Time it takes air to travel over and under an airfoil

LatitudeDancer said:
2nd: Equal Transit = booooo http://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html = false
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This link fails to address my issue with this, what greater intuition does this give a primary student? How can I explain why it happens if my student says, "hey that is interesting, why?"

In the link they even say, "There are many theories of how lift is generated. Unfortunately, many of the theories found in encyclopedias, on web sites, and even in some textbooks are incorrect, causing unnecessary confusion for students." However, nowhere in the rest of the document do they provide any evidence to back this claim.

Furthermore they don't explain why this happens, instead they say it does happen and spend the rest of the time proving why equal transit is not exactly right.

In one instance, "does not agree with the lift that we measure for a given airfoil." This is the closest thing I saw as to a "reason" to not explain equal transit. Somehow I doubt our primary students are going to be running the lift equations to determine what their theoretical lift is.



One thing people need to realize with aerodynamics is that for every situation encountered in flight there are multiple aerodynamic theories to explain them. Some of these theories are, of course, more right than others, but they are still theories, even this one.

When teaching a primary student, or any pilot IMO, the goal is to give them the simplest theory that you can explain and link through with other theories. Pending the theory will not give a dangerous conceptual understanding or in other words make them an unsafe pilot. The theory will never be 100 percent correct, if it were, it would be a law, not a theory.

Barring any new information here I do not see any legitimate reason to throw out the equal transit theory for this slightly more correct theory. Especially when no explanation has been given as to why it happens, with the acceptation the Kutta condition, which has yet to be dumbed down.
 
shdw said:
The theory will never be 100 percent correct, if it were, it would be a law, not a theory.
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The word "theory" in science has a very different meaning from the way we use the word in every day life. It doesn't mean a guess, a hypothesis, or wild speculation. The American Association for the Advancement of Science offers this definition:
In detective novels, a "theory" is little more than an educated guess, often based on a few circumstantial facts. In science, the word "theory" means much more. A scientific theory is a well-substantiated explanation of some aspect of the natural world, based on a body of facts that have been repeatedly confirmed through observation and experiment. Such fact-supported theories are not "guesses" but reliable accounts of the real world.
​
A theory never becomes a law, but theories may contain laws. The fact that germs cause disease is called Germ Theory, but it's universally accepted to be true, except by whackos.
 
tgrayson said:
The word "theory" in science has a very different meaning from the way we use the word in every day life. It doesn't mean a guess, a hypothesis, or wild speculation.
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The point was more that this theory, while containing fact, is likely still not the complete picture and will thus be revised, rewritten, or expanded upon in the future. I wasn't meaning to imply a guess, but instead another likely incomplete picture.

I find it interesting that you say theory will never become a law. When a theory is found universally true in all cases does it not become a law? Didn't many physical laws begin as theories, at least thats how my science classes made it sound. Where they just over simplifying the process?
 
I don't understand why the equal transit story is so prevalent when the truth isn't any more complex... What's wrong with teaching/learning it the right way from day one??
 
shdw said:
This link fails to address my issue with this, what greater intuition does this give a primary student? How can I explain why it happens if my student says, "hey that is interesting, why?"

In the link they even say, "There are many theories of how lift is generated. Unfortunately, many of the theories found in encyclopedias, on web sites, and even in some textbooks are incorrect, causing unnecessary confusion for students." However, nowhere in the rest of the document do they provide any evidence to back this claim.

Furthermore they don't explain why this happens, instead they say it does happen and spend the rest of the time proving why equal transit is not exactly right.

In one instance, "does not agree with the lift that we measure for a given airfoil." This is the closest thing I saw as to a "reason" to not explain equal transit. Somehow I doubt our primary students are going to be running the lift equations to determine what their theoretical lift is.



One thing people need to realize with aerodynamics is that for every situation encountered in flight there are multiple aerodynamic theories to explain them. Some of these theories are, of course, more right than others, but they are still theories, even this one.

When teaching a primary student, or any pilot IMO, the goal is to give them the simplest theory that you can explain and link through with other theories. Pending the theory will not give a dangerous conceptual understanding or in other words make them an unsafe pilot. The theory will never be 100 percent correct, if it were, it would be a law, not a theory.

Barring any new information here I do not see any legitimate reason to throw out the equal transit theory for this slightly more correct theory. Especially when no explanation has been given as to why it happens, with the acceptation the Kutta condition, which has yet to be dumbed down.
Click to expand...

stupid nasa, putting people on the moon and all. what were they thinking
 
shdw said:
This link fails to address my issue with this, what greater intuition does this give a primary student? How can I explain why it happens if my student says, "hey that is interesting, why?"

