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A wing flies to the left in the air (no wind). Particles A and B moves to the right in relation to wing.

  1. In which direction do particles A and B moves in relation to the ground? What is velocity of particles in relation to the ground?

  2. Don't forget, air is at rest (no wind) and the wing is moving, so why does particle C not move to the particle A, because the net pressure gradient is to the left and air always travels from high to low pressure?

(i think when observed from ground :

particle A moves up and left

particle B moves down and left

particle C moves down and left

and when wing pass all particles will move down and left)

enter image description here

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  • $\begingroup$ Related:[physics.stackexchange.com/questions/157038/… ] $\endgroup$
    – Auberron
    May 5, 2020 at 9:09
  • $\begingroup$ Instead of particle, you should write "dust particle" or any other suited formulation as air molecules move in all direction whereas dust particle average air molecule movements around them. It would remove ambiguity. $\endgroup$
    – Manu H
    May 5, 2020 at 11:54
  • $\begingroup$ You need to clarify your frame of reference. "From the ground" you would only see the perspective in your diagram if you were on a mountain top and an aircraft flew by at your elevation. $\endgroup$
    – Michael Hall
    May 5, 2020 at 16:00
  • $\begingroup$ @Rotor A is the one to focus on. It gets pushed up, and the wing runs past it before the lower pressure between it and the wing can pull it back down. In the process of trying to fill the vacuum, A gets pulled down and the wing gets pulled up. C is A a fraction of a second later. The process is constantly repeated as the wing hits "new" air, at the expense of drag. $\endgroup$
    – Robert DiGiovanni
    May 5, 2020 at 21:44
  • $\begingroup$ The wing velocity arrow is greater than your pressure gradient return arrow. At too high an AOA, the wing cannot "run away", and higher pressure indeed "catches up", breaking the lower pressure "bubble" over the wing. You actually see these as "reverse flow" arrows in many diagrams of stall towards the rear of the wing. Yes, they are now moving "forward" faster than the wing. $\endgroup$
    – Robert DiGiovanni
    May 5, 2020 at 21:53

4 Answers 4

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particle A moves up and left

No, up and right

particle B moves down and left

Yes

particle C moves down and left

Yes

The low pressure region first pulls in the particle A, so it initially moves in the opposite direction of the wing, and then the adverse gradient over the rear part of the wing will gradually accelerate it so it ends up moving a little in the direction of the aircraft. It also ends up moving down much faster than forward, which is how the lift can be much more than the drag.

On highly cambered wings the pressure may be higher than ambient everywhere on the underside, but most wings are not cambered that much and there is some reduction of pressure under them as well, so even the particles moving under the wing accelerate a bit to the right, opposite of the wing, and only then change to moving left along the flight direction.

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  • $\begingroup$ is velocity of A when first accelarate to the right,same if we meassure from wing or ground refernce frame? $\endgroup$
    – ROTOR
    May 5, 2020 at 12:47
  • $\begingroup$ @ROTOR, you asked about when observed from the ground, so I am saying what it is from the ground. From the point of view of the wing, all the air is moving to the right fast. But around the suction peak it is moving faster than free stream, so it's moving right even viewed from the ground (where the free stream is stationary). $\endgroup$
    – Jan Hudec
    May 5, 2020 at 13:39
  • $\begingroup$ @ROTOR, of course accelerations are the same in both reference frames, because we are considering wing in steady flight that is not accelerating relative to the ground. $\endgroup$
    – Jan Hudec
    May 5, 2020 at 13:40
  • $\begingroup$ if wing flying at 1000km/h and A=1100km/h looking from wing,then A=100km/h looking from ground? $\endgroup$
    – ROTOR
    May 5, 2020 at 15:04
  • $\begingroup$ @ROTOR, yes, just watch out for directions—when flying 1000 km/h LEFT, and A moves 1100 km/h RIGHT relative to wing, then A moves 100 km/h RIGHT relative to ground, yes. It is vector addition. $\endgroup$
    – Jan Hudec
    May 5, 2020 at 21:07
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The circulation theory of lift shows that the flow over a lifting airfoil comprises a steady flow against the direction of travel, superimposed on a circulation of air around the airfoil. It is this circulation which defines the lift, and is in turn generated by the pressure gradients (The pressure gradient you show is far from the most significant one, as most of the action takes place near the leading edge).

