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Originally posted by Harsh Penguin:
I think the comment that was made earlier about the important aspects that a pilot should know - for example - when I get too fast or too slow - is rather applicable.

I find that the most important safety advice is to stay away from the edges (of the air). The middle is pretty safe. The edges are where you find ground, water, cliffs, trees, condos, etc.

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Blade stall is something we can understand. For a Helicopter, blade stall Means go slower. Confusing - but let us stay away from retreating blade stall - laughs.

Hey, when you fly an aircraft whose wings are going in all directions at once - what do you expect?

So true. Follow the Manual and things should go well.

Very funny - Crazy wings.

you don't need a perfect understanding of how it does what it does; if you are not designing them - a general understanding is usually sufficient to keep from doing something that will make it fall out of the sky.

You know - that is why I started this subject, because it is irritating to teach a subject that you do not know comprehensively.

Anyone who has the privilege of teaching aviation (or teaching anyone anything professionally) should feel responsible for teaching it correctly. The Instructor Pilot (IP) should know the subject sufficiently if the student wants to take it further. That is why you are the Instructor Pilot. If not - he may as well learn everything from a computer or book. You have the students attention and the communicative ability to clarify in terms that he or she can understand. I can never back away from the subject and say - hey - not that important - lets move on. That is nothing more than saying "You have exceeded my knowledge level and I will cover it up by saying that we will move on" UGH!

If I was just another pilot - maybe I wouldn't care, but I am an Instructor Pilot and willing to study.

I want the instruction to be correct. There are many things that bother me in life - but one of the burdens that an Instructor Pilot carries with him for a life time - is "Did I teach my students correctly?" It is embarrassing when you realize that you taught a subject wrong - and with confidence - doubly embarrassing. They realize that you taught them wrong - and guess what - then everything you taught them is questionable. Rightfully so.

Now I have to admit - I taught what I was taught. So clearly there is a systemic problem, I don't blame myself entirely. At what point do we say - professionally - HEY - we're wrong. Lets fix it - simplify what we are teaching - and move on. Has that already happened? Maybe. Maybe the current Manual has taken ALL of this into consideration already and said - WOW - TOO MUCH - let us just simplify and move on.

Pilots - Don't like to be mislead. I am not sure anyone does. Teach it to me straight and let me figure out what I want to take with me in my Pilot bag of knowledge.

I am still going to persue this subject until I get it straight enough in my own head. Whether or not a student cares to know about it is something I don't care about nor will I push it that he/she understands it this deeply. At least - if he does - and there are many that do - I will feel better prepared.

I am not sure I want to touch the subject again - until I speak with this NASA person who wrote the article so that he can educated me fully - in simpler terms - so that I can teach it correctly to my students.

I would like to take it a little further and adjust the current publication of the FM 1-203 The Fundamentals of Flight The US Army's Helicopter Aerodynamics book). It appears to be teaching these false theories. I, for one, feel obligated to my students (and Myself) to teach correctly.

Especially if I am an Instructor Pilot. Oh - by the way - I am a Military Instructor Pilot. So to increase the problem - ALL - Military helicopter pilots are taught the incorrect theories. I was shocked when I read these NASA articles. First off - Military Helicopter Training is Superb. The People are Superb. However - the Manual and the People that teach helicopter aerodynamics - in regards to the production of lift - appear to be wrong. Ugh!

Very true; it took me over twenty years to get past the equal transit path "theory" that was taught to me in flight school. I rebelled against the idea right away which of course didn't endear me to the ground school instructor.

It wasn't until I got involved in the DDWFTTW thread that I finally was straightened out about how aero works - by ziploc (et al; must give credit to others too!).

Do stop in on that topic if you want to put your newly found knowledge to work on an intriguing aero brain-teaser. Be careful though, it can be an all-consuming journey.

I think a big part of the tendency to misrepresent the true physics of flight is to make sure pilots have a good grip on the consequences of losing lift due to pilot errors. Attempting to explain too much can get in the way of the important point: you need to keep air flowing over the wings at the correct speed and angle. Anything more is interesting to the aeronautical engineer, but not the pilot.

It's like driving a car: you don't need to know the physics of static and dynamic friction to be a good driver. You just need to know enough about grip, pavement variations, sufficient control inputs, and how to control skids and slides.

Look - I have over 3000 hours of Helicopter flight time and 4 combat tours. Not once have I ever tried to figure out why I was flying - while I was flying. Its natural. It just does. People have worked out the bugs and problems long ago. It is not about that. It is about being able to instruct the topic correctly.

