necoandjeff wrote:
> 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.

I don't have much time at the moment but you didn't click on the bottom 
link did you?
http://www.allstar.fiu.edu/aero/airflylvl3.htm

"The second description we will call the Popular Explanation which is 
based on the Bernoulli principle. The primary advantage of this 
description is that it is easy to understand and has been taught for 
many years. Because of it’s simplicity, it is used to describe lift in 
most flight training manuals. The major disadvantage is that it relies 
on the "principle of equal transit times" which is wrong. This 
description focuses on the shape of the wing and prevents one from 
understanding such important phenomena as inverted flight, power, ground 
effect, and the dependence of lift on the angle of attack of the wing.

"The third description, which we are advocating here, we will call the 
Physical Description of lift. This description is based primarily on 
Newton’s laws. The physical description is useful for understanding 
flight, and is accessible to all that are curious. Little math is needed 
to yield an estimate of many phenomena associated with flight. This 
description gives a clear, intuitive understanding of such phenomena as 
the power curve, ground effect, and high-speed stalls. However, unlike 
the mathematical aerodynamics description, the physical description has 
no design or simulation capabilities."

That last bit is not quite correct, the calculations used in fluid 
dynamics can be used but they are difficult to use.