dr.aero wrote: Back to angle of attack: so essentially what you're doing is coming up with a different definition for angle of attack, one that makes sense to you under the conditions of a specific subject, but will fall apart when you start going into different subjects.

Could the reader be credited with the intelligence to keep in mind that there are several different definitions of Angle of Attack, and to use the one that's appropriate for the work that they're doing at that time?

You had, by keeping the chord line relative to the shape of the airfoil, kept the CL line in the same position (roughly).

I did whatever I had to do to keep the Cl line in the same place on the graph, yes.

Look at the lift equation --> L = 0.5 rho V^2 S Cl -- you claim that the angle of attack had increased, resulting in more lift, and the Cl had stayed essentially the same. But in the lift equation you have no place to account for AoA separately. AoA and the coefficient line determines what coefficient of lift you're at and the Cl is what determines the lift! So if you make the Cl stay the same, by saying the angle of attack changes, you really haven't changed the lift at all. Does that make sense?

You draw a correct conclusion, but from something I didn't say. I didn't say the Cl was the same. I said the Cl curve was the same. Cl is a function of angle of attack. Change the angle of attack - and keep the same Cl curve - and you get a different Cl. That's what gives you the different lift. I hoped that was reasonably clear from the graph I sketched (remember Angle A and Angle B, and the two different values of Cl noted on the graph? Maybe it wasn't clear.)

Back to your quote I have at the top - you say "zero lift angle". That essentially means zero lift angle of attack (geometric). I explained that in detail in my last post (stating that it was the chord line of the clean wing) and you said you understood it. Knowing that, does that clear it up?

Alright. I understand now, that when you say zero lift angle you mean "relative to the zero lift line of the clean wing". When I say "zero-lift angle" I mean, relative to the zero lift of the wing section as it is, at that second. I will take your word for it that's not how the term is used by professional aerodynamicists. It's how I think Denker uses it (I'll email him to check), but, regardless, there does exist at every moment a physical quantity that is "the angle of the airflow relative to the zero lift line of the actual airfoil section as it exists at this precise second, clean or otherwise". That's the one I want to use. It's a quantity of interest to me, I'm allowed to use it, and I will use it - it's my explanation, after all. I'm much less interested in what name we give it. We can call it something else if the name clashes with a different physical quantity used in any given textbook. But - if you're going to critique my explanation, I'd like you to do it accepting that terms are used to mean what they're defined to mean in that explanation - not in an unrelated academic text. it's out of order to say "that's not the right definition of zero-lift angle". It's my explanation, and my definition. Let's pick a different name for it, if we need to.

If you want to give me a slap on the wrist for using "standard" terms in a non-standard way without explaining them clearly, then I'll accept it. I've learned from numerous discussions on the Internet and elsewhere to make sure that you all use terms in the same way otherwise, yes, you're going to be arguing two sides of two different arguments.

Thank you by the way - I appreciate your consideration, and particularly, your engagement with the technical details.

I just have a quick question for dr.aero

since you make repeated references to people's background in physics and/or aerodynamics may I ask what your experience is in these fields?

300_hour_wonder wrote:I just have a quick question for dr.aero

since you make repeated references to people's background in physics and/or aerodynamics may I ask what your experience is in these fields?

It's really not relevant. An explanation needs to stand up on its own feet, and be judged regardless of the bona-fides of the person making it. Usually the better qualified the person doing the explaining, the more easily they can make it do so. I don't mind if a coherent point of aerodynamics is explained to me by a seven year old, any of my old university physics lecturers, or the ghost of R.P.Feynman himself. I'll judge it on its own merits, as should anyone.

The funny part is that you begrundgingly admit that you are just being pedantic (and I quote):

I have admitted that we're arguing about definitions many times on this thread. And you've taken my quote out of context - it means what you'd like a reader to think it means!

Photofly...

Could the reader be credited with the intelligence to keep in mind that there are several different definitions of Angle of Attack, and to use the one that's appropriate for the work that they're doing at that time?

Yes. The different definitions of AoA include: geometric, absolute, effective, induced, etc. They all use a chord line based on clean wing though.

