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Guest Jimbofly

Bernoulli's principle - some myths

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Hi fellow pilots!I've been researching some information from NASA about how aircraft actually fly, and it turns out that lift generated from pressure differentials between the underside of the wing and the upper wing only accounts for about 2 percent of the lift!! The rest of the lift is actually generated from the reaction caused (Newton's third law of motion dictates that every action is directly opposed by a reaction of equal magnitude) by "pushing" the air downwards.Many experiments have been performed on wings and it has been found that the overall shape of the cross-section of an efficient wing works extremely well at displacing air downwards, hence causing a lift reaction in the wing.NASA have known about this over 50 years, which makes me wonder why most aerodynamics texts purport to state a "fact" when claiming that the pressure differential caused by the Bernoulli effect is what constitutes most of the lift.What do you think?James

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Hi James,Countless articles have discussed this subject.It appears that our friend Bernoulli has very little to do with lift generation by aircraft...Newton's principle (there is an equal and opposite reaction for each action) is the main reason for the cration of lift by an airfoil.

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>I've been researching some information from NASA about how aircraft actually fly, and it turns out that lift generated from pressure differentials between the underside of the wing and the upper wing only accounts for about 2 percent of the lift!! The rest of the lift is actually generated from the reaction caused (Newton's third law of motion dictates that every action is directly opposed by a reaction of equal magnitude) by "pushing" the air downwards.NASA have known about this over 50 years, which makes me wonder why most aerodynamics texts purport to state a "fact" when claiming that the pressure differential caused by the Bernoulli effect is what constitutes most of the lift.

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I think the reason it is taught, is that it was this main assumption that the Wright Brothers were using when they designed their wing. And because they were "successful" the argument sticks. As others have said, it does work to a point.Computer models and the like have shown that lift can be generated by not only Bernoulli's Principle, and Neutonian Physics, but a combination of the two. We have learned over many years that "lift" is a much more complex phenomenon than can be adequately explained by simply Bernoulli or Neuton.However, Bernoulli's Principle is adequate enough to explain to most of us, a very BASIC understanding of lift. It is also sufficient enough to teach future pilots the principles surrounding lift, and more importantly, departure from lift....stalls. The Link provided elsewhere in this thread is very useful and informative.

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>wonder why most aerodynamics texts purport to state a "fact">when claiming that the pressure differential caused by the>Bernoulli effect is what constitutes most of the lift.>>What do you think?James,I am afraid there is a lot of bull on the subject floating around. I work at NASA though my expertise has nothing to do with airfoil design but there are plenty of people around here who do things like that.The bottom line it is all matter of semantics. People like to take extreme views - Bernoulli principle or no Bernoulii principle. Neither is true. To say that Newtonian 3-rd law is mostly responsible also makes little sense since if this was true you could make efficient a wing out of a barn door. Also Bernoulli principle is derived from the Newtonian mechaniscs. The bottom line the lift is described best by Kutta-Zhukowsky theorem which states that lift = airspeed * circulation * density * span. This is a beautiful and simple law however circulation must be computed in itself which is not an easy task.You can grab any book on aerodynamics that will explain what all these terms mean.And I am afraid the lift is indeed the direct pressure differential between its bottom and upper part (regardless how it is generated).If you took a flying wind and integrate the pressure density by the area (the pressure diff would be very different in different parts of the wing) you would end up with total lift.Have you heard of term often used when reviewing tech. specs of an aircraf called wing loading - this is an average lift generated by the wing per square foot. This is also an average pressure differential between bottom and upper portion of the wing. So really it is not easy to popularize theory of a wing. A complex math/physics is involved here and anything short of full explanation that require a college course is bound to be an oversimplification.BTW, ever opened a book about sailing and theory of yachting ? They use Bernoulli all over the place. Are they wrong ? No. With completely different speed regime Bernoulli principle is absoultely sufficient. It also tells them how two boats sailing close in parallel may collide because of suction forces.Bernoulii is here and doing fine - it is not a 'myth'. It is good enough to 'popularize' things. Try to tell people about Kutta-Zhukovsky theorem ... ;) Michael J.http://www.reality-xp.com/community/nr/rsc/rxp-higher.jpg

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Pick up and read "Stick and Rudder" the author has a hand drawn diagram of a wing "beating' the air downward to produce lift. Bernoulli is grand but its just not that fancy.Good book for all pilots and simmers.Tony

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In the article the author states that the two molecules that seperate, one going under the wing and one over don't rejoin with each other. I was always taught that they DO rejoin since you cannot have a vacuum in nature. That is why the top molecule travels faster over the wingtop and rejoins those that were around it before the leading edge of the wing. What else would make the molecule travel faster (A speed limit sign with a higher limit?).The author also mentions that some wings are symetrical and therefore the distance over the top and bottom is the same. True, but when an airplane flys, the wing is not usually perfectly horizontal (i.e. angle of attack) and with the addition of flaps looses its symetrical shape.I was also taught that the majority of the lift generated by the wing was from the low pressure on top "sucking" the wing up and complemented by the high pressure underneath the wing push up.

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>In the article the author states that the two molecules that>seperate, one going under the wing and one over don't rejoin>with each other. If there is any single largest "myth" in the theory of lift it got to be the belief that upper and lower molecules must come together at the trailing edge. The author of this little article is correct. They don't rejoin. Moreover, contrary to intuition, it is the upper molecule that will arrive sooner !!!You are correct however about the lift - it is the pressure difference that ultimately provides the lifting force. But how these pressures come to being - that's where things are getting complicated.I like to use one word - circulation.Michael J.

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Thanks for the info, Michael! I now have a much better understanding of how lift is generated.I actually agree, it is a combination of a lot of factors which causes lift. It seems to me that the best way to visualise lift is when you see that the wing generally causes a downdraft with the air above the wing moving faster than the air below it. This downdraft causes the main pressure differential. This seems to work in concert with air striking the underside of the wing causing the action-reaction. When you combine all of these factors, you get a very strong lift force.Again, thank you, and I'll look into the lift = airspeed * circulation * density * span formula.James

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