Bernoulli's equation  
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Bernoulli's equation - 2/17/2004 1:01:32 AM   
elmerfud



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Joined: 6/19/2003
From: phoenix, AZ,
Status: offline 		Ok. If I have this correct,,,, in Bernoulli's equation,, he came to the conclusion(in a nutshell or laymans terms) that when the velocity of air is sped up…it will cause lower pressure.(over an airplane wing for example)
Or if the speed of liquid is sped up.. it would cause lower pressure.. (through a venturi in a pipe for example)
Hence..the airflow going over the symmetrical wing(top side in this case) will be going faster then the flat bottom side(also for this example). So low pressure will be above the wing. And higher pressure will be below the wing… causing it (the wing)to have lift. With me so far>?
So if the wing is symmetrical. On both sides… does that not make this equation for creating lift no good>? Because pressure would be the same on both sides>? No lift/same pressure on both top and bottom>?

photo has formula.(below)

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< Message edited by elmerfud -- 2/17/2004 1:04:07 AM >
       Post #: 1

RE: Bernoulli's equation - 2/17/2004 1:12:52 AM   
David Cutler



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From: Los Angeles, CA, USA
Status: offline 		The lift is created by a combination of angle of attack and the Bernoulli effect you mentioned.

Especially with a thick wing section, when you get a positive angle of attack, the effective centre of the leading edge moves downwards, thus making it a longer distance over the top of the wing compared with the underside.

I'm sure it's more complex than this, but I reckon that's why a thick section works better at slower speeds than a thin section. If you have a thicker leading edge then the forward-most point of the wing at positive angles of attack moves further down so greater lift is created by the differential between the top and the bottom distance.

-David C.

(in reply to elmerfud)
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RE: Bernoulli's equation - 2/17/2004 1:48:58 AM   
elmerfud



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From: phoenix, AZ,
Status: offline 		ah that makes sense... the "angle of attack"....i know if you increase the angle of attack. you get more drag though... so it takes more power to fly at same speed with the higher angle... thats why i would assume. they tell newbie`s to start with a flat bottomed wing.//// more stable at lower speeds... ect ect.

(in reply to David Cutler)
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RE: Bernoulli's equation - 2/17/2004 2:57:51 AM   
David Cutler



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From: Los Angeles, CA, USA
Status: offline 		True.

Actually, lift is proportional to drag in all circumstances.

That's why, when you make a turn by rolling with the ailerons, then applying up elevator there is a tendency to yaw away from the turn, and to make a fully coordinated turn you have to apply a little rudder towards the turn. The outside wing is creating more lift (hence the roll into the turn) and therefore, more drag, so it yaws towards the wing with the greater lift, that is, away from the turn.

-David C.

(in reply to elmerfud)
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RE: Bernoulli's equation - 2/17/2004 3:03:32 AM   
LouW



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From: Moreland, GA, USA
Status: offline 		This subject was beat to death last year, and if you look up the thread, I think you will find more than you want to know. Basically, Bernoulli was describing flow in pipes, not in a free field. Every wing section, both cambered and symmetrical, has an angle at which it produces no lift. If angle of attack is measured from this “zero lift” line rather the chord line it becomes obvious that there is no other explanation for lift other than angle of attack. Lift depends on asymmetrical flow regardless of the shape of the airfoil. The shape of the section only becomes important when it is desired to produce lift with a minimum of drag.

(in reply to David Cutler)
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RE: Bernoulli's equation - 2/17/2004 3:15:03 AM   
David Cutler



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quote:

Lift depends on asymmetrical flow regardless of the shape of the airfoil. The shape of the section only becomes important when it is desired to produce lift with a minimum of drag.


If that's true, then how come a ball moves towards the flow if you blow over just one edge of it? There is no angle of attack effect as you aren't blowing over the bottom and pushing it upwards. It must be caused by a lowering of the pressure in' free air' over the top.

-David C.

(in reply to LouW)
       Post #: 6

RE: Bernoulli's equation - 2/17/2004 4:30:52 AM   
Tall Paul



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Joined: 6/23/2002
From: Palmdale, CA, USA
Status: online 		" This subject was beat to death last year,.."
It truly was... The Bernoullis vs the Newtonians..
No one wins.

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RE: Bernoulli's equation - 2/17/2004 4:55:18 AM   
LouW



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From: Moreland, GA, USA
Status: offline 		Actually, Paul, both sides win. They are just two ways of looking at the same thing. One side sees only trees, the other side sees a forest. Both are right.

