Why is flawed aerodymics taught in flight schools?

[Guest Jimbofly]

Just wondering...I know that in most instances the flawed principles of Bernoulli's equations are used to explain lift in flight schools. Why is this so?Many people will argue till they're blue in the face that lift is explained with Bernoulli's equation, not knowing that what they were taught is incorrect. I'm certain that if the correct reasons for lift were taught, pilots would still not feel overwhelmed and there would be no confusion.Anyway, what are your thoughts?James

[Guest oyvindhansen]

What are the correct and incorrect explanations?-

[Guest Jimbofly]

Linky:http://www.regenpress.com/James

[LAdamson 240]

> I'm certain that if the correct>reasons for lift were taught, pilots would still not feel>overwhelmed and there would be no confusion.>Nobody, and I mean nobody................. seems to know the exact science of lift. A lot of speculation, theory, and application of what works; but still a whole lot of arguing on the subject.You can read theory of lift until you're blue in the face, then make your own assumptions and know that the truth lies somewhere between it all! :-hah I think I've been reading different reasons of what causes lift for about 40 years now....L.Adamson

[Guest Jimbofly]

True, but advanced aerodynamic theory attributes up to 2% of lift goes to our friend Bernoulli, which is negligible for the most part. I tend to go with how NASA define lift, but I do realise that there are varied explanations for lift. One thing that remains certain is that the explanation for lift that's taught in schools is the most innaccurate one. I find this strange.James

[michal 44]

>True, but advanced aerodynamic theory attributes up to 2% of>lift goes to our friend Bernoulli, which is negligible for the>most part. Sorry, I have to react to this because it is a major falacy. It is equally bad to dismiss Bernoulli as it is to say that it explains lift theory. Maybe we should start from the beginning.Lift is the result of pressure differential. If you were to integrate all pressures around the wing you would get the final force. Those pressures are directly related to speed of air flowing around the wing and are related through the Bernoulli principle. If someone gave you complete distribution of speeds of air flowing around the wing you could then use the Bernoulli principle to derive pressures and from that you could derive lift. Your answer would not be "exact" but would be very close (problem of so called boundar layer). The problem with Bernoulli principle is that is incapable of explaining why air flows around the wing the way it flows (faster above the wing then below,etc.). For that you need to go way beyond Bernoulli - you need Kutta-Zhukovsky's circulation theory which is very complex and is way beyond what a layman could understand.So in essence there is nothing wrong with using name of Mr. Bernoulli to "popularize" lift theory. Bernoulli's principle can be derived using even high school physics and you can show simple experiment like blowing air through two sheets of paper to impress your audience and illustrate this theory. But for someone who wants full explanation of the lift there is no other way but to use the full blown mathematical apparatus.The 2% that you mention above has nothing to do with Bernoulli but with simple Newtonian 3-rd law. Only 2% (or whatever the number) of the lift comes from particles of the air bouncing off the bottom of the wing and imparting (equal and opposite) force to the wing.I am amazed because this subject keeps returning to this forum and always with the same amount of misinformation.I work at NASA Ames and though I am not involved with different aspects of lift theory there are folks next door who do it 8 hours a day.By the way, I take issue with you writing "flawed principles of Bernoulli". If they were indeed "flawed" they would have disappeared from Physics textbooks very long time ago.Michael J.http://www.precisionmanuals.com/images/for...argo_hauler.gifhttp://sales.hifisim.com/pub-download/asv6-banner-beta.jpg

[Guest DreamFleet]

Right on the money, Michael! :-)Regards,http://www.dreamfleet2000.com/gfx/images/F...R_FORUM_LOU.jpg

[C-JEAN 1]

Hi, James. I learned those physic principles 2 times:1- in my Collegial diploma in Aerotechnics,2- in my real flying lessons.The best way to explain it was the first one. You understand immediatly, when you imaginea wing, as being a part of the wall of a **VENTURI TUBE** with infinite diameter. That diameter going towards the top of us. Take a venturi tube. We know the exteriorwall is straight, and the inside wall hasthe VENTURI shape/curve in it. Put the tube on its side. [ Hold it, or it will roll B-) ] Cut it axialy on the top.Then open it to make it straight. The straight exterior wall on the table,and the curved venturi shape facing you. We now have a wing. Underside is straigh,the top side is curved. Here is your infinite VENTURI. Blue skies.

