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Old Sep 14, 2012, 02:45 PM
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Originally Posted by Montag DP View Post
In many cases it is. Just do a google search on "airfoil pressure field" and you will see plenty of them.

Edit: or, look at the first entry in my blog, "Unsteady Panel Code."
Nice pictures !
I liked this gif as it resembles the pressure much as I see it in my minds' eye .
The AOA being much more relevant than anything else.
At least to me , as I use a lot of flat thin wing shapes small and large (1000sq in ) models The very quick pronounced low pressure bubble as the wing dips down -is important in aerobatic stuff
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Old Sep 14, 2012, 03:02 PM
Grad student in aeronautics
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Great find.
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Old Sep 14, 2012, 03:59 PM
Sink stinks
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Indeed, that site has some other cool visualizations. I like this one, which shows that in potential flow the drag of a 2D airfoil is 0. They do this by adding up all the force vectors from pressure.



Of course, in a real flow there will be drag due to skin friction and separation, and for a finite wing there is induced drag even in potential flow.
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Old Sep 14, 2012, 04:37 PM
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Sorry. DPATE ; you have not read your question.

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By definition lift does no work. Drag does, lift doesn't.
.................................................. .................................................. ........................
If you are a "grad student" and make a post in which you do not anaswer your own question by bringing in irrevalent data then I hope you are not a Physics Major.

Your question was, "Why does the card cause the air to move?"

Do you agree that air has mass?
Do you agree that for a statinary mass has to have a force act on it to be moved?
Do you agree that for a mass to be moved from the sationary state that work had to be done on that mass?

If you say "No" to any of these three questions then you do not have an understanding of Newtonian Physics.

Does F = ma ? ..... Is Work = Force x Distance? Please answer.
Once again - If a mass is accelerated from rest has a FORCE acted upon that mass?

If I lifted a 50 pound sack of flour from the ground up to the bed of a normal pickup truck - Did I Do Any Work? Answer this please.
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Old Sep 14, 2012, 05:07 PM
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I seem to have offended you; that was not my intention.

http://en.wikipedia.org/wiki/Work_%2...al_calculation
You have to take into account the direction of the motion and the direction of the force by using the dot product of these vectors. Since lift is defined to be perpendicular to the freestream flow it does no work on the object (because the dot product of perpendicular vectors is zero). Any work done on the object by the resultant aerodynamic force is via drag but not lift.
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Old Sep 14, 2012, 05:18 PM
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I seem to have offended you; that was not my intention.

http://en.wikipedia.org/wiki/Work_%2...al_calculation
You have to take into account the direction of the motion and the direction of the force by using the dot product of these vectors. Since lift is defined to be perpendicular to the freestream flow it does no work on the object (because the dot product of perpendicular vectors is zero). Any work done on the object by the resultant aerodynamic force is via drag but not lift.
Some would consider this to be a "trick" answer .
Something keeps the thing flying and lift is required -so why not include it as part of "work".?
I am one who just can't seperate lift from drag . I see both as simply the same thing - PRESSURE-with part of it doing one thing and part another but still nothing but differences in pressure at play.
I am a dropout -I admit it
and I still have the patents to proove it.
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Old Sep 14, 2012, 05:20 PM
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Any work done on the object by the resultant aerodynamic force is via drag but not lift.
And for the drag force to do any work, the wing must be moving. If you're sitting in the airplane or sitting outside the wind tunnel, the drag force does no work on the air in your reference frame.
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Old Sep 14, 2012, 05:39 PM
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Something keeps the thing flying and lift is required -so why not include it as part of "work".?
I am one who just can't seperate lift from drag . I see both as simply the same thing - PRESSURE-with part of it doing one thing and part another but still nothing but differences in pressure at play.
For very good reasons, the lift acting on a wing is DEFINED to be the component of the aerodynamic force acting in a direction that is perpendicular to both the wing's direction of motion and a line drawn from wingtip to wingtip.

The drag force (again for good reasons) is DEFINED to be the component of aerodynamic force acting opposite the direction of the wing's motion.

You can define them other ways, but keep in mind that if you do so, all of the behavior we're used to seeing in a familiar drag polar goes out the window.

With the above definitions, lift and drag act perpendicularly. This says nothing about their origins, only that the most certainly are not the same thing.

With the above definitions, and work done on an object defined as the force acting on the object times the displacement of the object in the direction of the force, the lift cannot by its definition do any work on the air (or the airplane).

