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Hey, Dave, the momentum explanation works great for aircraft, BUT....

There is a classic classroom demonstration in which the demonstrator  
connects a short length of plastic tubing to the stem of a funnel,  
drops a ping pong into the open end, and then asks the biggest kid in  
the class to blow through the tube to launch the ping pong ball.  The  
kid blows, but nothing happens - the ball stays put.  The demonstrator  
then turns the funnel upside down and has the kid try again - same  
thing.  As long as the kid continues to blow, the ping pong ball hangs  
unexpectedly in the funnel, defying gravity.  As soon as the kid stops  
blowing, it drops out.

I am no physicist, but I am having a real hard time explaining this  
effect in terms of momentum.  The force of gravity is downward, the air  
flow is downward, yet the ball doesn't fall.  The conventional  
Bernoulli explanation would be that air coming through the funnel is  
accelerated around the ball, resulting in faster moving air above and  
slower moving below, and since slower moving air exerts higher pressure  
the ball is pushed up.

As Natasha  says, Bernoulli doesn't make planes fly, flying planes make  
Bernoulli.  So what exactly is it that keeps that ball in the funnel?

Chuck

On Dec 3, 2006, at 10:51 AM, David Smith wrote:

> ISEN-ASTC-L is a service of the Association of Science-Technology  
> Centers
> Incorporated, a worldwide network of science museums and related  
> institutions.
> *********************************************************************** 
> ******
>
> 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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