Angle of attack in a climbing turn

[photofly]

because a downward deflection of aileron will increase the AoA on that section of wing and possibly get it to stall...

Alternatively you could say that a downward deflection of the aileron, while maintaining the same AoA on that section of the wing, transitions the wing section to a changed airfoil shape whose new and different Cl curve has a lower critical angle of attack than that section of the wing is maintaining, and possibly gets it to stall...

It seems a long winded explanation though.

[Cat Driver]

Of course an aileron will stall if used aggressively in the wrong speed envelope.....how many of you have experienced the shock on the airframe that aileron snatch produces?

[dr.aero]

I really get depressed thinking about how all these aerodynamic questions, I guess it is because I did not have the interest in digging so deep into the theories surrounding these issues.....

You really don't need to dig this deep. I wouldn't get anywhere near all the stuff I've been talking about with a PPL student or a CPL student unless they were interesting in learning more.

However, if you're trying to prove a 'theory' wrong you need to go to the complex part of it to demonstrate the flaws. I haven't even gotten to the complex part of it yet - and frankly, I don't think I will as it's pointless to discuss it when both parties don't have the same backgrounds. I've explained in another one of my posts why it's incredibly difficult to prove someone wrong in this case because their 'theory' isn't based on facts that they understand, it's based on their beliefs.

I'd still tell them you shouldn't pick up a wing drop in a stall with aileron (and I'd also tell them not to "pick up" a wing drop with rudder either - more on that if you'd like) because a downward deflection of aileron will increase the AoA on that section of wing and possibly get it to stall which will drop you out of the sky pretty fast into a very unusual attitude or into a spin.

Thank you -- I think that's all anybody was trying to say.

Oops! I made a mistake - I've been thinking too much about angle of attack. I meant to say that the downward deflection of aileron will increase airflow separation (or change the CL - if you want to think in those terms) bringing the wing section, and therefore the whole wing, closer to the point of stall. I edited my post to make that correction. Sorry for the confusion.

It seems a long winded explanation though.

Photofly, because something requires a long explanation doesn't make it incorrect. That philosophy has contributed to instructors who aren't qualified to teach a subject has decided that they can simplify an explanation to provide a simple and clear explanation for their students - and most of the time the simplfied explanation then becomes incorrect as the instructor, who isn't qualified, didn't know how to simplify correctly.

Of course an aileron will stall if used aggressively in the wrong speed envelope.....how many of you have experienced the shock on the airframe that aileron snatch produces?

Aileron snatch - that brings up thoughts of icing...

[Cat Driver]

Aileron snatch - that brings up thoughts of icing...

No, it is a very abrupt stall of the aileron caused by abrupt full aileron input to exit a steep turn.

It is also only produced in certain aircraft in very close to the edge flying..therefore few pilots will have experienced it.

. I've explained in another one of my posts why it's incredibly difficult to prove someone wrong in this case because their 'theory' isn't based on facts that they understand, it's based on their beliefs.

My " beliefs " are based on having done the experiment and observed the results.....many thousands of times.

My beliefs and understanding of how an airplane flys have been rewarded with licenses issued to do said flying......up to and including a JAR unrestricted airdisplay authority in Europe......I passed the tests to hold said license for eight years....

...does that count?

[photofly]

You know what? I think there's a misunderstanding between us about this. Give me 10 mins to draw some diagrams and give you a good, rational, explanation of how an aileron works, in terms of angle of attack. I'd hate to waste all this hot air to find that we're not actually promoting or opposing the same thing.

[Trematode]

Oops! I made a mistake - I've been thinking too much about angle of attack. I meant to say that the downward deflection of aileron will increase airflow separation (or change the CL - if you want to think in those terms) bringing the wing section, and therefore the whole wing, closer to the point of stall.

Again, I think you're arguing semantics. Your first explaination was just as valid, and will make more sense to a student who hopefully already understands flow separation due to increases in AoA, and things like washout -- I think it's telling that it was the answer you produced as the first response.