NO - I do not need to know how lift is created.

In fact, we have argued if aerodynamics should even be taught. It really should be taught, but some will argue - WHO CARES? It flies just fine without me knowing how. Just like the operation of an engine. DO I really care how it works as long as I go where I want and how i want to do it. I fly it fine without knowing why?

That is all fine and well - but it still is shameful that the simple fact remains - that we are teaching a subject wrong and no one seems to really care.

Lets simply explain what we are saying correctly. Is it that much of a quantum leap to paraphrase the correct theory/Phenomona appropriately. Why is it that we keep turning away from the education of the subject. I don't care if I do or don't need to know it for aviation. I simply want to understand the phenomena clearly. What is going on that creates lift = flight. Does it get sucked up or pushed up. I still do not think that we have agreed to the original question as to what answers this question clearly. Let alone the fact that I am more confused than I was when I started this thread - laughs.

ZIPLOC - has certainly been helpful in clarifying the theories that I understood, but roofinguy and his links sent me down Alice's Hole.

Did we ever come to an agreement as to whether or not the airfoil is sucked or pushed up? Let alone how Lift is created now....ugh. I love it - this is how education works. Exhaust all angles until something is left that sticks.

Looking for a simplified phrase that sums up lift (Correctly) and whether or not it is sucked or pushed up. Any takers?

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Originally posted by Harsh Penguin:
You know - that is why I started this subject, because it is irritating to teach a subject that you do not know comprehensively.

That is a noble goal, but there are many subjects that could never be taught by anyone if they held themselves to that standard. I'm reminded of something a friend told me when he was getting his PhD... it's not about learning all there is to know about a subject; it's about becoming more comfortable with how little you know.

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Anyone who has the privilege of teaching aviation (or teaching anyone anything professionally) should feel responsible for teaching it correctly.

I agree. But I fear you and I are in the minority in that respect. I think most flight instructors and ground school instructs strive to teach good piloting and navigation skills. Few seem to understand aero or care enough about it to do what you're doing now.

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I can never back away from the subject and say - hey - not that important - lets move on. That is nothing more than saying "You have exceeded my knowledge level and I will cover it up by saying that we will move on" UGH!

I find nothing wrong with saying "you've exceeded my knowledge". Trying to cover that up however is annoying at the very least.

I had a physics teacher in high school that I really like and admire. He wasn't the greatest physicist in the world, but he sure was a good teacher. And he wouldn't hesitate to tell you the bounds of his knowledge or where to go to learn more.

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I want the instruction to be correct.

Agreed. Not knowing is one thing. Teaching things that are simply false bothers me much more.

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Pilots - Don't like to be mislead. I am not sure anyone does.

Really? You should come join us on the religion thread

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I am still going to persue this subject until I get it straight enough in my own head. Whether or not a student cares to know about it is something I don't care about nor will I push it that he/she understands it this deeply.

I recommend you learn it and teach it accurately, but you really don't have to get into a level of detail that will cause them to lose interest. They're there to learn to fly. Understanding some aerodynamics is a very good thing - but keep in mind people get PhD's in the subject.

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I am not sure I want to touch the subject again - until I speak with this NASA person who wrote the article so that he can educated me fully - in simpler terms - so that I can teach it correctly to my students.

Understood. If I were to simplify the whole thing to a sentence or two, I'd say...

Lift is caused by faster moving air over the wing. Faster moving air exerts less pressure (thus lower pressure on top fo the wing than on the bottom). The wing diverts the flow downward (both above and below the wing). We can compute lift by summing up the pressure differential OR by computing the momentum imparted on the diverted flow. What we can't easily do is explain exactly *why* the air moves faster over the wing - suffice it to say all the simple explanations are wrong.

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I would like to take it a little further and adjust the current publication of the FM 1-203 The Fundamentals of Flight The US Army's Helicopter Aerodynamics book). It appears to be teaching these false theories.

That's a great goal.

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Oh - by the way - I am a Military Instructor Pilot. So to increase the problem - ALL - Military helicopter pilots are taught the incorrect theories.

I'm sure you noticed my comment about talking aero with military pilots. That is not meant to disparage military pilots. They tend to be very self confident, and they know what they're taught. So we shouldn't teach them things that aren't true.

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First off - Military Helicopter Training is Superb. The People are Superb. However - the Manual and the People that teach helicopter aerodynamics - in regards to the production of lift - appear to be wrong. Ugh!

We're in complete agreement.

Incidentally, there are plenty of very bright aerodynamicists at NASA. But they are people too. Some get things wrong sometimes.

