Angle of Attack

[Lee-a-Roady Moor]

On a C150 for example, the stalling angle of attack is around 16' clean. There will be a corresponding Indicated airspeed, but while this is a good guide, it is only that.

With flaps extended, the Indicated stalling speed will decrease, but what happens to the stalling angle?

My understanding is that the stall is dependant on the Critical Angle being exceeded and that that angle doesn't change, ie: remains at 16' even with flap extended.

Is this correct or will the angle decrease with flap extended, say to 15' (in this example), and thus the wings will stall at a lower angle of attack.

[Raf-CSB3]

Hi,

Your aircrat stall when happen the critical angle of attack. With or without flap, the angle is the same. We are unable to know this angle in the airaft so it's for this reason we talk about stall in reference of speed.

Good reference for that is this book: mechanic of flight of A.C Kermode.

Raf

[Snigs]

The wing will stall at the same AOA. Remember that the AOA is measured relative to the chord of the aerofoil, generally a line drawn from the leading edge to the trailing edge. Put the flap down and the chord relative to the fuselage changes. This enables the a/c to fly slower (i.e. producing more lift) at a certain AOA.

It's better demonstrated by diagrams, but I hope you get the gist of it. :)

[Say again s l o w l y]

When flap is put down, the aerodynamic 'shape' of the wing changes, so why do people assume that the stalling AoA stays the same? Surely it changes a bit?

I don't know if this is actually true, but could some of the more learned members of this forum explain it in detail?

[slim_slag]

Yeh, I thought it changed too, but plenty of websites give all three possibilities. Surprisingly a search on angle attack stall flaps in the tech log forum suggests this hasn't been discussed before. So a new subject on the proon, how delightful. There are one or two people on tech log who command respect, perhaps somebody can go get them?

[Tinstaafl]

Typically trailing edge devices eg flaps cause the critical/stalling AoA to reduce slightly compared to the clean configuration. This is offset by the overall greater coefficient of lift that the wing gains with flap extended.

End result is a slightly lower crit. AoA but also a lower speed that corresponds to this angle.

[homeguard]

Lowering of Flap changes the Angle of Attack of an aerofoil in just the same way as any other flap; Ailerons, Elevator and Rudder. Unless the flap was to extend the full length of the aerofoil it will increase the angle of attack only where it is placed along the span of the aerofoil.

Thus it is perceived that the attitude of the nose, say to the horizon, or any other reference, is lower at the point of stall than when the wing is 'clean'. Although the IAS is most likely lower owing to the benefits from increased surface area and in the case of 'Fowler Flaps' a slot that is formed re-energising the airflow. The critical Angle of attack of the flap area just the same will be reached before the rest of the wing which will not yet have attained the critical angle pertaining to it.

[BEagle]

So much nonsense!

When a clean wing is stalled, it does so at an unique angle. Extend flap and the whole lift curve slope (Cl v. AoA) changes. The angle of attack at which the wing with flap extended stalls will also change.

Don't worry about the re-energised airflow or any other pseudo aerodynamic terms; think of the Cl v. AoA curve as the wing's 'fingerprint' - and that it has a different one with flap extended!

For most a/c, the stall occurs at a lower AoA and higher Cl value with flap extended than when clean.

[MEI]

BEagle,

I always take your posts in high regard. Keep up the good work. Thanks for the countless amounts of info.

MEI

[Lee-a-Roady Moor]

Thanks for all the info.

I can accept that with flaps extended, the chord effectively changes and thus the camber also - hence a 'new shaped' aerofoil; and that 'new' aerofoil could be designed with a different AoA.

But I could also accept that an aerofoil is designed to stall at a particular angle, and while flaps reduce the indicated speed at which the stall occurs, that angle would remain the same.

I think there are a lot of arguments in favour of either theory, but it doesn't seem to be documented terribly well - at least in the various places I've looked.



:ok:

[slim_slag]

Yes, Lee-a-Roady, it's confusing. Aerodynamics is something even the experts argue about, I don't have time (or the aptitude) to become an expert, so have to rely on published materials or people I think I can trust.

But the published materials are either too scientific, or just too simple. I've always liked See How It Flies (http://www.av8n.com/how/) by John Denker, it's readable and pitched at my level, it's on tne Internet so not too heavy to carry around, and he appears to talk sense. But now I find out he is wrong, because he says (http://www.av8n.com/how/htm/vdamp.html#sec-flaps-lower-stall) Extending the flaps gives the airfoil a shape that is more resistant to stalling. That means, among other things, that it can fly at a higher angle of attack without stalling,

And be produces pretty diagrams to demonstrate this.

