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Old Dec 05, 2001, 07:45 AM
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United States, OH, Cincinnati
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How does a fully symmetrical airfoil generate lift?

Ok, so I understand Bernoulli's (sp?) principle of a reduction in pressure as the velocity of a fluid increases. On a semi-symmetrical and a flat bottom foil it is easy to see that the top surface is longer than the bottom, hence lift is generated.

But....on a fully symmetrical foil the distances are the same, so how is lift generated? I'm guessing the angle of attack of the wing has something to do with the answer.

Anyone?

Thanks, Jon.
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Old Dec 05, 2001, 08:18 AM
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Canada, NS, Lunenburg
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Angle of attack has everything to do with the answer. Even a flat plate at a positive angle of attack generates lift when moving through the air.
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Old Dec 05, 2001, 08:30 AM
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ChrisP's Avatar
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Re: How does a fully symmetrical airfoil generate lift?

Quote:
Originally posted by AndrewsJr
But....on a fully symmetrical foil the distances are the same.....
Yes if you just measure the aerofoil, but no if you measure the distance travelled by the air.
A symmetrical aerofoil only produces positive lift when it is at a positive angle of attack (LE higher than TE).
The airstream splits between top and bottom at the stagnation point which is in front of and below the centre of the nose radius.
The distance 'over the top' is therefore greater than 'over the bottom'.
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Old Dec 05, 2001, 08:38 AM
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Reading the replies, you can also see why flat bottomed wing is more efficient at low speeds than a fully-symmetrical...less angle of attack. That is the reason you see alot of electrics and gliders, with a flat bottomed, semi, or undercambered wing, less power and drag for the same lift. On an IC powered plane, you have all the power in the world to fly around a fully sym wing.
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Old Dec 05, 2001, 08:53 AM
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Australia, ACT, Kambah
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Check this out for a good description of how lift is generated.

http://www.monmouth.com/~jsd/how/htm/airfoils.html

or http://www.grc.nasa.gov/WWW/K-12/airplane/short.html

Both have interactive Java applets to help you out and can tell you in very straightforward tems more than you ever wanted to know

Bernoulli and distances over the top and bottom are oversimplistic and often misleading explanations, or fallacies as the NASA site puts it
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Old Dec 05, 2001, 08:57 AM
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Bernoulli's Principle contributes only in a slight way to the generation of lift. The reason things fly has much more to do with Newton's third law of motion: that for every action there is an equal and opposite reaction. Rockets lift off as an opposite reaction to all that energy going out the bottom. As pointed out above a plane's angle of attack is very important in the generation of lift. A few positive degrees of angle causes the air to be pushed down at the back of the wing and the opposite reaction is that the wing is pushed up. As a plane develops more speed the force becomes greater and the elevator can be used to cause the whole plane to point up more and increase the angle of attack. With sufficent speed (power) you can go vertical. That is why fighter jets can fly with such short wings, lot's of power/speed. To high of an angle of attack for your planes speed and the adhesion of the air over the wing surface is broken and your plane stalls as it loses the downward push of the air which generates the upward push on the wing.
By the way the Bernoulli Principle assumes that the split ait from the front of the wing has to get to the rear of the wing at the same time whether it goes over the top or the bottom of the wing and thus with the correct airfoil generates lift. That assumption doesn't check out. Hope that helps. Mike Heer
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Old Dec 05, 2001, 09:05 AM
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>>you can also see why flat bottomed wing is more efficient at low speeds than a fully-symmetrical...less angle of attack<<

So, would this explain why two similar planes (both with identical gearing, motors, batteries) have different flying times? My 24 oz flat-bottomed trainer flies a lot longer than my 18 oz symmetrical-wing acrobat plane. It appears that I need more throttle to keep the symmetrical plane aloft, whereas, I can dramatically cut the throttle on the flat-bottomed plane and it retains its altitude.
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Old Dec 05, 2001, 09:18 AM
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Despite all the hoopla about airfoils and the like, when a model gets small enough, it really does not perform like a full scale would. You can scale down the airfoil, but you can't shrink the air molecules that you fly in. I once flew my rudder and elevator trainer with the wing backwards, trailing edge forward to prove this point. Funny thing is, it really did not seem to care too much about the wing being backwards in flight. Floated in for landings, and generally flew fine.

James
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Old Dec 05, 2001, 09:50 AM
eschew obfuscation
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Quote:
Originally posted by Admiral_Red
Floated in for landings, and generally flew fine.

Did I pull up while inverted?
James
Your .sig somehow seems appropriate at the this moment. ;-)
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Old Dec 05, 2001, 10:14 AM
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When you have been flying for 20 years, you tend to do some silly stuff for kicks. I only performed the high speed inverted landing once, didn't have the power for a go around
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Old Dec 05, 2001, 10:45 AM
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Quote:
Originally posted by Michael Heer
Bernoulli's Principle contributes only in a slight way to the generation of lift. By the way the Bernoulli Principle assumes that the split ait from the front of the wing has to get to the rear of the wing at the same time whether it goes over the top or the bottom of the wing and thus with the correct airfoil generates lift.
I thought that Bernoulli only established the relationship between air velocity and pressure, or am I wrong ? I didn't think that he was at all involved with lengths of streamlines over aerofoils. But please correct me if I'm wrong.

When I was at university in the mid/late 60's studying Aeronautical Engineering, we spent ages on the Joukowski transformation of the pressure around an aerofoil into a circle. I guess Joukowski must be completely out of fashion now as I have never heard anything about this ever again ! And oh all the hours on boundary layer theory. And every time I stir my coffee I still think of free vortices and forced vortices.

If you tell me that Newton is more relevant I believe you. At least he and Reynolds were British, God bless them !

No wonder I fled the insanity and joined the car industry.
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Old Dec 05, 2001, 11:07 AM
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Searching for "symmetrical"

This is a popular subject... there's 5 threads on it in Modeling Science.. search for "symmetrical".. to avoid re-inventing old wheels..
.
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