necoandjeff wrote:
> Brett Robson wrote:
> 
>>>I
>>>said that the wind doesn't push the boat. You're right that it is
>>>high pressure air pushing under the airfoil, which is not fully
>>>countered by the lower air pressure above the air foil, that moves
>>>the boat. But that is a force that is (roughly) at a right angle to
>>>the direction of the wind. So when I say the "wind" isn't pushing
>>>the boat I am not only practically right, I am technically right.
>>>The movement of the wind above and below the airfoil merely creates
>>>the pressure differential through a phenomenon known as Bernoulli's
>>>Principle. The wind itself doesn't do the pushing.
>>
>>Where does the energy come from?
> 
> 
> The kinetic energy (vibration) of the molecules in the air, which vibrate
> from heat and strike the underside of the airfoil. Because the air molecules
> are "stretched out" from the curved flow of the air across the top of the
> air foil, the total kinetic energy of molecules striking that side isn't
> enough to counterbalance the force under the airfoil. But this is all quite
> different from saying the sail is "pushed" by the wind which is blowing
> (roughly) at a right angle to the sail. We both basically understand what is
> going on here from the perspective of physics. You were simply wrong to say
> that I was technically wrong by saying that the wind doesn't push the sail,
> unless for some reason you aren't making a distinction between "wind" and
> "air."
> 

You've got several concepts confused here. Kinetic energy is 
movement, not vibration (that's not quite correct but let's drop 
that hot potato). A vibrating particle at rest has no kinetic 
energy (sort of). In a fluid (gas, liquid and plasma) heat is not 
molecules vibrating, heat is molecules moving, literally having 
kinetic energy. You may be thinking of Browian motion which is 
the movement of macro objects impacted by molecules.

If you have sailed you would know that the most important 
telltales (bits of string on the sails) to keep flying  are the 
ones at the windward side at the back of the sail.

Any airfoil whethert flat or not, with postitive angle of attack 
the kinetic energy of the flow is creating a force on the 
underside of the aerofoil. By bending the fluid flow downwards 
(angular acceleration) the aerofoil can absorb the kinetic energy 
of the fluid (in the case of a sail). (Fluid not molecules) It is 
actually the force of bending the wind that propells a yacht.

If you agree that that an aerofoil creates lift by absorbing 
kinetic engery of the fluid (in a sail) or the reaction of it's 
own forward kinetic engergy in a wing (Newton's 3rd), then surely 
this is pushing. This is important as many people such as Byrce 
above think that a wing or sail is being sucked or pulled.

Do this experiment. Hold a peice of light paper or plastic film 
to just below your mouth and against it. Blow across the top of 
the film, you are creating a low pressure so it should rise from 
the high pressure below. Was Bernoulli on drugs?


> 
>>You probably learnt that at highschool.
> 
> 
> Nope. I had to learn it to get my private pilot's license actually.
> 
> 
>>However the Bernoulli
>>Principle/Effect is a specific application of Newton's laws of
>>motion. Bernoulli only explains what you see and doesn't explain
>>what is happening at all. It is actually particles (ie air
>>molecules) hitting the back of the sail that provides the force.
> 
> 
> Yes, and those molecules are hitting the bottom of the sail because of their
> kinetic energy (vibration from heat), not because of the wind.

No, that is completely incorrect.

> If you were
> to stretch yourself out inside the curved underside of the sail, closest to
> the mast, do you think you would feel the force of the wind hitting you?

That is the barrier level and is an area of high pressure not 
movement. The individual molecules are not hitting the sail but 
against each other, passing on the energy in a chain of bouncing 
molecules.


> 
>>A similar effect is molecules of air striking the inside of a
>>ballon that keeps it inflated.
> 
> 
> Hmmm. So if it is the "wind" pushing the sail, then where does the "wind"
> inside the balloon come from?

The kinetic energy of all the molecules.


> 
>>>And for the sake of completeness, there are times (such as when you
>>>want to go exactly in the direction of the wind) that you might let
>>>the sail out at a full right angle to the boat and allow the wind to
>>>push you, but it is by using Bernoulli's Principle and treating the
>>>sail as an airfoil that you can get the greatest speed in a sailboat.
>>>
>>
>>If you ignore Bernoulli and concentrate on Newton you will
>>realise that the only difference is that on a reach you have a
>>partial vacuum on the windward side, but on a run you have to
>>push the air away from the front of the sail.
> 
> 
> I completely disagree that "Newtonian physics" is the best way to teach
> someone the principle of how an airfoil works. And the author of the
> textbook I read when studying for my pilot's license apparently disagreed
> with you as well. 

The Bernoulli effect is an abstraction and is in fact misleading 
and wrong as normally applied to lift. The Bernoulli Effect 
explains conservation of engery, not transfer of energy. You are 
being taught the result of an aerofoil, not the cause. For a 
pilot it might be a useful abstraction, but not for a 
aeronautical engineer, the numbers do not add up.