In the link they even say, "There are many theories of how lift is generated. Unfortunately, many of the theories found in encyclopedias, on web sites, and even in some textbooks are incorrect, causing unnecessary confusion for students." However, nowhere in the rest of the document do they provide any evidence to back this claim.

Furthermore they don't explain why this happens, instead they say it does happen and spend the rest of the time proving why equal transit is not exactly right.

In one instance, "does not agree with the lift that we measure for a given airfoil." This is the closest thing I saw as to a "reason" to not explain equal transit. Somehow I doubt our primary students are going to be running the lift equations to determine what their theoretical lift is.



One thing people need to realize with aerodynamics is that for every situation encountered in flight there are multiple aerodynamic theories to explain them. Some of these theories are, of course, more right than others, but they are still theories, even this one.

When teaching a primary student, or any pilot IMO, the goal is to give them the simplest theory that you can explain and link through with other theories. Pending the theory will not give a dangerous conceptual understanding or in other words make them an unsafe pilot. The theory will never be 100 percent correct, if it were, it would be a law, not a theory.

Barring any new information here I do not see any legitimate reason to throw out the equal transit theory for this slightly more correct theory. Especially when no explanation has been given as to why it happens, with the acceptation the Kutta condition, which has yet to be dumbed down.
Click to expand...

Actually, there are NOT "many theories". It has been well established what causes lift for about 100 years now. There is no controversy outside of discussions among lay people (which I include many in this group).

There have been several threads here on this topic, rather than retype it all, do a search.

Here are a couple, to save you time:

http://forums.jetcareers.com/technical-talk/76196-bernoullis-principle-airflow.html

http://forums.jetcareers.com/technical-talk/77027-boundary-layer-separation.html

http://forums.jetcareers.com/technical-talk/34976-why-does-airplane-fly.html

http://forums.jetcareers.com/technical-talk/34813-seagull-help.html

There are more, but this will get you started.
 
LatitudeDancer said:
stupid nasa, putting people on the moon and all. what were they thinking
Click to expand...

If you do not have anything constructive to reply with, than why even waste our time replying?



Seagull, I have been reading since last night and am still reading, but fear not a reply is coming. Also if you didn't catch this already, I am not disagreeing with this theory, in fact I agree with it completely. Instead, I only question its necessity for presentation to the primary student (or any pilot).


E_Dawg, I will address that question as well when I reply to seagull after I finish his reading assignment and do some more of my own research.
 
shdw said:
If you do not have anything constructive to reply with, than why even waste our time replying?



Seagull, I have been reading since last night and am still reading, but fear not a reply is coming. Also if you didn't catch this already, I am not disagreeing with this theory, in fact I agree with it completely. Instead, I only question its necessity for presentation to the primary student (or any pilot).


E_Dawg, I will address that question as well when I reply to seagull after I finish his reading assignment and do some more of my own research.
Click to expand...

Are you asking why we are taught aerodynamic theory at all?
 
shdw said:
If you do not have anything constructive to reply with, than why even waste our time replying?
Click to expand...

Well I'm sorry, sky god of the universe. Didn't mean for me and my beer to waste your time. Silly us.

Side note: I'm going to put money on you're from above the Mason-Dixon
 
poser765 said:
excuse me...someone with authority...would you be so kind as to replace the "senior member" under my name on the left with the above quote. Thank you.
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Touche, that is the best thing I have heard anyone call someone in a long time!

Kudos to you latitude, you stopped too soon though, you were still able to type.


Seagul: No, only why it is such a big deal to teach equal transit or this theory. I only ask because honestly are they even going to remember it? Likely not, and if they are interested in it they will ask and then by all means start them off in the right direction. Just seems like a lot of fuss for a very small issue. I am still working on my research by the way, but for now it is bed time I will post tomorrow.
 
seagull said:
Actually, there are NOT "many theories". It has been well established what causes lift for about 100 years now. There is no controversy outside of discussions among lay people (which I include many in this group).
Click to expand...

Let me start by saying, I do not fully understand many of these theories but I will post them to show you what I was referring to. Also what causes lift has been understood, that is the difference of pressure, I am not claiming it is not. What has been under constant debate is all the intricacies involved and how to calculate for each to be able to accurately measure lift.

We have discussed two here, equal transit and the more correct theory with circulation accounted, does this have a name?

We also have the Thin Airfoil Theory.

Here is someones attempt to improve that theory: Improved Thin Airfoil Theory.

Here are some theories that are believed to have some effect in lift production, in no particular order:


In my search/readings I stumbled upon discussion on a physics forum with regards to newtons third law. Some of you may find it interesting, Forum Link.



