Ailerons - theory of operation, and a general discussion of lift

None of the engineers here mentioned the coanda effect.
Which I think is the most prevalent reason a wing flies.
It makes liquids and gases stick to convex surfaces when flowing over them. Everyone knows what happens to a spoon when held under tap water.

Ted wrote:

One thing is for sure the downward turning does not account for the lift mathematicaly.

I think that depends on what you mean by “downward turning”. If you draw a vertical plane parallel to and ahead of the leading edge, and another vertical plane parallel to and behind the trailing edge, isn’t the net rate of change in momentum of the air flowing between the two planes equal to the lift? Some of that net change is the contribution of the upward movement (compared to the freestream) before the leading edge, of course. But the momentum theorem should still apply.

I’ve never given a thought to any theory when flying. Shouldn’t an EASA LPPL candidate be required to justify his actions in terms of Navier-Stokes (who/whatever they are) while demonstrating a medium rate level turn?

bookworm wrote:

I think that depends on what you mean by “downward turning”. If you draw a vertical plane parallel to and ahead of the leading edge, and another vertical plane parallel to and behind the trailing edge, isn’t the net rate of change in momentum of the air flowing between the two planes equal to the lift?

a difficult choice of words, and I agree that the net rate of change in momentum equals the lift and also the induced drag. If you apply the theorems correctly you get the right answer.

IMHO the ‘deflection’ approach while seductively simple, is not quite so simple when you try and visualise it, as shown in the smoke video that you originally posted and even more difficult to calculate and measure. Where as simply measuring the pressure changes in a wind tunnel, a good estimation of the size and shape of the wing and (including the ailerons) can be made.

EuroFlyer wrote:

None of the engineers here mentioned the coanda effect.
Which I think is the most prevalent reason a wing flies.

How does a plane (or spoon) fly upside down?

It is ironic that very few aviation textbooks actually mention the scientists that attempted to describe lift.

Ted wrote:

and also the induced drag

In my post above I should not have implied lift = induced drag, the discussion on vertical planes caused a brain fart, and I was thinking about something different.

I had best get back to work.

EuroFlyer wrote:

It makes liquids and gases stick to convex surfaces when flowing over them. Everyone knows what happens to a spoon when held under tap water.

That example is NOT the Coanda effect. It simply show the water sticking to the spoon. Rub the spoon with wax or something water repellent, and the effect is gone. Which is the reason people don’t use the term Coanda effect. It’s too easy to misconceive. The (real) Coanda effect is caused by the rotation of the fluid that always exists in the viscous sublayer around a fully submerged body, and is indeed one key element of why lift is possible. Lift cannot be created without that rotation in the viscous sub-layers.

Ted wrote:

however you can derive his formula from newtons second law

How? Bernoulli equation is a simple form of conservation of energy. Newton second law is simply F = ma. Bernoulli is more general than Newton, more fundamental. Force is the time derivative of momentum (energy), and is therefore not a conservative law. You can derive Newton from Bernoulli, but the other way makes no sense, because you need the concept of conservation, which is what Bernoulli is all about.

LeSving wrote:

How?

Using google

my comment was offered in the spirit of ‘shutup and calculate’

Ted wrote:

IMHO the ‘deflection’ approach while seductively simple, is not quite so simple when you try and visualise it, as shown in the smoke video that you originally posted and even more difficult to calculate and measure.

I agree. Denker explains the usual fallacy well here.

Ted wrote:

Using google

my comment was offered in the spirit of ‘shutup and calculate’

My point was that the only way of doing it is to integrate Newton’s second law (integration being the “opposite” of derivation).

This integral equation:

requires that the energy between a and b is conserved for it to make any physical sense whatsoever. That requirement IS the equation itself that you want to derive. The conservation laws may seem trivial. Mass, energy, momentum etc have to be conserved. But they are the very basic fundamental principles in fluid dynamics, and have to be included every time.