Take away the steady flow as you have done, and you are left just with the circulation. The circulation flows up over the leading edge, back over the upper surface, down behind the trailing edge and forwards again underneath. You will see A, B and C moving accordingly. For example A has probably already completed its small forward motion (if any) and much of its upward motion, and is well into accelerating backwards. C is just slowing down, possibly ready to turn down, while B is moving forwards and possibly accelerating.

Of course a given particle does not follow the full circle, you can think loosely of a circulatory loop as a string of particles all moving together (although as stated it is a flow component and the particles are also transiting across the loop).

The flow at the front is fairly well confined, the further back you go the more diffuse it becomes, hence the trailing downwash reaches a great deal further than the upwash at the leading edge.

A similar circular motion is sometimes seen when an object floating on the surface of water is subject to waves passing by.

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  • $\begingroup$ "Circulation" of a wing is probably more like a low pressure system, with the front sweeping behind it, than a hurricane because of the forward movement of the wing. But new air constantly feeds in from in front, likely splitting at the stagnation point. The flow off the bottom, at proper AOA, seems to help keep pressure low at top rear wing. This is what breaks down at stall. $\endgroup$
    – Robert DiGiovanni
    May 5, 2020 at 20:55
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This is a good thought to consider the wing actually strikes the air in real flight, whereas in a wind tunnel it is the opposite, but the resultant flow patterns are the same.

From the ground, particle A is pushed up and forward, B pushed down and forward, but in that time the wing moves father forward, so relative to the wing, "flow" is backwards.

A moving airfoil at positive AOA can be considered a "pump", creating a "circulation" and "downwash" relative to the wing. Therefor, at any Time 0 to Time X, particle A will be farther behind the wing than particle B, and particle C behind and lower than both from downwash any way you look at at.

Do Time 0 to Time X for A, B, C, and the wing. You will find point of reference will not matter. Their relative positions will be the same at Time X, allowing wind tunnel designers to breathe a sigh of relief.

In your diagram, higher pressure (and slower "flow") should be underneath the wing. The situation you show is what happens when the wing is at too high an Angle of Attack and higher pressure "leaks" over to the top. This also happens at wing tips and is "bad", causing loss of lift and more drag. When this happens at the trailing edge, it is called a "stall".

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  • $\begingroup$ situation on my picture is allways happening on wing,because pressure on leading edge is allways lower then pressure on part of wing close to trailing edge..but velocity of A,B,C are slower when we look from ground than we look from wing? $\endgroup$
    – ROTOR
    May 5, 2020 at 11:25
  • $\begingroup$ @Rotor from the ground the velocities are W - A, W - B, W - C, from the wing they are - A, -B, - C. From the ground the forward motion is W > B > C > A. From the wing the relative rearward motion is A > C > B > W. No need to confuse yourself, just use the wing frame of reference, and see the "circulation" towards the "wake" of the wing. lowest pressure is above and behind the wing, exactly where the drag vector is. $\endgroup$
    – Robert DiGiovanni
    May 5, 2020 at 14:55
  • $\begingroup$ So, please move low pressure further back, and high pressure underneath the wing. $\endgroup$
    – Robert DiGiovanni
    May 5, 2020 at 14:57
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Forward.

A wing is pushing the air out of its way. It pushes the air in the direction it moves itself.

Same way if you stir water in a cup with a teaspoon, the water will move in the direction you move the spoon. It will move slower than the spoon so with relation to the spoon will appear moving backwards.

Or, another comparison: if a locomotive is pushing rail cars "out of its way", they obviously move the direction the locomotive moves, with relation to the ground. If a bulldozer is pushing a pile of sand, the sand is moving the direction bulldozer does, even if some spills into sides.

As the wing is also a wedge, some air will also move up and down is it makes its way through it.

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    $\begingroup$ It's not that easy, and it's not that easy for cars either. $\endgroup$
    – Jan Hudec
    May 5, 2020 at 12:28
  • $\begingroup$ Never seen cars moving the opposite direction than locomotive, however. $\endgroup$
    – h22
    May 5, 2020 at 12:31
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    $\begingroup$ True… It's because it's actually a bad analogy. $\endgroup$
    – Jan Hudec
    May 5, 2020 at 12:33

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