You draw a correct conclusion, but from something I didn't say.

You say that after bolding my: "if you make the Cl stay the same. But you say: "I did whatever I had to do to keep the Cl line in the same place on the graph, yes."

That is confusing.

Change the angle of attack - and keep the same Cl curve - and you get a different Cl.

That is correct. But you DIDN'T keep the same Cl curve!! You moved the Cl cure AND moved to a different AoA.

This is really hard to explain clearly - which is frustrating me. If you and I could sit in front of a whiteboard and actively interact, I could explain this and I'm quite sure I could clear it up for you!

That's the one I want to use. It's a quantity of interest to me, I'm allowed to use it, and I will use it - it's my explanation, after all.

When you're discussing a technical subject and you're using a different definition than someone else is using, don't expect them to continue with your discussion - regardless of who is using the correct definition or not.

You don't understand what I'm trying to teach and it's much easier to teach this if you will just accept the definition I'm using until you can see why I'm using this particular definition and why aerodynamic engineers also use it. I can't explain this correctly if you refuse to just accept the definition I'm telling you should be used in this case. I hope you understand.

It's really not relevant. An explanation needs to stand up on its own feet, and be judged regardless of the bona-fides of the person making it. Usually the better qualified the person doing the explaining, the more easily they can make it do so. I don't mind if a coherent point of aerodynamics is explained to me by a seven year old, any of my old university physics lecturers, or the ghost of R.P.Feynman himself. I'll judge it on its own merits, as should anyone.

I couldn't have said it better myself!

the reason I ask is when you make a broad statement like all aircraft desginers use this method but don't back it up with any proof I just was interested where the statment came from

back to the topic at hand

I think this kinda sums up what has been going on here its been borrowed from another site

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:

the reason I ask is when you make a broad statement like all aircraft desginers use this method but don't back it up with any proof I just was interested where the statment came from

If I ever was interested in providing proof of that statement, how would I prove that to you?

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:

That's an excellent quote! It's too bad you just don't see what it actually means. All it's saying is that it's going to use absolute angle of attack instead of geometric. Nothing wrong with that. If you look back I provided expanded definitions of both geometric and absolute AoA.

We're not debating that. We're talking about using the chord line or zero lift line for a clean wing profile and I'm saying that you always keep it relative to the clean wing and photofly is saying that you don't!

sorry my bad I had some cut and paste issues

here is the rest of the quote


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.

I'm saying that you always keep it relative to the clean wing and photofly is saying that you don't!

Hurrah. We've distilled this down to the core of the argument. It took a long time, but we got here. You just won't let me define my own quantity of interest, regardless of what I want to call it.

The different definitions of AoA include: geometric, absolute, effective, induced, etc. They all use a chord line based on clean wing though.

My one doesn't. I expect the intelligent reader to take it in his stride. I expect the really intelligent reader to realise quickly how it makes good sense from a pilots point of view. I won't say what I think of the kind of reader who isn't flexible enough to accomodate the idea and wants to terminate the discussion purely on that basis.

That is correct. But you DIDN'T keep the same Cl curve!! You moved the Cl cure AND moved to a different AoA.

I move the Cl curve so it's superimposed over the old Cl curve. (Before I move it, it's the same shaped line - near enough - in a different place on the graph. Now it becomes the same curve but in the same place. I do that by an "arbitrary" choice of the reference angle. I've more to say about that.

Let's look at this choice of reference angle for the Cl curve: In fact it's not in the least bit arbitrary. I pick something that's hard for an engineer to predict on paper (don't give a sh*t) but very easy for a pilot to sense (care very much) and understand how control inputs affect (love that fact). Which is what makes the accompanying explanation of the action of the aileron valuable and appropriate for this forum.

