Brett Robson wrote:

>> 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.

You're right. My use of the word "vibrating" was wrong. I was thinking of
the molecules literally flying to and fro from their kinetic energy and it
came out as vibrating. Of course I don't mean to convey that the molecules
sit there and vibrate like a guitar string and are otherwise stationary.
Delete the parenthetical and replace the second "vibrate" with "move" and
what I said is otherwise fine.

> 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.

I have sailed and of course you are right. What does that have to do with
our discussion as to whether the wind is pushing the sail in the direction
of lift?

> 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.

But it is also important not to confuse people like the OP by suggesting
that the wind is pushing the sail. It isn't, it's the kinetic energy of the
air on the underside of the airfoil that is not sufficiently countered by
the kinetic energy of the wind on the upper side of the airfoil. When you
say the wind pushes the sail, most people mistakenly think that the sail is
being propelled in the direction of the wind. Although that may happen when
you are sailing with the wind at your back, it is not the case at all when
you sail into the wind. I think we're just mincing words here.

> 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?

That's funny that you mention that particular experiment because it exactly
demonstrates my point. The reason this is experiment is so counterintuitive
is that the pressure from below is not being created by the force of the air
being expelled from your mouth, as you suggest when you say the wind is
pushing the sail. It is strictly from the kinetic energy of the air on the
other side of the paper that suddenly finds itself able to overcome the
pressure on the mouth side of the paper, because propelling air across the
top of the airfoil creates a low pressure area.

>>> 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.

Again, take out "vibration" and what I said was completely right. As the
mouth and paper experiment demonstrates, wind doesn't even need to hit the
underside of the airfoil to cause the lift. You have approximately 15 pounds
of force pounding against each square inch of the underside of the airfoil
in the complete absence of any wind. All you need to do is lower the
pressure on the top side and you have lift. It's got nothing to do with the
wind itself doing any of the pushing pushing.

>> 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.

Exactly, which has nothing to do with "pushing" of the wind.

>>>> 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.

The Bernoulli effect doesn't even necessarily have to be associated with
airfoils. It is simply the principle that a fluid that is accelerated loses
pressure. It happens in a pipe that narrows too. The drop in pressure is
what allows the 15 lbs per square inch of pressure on the underside of the
airfoil to lift the airfoil along with anything attached to it. What a pilot
or sailor should know is basically how an airfoil does what it does. And it
does what it does by creating a pressure differential on either side of it
that results in lift. Physicists, and maybe even aeronautical engineers may
want to know more detail about the physics behind the pressure differential,
but it is more than the average pilot or sailor needs to know.

Jeff