It's not like you'd be sacrificing that little bit of convention in vain. Usually the simplest (valid) explaination is best.

[dr.aero]

No, it is a very abrupt stall of the aileron caused by abrupt full aileron input to exit a steep turn.

It is also only produced in certain aircraft in very close to the edge flying..therefore few pilots will have experienced it.

Hmm.. haven't heard of that before. But aileron snatch due to icing is real - it can cause loss of control.

My " beliefs " are based on having done the experiment and observed the results.....many thousands of times.

My beliefs and understanding of how an airplane flys have been rewarded with licenses issued to do said flying......up to and including a JAR unrestricted airdisplay authority in Europe......I passed the tests to hold said license for eight years....

...does that count?

That comment wasn't directed at you. We're not on the same page. I'm at a loss of words to explain myself at this point. This has nothing to do with a person passing exams and being a good pilot and knowing the result of control inputs as he/she flies. I'm sure you do and I have no evidence to say that you aren't a fantastic pilot.

You know what? I think there's a misunderstanding between us about this. Give me 10 mins to draw some diagrams and give you a good, rational, explanation of how an aileron works, in terms of angle of attack. I'd hate to waste all this hot air to find that we're not actually promoting or opposing the same thing.

I totally agree! I look forward to a good discussion about this. Being on the same page is important.

[photofly]

Alright, ladies and gentlemen - a basic explanation of how an aileron works. Nothing here about angle of attack, nothing here about vorticity, spanwise distribution of circulation, and not even any equations.

Let's begin:

Here's a wing section. It has a moderate camber, and I've drawn on the picture the chord line. I've also drawn on the picture a line I'm going to call the zero-lift line. If air flows past this wing section parallel to that line, no lift is generated. You'll notice that because of the camber, the zero-lift line makes a slight angle with the chord line.

wing1.png (14.99 KiB) Viewed 1915 times

Let's call it Wing 1.

Here's a graph of the coefficient of lift of Wing 1 plotted against the angle the free-stream flow makes to the zero lift line. The graph passes through the origin, of course it has to, because of how I define the zero-lift angle:

graph.png (20.04 KiB) Viewed 1915 times

Exhibit 2 is another wing section. Let's call it Wing 2. It's solid, like Wing 1, and it's a similar shape except that the trailing portion has been bent downwards by 10 degrees or so. because of this bend the Wing 2 as a whole has a lot more camber than Wing 1. I've drawn the chord line, and I've drawn the zero-lift line. Because of the extra camber the zero-lift line makes a much bigger angle to the chord line than Wing 1.

wing2.png (22.88 KiB) Viewed 1915 times

Here's a graph of the coefficient of lift of Wing 2, plotted against the angle the free-stream flow makes to the zero lift line. It's very similar to the same graph for Wing 1 - there are some differences at the top, near the critical angle of attack, because it has a different shape, and extra camber. But one of the lessons of all those NACA curves is that for the most part, the shape of the wing has only a very slight effect on the shape of the Cl curve. Certainly nothing you'd notice in a hand-drawn sketch like this.

graph.png (20.04 KiB) Viewed 1915 times

To be continued ...

[photofly]

Continuing on ...

Here's a new wing - let's call it Wing 3. It's the same shape as Wing 1 except the rear portion is hinged so it can swing up and down. Here's the wing with the rear portion straight, in flight. I've drawn the free stream direction, the zero-lift line, and the angle between them (Angle A). If I want to know the coefficient of lift for that configuration I can look it up on the graph I posted earlier.

wing3.png (15.87 KiB) Viewed 1915 times

And here it is again, with the hinged part in the down position.

wing4.png (17.96 KiB) Viewed 1915 times

By curious coincidence it now has exactly the same shape as Wing 2! I've drawn on the picture the free stream direction, and the new zero-lift line (which is the same as the zero-lift line for Wing 2 as is has the same shape) and I've marked the angle between them as Angle B. I can look up the new coefficient of lift for this configuration on the Cl graph for Wing 2.


