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Not bad for a chemist ;-)

Natasha has it right.  There is no requirement that parcels of air
reunite at the tail edge of the wing.  Not only that, but Bernoulli
would fail to explain the many planes that fly quite successfully with
symmetrical airfoils (same path length top and bottom), not to mention
inverted flight.  

Lots more correct info can be found at
http://amasci.com/wing/airfoil.html  

A very practical explanation can be found by getting out of pressure
arguments altogether and looking at momentum change.  As long as there
is a net downward momentum change experienced by the air, there will be
a net upward momentum change experienced by the plane.  Planes are
massive, compared to air, so a lot of air has to change its momentum to
keep a plane from falling, which is another way to see why planes have
to fly fast to stay in the air.

Dave

David L. Smith, Ph.D.
Director of Professional Development
Da Vinci Science Center, Allentown, PA 
http://www.davinci-center.org

Engage, Explore, Share



> -----Original Message-----
> From: Informal Science Education Network 
> [mailto:[log in to unmask]] On Behalf Of Natasha Aristov
> Sent: Saturday, December 02, 2006 6:00 AM
> To: [log in to unmask]
> Subject: Flying and Bernouilli's principle
>
> Uh-Oh.
> 
> Bernouilli's principle states, or rather observes, that the pressure 
> in a moving fluid is lower than the pressure in a standing 
> fluid.  This is hard to believe at first glance, but I use the 
> analogy of cars on a freeway:  when they are moving fast, the 
> distance between them is larger (low pressure), when they are in a 
> traffic jam, the "car-fluid pressure" is high.
> 
> So much to Bernouilli.
> 
> The only ONLY way to get anything to fly is by making the air 
> pressure below the object higher than the air pressure above the 
> object.  It has nothing to do with wing SHAPE -- the wing shape can 
> help make flying more efficient, but it is not decisive in whether a 
> thing will fly.  Check out some pieces of trash one day on a 
> windy day.
> 
> So:  you set up a pressure differential above and below a plane 
> wing.  You do this by moving the plane through the air at very high 
> speeds so that the air gets shoved up under the forward part of the 
> wing.  The wing blocks (part of) the air from flowing behind 
> it:  So  you have high pressure below and low pressure behind.
> 
> 
> SO.... it turns out (it is a CONSEQUENCE, not a cause!!!) that the 
> air particles below the wing are slower than the air 
> particles above the wing.
> 
> 
> Still don't believe this?:  a terrible, common "explanation" of 
> flight  is that because the wing is shaped the way it is, the air 
> above the wing has a longer distance to travel than the air below the 
> wing.  So it moves faster to keep up.
> 
> WHAT????  How does air know how far it has to go?  And ... why does 
> it need to keep up with the other air????
> 
> Bernouilli is OK, but it doesn't make planes fly.  Flying planes make 
> Bernouilli.
> 
> 
> 
> But what do I know?  I'm just a chemist.
> 
> Natasha
>

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