Now this is a pretty simple question, what does somebody like me who should understand all this stuff, but is hearing conflicting stories from people who definitely understand it better than me, tell our students?

Maybe we are getting confused between 'angle of attack' and 'pitch angle' ??

[homeguard]

I too think that John Denker is one of the best at keeping simple and here is a quote from him, which gives some food for thought as to why one may confuse one explanation to be in conflict with another;

ABSOLUTE VERSUS GEOMETRIC ANGLE of ATTACK

As mentioned in connection with figure 2.1, we are free to choose how the angle-of-attack reference stick is aligned relative to the rest of the wing. Throughout this book, we choose to align the reference with the zero-lift direction. That means that zero angle of attack corresponds to zero coefficient of lift. According to the standard terminology, the angle measured in this way is called the absolute angle of attack.

Some other books try to align the reference with the chord line of the wing. The angle measured in this way is called the geometric angle of attack.

If you try to compare books, there is potential for confusion, because this book uses “angle of attack” as shorthand for absolute angle of attack, while some other books use the same words as shorthand for other things, commonly geometric angle of attack. To make sense when comparing books, you must avoid shorthand and use the fully explicit terms. In particular, to convert from one system to another:

absolute angle of attack = geometric angle of attack + k
geometric angle of attack = absolute angle of attack - k

where -k is the X-intercept of the graph of the coefficient of lift according to the “geometric” scheme, in which the angle is measured relative to the chord line. The X-intercept is always zero in this book.

Also note that there are many possibilities, not just absolute versus geometric; the choice of reference is really quite arbitrary. It is perfectly valid to measure angles relative to any reference you choose, provided you are consistent about it. (Aligning the reference stick with the fuselage is useful in some situations)

Using the chord as a reference works OK if you are only talking about one section of a plain wing. On the other hand: On typical airplanes, the chord of the wing tip is oriented differently from the chord of the wing root. Which one should be considered “the” reference? When you extend the flaps, the chord line changes. Most books that choose to measure angle of attack relative to the chord line violate their own rules when the flaps are extended, and continue to measure angles relative to where the chord of the unflapped wing would have been. That is illogical and creates confusion about how you should use the flaps. This is one of the reasons why it is advantageous to think in terms of absolute rather than geometric angle of attack. Thinking about geometric angle of attack would be advantageous if you were building an airplane, or conducting wind-tunnel research on wing sections. Engineers can look at a wing section and determine the geometric angle of attack.

In contrast, if you are piloting the airplane, geometric angle of attack has no advantages and several big disadvantages: it’s hard to define, it’s hard to perceive, and it doesn’t tell you what you need to know anyway! We care about coefficient of lift, which is proportional to absolute angle of attack over a wide range (i.e. not too close to the stall). Each degree of angle of attack is worth about 0.1 units of coefficient of lift.

The simple rule “pitch plus incidence equals angle of attack plus angle of incidence” is always mathematically valid, no matter what reference you’re using to measure angle of attack. (That’s because the arbitrariness in the angle of incidence cancels the arbitrariness in the angle of attack.) But if you want the rule to be useful in the cockpit, especially in situations where flap settings are changing (as discussed in section 5.5), you need to focus on absolute angle of attack.

Summary
Trim for angle of attack! Trim for angle of attack!

Pitch attitude is not the same as angle of attack. Angle of attack is what really matters. You can observe pitch attitude and direction of flight as a means for controlling angle of attack.

The airspeed indicator gives you quantitative information about angle of attack (except near the stall). If the aircraft is producing a non-standard amount of lift, many (but not all) of the critical V-numbers must be corrected. The percentage change in speed is half the percentage change in weight.

END OF QOUTE

A previous contributer to this thread has already posted the link to John Denkers book and for those who doubt the value of Slots and the re-energising of airflow may also wish to give it at least a fleeting a glance.

Please note that I have edited parts of the above but only to remove references to graphs etc that cannot be seen here.

[BEagle]

The angle of attack is the angle between the chord line and the relative airflow. Period.

The chord line is the line which joins the centre of curvature of the leading edge to the centre of curvature of the trailing edge. Hence it will change with tab angle (flap extended).

Personally I find Denker is merely confusing a very basic concept.

There are also older terms in existence such as 'angle of incidence' and even 'rigging angle'. And as for 'deck angle'.....!

K I S S!

[Sleeve Wing]

Thanks again, BEags, for a breath of fresh air.
Since I first read Kermode 'n' years ago, I have struggled regularly with contributions from various well-meaning but self-serving gentlemen who wish to reinvent the wheel.
Whilst training, all my notes on the theory of supersonic lift were turned on their heads !
All that happens, as we see, is that it produces more and more confusion.
Let's get back to basics, gentlemen, and then we won't mislead our students.
Rgds, Sleeve.

