the price, regardless of what the indicators display or the control system is designed for. See the AF447 threads.

Gee, I would agree in principle, but even minor cross-control (uncoordinated flight) could rob the pilot of a warning buffet. I learned that lesson in a few memorable seconds in an old PT-22, fifty years ago. My instructor asked for a stall with about 1/3 rudder, and we did the prettiest snap roll :eek: with NO buffet warning, only a knot or two above Vso.

[gums]

Good point, Barit. And Love had another one.

One of the things I learned from my instructors ( over 50 years ago, same as you) was the "feel" when "close" to that arena where you could snap roll with a lotta rudder and maybe a reasonable amount of back stick. Never saw one when at a fairly low AoA, but the bent wings I flew wouldn't do it before adverse yaw kicked in and such, plus a lotta shaking/burble.

Love's point about wind tunnel tests of wings are great, and when wings are mounted on an airplane, things change. Just look at the Viper and Hornet AoA stuff ( F-16 and F-18). The vortex "lift" kept the air flowing over the wings longer than planes with no leading-edge extensions/strakes. Prolly also got some lift from the fuselage. However, we're talking about 20 - 30 degrees or more AoA, and the average general aviation dude or airline pilot would never see that part of the "envelope".

My main point is that during training you must fly the plane close to the limits and try to get the "feel" just before the plane goes outta control. Many of the heavies can't allow this during training, and I do not know what can be done about it.

Some folks talk about "zero" AoA to gain energy and recover from a stall. Prolly true, but the certain method is zero gee, which you can feel without any stupid indicator. Don't think the SLF's would like that procedure, but it's damned hard to stall if the plane is not producing lift, ya think?

[laz219]

Been some really interesting replies in this thread, as I'm currently studying aerodynamics it's great to read all the bits about 'how things work in the real world' rather than the perfect examples they seem to use in the textbooks.

[roulishollandais]

1. Thank you,Henra for that link to old documents (1930')
http://naca.central.cranfield.ac.uk/...report-460.p

2. Do you know or does anybody knows where Prandtl's closed curves can be found on the web : lift and drag on the two axis as mesured with dynamometers, for the whole range of angle of attack (-180,+180°) at constant speed ?
I had these precious books for all the studied profiles, but they have been stolen... :(
On these curves it is easy to understand that stall is not so wild, but aircraft stability and changing speed may do (or not do ) it wild.

3. We often read posts opposing flying speed and flying gee, or Cl max and deplacement of center of lift, as coming from different paradigms. Thanks to Gums who is reminding or teaching us how gravity feeling, hence gee is an accurate mean to pilot speed, stall,aso for well taught pilots with aerobatics :Nz is the derivate of the speed, so it tells us the speed tendance, it previews speed modification, so the best way to pilot speed uses gee !

Similar math reading of cl max does not emphasizes the fact that cl is maximum at that point, but that left of that point lift increases with attitude, and starts to decrease with attitude passing at the right size of that point.

It is mandatory to the airline pilot to understand the tendance (mathematic derivation) to preview the flight. Hiding how FBW ( in normal law) elevator is a math integration of the side stick position or pressure, and Airbus THS is a math integration of elevator are pedagogic holes, with heavy issues for flight safety. The requested level of math is elementary math. Piloting anticipates the plane 's movement, you need to know the tendance.

[Peter H]

roulishollandais
Do you know or does anybody knows where Prandtl's closed curves can be found on the web?
... I had these precious books for all the studied profiles, but they have been stolen

Sounds interesting, what were the books called?

[roulishollandais]

@Peter H, others
I no more remember the tittle of the books.
They are written in German, the tittle could be something like General aerodynamic works (Generale aerodynamischen Studien ???)
As the author is now well known some German University have probably the original books recovered with dust.Perhaps the name of Prandtl was only inside. The books were green middle sized, , perhaps 100-200 pages for each book.
I already searched on the web some of the curves, but without success. I did not tried to find books again. Mine were in the library from my ornthologist oncle and I was surprised to find them after he died. Nobody in the family was interested,they seemed not modern science ! For me it was a treasure to teach my new pilots.
Very pedagogic works for today pilots.. and for aviation administration, BEA, etc. who seem to have very raw and low documentation about stall (AF447 final report !!)

Decade after decade the knowledge taught in the books was poorer, often false in french litterature, included to teach airline pilots and ENAC APTL preparation... And finaly really ignorant airline pilots diving in the Ocean. :ugh:

[Peter H]

roulishollandais, you said books. Are we looking for several books, or several volumes belonging to a single title?
How many books were there?