The reference line I consistently choose to measure the angle of the airflow against, is, quite clearly, the line which would give zero lift in the configuration of the wing, or section at that time. As a pilot I care very much indeed about the angle my airflow makes with this line since my safety in the air depends enormously upon it, and not at all about any other "Angle of Attack" Why? Because if I have an appreciation of what this angle is and how this angle changes in flight for different parts of the wing I have a very good handle on:

- How much lift I'm generating: When this angle is zero, I have no lift. At equal airspeeds, and away from the stall, my lift is in proportion to this angle.
- How much drag I'm generating: when this angle is zero, I generate no induced drag. As this angle rises, for equal airspeeds my drag goes up.
- How close to the stall I am: when this angle gets to around 15 degrees, my coefficient of lift reaches a maximum. Flaps up, flaps down, ailerons up, ailerons down. Somewhat higher, with flaps down - that's their effect. All the complicated shifts in the Cl curve that I don't know about and can't calculate in the cockpit disappear.
- What the airflow around the wing section looks like: tell me the value of this angle and I can sketch the airflow pattern - I don't need to know anything at all about the wing section - I can treat it like a perfect black box.

If I had a cockpit display that would indicate my choice of any single angle of attack (or angle of anything related) in flight, this would be the one I would pick, hands down. I'm sure you would too, if you give it some thought.

So there you go: as far as understanding the aerodynamics of control inputs goes I believe it's a much more useful concept to a pilot than any (other) definition of Angle of Attack. I'm afraid I don't really care if aerodynamic engineers fall off their draughting stools with self-righteous indignation that I should want to consider it.

Colonel!...

http://www.avweb.com/blogs/insider/AVwe ... 911-1.html

Written 2 days ago. Interesting that stalls are at the top! It's quite unfortunate but I don't see much of a change happening to that considering the amount of misinformation about stalls and aerodynamics that gets passed on on forums like this and in classrooms around the world by instructors who aren't qualified to teach these subjects.

One of the problems is that most pilots think they have a perfect understanding of all aerodynamics and most examiners aren't qualified to determine if they do or don't. It's quite sad actually. We're throwing people into the pilot seat who shouldn't have a pilot license.

You may be able to train someone to pass a Transport Canada flight test without going into depth on aerodynamic theory (you actually don't need to teach anything on aerodynamics; the benefit of rote learning!) but I find that attitude to be just what we need to stamp out if we're going to make a difference in the quality of pilots we're turning out.

300 hour...

And you still don't get what the debate is about! Are you going to answer my question about providing proof for you? Or have you decided that you won't want any proof now?

Photofly...

Hurrah. We've distilled this down to the core of the argument. It took a long time, but we got here.

I knew what the core of the argument was pages ago...

I expect the really intelligent reader to realise quickly how it makes good sense from a pilots point of view.

It doesn't! You still don't see how it doesn't make sense yet. I've been trying and trying for pages to explain it to you but you're refusing to work with the definition I'm using. This is going absolutely nowhere. I already can see you have a proper understanding of what happens when you deflect an aileron with regard to the lift and that deflecting an aileron brings it closer to the stalling AoA. You do understand that. How about we just leave it there? We're obviously not going to get past this definition issue.

You are refusing to use the correct definition and I refuse to start teaching something incorrectly. We need to drop this.

It doesn't! You still don't see how it doesn't make sense yet. I've been trying and trying for pages to explain it to you but you're refusing to work with the definition I'm using.

Because the very thing that makes sense is to not use your definition!

I originally thought that your grievance was my referencing AoA to the "instantaneous" zero-lift line, rather than line based on the clean wing chord.

It transpires that you refuse to let me define any quantity at all based on the "instantaneous" zero-lift line. And the only reason you can give me is that it will upset an aerodynamic engineer because it doesn't appear in his text book. Too bad for him, I don't accept that as a valid argument.

Photofly...

I find it amusing that you provide this picture to support your argument: download/file.php?id=7079

And you also stated that the critical angle of attack changed! But then when I showed my picture from my textbook (showing the same type of Cl line movement as in your picture you posted), you dismissed it as being wrong and proceeded to draw two Cl graphs with the EXACT same line for flaps up and flaps down. Do you see your contradiction?! Compare your graphs that you drew to this picture you attached: download/file.php?id=7079

Why are they different?! I'll help you out... one of them is wrong!

Photofly...