[slim_slag]

Apologies for being inquisitive. So sleeve wing, what's your answer to the original question, and why is Denker wrong (if that is what you are saying)?

[bookworm]

The angle of attack is the angle between the chord line and the relative airflow. Period.

The chord line is the line which joins the centre of curvature of the leading edge to the centre of curvature of the trailing edge. Hence it will change with tab angle (flap extended).

While that may be "simple" and self-consistent as a convention, it is not the way that aerodynamicists treat flap.

The convention in the classic texts (such as Abbott and von Doenhoff) is to use the chord line of the aerofoil with flap retracted as the zero of angle of attack. This has the advantage that the zero reference does not change relative to the aircraft geometry when configuration changes.

Using that convention, trailing-edge flap extension
increases the maximum lift coefficient but tends to reduce the AoA at which that maximum occurs. Leading edge device extension also increases the maximum lift coefficient and tends to increase the AoA at which that maximum occurs.

[Sleeve Wing]

slim_slag.
Suggest you compare the responses from BEagle and tinstaafl with those of homeguard and bookworm.
Each is correct but we're trying to teach pilots here.
Let's keep it simple and remember they're flying 'em, not designing them.
:ok:
Sleeve.

[slim_slag]

Well sleeve wing, you speak in riddles and don't answer the simple question posed, but I think you are saying 'Work it out for yourself'. A good approach to take, better than spoon feeding....

So I've read the posts and thought about it, and my answer is (drums please, maestro).

It depends where you measure it from.

Er, I think...

And Denker is correct, both ways.

http://www.av8n.com/how/img48/flap-lift-incidence.png

Thanks to bookworm for making the light go off over my head, and homeguard for picking the other text. And hell, the rest of them too. I remember when Denker first put his stuff on the INternet it was discussed at length on the good old usenet rec.aviation groups. Other 'experts' who had written books had things to say, and Denker changed some things. So there has been peer review of sorts, and if somebody publishes then I presume they know more than somebody who hasn't. Anonymous bulletin boards like PPrune don't count as a publication. Anyway...

I think most students don't even want to know how the thing flys, never mind how it is designed. Some of them don't even know what flaps are used for! But you do get the occasional one who wants to know more, and I think telling them they don't need to know about that stuff because I don't understand it myself is a bit poor. :)

Cheers

[machonepointone]

Quite a lot has been written on this subject - a lot of it wrong and misleading, especially what John Denker has written. To state that

“Extending the flaps gives the airfoil a shape that is more resistant to stalling. That means, among other things, that it can fly at a higher angle of attack without stalling.”

is nothing short of utter nonsense and displays a woeful lack of understanding of the subject.

First of all, let’s look at the angle of attack at the stall with and without flaps, and to do that it is necessary to look at why a wing stalls in the first place. Across the upper surface of the wing the airflow initially speeds up to the point where the laminar flow breaks down. This is the transition point. From then on it is slowing down to the free stream speed which in theory it will reach at the trailing edge of the wing. However, since lift is dependant on the difference in pressure between the upper and lower surfaces, it is necessary to consider what is happen on the lower surface.

Given that the pressure here is higher than on the upper surface, what happens at the trailing edge is that the air tries to move towards the lower pressure, thereby giving rise to a small but significant flow of air forwards. This meets the turbulent air which is slowing down and further decreases its speed such that it causes the airflow to break away from the upper surface of the wing before reaching the trailing edge. The point at which this occurs is known as the separation point. The area of the wing behind the separation point is therefore not producing any lift.

As the angle of attack is increased, the difference in pressure between the upper and lower surfaces is increased. At the same time the transition point moves forwards so the airflow starts to slow down from a point further forward on the wing. The greater pressure difference means that the air from the underside of the wing has more energy to move forward along the upper surface of the wing. The combination of the forward movement of the transition point and the increased energy of the air from the lower surface causes the separation point to move forward, thereby causing progressively more of the wing to stop producing lift. There comes a time when it has moved forward far enough that the loss of lift now causes the wing to stall.

A cambered airfoil gives a higher coefficient of lift at a given angle of attack than a non-cambered airfoil due to the greater difference in pressure between the upper and lower surfaces. This therefore means that at any given angle of attack, the separation point of a cambered wing is further forward than on a non-cambered wing. Therefore, the separation point has less distance to move forward to reach the point where the wing will stall resulting in a stall at a lower angle of attack (albeit with a higher coefficient of lift). This combination results in a stall at a lower angle of attack but at a lower airspeed as well. All that lowering trailing edge flaps does is to increase the camber of the airfoil.

slim_slag, your right hand diagram accurately represents what I have just been saying. The left hand one is representative of the difference with and without slats or slots.

