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If I understood it correctly, the shape of the wings and/or propellers generates lift/thrust with the difference in pressure in both sides of the wings/propellers; where the lower side has higher pressure airflow and the uper side has low pressure airflow.

Illustration explaining how lift is generated

With this in mind, I was wondering if it is possible to generate an area of low pressure around the upper part of the an aircraft without the moving balloons, wings or propellers/rotors.

A "static lift" is the best way I could put it.

So, would such thing be possible? Or lift would only be achieved with the airflow that wings already work around?

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    $\begingroup$ For what it's worth, that picture is totally inaccurate. $\endgroup$
    – Tanner Swett
    Commented Jan 22, 2023 at 5:52
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    $\begingroup$ The closest thing to a "static lift" would be a balloon with a heated air raising due to buoyancy force. But you have said "no" to a balloons, so... $\endgroup$
    – Agnius Vasiliauskas
    Commented Jan 23, 2023 at 8:17
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    $\begingroup$ This section on the Wikipedia article about lift en.wikipedia.org/wiki/… explains why lift isn't generated the way you think it is. $\endgroup$
    – Vorbis
    Commented Jan 23, 2023 at 8:44
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    $\begingroup$ You don't understand it correctly. The shape of the wing doesn't generate "lift/thrust with the difference in pressure" (whatever that means). Difference in pressure is the result of difference in velocity across the wing. Difference in velocity is the result of air being viscous. Even if you don't have a wing, there is still difference in pressure across the air FLOW - the stagnant air doesn't flow, thus there is no pressure difference in it (neglecting hydrostatics). That all comes from the Navier-Stoke's. In order to understand all that, you have to take an advanced fluid mechanics course. $\endgroup$
    – Ivan Nepomnyashchikh
    Commented Jan 23, 2023 at 17:51
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    $\begingroup$ The high speed/low pressure vs. low speed / high pressure in the picture is correct. However, the distance is almost irrelevant and the flow cannot be parallel to the ground both before and after the airfoil. The streamlines are more like this - note the airfoil is symmetric. For pictures with wind coming horizontally from left, try hyperphysics.phy-astr.gsu.edu/hbase/Fluids/airfoil.html Note how it turns down after the airfoil. $\endgroup$
    – Vladimir F Героям слава
    Commented Jan 23, 2023 at 19:52

5 Answers 5

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The cartoon is missing a key feature: the flow beyond the wing is downward. This is necessary to create lift. The lift force is balanced by a force on the air, Newton's third law in action. This force accelerates the air downward. So, no, you cannot cannot generate lift statically.

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    $\begingroup$ Balloons would like to have a word with you. They generate lift statically! Also, my cat, which is sitting on a sofa and not moving seems to not be falling into the core of the earth despite gravity. ;) My point is, imagine a device that lowers pressure above the wing; the wing goes up even if it is not moving forward. $\endgroup$
    – Yakk
    Commented Jan 24, 2023 at 1:29
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    $\begingroup$ @Yakk I believe the op was interested in open structures in air. That's how I read the question. Closed structures and solids are off the table, I believe. Even boats. $\endgroup$
    – John Doty
    Commented Jan 24, 2023 at 1:35
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If you have a difference in pressure, then fluid will flow to equalise that pressure unless restricted by a barrier. If you have a barrier then that's a balloon, if you don't have a barrier than you don't have a static system.

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    $\begingroup$ Suppose you had lasers that heated the air directly above a plate at a rate fast enough that it kept up with outflow. Would the region of low pressure cause lift? The air flows, but the plate (the wing) doesn't move... $\endgroup$
    – Yakk
    Commented Jan 24, 2023 at 1:39
  • $\begingroup$ @Yakk sure but it wouldn't be a static system $\endgroup$
    – Pioneer_11
    Commented Feb 6, 2023 at 3:41
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The purpose of wings is to generate this lift in a dynamic situation.

If there is a low pressure area and a high pressure area in a system and they are connected, then air will flow from the high pressure area to the low pressure area. Air will go around the wing from the bottom to the top.

Something would have to be done to prevent it. There is a real life example of this: air ride suspension. On large trucks, a volume of high pressure air is trapped inside a rubber shock absorber so that it is not connected to the outside air and cannot travel from high pressure to low. Thus we have high pressure below the top of the shock, and low pressure above it. This generates lift, and is used to keep the semi trucks from falling to the ground.

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It's worth noting that the diagram you've shown is a common misconception; it's almost identical to the one on this NASA webpage explaining what's known as the "transit time" myth. In fact, it's the exact image given under the "Misconceptions" section of the Wikipedia page on the Bernoulli principle.

In short, the myth states that because the upper surface is longer, air must travel faster to cross it in the same time, and thanks to Bernoulli's principle we know that air travelling faster is at lower pressure, and this low pressure therefore exerts a force that draws the wing upward.

This is actually wrong twice. Firstly, there is no particular reason why the air travelling over the top surface should have to take the same time as the air going underneath; indeed, it's pretty simple to put a wing in a wind tunnel and release a puff of smoke ahead of it, watch the smoke puff split into two pieces at the wing's leading edge, and observe as those pieces reach the trailing edge at different times.

Secondly, this gets the cause and effect of Bernoulli's principle backwards. If you apply Bernoulli's equation to the wing-in-airflow scenario, you can actually show mathematically that the air flowing over the top surface of the wing must take less time than the air passing underneath.

Either way, the explanation is actually much simpler. In your diagram, the airflow somehow returns to being perfectly horizontal behind the wing. In reality, air that strikes the wing horizontally at the leading edge ends up leaving the trailing edge with some downward velocity (and again, you can derive this mathematically from conservation of momentum). For the air to have gained downward velocity, it must have been acted on by a net downward force. If the wing has imparted some downward force on the airflow, then we know from Newton's Third Law that the airflow must be exerting an upward force on the wing.

It's true that there are pressure effects caused by the change in effective cross-section area of the various airflows, and these do affect the wing's ability to exert that downward force, but by and large the lift force can be explained in terms of momentum and Newton's Laws.

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Wait, you want to move something upward against the pull of gravity by differentially lowering the pressure on its upper surface?

Uh, have you considered sticking a straw in a milkshake and sucking the shake into your mouth?

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    $\begingroup$ If only we could do this without the straw and the mouth. ;) $\endgroup$
    – Yakk
    Commented Jan 24, 2023 at 1:29
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    $\begingroup$ @Yakk — Supposedly, Guglielmo Marconi once explained his invention thusly: “Imagine a very large cat, with its tail in Rome and its head in New York. If you stepped on its tail in Rome, it would meow in New York. Radio is like that, but without the cat.” $\endgroup$
    – Michael Lorton
    Commented Jan 24, 2023 at 1:33

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