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RE: Bernoulli's equation - 2/17/2004 12:58:16 PM   
dick Hanson



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From: slc, UT, USA
Status: offline 		Lift is the drag in the direction you wanted
Drag is lift in the direction you didn't want.
Yin and Yang

_____________________________

I am watching you
Libby

(in reply to LouW)
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RE: Bernoulli's equation - 2/17/2004 3:47:33 PM   
banktoturn



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From: Bloomington, MN,
Status: online 		LouW,

I don't know what particular problem Bernoulli had in mind when he did his work, but the "Bernoulli equation" is valid for flow around a wing, as well as for flow in pipes. In particular, one can use the Bernoulli equation to calculate the variation in pressure as a result of variations in velocity, along a streamline. As long as you apply it along a streamline, this works for pipes, wings, sneezes, ... whatever. Pressure really does go down when velocity goes up, along a streamline, and this phenomenon really does explain one of the main reasons wings generate lift. It is not correct or useful to say that angle of attack is the only explanation of lift. It is also incorrect to assert that Newton explains lift and Bernoulli doesn't. It is correct to note that a symmetric wing must be operated at a positive angle of attack to generate lift.

banktoturn

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RE: Bernoulli's equation - 2/17/2004 3:50:44 PM   
David Cutler



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quote:

ORIGINAL: banktoturn

LouW,

I don't know what particular problem Bernoulli had in mind when he did his work, but the "Bernoulli equation" is valid for flow around a wing, as well as for flow in pipes. In particular, one can use the Bernoulli equation to calculate the variation in pressure as a result of variations in velocity, along a streamline. As long as you apply it along a streamline, this works for pipes, wings, sneezes, ... whatever. Pressure really does go down when velocity goes up, along a streamline, and this phenomenon really does explain one of the main reasons wings generate lift. It is not correct or useful to say that angle of attack is the only explanation of lift. It is also incorrect to assert that Newton explains lift and Bernoulli doesn't. It is correct to note that a symmetric wing must be operated at a positive angle of attack to generate lift.

banktoturn


FWIW, I agree totally!

-David C.

(in reply to banktoturn)
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RE: Bernoulli's equation - 2/17/2004 4:18:28 PM   
JimTrainor


 

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From: Ontario, ON, CANADA
Status: offline 		Mr B.'s equation is an energy balance.

Potential (static pressure)
+
Kinetic ( 1/2 row V^2 )
=
Constant

... as such, of course it applies anywhere and everywhere energy is conserved. It easier to apply to pipe than it is to a free flow. Applying it to a sneeze may be grand challenge computing territory.

< Message edited by JimTrainor -- 2/17/2004 4:20:11 PM >

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RE: Bernoulli's equation - 2/17/2004 4:31:10 PM   
banktoturn



Posts: 700
Joined: 8/22/2002
From: Bloomington, MN,
Status: online 		Jim,

Yes, it applies anywhere and everywhere that energy is conserved, but you have to make sure that you are dealing with the same 'piece' of air, which is why one can only take advantage of the relation along a streamline. It is 'easy' to apply when you know the path of a streamline. For attached flow around a wing, this is easy, since we know that one streamline follows the surface of the wing, from the leading edge (stagnation point) to the trailing edge. For the turbulent flow in a separation region, it isn't feasible. Same for pipe flow (meaning that there are situations in which the paths of streamlines are known, and there are situations in which they aren't).

banktoturn

< Message edited by banktoturn -- 2/17/2004 5:16:03 PM >

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RE: Bernoulli's equation - 2/17/2004 4:48:36 PM   
David Cutler



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From: Los Angeles, CA, USA
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quote:

Applying it to a sneeze may be grand challenge computing territory.


Oh I dunno. Somebody at our flying field recently sneezed and his head imploded from excess pressure!



-David C.

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RE: Bernoulli's equation - 2/17/2004 5:48:01 PM   
LouW



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Joined: 1/1/2003
From: Moreland, GA, USA
Status: offline 		Actually the Bernoulli equation is a mass balance (conservation of mass). It simply states that within the confines of a stream tube , the mass passing through each section must be equal. Obviously if the section gets smaller, the velocity of the mass must increase. This increase in velocity can only happen if there is an unbalanced force. To exist, this force demands a lower pressure at the smaller section in order to accelerate the mass. It is only valid for a fluid that is incompressible (which air is not) and inviscid (which air is not), and for flow confined within a stream tube. Within the limits of its’ derivation, it is an extremely useful concept.

It is not a matter of whether Bernoulli or Newton is right. In fact they are simply two ways of looking at the same thing. If you are designing wing sections and need to explore the pressure field around an airfoil, Bernoulli is the only way to go, but if you are flying an aircraft that is already designed and built, Newton gives a better understanding of what is going on.

In fact the air is not flowing at all. It is essentially still, and the airplane is moving through it. This doesn’t make any difference as far as the pressure distribution around the wing is concerned, but is highly significant to a pilot, who is trying to move along a certain path through the air.

In either case, the overly simplified explanation of lift, usually seen in popular books involving a flat bottom airfoil, is not quite accurate. Bernoulli works just as well for a flat plate moving at some angle of attack as it does for a Clark Y. As I stated before, if angle of attack is related to the zero lift line (rather than the chord) it becomes obvious that it is really the secret of lift.

(in reply to JimTrainor)