What makes lift

[InsyleM]

Hello Everyone.
For years Ive been taught and told, even at A&P School 2007 - 2008 that lift is from Bernelli's principal. It seemed logical and what not. My brother then after takeing some physics classes told me that is incorrect...

Well, I know wikipedia isnt always 100% accuarte, but here is what I found.
I figure if I post this in the new comers section maybe some of the new people can lean I know I learned and im not new Ive been on this sight since the last melinium (and I still can't spell)
http://en.wikipedia.org/wiki/Lift_%28force%29

[Flying Officer Jevans]

Your brother is quite correct. In fact, that wiki article seems to do quite a good job of explaining it correctly - it's clearly been written by a physicist!

We did have a long discussion on this topic in the Aircraft Customisation forum a few months ago - and we also eventually arrived at the conclusion that Bernelli was not a good way of explaining it.

[mgh]

Agreed. Wikipedia gives a sound overview of this subject.

[Brett_Henderson]

Given enough thrust, even a barn door will function as a wing; with all the lift being generated by a control-surface-like AoA. And if you look at every diagram used to 'discredit' Bernoulli's Principle... you'll see that they all have an airfoil cross-section with a geometric, POSITIVE AoA... i.e.. barn door lift.

Every explanation that credits the wing shape for the downward air-flow redirection, does not account for the UPWARD redirection that must also occur at the leading edge. Since we all know that in physics, nothing is 'free' .. there's no way to just re-direct air and get a net force out of it. If that were the case, we could roll a wing into a circle; creating a venturi-like tube, and get a net thrust gain, by just moving it through the air.

It all boils down giving up airspeed to drag, in return for lift. Bernoulli's principle will allow that to occur with an absolute ZERO AoA. However, it would be a fraction of the lift needed to make an airplane fly.. The rest comes from barn door lift via positive AoA.

Now.. the trick is to get an AoA without a 'stall'... and this goal is dependent on the available thrust. A modern jet fighter has what (for the sake of this discussion) might as well be barn door wings.. But a C172 relies on Bernoulli (not just a rounded, smooth cross-section) to get away with its AoA.

[Flying Officer Jevans]

Exactly. Most 'explanations' of lift use either the AOA principle or Bernoulli, not the correct explaination, which is actually a combination of both. Both 'barn-door' and 'Bernoulli' lift occur, just at different times with different shapes.

The true explanation can only involve a study of computational fluid dynamics and the Euler/Barnard-Stokes equations for flow. The wikipedia article is actually quite good; it's better than I remember.

[mgh]

What is "barn door" lift?

[Flying Officer Jevans]

Lift created by the simple deflection downwards of air from the bottom surface of the wing, and so could be created by a flat( barn-door) wing with positive AOA.

[mgh]

Lift created by the simple deflection downwards of air from the bottom surface of the wing, and so could be created by a flat( barn-door) wing with positive AOA.

And how does that account for the effects of the flow over the top of the wing?

[Flying Officer Jevans]

It doesn't. That's the point. Neither the 'barn-door' explanation nor pure Bernoulli correctly explain lift.

[jeroen79]

Isn't the 'true' origin of lift simply asymmetry?
Air flows differently over the top and bottom surfaces and therefore the forces on top and bottom are different, resulting in a net lateral force called lift.