It transpires that you refuse to let me define any quantity at all based on the "instantaneous" zero-lift line. And the only reason you can give me is that it will upset an aerodynamic engineer because it doesn't appear in his text book. Too bad for him, I don't accept that as a valid argument.

The problem comes when you use your different "language" to explain a picture off the internet - such as the one you attached with the different flap positions. Most pictures off the internet, like the one you had attached on page two of this thread, follow the rules of aerodynamic engineering and when you try to explain to someone else using those pictures I really hope they have enough intelligence to see that the pictures don't add up with your explanations!

So according to your vision of flight training dr.aero. unless a person has a deep understanding of aerodynamics and mathematics they can not be a real pilot in your opinion?

So according to your vision of flight training dr.aero. unless a person has a deep understanding of aerodynamics and mathematics they can not be a real pilot in your opinion?

Not at all, Cat.

It's about having a correct understanding. The aerodynamics doesn't have to be university level for a pilot at all, but what does have to be taught needs to be correct!

OK, this is technical stuff. That's good.

dr.aero wrote:Photofly...

I find it amusing that you provide this picture to support your argument: download/file.php?id=7079
And you also stated that the critical angle of attack changed! But then when I showed my picture from my textbook (showing the same type of Cl line movement as in your picture you posted), you dismissed it as being wrong and proceeded to draw two Cl graphs with the EXACT same line for flaps up and flaps down. Do you see your contradiction?! Compare your graphs that you drew to this picture you attached: download/file.php?id=7079

Why are they different?! I'll help you out... one of them is wrong!

There's no contradiction. The graphs are different, and they're both correct.

The graphs are different because the x-axis variables are different in each case.
The scan from the NACA report has section angle of attack on the x-axis variable.
My sketch graph has the "angle between the free stream flow and the instantaneous zero-lift line" as the x-axis variable.

It's axiomatic that any graph of Cl vs. "angle between the free stream flow and the instantaneous zero-lift line" must pass through the origin. I don't think you'd disagree.

If you change the x-axis quantity on my sketch to "section angle of attack" then the lines for the wing with the aileron up and down will slide apart and it will match the NACA graph, in substance.

If instead you redraw the the NACA data so that the x-axis is "angle between the free stream flow and the instantaneous zero-lift line" then all the lines on the NACA graph will slide sideways and pass through the origin, just as I sketched.

There's a difference in the height of the different curves of the NACA graph; the NACA curves are for a flap that descends up to 60 degrees. One of the effects of increasing the camber of a section by lowering the flap is to increase the peak value of Cl. My sketch curve was for an aileron, which descends only 10 degrees. If you look at the NACA graph for the flap extended 10 degrees you'll see the peak value of Cl hasn't change very much. I took a liberty in not showing this difference in my sketch, because it's not significant, although we discussed it.

I don't recall dismissing the picture from your textbook as wrong. I recall saying it was a valid explanation. I also recall saying that it wasn't the only valid explanation.

The problem comes when you use your different "language" to explain a picture off the internet - such as the one you attached with the different flap positions.

I didn't explain the NACA graph in different language. I think you misunderstood the point I was illustrating by posting that graph. I'm happy to have another go at making the point, although I think it's a point that you and I both accept as true, at this point. I was still, at that time, trying to work out what your objections were.

Photofly...

The graphs are different because the x-axis variables are different in each case.

They're different? So in one of them the origin isn't actually at zero on the x-axis? Geez! I'm done with this crap.

Here's a reference to show you're wrong to do that: http://en.wikipedia.org/wiki/Origin_(mathematics)

Quote from that wikipedia reference: "The coordinates of the origin are always all zero, for example (0,0) in two dimensions and (0,0,0) in three."

But I'll be waiting for you to come back and tell me that you're not wrong, you're just doing it differently!

If you adjust the x-axis variable of either graph to match the other, the lines will coincide, and the graphs will no longer be different.

Whoever taught you math has failed you horribly.

Of course the what one teaches has to be correct.....

.....and as I have stated before you can teach an illiterate how to very skillfully fly an airplane.

One of my students was an astronaut and a test pilot for Airbus.....we flew together for two years and never did either of us get into the depth of aerodynamics that is being discussed here.