Take the drag away and you don't need to supply any power (or rate of work) to keep a glider flying indefinitely. A glider certainly needs lift to keep flying, but the lift requires no work (or power).
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Old Sep 14, 2012, 06:31 PM
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No disagreement with all that -
At a glance tho- it seems odd that lift is not considered work- just seems that "lifting" is work
In any other context -it is work.
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Old Sep 14, 2012, 06:45 PM
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If you carry something heavy across a room, you might call that "work". But using the precise definition of work (to describe changes in energy), you only do work on an object you carry if you put it down above the height from which you picked it up.
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Old Sep 14, 2012, 06:48 PM
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With the sideshows of late, any focus this thread might have had has eroded. I'm going to make an effort to get back to what I think is the fundamental question here:

Do wings in steady flight generate lift purely by transferring momentum to the air, or are there circumstances where other mechanisms are involved?

It has been suggested that: "The wing develops lift by transferring momentum to the air. Momentum is mass times velocity. In straight-and-level-flight the momentum is transferred toward the earth. This momentum eventually strikes the earth. If an airplane were to fly over a very large scale, the scale would weigh the airplane. This should not be confused with the (wrong) concept that the earth somehow supports the airplane. It does not. Lift on a wing is very much like shooting a bullet at a tree. Lift is like the recoil that the shooter feels, whether the bullet hits the tree or not. If the bullet hits the tree, the tree experiences the event, but has nothing to do with the recoil of the gun."*

Consider how the gun/bullet/tree analogy applies to a rocket that has just lifted off from the moon. Consistent with the above discussion, we imagine the rocket's propellant as a stream of bullets. As the bullets hit the moon, they are absorbed inelastically (brought to rest in the moon's surface rather than bouncing back up). If the bullets are spaced such that there is no physical contact between them in flight, it should be apparent that the impact of a bullet with the moon's surface in no way influences following bullets. As long as it operates in a vacuum and its propellant collides inelastically with any nearby surface, the rocket's performance is independent of altitude. In this scenario, the gun/bullet/tree analogy provides an accurate description of the underlying physics.

If the gun/bullet/tree analogy accurately describes a lifting wing, then a wing's performance should also be independent of its altitude. It is well documented that a wing's performance is significantly affected by proximity to the ground. Unlike a disconnected bullet that flies the same path to the ground regardless of its predecessor's fate, air influenced by a wing is very much connected to the surrounding air. The path that air takes when influenced by a wing is different depending on how close the surrounding air is to the ground. Wings are not rockets, and it should be apparent the gun/bullet/tree analogy applied to a wing provides a very misleading description of how a wing develops lift.

*"Understanding Flight", David F. Anderson and Scott Eberhardt, McGraw-Hill, 2001
So is there vehement agreement that using the above gun/bullet/tree analogy to describe a lifting wing is complete nonsense?
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Old Sep 14, 2012, 07:00 PM
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If you carry something heavy across a room, you might call that "work". But using the precise definition of work (to describe changes in energy), you only do work on an object you carry if you put it down above the height from which you picked it up.
Caerful--I said LIFT, not carry.
Like pushing on a fixed object - if it does not move -no work has been done . Also moving an object. from one point to another constitutes work..
The semantics get tricky----
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Old Sep 14, 2012, 07:20 PM
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Caerful--I said LIFT, not carry.
Like pushing on a fixed object - if it does not move -no work has been done . Also moving an object. from one point to another constitutes work..
The semantics get tricky----
Moving an object from one point to another constitutes work only if the energy of the object changes. In the case of an object subject to gravitational forces, that means its height must change.

When you "lift" something you are moving it in the direction of the force you are applying to overcome gravity. In doing so you are doing work. A wing, by definition, does not move in the direction of the "lift force".
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Old Sep 14, 2012, 07:51 PM
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You make two seperate observations -- a car requires work to move i,t' (ever pushed a car to start it?)
Lift is the basis of Watts expression "horsepower "
( lifting water over a time period was the comparison)
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Old Sep 14, 2012, 08:00 PM
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When you push a car, the force you are applying is in the direction of the car's motion... it does work. A wing's lift force is perpendicular to the motion of the wing... it cannot do any work. When you lift water, you move it in the direction of the applied force. A wing does not move in the direction of the lift force.
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