Bernoulli's Principle
 

Hi all,

A question that I am hoping you can all help me with please.
I am learning to fly and have been learning about Bernoulli’s principle.
The principle as I understand it is that the air travels faster over the top of the wing therefore creating a low pressure. This low pressure enables the air passing under the wing to effectively be pushing against a lower pressure area and therefore more effective thus helping lift.

My question is why does the air travel faster over the curved surface? I was told that it’s to get to the back of the wing at the same time as the air that goes underneath the wing but what is it that means it has to get to the back at the same time? Why does it have to travel faster? Why can it not travel at the same speed?

All opinions and advice gratefully received.

Regards.

From Wikipedia:

The top surface air reaches the trailing edge before the air underneath.

His experiments in the 1730s observed that water flowing through pipes of varying diameter was speeded up as it entered a segment of smaller diameter, i and its pressure decreased.

If an instructor told you that then they should be shot. That's the Equal Transit theory and is just plain wrong.

So what?

I have never seen any real evidence that a detailed knowledge of Bernoulli is of real benefit to either a PPL or their instructor.

G

Bernoulli seems to be falling out of favour anyway;-
The Airfoil Misconception in K-6 Textbooks
Airfoils and Airflow [Ch. 3 of See How It Flies]

It used to annoy me that some people just couldn't get lift theory, but if you told them it was lift fairys that kept the aircraft up they grasped it instantly.

You can actually do the whole of the PPL with lift fairy theory.

Critical angle of attack = angle which the lift fairys fall over so the aircraft stalls.
etc.

O well didn't make the slightest difference to them flying the aircraft they did get a brief though that on no account were they to tell anyone else about lift fairy theory.

Bernoulli and Newton were both 17th century (or so) scientists and were trying to understand why stuff behaves like it does. And they did a good job progressing science. In fact, a lot of their principles have been used to teach science to people.

But after their deaths, science has moved on, and both have been proved wrong, either in generic cases or in corner cases.

As I understand it, the currently considered "good" explanation of air flow around a wing, and the resulting lift and drag, is based around an equilibrium equation involving five different factors (like pressure, temperature and speed) in some sort of integral. But that equation is very hard to visualize if you don't have an extremely good grasp of math, and is too hard to calculate by hand anyway. That's why we have computers and CFD.

So anything from the Lift Fairies, via the Equal Transit theory, to Newton (Impact Lift anyone?) to Bernoulli (high speed = low pressure) is essentially wrong to some degree. And is best used as an approximation of what happens only.

Its all to do with these equations.

Navier

They give engineering students nightmares until you discover that they just zero out the hard stuff.

And there still isn't a computer thats powerful enough to solve the whole lot.

They are pretty good at doing 2D linear models but as soon as its gets turbulent your into wag land. When your into 3D nonlinear and transient dynamic flows it produces some very pretty pictures and gives you an idea of what might happen but thats about it.

This is probably the best theory so far... and is supported by movies such as Tinker bell. ;)

Best not to ask how fairies fly in the first place though.

saycheese - one poster has already said the important thing, the explanation you seek refers to the equal transit theory which is not held in high regard because it leads to the very question you ask which is not answerable in those terms. (ie the overall explanation is really wrong/poor).

Google the coanda effect for more info. My understanding is that lift is caused by a combination of the air flowing over the upper surface sticking to it (coanda effect) and thus following the curve downward at the back (and thus being accelerated downward) and also the air striking the lower surface of the airfoil being accelerated downward by virtue of its shape.

The two accererations/deflections produce a reaction (newton) on the airfoil in an upward direction.

Stalls occur when the air on the top unsticks.

I expect someone who is better informed/has the correct instructors explanation can put some more detail to this...

Here is a link to a paper by Prof Babinsky who made the video I posted earlier.

http://iopscience.iop.org/0031-9120/.../pe3_6_001.pdf

I have some reason to consider this explanation plausible, at some level.

Lift fairies? Pah - it's all about sky hooks.

That doesn't seem to bother the people that get it with lift fairy theory.

Say Cheese

I could not agree more.

As a pilot to be you should not concern yourself one jot with why a wing produces lift but become the world's greatest expert on what factors determine the lift of a wing and which of those are available to the pilot control the lift.

That is what will keep you alive - especially when turning onto finals.

John,its just that a bit of knowledge of it can help with the CAA exams!

Yet the proof is simple, and you can "experiment" when sitting on the lavatory, if the equipment is to hand.

If there is awall-mounted loo-roll, and the end is hanging down, blow between the paper and the wall, parallel with the wall.

You are adding air and might expect the paper to move away from the wall. It doesn't; it moves towards the wall as the faster air stream lowers the air pressure on on side of the paper and the air pressure in the room acting on the other side of the paper pushes the paper towards the wall.

Flight training should be undertaken in the lavatory!

I just teach 'draught through a door'-studes get concerned if you go to the lavatory with them