Take the 2-minute tour ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.

I understand that the Bernoulli effect is a flawed explanation for the cause of lift, and does not cause much at all, but how much?

Is there any experimental data on the force caused by the Bernoulli effect? Maybe implicitly through data of the pressure difference between the top and underside of an aeroplane's wings. After that, I assume I could (crudely approximating the pressure to be acting perpendicularly to the flight direction) use $\Delta P A$ to work out the net force on the plane.

Perhaps there is another way to quantitatively analyse the extent to which the Bernoulli effect causes lift.

Edit: see this short cartoon (content similar to Mike Dunlavey's answer).

share|improve this question
    
Yeah, that video is pretty good. The only question is it refers to the Coanda effect above the wing. Denker explains why that's not right. –  Mike Dunlavey Jun 21 '13 at 12:39
    
related: physics.stackexchange.com/q/63039/4552 –  Ben Crowell Jun 21 '13 at 13:26
add comment

2 Answers

up vote 10 down vote accepted

There's no problem with the Bernoulli effect, only with the way it's understood and explained. It's usually explained with mistakes, like the need for asymmetrical airfoil and equal flow time above and below, and without mentioning the need to deflect the direction of airflow.

Here's the best light-math explanation I've seen. Also study this section that directly answers your question.

EDIT: It is easy to find wrong pictures like this: enter image description here

as opposed to a correct one like this (from the link above):

enter image description here

So the answer to your question is: All of the lift depends on the Bernoulli principle, because speed and pressure are in trade-off, but the physics need to be correctly understood.

share|improve this answer
    
It is equally true that all the lift depends on the change in airflow direction. You have to calculate one or the other but not both. –  Marty Green Jan 18 '13 at 17:53
1  
@mike Thanks Mike. I'm not used to people agreeing with me. –  Marty Green Jan 18 '13 at 18:46
1  
@Alyosha: There really isn't a larger thing. If you really want basics, you can go back to Newton's laws. Bernoulli's principle is just a consequence of Newton's laws. –  Mike Dunlavey Jan 18 '13 at 19:47
1  
@Alyosha: Pretend you're a wing. You can only be lifting if the pressure underneath exceeds the pressure above. Then pretend you're a slug of air. This wing comes along, cuts you in half, then pulls/pushes you downward. Same thing. –  Mike Dunlavey Jan 18 '13 at 22:05
1  
+1 Very good answer. I've seen many comments/answers on this site that unnecessarily bash Bernoulli because the author equates Bernoulli with the incorrect "equal time" explanation. Glad to see someone else trying to set the record straight. –  OSE Jan 27 at 20:17
show 6 more comments

I cannot say exactly how much the Bernoulli Effect contributes to lift but it is not much. Cambered wings and barn doors fly inverted. Air speed increase is the same as the increased distance ratio. not enough to produce much lift. The wing tries to generate a ring vortex. This gets thwarted because the roll up around the trailing edge, called the "starting vortex" is shed before takeoff. This leaves, in plan view, a horseshoe shaped vortex consisting of the "bound vortex" and the two tip vortices. Stretch your mind here! The "Bound Vortex" is not convection, it is diffusion. It is a molecular knock on effect and travels at sonic speed around the wing forward underneath and stream direction on top and rolls up around the tips. Some air follows the diffusion around the tips and that is convection. The pressure above a wing is not reduced because of accelerated air, The pressure is reduced first by the bound vortex which then accelerates the air. The wing is a pump. Air from 18 feet above a Cessna 172 is accelerated down at 5 tons/second in normal flight. Thats how Newton is involved.. For every action..... Before responding check a text book on aerodynamics. See also www.newfluidtechnology.com.au "The Coanda Effect and Lift".

share|improve this answer
2  
++ Your PDF is a much better explanation than your answer here. When you denigrate the Bernoulli Effect, you're referring to the version commonly and incorrectly taught. With some editing, this could be a nice answer. –  Mike Dunlavey Jun 21 '13 at 12:54
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.