I'm out of this now because it is obvious my thoughts on flight training and yours are quite different.

One more comment though, both Andrew and I post here and express our thoughts on the subject of flying backed up by what and who we are.

Cat...

One of my students was an astronaut and a test pilot for Airbus.....we flew together for two years and never did either of us get into the depth of aerodynamics that is being discussed here.

I'm out of this now because it is obvious my thoughts on flight training and yours are quite different.

I've mentioned this about twice now but I'll do it again.

There is no need for a pilot to get into this depth of aerodynamics!! I can't make that any clearer. The only reason we've gotten into this depth is because of a question, initially posed by photofly, asking to get an explanation of why the outside wing has a higher angle of attack in a climbing turn. I provided a clear explanation of that in my first post on this. From there it went into discussion about poster on here using incorrect terminology and that's where all this crap is coming from.

You agree with me that what is taught needs to be correct and then you say that "it is obvious my thoughts on flight training and yours are quite different." - Whaaat??? Where did that come from?

Let me see if I can be clearer, then.

They're different? So in one of them the origin isn't actually at zero on the x-axis?

The physical variable that's plotted on the grap, as the x-axis variable, for the NACA graph, is not the same variable along the x-axis in my sketch graph.

My graph sketches Cl (y-axis) vs. "angle between the free stream flow and the instantaneous zero-lift line" (x-axis)

The NACA graph plots Cl (y-axis) vs. section angle of attack (x-axis)

The variable along the x-axis is different. Section angle of attack is not the same as "angle between the free stream flow and the instantaneous zero lift line". That's why the NACA graph doesn't match mine.

The difference betwen "angle between the free stream flow and the instantaneous zero-lift line" and "section angle of attack" is a fixed quantity as long as the airfoil section remains unchanged. It varies according to changes in the airfoil, such as a by lowering a flap or aileron. It's therefore clear how to redraw the data from one graph in a manner that's consistent with the other. If you do that the graphs will be sustantively the same.

Whoever taught you math has failed you horribly.

Can I politely suggest that you start making an attempt to understand what I'm saying, rather than making assumptions about my education?

Photofly...

Just answer the question - does the origin in both of your graph have an x and y value equal to zero?

Cat...

I gotta ask - do you think photofly is correct?

dr.aero wrote:Photofly...

Just answer the question - does the origin in both of your graph have an x and y value equal to zero?

Both my graph... that's a singular and a plural in the same sentence.

The origin of my sketch graph has a Cl value of zero, when the angle between the free stream flow and the instantaneous zero-lift direction is zero. It passes through the origin.

I define the instantaneous zero-lift direction of an airfoil section, as, in the configuration as it is at that moment, to be the angle of the free stream flow that generates no lift, or, equivalently, that direction of free stream flow for which the coefficient of lift is zero.

It's therefore axiomatic that a graph of Cl vs the angle between the free stream flow and the instantaneous zero-lift direction passes through the origin. It cannot be any other way.

It's true for a wing with an aileron up, and it's true for a wing with an aileron down. All graphs of Cl vs "the angle between the free stream flow and the instantaneous zero-lift direction" pass through the origin. A curious feature of aerodynamics is that they all have very closely the same gradient - about 0.1 units of Cl for each degree.

You can see as clearly as I can where the (0,0) point on the NACA graph is.

Person claiming to have a PhD in Aeronautical Engineering wrote:

if we're going to make a difference in the quality of pilots we're turning out

I have no idea as to the quality of your students - I
strongly suspect they are horribly confused - but I
will gladly put mine up against yours.

Here's a student pilot of mine, with a helmet cam on,
flying wing on me:

http://www.youtube.com/watch?v=RVXWiIrTZlg

Here's a still shot of him flying formation:

http://www.pittspecials.com/images/eric_form1.jpg

He obtained his Statement of Aerobatic Competency,
valid for Solo and Formation Aerobatics at Level 3
(500 feet) on all Pitts variants, at the age of 19.

What is your Statement of Aerobatic Compency
endorsed for? What acts? What level? What aircraft?