FYI a world-wide library search for books in German written by L Prandtl can be found at:
Results for 'au:L Prandtl' > 'German' [WorldCat.org] (http://www.worldcat.org/search?q=au%3AL+Prandtl&fq=ln%3Ager&qt=advanced&dblist=638)
Do any of these titles trigger your memory?

Peter H

From the limited information so far -- and not having any German -- I'll just observe that Prandtl published a multi-volume work in German called
Ergebnisse der Aerodynamischen Versuchsanstalt zu Göttingen.
... which does seem to have the green covers you mention.

Text from [all?] four volumes can be found at:
http://tinyurl.com/kgk3c6j
http://tinyurl.com/ljws7un
http://tinyurl.com/kubffpc
http://tinyurl.com/lhl7v4r

... is this of any interest?

[roulishollandais]

@Peter H
Thank you very much ! :D
My books effectively are
"Ergebnisse der Aerodynamischen Versuchsanstalt zu Göttingen"
Different "Lieferung" !
Congratulations for all these references I will read again 30 years after I discovered them (I was only begining to feel me allowed to have a critical analysis of official aerodynamics theory)
Without surprise I already found something wrong :}we discussed in the Thread about Lift !
We have to sort what is worth from these old studies who were forgiven with dispersion of german scientists : someone had to fly away nazism, others have been made prisoners and worked on other subjects.

[Peter H]

roulishollandais,

A pleasure to re-unite you. I'll also mention a couple of web sites
that you might find of interest.

The open-acess archives of the university of Goettingen:
Open Access (http://www.sub.uni-goettingen.de/elektronisches-publizieren/open-access/)

The vintage gliders bookcases of Scale Soaring UK:
Scale Soaring UK - Book Case 2 (http://www.scalesoaring.co.uk/VINTAGE/Books/BookCase2.html)

[WeekendFlyer]

Sometimes it helps to use the equations:

Lift = nW = 0.5 * density * wing area * speed^2 * CL

where n = load factor (number of g) and W = weight = mass x gravitational acceleration

Put simply, to sustain flight at a given load factor CLmax determines the minimum speed you must have. In the equation above you can use either TAS with actual air density or EAS with sea-level density. The relationship between speed and g is thus:

n proportional to V^2.

This is plotted on a V-n diagram; google for one if you need reminding!

CLmax is a factor of AOA and wing configuration, nothing else. It is the maximum CL the wing can produce and it occurs at the critical AOA specific to each configuration (i.e. flap and slat settings). However, what happens if you exceed the critical AOA varies massively depending on the aircraft: most GA aircraft will just pitch down a bit or "porpoise", perhaps with a bit of wing drop. Deltas and highly swept wing aircraft tend to increase sink rate and not much else. Many twin propeller aircraft and some swept wing jets will flick-roll on to their back and try to kill you, hence the need for a stick-pusher. But the common factor is that for every aircraft there is an AOA above which no more CL is available, so to sustain non-stalled flight you must reduce AOA.

CL vs AOA graphs can vary hugely with wing shape, even for an identical wing section, due to complexities such as spanwise flow. However, in simple terms the critical AOA is the point at which there is a reversal of the gradient of CL with AOA. On some aircraft the change is gentle, on some it is severe. The worst I know of is the F104 Starfighter, where the reduction of CL was extremely rapid above the critical AOA, earning the aircraft the nickname of "widowmaker" because a stall during the finals turn was often fatal.

When looking at the whole aircraft, remember that if it is longitudinally stable in pitch without a FBW computer helping you, then there is a down-force on the horizontal stabiliser. Thus for >1g flight the down-force will increase, needing greater CL from the wing to achieve a given load factor than if there was no tailplane. To avoid the pilot having to think about this, stall is determined by AOA. After all, even simple GA aircraft stall warners are activated by AOA exceeding a set angle, thus moving the vane on the leading edge.

Hope this helps, or at leasts adds to the discussion!

[roulishollandais]

@Peter H
Thank you for these two very inteesting new links. :)
The open-acess archives of the university of Goettingen: Open Access
The vintage gliders book cases of Scale Soaring UK: Scale Soaring UK - Book Case2

They are both in that open access/sharing/published information movement ICAO choosed to increase Flying skill and freedom of trade.

Since some decades someone would like to do secrets with these informations. Sometimes patent Offices accept to privatize already public knowledge and traditional culture or names...

I hope that in the future ICAO after WWII spirit will be stronger again, helped by the web and communities like PPRuNe against today's tendancy to hide information : CVR after crash, algorithms of flight laws, name of report writer ; the latter shows that such secrets don't protect writer's/searcher's/inventor's rights. The real State and Defence Secrets are better protected when comunication rules are not the result of obscurity and tiranny.
Civil aviation development is a Precious Gift of the Military world.
Thanks so much for sharing here and in the shadow,:D