Brett Robson wrote:
> necoandjeff wrote:
>
>>
>>> 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.
>>
>
> I don't think you did the experiment. Hold the paper /against/
> your bottom lip so that none of your breath goes underneath. Fold
> it slightly in a convex if you have to. It doesn't matter how
> hard you blow you cannot create enough low pressure to life the
> page as you might expect from Bernoulli. You can only get it to
> work by blowing underneath.
>
> Don't worry, Einstein got it wrong too!

What? I did the experiment many times as a kid and I just tried again now
for the first time in about 20 years. Even without your breath hitting the
under side of the paper, blowing across the top of it lifts it up. Perhaps
*you* haven't conducted the experiment. Look at the diagrams on this page as
an example of how to conduct the experiment in a way so as to ensure that
none of the air you are blowing hits the underside of the paper.
http://www.allstar.fiu.edu/aero/Experiment1.htm.

>> 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.
>
> no it creates lift from bending the air, the force underneath is
> from angular accleration from the kinetic energy of the airlow.
>
> How do flaps work?

I'm not saying that a positive angle of attack will not also create a force
against the wing. And flaps work exactly in this way. But much of the lift
of a wing, or a sail, is created by the pressure differential, regardless of
the presence or absence of wind hitting the bottom of the airfoil. Why do
you think you can achieve such a greater speed while sailing into the wind
than you can with the wind to your back, if the lift is being generated
because of wind hitting the sail and pushing it?