Let's superimpose the CL graphs for Wings 1 and 2, and simultaneously mark on them the angles A and B, to see how the coefficient of lift changes for Wing 3 as I lower the rear hinged portion. The two graphs are so similar (especially when sketched by hand) that there's no real difference between them!

graph2.png (22.91 KiB) Viewed 1915 times

And lo, there, in graphic detail is the difference in the Cl between the two configurations of the wing. If the Cl increases when I lower the rear section of the wing, then the lift increases. If that were the outboard part of a long wing then I'd generate a rolling moment and able to manoeuvre the aircraft about it's longitudinal axis.

[Trematode]

Interesting, that difference between angle A and B. I wonder what somebody might call that.

[photofly]

Interesting, that difference between angle A and B. I wonder what somebody might call that.

Don't troll. It's not helpful.

[dr.aero]

Again, I think you're arguing semantics. Your first explaination was just as valid, and will make more sense to a student who hopefully already understands flow separation due to increases in AoA, and things like washout -- I think it's telling that it was the answer you produced as the first response.

It's not semantics - it's definitions. It may not seem apparent at this point why it's important to use the definition I've stated but if you were to look any further into aerodynamics it would become more apparently why the aerodynamic engineers have decided on the particular definition that they use.

I would argue that, as a pilot, you need to have an accurate understading of the result of a control deflection on the handing characteristics of the airplane. If you explain the AoA as increasing if aileron is deflected down, the result should be the same in this case. However, the end doesn't justify the means. And I guess that's essentially what I'm 'fighting' about. When I was a student I never agreed with being taught something one way and then once I had an understanding of that, I'd be taught it wasn't exactly correct and for the more advanced stage of the course you're going to have to learn the correct way. It was frustrating and as my philosophy, I won't knowingly teach a student something that I know to be wrong - I also don't believe any argument that supports teaching something incorrectly so a student can learn. I had a Class 1 instructor tell me, during my flight instructor rating, that I should teach the forces in a turn wrong (that all forces are balanced) "because it's easier to learn". That's absolutely ridiculous! First off, it's completely wrong and secondly, I would argue it's actually easier to learn as the correct way has less forces depicted.

So, you may not see the reason why you should be learning that AoA doesn't change based on aileron deflection and you may still get the same understanding of the result of control deflections on the handling characteristics of the airplane, but the end doesn't justify the means and I believe it applies in this case. There is absolutely no reason to teach something incorrectly in my opinion. Pilots only require a simple explanation and basic understanding of how the control deflections affect aircraft handling and that can be achieved. However, I won't lie to a student.

I could explain that air is like bullets and explain how the control deflections affect the aircraft handling and for a PPL student, they would come out with the exact same understanding of the results of control deflections on flight! So why should I tell a PPL student that air is actually a fluid? Fluid interactions with an airfoil are harder to visualize than a bullet's interaction.

In summary: you probably have the same understanding as I do of how a pilot's action in the cockpit will affect the airplane in flight. However, the end doesn't justify the means.

[dr.aero]

Photofly... I appreciate the effort you put into your explanations. I'm just not sure I have the energy to go through them and respond to all your points. If you read my last point you might understand where I'm coming from.

A quick summary though: As I've mentioned before this is really about definitions. Now, in your diagrams you've make your CL curve relative to your zero-lift line. Nothing wrong with that. Later you compare the CL graphs of the clean wing and the wing with an aileron deflection. You say that there is hardly any noticeable difference between the two, and I agree, there isn't. The reason there isn't is because you've changed the position of the zero-lift line!! Go back to that graph where you showed me different CLs for flaps. If they made that graph relative to the zero-lift line and then moved the zero-lift line so that it was the line that produced zero lift in the specific configuration and NOT the line for the clean wing - you would notice that the CLs would practically fall on each other, just like you've diagramed in your drawings. This is a perfect example of why aerodynamic engineers have defined those chord lines based on the clean wing!!

[Trematode]

Aero -- I'm sorry but the students are not going to be plotting a C of L curve for any of the wings they are going to be flying.