As far as the attitude at the stall is concerned, as BEagle so rightly states, the angle of attack is the angle between the relative airflow and the chord line, the latter being measured from the leading edge to the trailing edge of the wing. By lowering flaps the trailing edge is lowered thereby increasing the angle of attack of the wing at that attitude. Therefore, the nose does not need to be raised so much to reach the stalling angle of attack, ergo a lower attitude at the point of the stall.

[bookworm]

“Extending the flaps gives the airfoil a shape that is more resistant to stalling. That means, among other things, that it can fly at a higher angle of attack without stalling.”

is nothing short of utter nonsense and displays a woeful lack of understanding of the subject.

But his statement is consistent with his own "absolute angle of attack" definition, is it not?

[Croqueteer]

BEagle, small point, but angle of incidence is not an older term, but is the angle between the cord line and the fore and aft datum line of the fusilage.

[BEagle]

Depends - the term I first learned was 'The angle made between the fuselage datum and a straight edge placed against the lower surface of the mainplane'. That was actually 'Riggers' angle of incidence' and stemmed from the days of undercambered aerofoils! The same book also explained why RAF roundels should not be painted so that they overlay upon the ailerons!

Unfortunately many people confuse 'angle of incidence' with 'angle of attack' - hence in flying training it is normal just to stick with AoA!

[Croqueteer]

BEagle, angle of indidence, or rigger's angle, is still very much with us. Try rigging a biplane or a high winged aircraft with adjustable struts without it!

[3 Point]

Seems to me we are all having a heated agreement here! I've not read anything on this thread which is "wrong" just confusing.

The "angle of attack" can be defined an any way to suit the purpose of the individual making the definition. It may suit an aerodynamicist (I said may - I'm not one!) to use a definition similar to Mr Denker's "absolute angle of attack"; I've not read his book but thanks to homeguard for the extract. I would think that even in a wind tunnel the fact that the datum is related to the point where the Cl =zero would create problems; wouldn't this point depend on reynolds mumber, skin surface quality, position of high lift devices and the angle at which the aerofoil is presented to the airflow?

The other definition, with which we are more familiar in the UK is the one which uses the wing chord line with high lift devices retracted and retains this datum regardless of the subsequent movement of those high lift devices.

If you prefer to use Denker's definition then the wing would indeed stall at a higher angel of attack with trailing edge devices deployed however, because we are now using a diffefent datum we are really comparing apples with oranges - both useful in their own way but you can't make tequila sunrise with apple juice!

As a pilot I much prefer the use of a fixed datum corresponding to the wing chord line with high lift devices retracted. Two reasons; firstly the use of this convention allows a more intuitive correlation from the pilot's viewpoint between pitch attitude and "angle of attack" (please don't anyone dive into explanations of the effect of load factor, wing loading, effects of power etc, I said intuitive corelation, not fixed relationship!). Secondly; if the aeroplane has an angle of attack indicator (I have flown with such devices for 20 years - the most useful instrument in the cockpit) there needs to be some fixed datum for that. On an aeroplane which indicates (on a HUD or PFD) both the pitch attitude index (like the aircraft symbol on a normal artificial horizon) and a flight path vector (the direction in space the aircraft is travelling or, the direction from which the relative airlfow is approaching) one can see the angle of attack clearly and it's most useful.

Mach 1.1; You quote Beagle thus "as BEagle so rightly states, the angle of attack is the angle between the relative airflow and the chord line, the latter being measured from the leading edge to the trailing edge of the wing. By lowering flaps the trailing edge is lowered thereby increasing the angle of attack of the wing at that attitude." Thats not what I understod him to say. He argues for the point of view which defines AoA measured from a fixed datum corresponding to the geometric chord line without high lift devices and for the constant use of that datum regardles of the use of flaps etc.

So. The variable datum (ie Denker) is correct and in this case the wing would be at a higher "angle of attack" when it stalled - the aircraft would be at a lower pitch attitude however (assuming constant mass and level flight) because the AoA datum has changed with the movement of the flaps. While this may be correct, it is not very useful for pilots and the fixed datum version, equally correct, but in my view much more useful to us pilots would have us reach the stall at a lower angle of attack (because in this case the datum from which we measure the angle has not changed) which would also ocurr at a lower pitch attitude. In either case, at the point of stall the wing would be producing a higher Cl than with no flaps deployed and the ias at this point would be less.

3 point