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!


> 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?