For aerodynamicists it is important for them to understand how their wing as a whole responds to the use of high lift devices and ailerons, they need a reference chord line to plot the graph -- for a student trying to understand the concept of washout, or why deflecting an aileron down in an incipient spin is bad, a C of L curve for the whole wing doesn't tell them jack.

They need to know that the outboard section of the wing is more stalled than it was before they added the aileron -- you said it yourself: Deflecting it causes more flow separation. Well what causes that flow separation???? The answer is that particular section of the wing is now flying further BEYOND its critical angle, because they moved the aileron down. In effect, they have increased the Angle of Attack on that section of the wing. These are terms they can understand. If you can't see the benefit in being able to explain it that way, instead of stubbornly clingling to conventions more appropriate for a course in aerodynamics, then we will never see eye-to-eye.

YES, they have also changed the camber, and the coefficient of lift for that section, and indeed the entire wing as well. But they are not aerodynamicists, and the 14 year old you are trying to get PGI before your training flight would be much better served by an instructor that can teach it in whichever way he most readily absorbs it.

you may not see the reason why you should be learning that AoA doesn't change based on aileron deflection and you may still get the same understanding of the result of control deflections on the handling characteristics of the airplane, but the end doesn't justify the means and I believe it applies in this case. There is absolutely no reason to teach something incorrectly in my opinion. Pilots only require a simple explanation and basic understanding of how the control deflections affect aircraft handling and that can be achieved. However, I won't lie to a student.

I'm sorry man, there's a difference between "incorrect" and "different". You are arguing for convention; one specific way of teaching -- I have no problem teaching students either way. I'm sure many aerodynamicists can happily discuss what may be happening at a specific section of an airfoil -- by deflecting an aileron or deploying a flap, you are changing the shape of that airfoil section, it is no longer the same and if your aircraft is still moving in the same direction, that section is no longer generating the same lift, it no longer has the same camber, it no longer has the same chord, it no longer has the same angle of incidence, it no longer has the same angle of attack!

The question we've been debating is whether or not it is appropriate to examine that isolated section of wing on its own. You are insisting we can't, and have to view the changes in the context of the wing as a whole -- but a simple C of L curve doesn't tell you anything about how the wing changes along its span. Sometimes this information is important to get across to the student! Not only that, but sometimes by breaking the wing down into sections it becomes more intuitive for them to learn!

[dr.aero]

Aero -- I'm sorry but the students are not going to be plotting a C of L curve for any of the wings they are going to be flying.

When did I EVER say that I was going to have a student do that? They also aren't going to be plotting AoA. What does that have to do with any of this?

Your argument is ridiculous! You're trying to say that CL is so much more complex and only needs to be understood by aerodynamicists and the like and AoA is much simpler and suited for students. They're both on the same level!!! - look at the lift equation. Do you see a value for AoA? No. Only CL. A CL graph shows the CL vs the AoA.

[Trematode]

Look at the lift equation. Do you see a value for AoA? No. Only CL. A CL graph shows the CL vs the AoA.

If a student asks how an aileron works, you can show him the formula for lift, but that doesn't tell him the whole story -- as photofly illustrated, the AoA for that section of the wing is also increased. If you lump that increased lift (from the camber, and the local AoA) into the overall CL curve for the wing, a student might not get the whole picture.

How are you going to explain to a student why the outboard section of a wing becomes more deeply stalled if they try to use aileron to pick it up in a spin, without mentioning the AoA in that section of the wing?

All I am saying is that there are benefits to departing from convention in some cases. Strict adherence may not always be the best course of action, because the student wont have an entire semester to get the entire precise picture.

[photofly]

All I am saying is that there are benefits to departing from convention in some cases

It's not even departing from convention, merely using a different convention. The angle between the free-stream flow and the zero lift line is not a new invention of mine, or even Denker's. It's in the dictionary as "Absolute Angle of Attack". It might not appear in your favourite aerodynamics text, and it might be of no use to aircraft designers - but it's certainly of use to pilots. For reasons I demonstrated above. Among other things it makes explaining the correct aerodynamics around a lowered aileron simple.