The point here isn't to say that we don't know how lift is formed, on the contrary, we do. We just don't know all the factors involved in it and digging into aerodynamics, for each question I answer I stumble upon 5 more I cannot answer. Members on this forum have criticized me up and down for not always following the KISS method of teaching. Well when it comes to lift, this is one area where I believe the simpler the explanation the better the result.

In one of your links you gave fish explained lift as he would to a student. Notice, no mention of equal transit or the more correct theory. The point is, and my question still stands unanswered, what better insight does this knowledge give a student with regards to making them a safer/better pilot?

IMHO all that needs to be known about lift for a pilot is what the FAA requires so they can past the test and a thorough understanding of what they can do to control it. Anything beyond that, to me, is nothing more than a CFI trying to show the student how much they know.



E_Dawg, for the mere fact that most of the books they will read will contradict what your saying if you teach it "the right way." All this will do is provide confusion which you will need to spend time and brain power explaining away. Wasting brain space on knowledge that, barring any new insight, will not give the student any greater ability to fly the aircraft.

By all means, if the student is curious about lift and wants to delve deeper, start them in the right direction as best you can. If they get hung up on an area with regards to flight, try to explain it away in the most truthful way possible. But to offer unnecessary information, just doesn't seem prevalent, to me.



What I do to explain lift:

I show bernoulli the typical way with a narrowing pipe, then erase the top half and make the bottom half into a wing. I then show the low pressure compared to the high pressure. Next I show the lift formula, tell them that is too complex and to ignore it giving them Lift = Speed + AOA. Then we discuss how these two things are what we can control and how they interact.
 
shdw said:
  • Thin Airfoil Theory.
  • Improved Thin Airfoil Theory.
  • Newtonion
  • Circulation
  • Bernoulli
Click to expand...


These really aren't different theories of lift. Thin airfoil theory is essentially a way of determining the circulation around the airfoil and provides a means of calculating other interesting characteristics of airfoils that don't fall out of the basic circulation theory. So it's an extension of circulation theory, rather than a replacement of it. Circulation theory depends on the Bernoulli principle, so Bernoulli can't be a competitor either. And the Newtonian theory of lift just looks at lift through the airfoil's action on the air, rather than the pressure distribution that produces it.

In contrast, the Magnus Effect isn't involved in lift production on an airfoil; it's more of an analogy of how circulation works. Neither is the Coanda Effect involved in lift production on a standard airfoil, but there are some exotic techniques that make use of it.

I agree that it may not be fruitful to teach a private pilot this stuff, unless he shows a particular interest, but I do think it's better to leave out details rather than giving him incorrect ones. Law of Primacy, you know.
 
tgrayson said:
These really aren't different theories of lift.
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Listen to TGrayson, he is 100% correct in his reply. The quote above is ESSENTIAL, as one of the most common problems is that people do not understand this. These are NOT competing, nor is lift "part of" one of these and another "part of" another (something you hear sometimes).
 
Yes, I realized that and had to reread my own post because I worded that improperly. I even contradicted myself in the post:

What has been under constant debate is all the intricacies involved and how to calculate for each to be able to accurately measure lift.
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Then I went on to say the different theories part, whoops. I meant different theories used to calculate lift, is that wrong? Just trying to point out that the hole goes much deeper than "lift is difference of pressure."
 
shdw said:
Just trying to point out that the hole goes much deeper than "lift is difference of pressure."
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Well, not really. Lift IS the difference in pressure and that's all it is. There isn't any more. No pressure differences, no lift. The theory is merely how to calculate those pressure differences.
 
shdw said:
Yes, I I meant different theories used to calculate lift, is that wrong? Just trying to point out that the hole goes much deeper than "lift is difference of pressure."
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If I understand you correctly, yes, that is wrong. Different methods to calculate lift is not relevant. You can measure the height of a tree using a sextant and a tape measure off the ground, or you can climb the tree and run the tap straight down the side, or use a very precise altimeter between the top and bottom, and few other ways, but in this example, it is a lot easier to make the measurement from the ground with the sextant than it is to climb the tree using either method. The tape from the top is probably the most accurate. Which you choose depends on the accuracy required for the job.

What I am saying here is that some methods of measuring lift are easier, based on the data set, than others, and some are more accurate. Which you choose depends on your needs, but to raise the issue with a student just confuses things.

KISS. An object moves for one reason, and one reason only. It is pushed. To counter the acceleration of gravity, you need to PUSH the wing up. Using the "Newtonian" method to measure the change in mass airflow doesn't explain how the wing is getting PUSHED up, it just is a way to measure it, that may be useful for certain applications. The ONLY explanation for what is PUSHING the wing is the pressure differential. That is WHY. The rest just measure the "why".
 
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