I built a wind powered vehicle that goes directly downwind much faster than the wind (to settle a long running debate). I've had high level NASA aero PhD's assure me it was absolutely impossible. To their credit, none of them stuck to their guns (as some others have done).

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Originally posted by Harsh Penguin:
Did we ever come to an agreement as to whether or not the airfoil is sucked or pushed up?

Unfortunately you've hit upon a question that's very simple to ask, but not quite as simple to answer. Not because we don't understand and agree as to what's really going on, but because the answer gets wrapped up in semantics and philosophy.

To simplify it (if I can) the answer is - "the wing is pushed up - NOT sucked up". But the reason that gets tricky is that NOTHING can be "sucked up" in reality. You can simply reduce the pressure above so that that greater pressure below is able to push it up.

That being said, you could ask why the word "suck" exists if nothing can actually be sucked up. But in common parlance, it's easier to think of our vacuum cleaners as sucking the dirt up.

That is a noble goal, but there are many subjects that could never be taught by anyone if they held themselves to that standard.

Yes - Noble - perhaps not realistic though, When you consider the scope of subjects that can be covered. I guess that I just want to nail this one down because it seems so fundamental to flying. I have a hard time with its ambiguity.



I think most flight instructors and ground school instructs strive to teach good piloting and navigation skills. Few seem to understand aero or care enough about it to do what you're doing now.

Perhaps, not saying that these subjects don't deserve appropriate attention; however, when operating frequently in the Height/Velocity Danger area in a single engine helicopter it would bode well for every scout pilot to know what altitudes and airspeeds are necessary to still give you a chance for survival. This situation, autorotations and a plethora of other helicopter unique aerodynamic scenarios is all connected to aerodynamics in some degree or another. To understand them - I certainly feel has a stronger pilot at the controls. Not saying that it will make a great difference - but it may.



I find nothing wrong with saying "you've exceeded my knowledge". Trying to cover that up however is annoying at the very least.

I agree with this also. I always respect people - who are still great teachers - that simply are truthful when it comes to a certain point. They say it - I don't know." I like it when that happens because it validates everything else that they have taught. At least what they have taught is correct and I can accept that.

Really? You should come join us on the religion thread

Total laughs - and NO way!

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To simplify it (if I can) the answer is - "the wing is pushed up - NOT sucked up". But the reason that gets tricky is that NOTHING can be "sucked up" in reality. You can simply reduce the pressure above so that that greater pressure below is able to push it up.

This is simple enough. I like it. Thank you for writing it. What difference does it make that the upper air actually beats the lower air to the trailing edge of the airfoil?

If this pressure differential still creates lift, where is the false teaching other than the upper air molecules beating the air molecules on the bottom - instead of meeting it at the same time. So what? What does that really mean in the great Lift equation quagmire?

It appears that most of what is taught is close to being truthful with exception of the airflow over the top. This leads me to believe that at least Bernoulli's principle is correctly being incorporated into this equation. (I will have to ponder the Venturi effect a little more.) It is just a matter of what is actually happening with the principle. Bernoulli's principle still applies - just with a greater degree of effect. Since the airflow is faster than the "Equal-Transit" theory suggests there is even LESS pressure over the top of the airfoil facilitating an even greater upward push due to the differential of pressures. I can live with that. That seems to be an easy adjustment for instruction.

NO - the molecules do not meet at the same time.

In fact, the upper molecules move significantly faster than once thought, increasing Dynamic pressure, resulting in an even greater net lift production.

The "Turning" seems to be another term to describe down wash or induced flow. This turning phenomena is accelerating the air over the airfoil downward at an angle commensurate to the pitch angle.

The upward push of the airfoil seems to come from the oncoming air pressure striking the bottom profile of the blade. So this combination of increased dynamic pressure atop the airfoil coupled with the air pressure striking the bottom half of the airfoil allows the blade to easily lift upward. Simply because there is little or no resistance to the upward thrust. I do not see a Sucking or Pulling up event here. Rather an upward pushing. With that being said I would gravitate more towards a pushing theory versus and upward sucking theory.

Tell me where I am going wrong in this explanation, or do I have it correct?

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Originally posted by Harsh Penguin:
If this pressure differential still creates lift, where is the false teaching other than the upper air molecules beating the air molecules on the bottom - instead of meeting it at the same time. So what? What does that really mean in the great Lift equation quagmire?