[Guest DNelson]

One thing that remains certain is that the explanation for lift that's taught in schools is the most innaccurate one. I find this strange.If you're talking about that "principle of equal transit time", or whatever it's called, I agree 100%. It's easy to show by experiment that air flowing over a wing is at a higher velocity than air flowing under a wing. However, one of the reasons given for that velocity increase in some of the science texts is absurd. It says something like, since the travel distance over the cambered upper surface is farther than the straight-line lower surface, the air going over the upper surface must speed up so it gets to the trailing edge at the same time as the corresponding air particles on the lower surface. One small problem with this explanation is that it requires the air molecules to know, in advance, what the distance over the wing is, and it requires the molecules to be able to perform velocity/time/distance calculations instantly. :-lol :-lol :-lol And I know of no principle of physics or gas dynamics that requires the particles to reunite at the trailing edge.I still remember hearing this explanation in 6th grade science class. Coincidentally, I'd been to an air show with my parents a week or two before the subject came up, and was intrigued by the inverted flight. I asked the science teacher, if that explanation was correct, how can the same airplane fly both right side up and inverted? He had no answer. ;)

[michal 44]

> It says something like, since the>travel distance over the cambered upper surface is farther>than the straight-line lower surface, the air going over the>upper surface must speed up so it gets to the trailing edge at>the same time as the corresponding air particles on the lower>surface. Correct, this is often what is wrong in popular explanations of lift theory. In fact the air molecules that travel above the wing get to the trailing edge before their counterparts that go below the wing. Again, the full explanation of how the air flows around the wing is part of Kutta-Zhukovsky's circulation theory and no amount of 'common-sense talk' can make this subject approachable to a layman.Michael J.http://www.precisionmanuals.com/images/for...argo_hauler.gifhttp://sales.hifisim.com/pub-download/asv6-banner-beta.jpg

[LAdamson 240]

> I asked the science>teacher, if that explanation was correct, how can the same>airplane fly both right side up and inverted? He had no>answer. ;)Even if the wing is symetrical (airfoil same, top & bottom), the wing still needs be be flown with a positive angle of attack to the incoming airflow. With a positive angle of attack, the air will still flow a greater distance over the top of the wing.But it doesn't matter! :D I subscribe to my "water ski" theory. I waterskied for many years, and that feel of being pushed upwards is much the same as the feel of a planes lift. And that water compressing under my slalom ski's positive angle of attack is what kept me up. I figure its the same for lift. The prop pulls me over that compressable air, just as the boat's prop pulls my ski over the compressed H2O. The shape of the airfoil is there to determine drag effeciency, as does the aspect ratio. At least that's my theory, and I'm certain it's correct!!! :-hah L.Adamson

[Guest DNelson]

Any kid that's stuck his outstretched hand out of a car window that's moving 80 mph, then tilts his hand up and down, knows the relationship between angle of attack and lift. :)

[michal 44]

>Nobody, and I mean nobody................. seems to know the>exact science of lift.Larry, mankind knows the "exact" science of lift since about circa 1930 (give it plus or minus a few years). I would expect nothing less considering that the present day Physics works on 11-dimensional universe, string theory, black-holes and the Big Bang. Believe me they represent challenges of a few order of magnitude greater than lift theory. ;)If you want to blame someone for amount of "arguing" on the subject - it is the popular media or other "well meaning" people who want to inject their own egos into the discussion. You won't find them arguing about the string theory - it is well beyond their grasp, but lift - hey, it is something so tangible that you will find 'experts' even among peasants on collective farms - I had opportunity to listen to one such guy .. :-lolMichael J.http://www.precisionmanuals.com/images/for...argo_hauler.gifhttp://sales.hifisim.com/pub-download/asv6-banner-beta.jpg

[Guest Jimbofly]

Sorry I mis-represented what I meant. I know Bernoulli's principle is true and not flawed, I just meant in terms of lift it isn't the main reason for the cause of lift.Many NASA texts I've read say that the majority of lift is caused by Newton's 3rd law, about the actual deflection of air downwards, moreso than Bernoulli's law. I provided the link which explains this very convincingly.Oh, and I have a basic understanding of circulation theory, not the actual mathematics involved, just the principles it's derived from. I certainly feel that circulation theory along with the application of Newton's 3rd law account for the vast majority of lift, and that Bernoulli only accounts for a max 2%. The equal transit theory is simply wrong, but it is taught as gospel in many schools.James

[LAdamson 240]

>Larry, mankind knows the "exact" science of lift since about>circa 1930 (give it plus or minus a few years). I would expect>nothing less considering that the present day Physics works on>11-dimensional universe, string theory, black-holes and the>Big Bang. Believe me they represent challenges of a few order>of magnitude greater than lift theory. ;)When the Big Bang theory is proved to me, without any doubts whatsover; then I'll believe that someone really knows the "science of lift". :D But until then, the subject is still open for serious debate, just as it's been for a 100+ years! :)Afterall, back in the 30's, scientists and engineers knew little about compressibility, using swept wings, etc.L.Adamson