This is a perfect example of why aerodynamic engineers have defined those chord lines based on the clean wing!!

I totally agree with you. And it's also a perfect example of why for pilots it makes more sense to use an Angle of Attack based on the zero-lift line. We agree the aerodynamics is the same in both cases. If a pilot is also an aircraft designer then he's certainly wise enough to appreciate the difference and know which AoA simplifies whatever he's trying to do. Do remember, though, that this is a forum for pilots, not aircraft designers. If you espouse dogma that's appropriate to aircraft designers but makes things more complicated for pilots you should expect to be challenged.

Overall, I don't really want to talk about definitions, I want to talk about aerodynamics, and there's no difference between us on that. You like to quote from R.P.Feynman. I'm sure you'll be aware of his feelings on the difference between knowing the name of something, and knowing how it behaves, and which is more important.

[Trematode]

I totally agree with you. And it's also a perfect example of why for pilots it makes more sense to use an Angle of Attack based on the zero-lift line.

I don't even think it's that. I think Dr. Aero -- probably from his academic experience -- would prefer to have a single reference for the entire wingspan.

Whether it be the chord line or the zero lift axis is irrelevant. He wants angle of attack to be the difference between the free stream flow, and the zero-lift line for the entire wing, not just the one section to which the aileron, or flap is attached.

By breaking the wing down into sections and having different reference chord lines for each, he feels we are muddying the concept of angle of attack, and I can understand why. But I can still see the benefit of doing this when it comes to conveying information to students.

[photofly]

don't even think it's that. I think Dr. Aero -- probably from his academic experience -- would prefer to have a single reference for the entire wingspan.

If that's what he wants, it makes even less sense to base it on the chord line of the clean wing section. Real wings are built with washout, so the chord line of the clean wing changes spanwise. His engineering definition of AoA requires qualification by a note of the station at which the reference chord line of the clean wing is to be taken.

By contrast, the concept of Absolute Angle of Attack extends gracefully to the "real" wing with washout. A washed-out wing also has a zero-lift direction, just like a wing-section.

Shall we have a new thread entitled "Pros and cons of different measures of Angle of Attack"? That would be interesting. I can think of several reasons why Pilots might prefer to consider Absolute AoA to have primacy.

[Trematode]

If that's what he wants, it makes even less sense to base it on the chord line of the clean wing section. Real wings are built with washout, so the chord line of the clean wing changes spanwise. His engineering definition of AoA requires qualification by a note of the station at which the reference chord line of the clean wing is to be taken.

By contrast, the concept of Absolute Angle of Attack extends gracefully to the "real" wing with washout. A washed-out wing also has a zero-lift direction, just like a wing-section.

again, I think he knows about washout, and when he says geometric chord line it's just a reference for the entire span that you can plot the coefficient of lift/AoA against, just as the zero-lift axis would be.

I don't think he's really arguing between angle of attack and absolute angle of attack. I think he just doesn't like you to examine that one section of the wing with the aileron on it, and consider it independent of the rest of the wing. He wants to take the changes of lift caused by the aileron deflection, and lump them in with the overall CL for the wing in its entirety.

This is great, but doesn't tell you anything about AoA at specific wing sections, which is usually more important to a pilot because they have to understand how and when their wing is going to stall, and at what specific locations.

[photofly]

I think he just doesn't like you to examine that one section of the wing with the aileron on it, and consider it independent of the rest of the wing. He wants to take the changes of lift caused by the aileron deflection, and lump them in with the overall CL for the wing in its entirety.

No, I think you misunderstand him. I believe he considers that it's only appropriate to consider the AoA of just the aileron section to remain unchanged.

I find that philosophically unsatisfying because, in terms of lift, drag or airflow, there's no difference between a wing with a lowered aileron, and wing whose fixed rear section droops 10 degrees in the clean configuration. According to dr.aero's rigid definition they have different angles of attack since one is clean and one is not. They therefore must have different lift curves to explain the fact that they generate the same lift (since they have the same shape.) I'd rather say that since they have the same shape they have the same lift curve, and the same AoA. I suspect most rational people, engineers and scientists among them, would at least see the logic in that point, which is not apparent to dr.aero.