The equal transit time theory seeks to explain why the air travels faster over the top of the wing, and it gets it completely wrong. Remember there are two parts to this thing:
1) Computing/explaining the velocity profile over the wing
2) Computing the lift and drag that results from that velocity profile.

Item 1 is clearly the more difficult one by far. The equal transit time theory suggests a trivial answer to it - but that answer is not only wrong in its description as to why it happens, but also gives a result that will compute the wrong lift by a long shot.

Make sense?

OK - I am not interested in computing the velocity at this point - just explaining.

So the equal transit theory is trying to eliminate the Venturi effect in regards to air flow acceleration and replace it with its own explanation? Or is it still incorporating the Venturi effect in its process?

OK - I understand that when trying to compute the total NET force of lift that the velocity of the airflow makes a huge difference. I got that. I would like to focus on the why the airflow is accelerating now. To Venturi or not to Venturi?

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Originally posted by Harsh Penguin:
OK - I am not interested in computing the velocity at this point - just explaining.

Understood. The danger is in explaining that the air speeds up because it has further to travel. That isn't really why it speeds up.

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So the equal transit theory is trying to eliminate the Venturi effect in regards to air flow acceleration and replace it with its own explanation? Or is it still incorporating the Venturi effect in its process?

The venturi explanation is also incorrect. You really have to solve the Navier-Stokes equations to compute the airflow around the wing.

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I would like to focus on the why the airflow is accelerating now. To Venturi or not to Venturi?

That gets complicated. That's the part I mentioned I'd never truly heard an intuitive and correct explanation for. The equations themselves are reasonably intuitive. They simply enforce things like conservation of momentum. But when you put it all together, there's no simple explanation like "the air had to go faster because it had further to go".

I can try to get into it a bit deeper if you like, but am on the way out the door to go skiing now.

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Originally posted by ziploc:

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I would like to focus on the why the airflow is accelerating now. To Venturi or not to Venturi?

That gets complicated. That's the part I mentioned I'd never truly heard an intuitive and correct explanation for. The equations themselves are reasonably intuitive. They simply enforce things like conservation of momentum. But when you put it all together, there's no simple explanation like "the air had to go faster because it had further to go".

I can try to get into it a bit deeper if you like, but am on the way out the door to go skiing now.

That's about where I got to when trying to get answers.

F=MA, so in order for the molecules of air to accelerate they had to be acted on by a force. The equal transit theory is noticably void of any source for that force.

At the risk of confusing things, I'll explain how I visualize what's happening.

As the wing passes through the air, it moves the air out of the way and down (drag and lift). That increases the air density below the wing. It also creates a pocket of decreased air density above and to the rear of the wing. Since pressures equalize, the surrounding air provides the force needed to push the immediate air into the low pressure pocket, thus accelerating the air over the wing in the direction of that pocket.

The pressure over the top of the wing is lowered both by the decrease in density and the acceleration of the air. The pressure under the wing is increased by deflection and drag. The net result is a change in momentum of the air in a downward direction.

When ziploc gets back he can tell me where and why I messed up.

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Originally posted by oldguy1:

As the wing passes through the air, it moves the air out of the way and down (drag and lift). That increases the air density below the wing. It also creates a pocket of decreased air density above and to the rear of the wing. Since pressures equalize, the surrounding air provides the force needed to push the immediate air into the low pressure pocket, thus accelerating the air over the wing in the direction of that pocket.

I think that's essentially true. And it brings up an interesting point that I think a lot of folks miss. We can say the faster moving air exerts less pressure (which is true) or we can say that the lower pressure causes the air to move faster (for the reason you explained). We want a simple cause and effect relationship, but we can't really say that one causes the other. We can only say that when we solve the Navier-Stokes equations for the airflow, we will find faster moving, lower pressure air on top of the wing. And yes, this will be consistent with Bernoulli (faster moving air exerts less pressure) and Newton (air caught between high and low pressure regions will be accelerated).

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The pressure over the top of the wing is lowered both by the decrease in density and the acceleration of the air.

At subsonic speeds air is generally treated as incompressible. You don't really see meaningful density changes until you approach mach speeds (but there's no hard line. You sometimes account for compressibility effects as low as mach 0.3).

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The pressure under the wing is increased by deflection and drag. The net result is a change in momentum of the air in a downward direction.

There's no question that there's a change in momentum, and that that accounts for lift (and at least a portion of drag). "Deflection" is considered a false explanation, but I have to admit, I don't know exactly what to make of that. The problem here being that "deflection" is just a word that we define. The flow does change directions, and this causes a force on the wing just as the force of the wing on the air causes the change in direction of the flow.