I believe dr.aero's point is that when designing an aircraft it's important to use an AoA that's fixed relative to the spars, ribs, etc (and for a fixed angle of attachment) fixed relative to the fuselage. I accept that point as far as it goes. It's nonsense then to extend it to say that every one else in the world who isn't designing an aircraft can use only that quantity even when it complicates what they're trying to learn. It's just plain rubbish, frankly. If I'd tried to use that argument with a physics professor at university I'd (correctly) have been thrown out of the room.

More to the point he's unable to give me any example of interest to pilots where the Geometric Angle of Attack is a more attractive measure. The best he can do is to say "when you go on to study more". Let's hear what we would hear "when we go on to study more" right now. I am actually truly interested to hear why an aircraft designer prefers that measure, albeit, as a pilot, and not a designer, I willingly take dr.aeros' word for it.

[Trematode]

I believe he considers that it's appropriate to consider the AoA of just the aileron section to remain unchanged.

By his definition of AoA -- something that's relative to the entire wing, not just the aileron section -- it would remain unchanged.

That's what I was trying to say. I don't think he's even allowing himself to consider separate sections of the wing can have differing angles of attack, because by his definition AoA is somthing that's strictly referenced to a single point relative to the entire wing, not a single section of wing.

I can see why he would want to stick with that definition, because it works great in describing the lifting properties of the wing -- with washout, and differing camber along the span -- as a whole. But for a student pilot to understand how the wing behaves, I think you have to be able to explain what's happening in localized sections of the wing as well. His definition doesn't help much with that.

[photofly]

I don't think he's even allowing himself to consider separate sections of the wing can have differing angles of attack .... because it would work great in describing the wing

No it wouldn't. You'd have a lift curve that varied at every station, even where the wing section is the same NACA airfoil - a continuum of different lift curves, so to speak. Even an idiot can see the situation is more simply described by a single lift curve and an angle of attack that varies spanwise. Good science is about making complex things simple. Not about making simple things complex.

While the Wright Brothers invented wing-warping, literally twisting the wing-tips to change the lift distribution, is he seriously trying to tell me that, by any measure, the AoA of the wingtips isn't changed? Or do we allow that as a change in AoA, yet suddenly because I hinge the rear section of the wing that section instead of twisting it, that doesn't change change AoA? If so, these are distinctions without merit, and I dismiss them.

[Trematode]

Good science is about making complex things simple.

But that's exactly why an aircraft designer or aerodynamicist might use his definition for certain tasks.

All the changes along the span are expressed as a sum total coefficient of lift, and if you plot it next to alpha, you can see exactly how CL is affected by changes in AoA, and you can plug that into the lift equation.

It's making a complex wing a simple curve.

But again, that does not tell the whole story, and is the reason I am taking issue with what he's saying. It's not that I disagree with either of you -- I'm saying you can think of it both ways.

In his version, a change in aileron deflection, creates a change to the C of L curve. Same thing would happen with flap deployment. If he wants to think of it that way, the AoA of the entire wing doesn't change -- just the shape and position of the CL curve.

What I am saying he's missing, that is probably more important to a student, is that the CL curve changes precicely because individual sections of the wing are changed -- their camber, and angles of incidence (and therefore AoA) are altered. We both know it, and I suspect he knows it as well -- it's just that his restrictive definition of AoA (which he says can only relate to the entire span) doesn't allow him to readily acknowledge it.

[dr.aero]

Good science is about making complex things simple.

No, it is not! That is representative of the level of ignorance about science in this thread.

What I am saying he's missing, that is probably more important to a student, is that the CL curve changes precicely because individual sections of the wing are changed

You seriously think I don't know what affects the CL? Wow. I love how you call my definition of AoA "restrictive" too.

I think I'll tip my hat, it's becoming worthless to continue.

Cheers!