Why a simple theory of lift like the one described below, is not accepted?
If you don’t agree with the following, please just skip this question and do not delete or downvote it, because it is a legitimate, scientifically sound explanation of lift.
Common sense theory of wing lift fundamentals:
Assume an airplane moves with the right speed and proper angle of attack.
The wing pushes the air in the direct path of its angle of attack. After the wing passes, the air previously in front of the wing, ends up down below the trailing edge. Air when pushed compresses, thus creating a higher-than-ambient pressure. The higher pressure then propagates and dissipates with the speed of sound.
The upper surface of the wing moves away from air, thus creating a lower-than-ambient pressure. Air from around then rushes in to fill the void (lower pressure), also with the speed of sound.
The pressure difference translates in a force pushing the wing back up. That force is Lift plus Induced Drag.
There is more to it of course, especially about what a proper angle of attack is, and what the right speed is.
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Arpi, have you studied any of the standard texts on lift-generation by cambered wings? – niels nielsen Sep 13 '22 at 01:58
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@niels nielsen Niels, currently there is no consensus on the explanation of lift. Yes, I studied all the texts. I have been doing nothing else in the past year since I am retired. – Arpi Sz Sep 13 '22 at 02:41
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Can you please edit your question to show how your theory of lift can be used to arrive at the same predictions of lift, drag, and their variation with angle of attack that currently accepted theories do? – TimWescott Sep 13 '22 at 02:51
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@TimWescott Hi, Tim. Yes I can edit my question, I can explain everything about lift but, ... which currently accepted theories are we talking about? because there is no consensus on the explanation of lift. – Arpi Sz Sep 13 '22 at 03:32
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2regarding lift explanations, there is a consensus, otherwise computational fluid dynamic models of airflow over wings couldn't be written and run on computers. Suggest you post your questions on the Aviation stack exchange. there are some professional aerodynamicists there who are friendly and helpful. -Niels – niels nielsen Sep 13 '22 at 04:11
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@nielsnielsen Niels, if you believe there is a consensus, please send me some links to websites which state that. – Arpi Sz Sep 13 '22 at 06:09
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1Two general comments: 1) the Web is a great source of information, but it is not always the most authoritative. Therefore, judging the existence or not of consensus about the theory of lift on the basis of web pages may be misleading. 2) the theory of lift is a complex subject, and hand-waiving arguments cannot substitute more formal treatments. – GiorgioP-DoomsdayClockIsAt-90 Sep 13 '22 at 06:18
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@GiorgioP is right; a good intro would be the wikipedia page on lift generation. – niels nielsen Sep 13 '22 at 16:29
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1This is a more than book length subject. Google for books with titles like "Theory of Wing Sections" and go from there. Note that pilots and airplane modelers and other non-engineers are given non-technical, hand-wavy explanations of lift that are sufficient to fly a plane, but which aren't sufficient to do serious work with airfoil or propeller design. That population does argue over interpretations, because those theories lack the mathematical foundations that make them complete. Please show the math that makes your theory better than the accepted practice of decades. – TimWescott Sep 13 '22 at 16:33
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@ArpiSz https://physics.stackexchange.com/a/741007/176092 Go and see my explanation. Comments are welcome. – enbin Jan 01 '23 at 01:19
3 Answers
Pressure is the result of momentum exchange between gas particles and a surface. Even in a gas which has no bulk velocity vector, each particle travels in a straight line until it collides with either a surface or another particles. When a particle collides with anything it can’t continue on its original trajectory therefore per rules of classical mechanics it must undergo an exchange of momentum to change its velocity vector. The rate of impulse of many of these particles is simply expressed as a pressure. This simplification is used since we are only interested in the net effect on the surface/body and not the individual particles. When the bulk / free stream gas has a net velocity vector the added kinetic energy of the particles will be accounted for when they collide with a surface. This results in an increase in the impulsive delivered by individual particles as well as a net increase in the total effect the gas has on the surface.
The lower pressure on top of the wing is due to the collisions on top of the wing not needing to redirect as much momentum as the bottom. In fact the bulk velocity vector of the fluid on top of the wing points away from the surface reducing the relative speed a gas particle will strike the surface with. For speeds much lower than Mack 1 compressibility is negligible due to the speed of the random motion of particles being much much higher than the speed of the bulk flow.
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Thanks for the answer. When explaining lift we use air pressure as the mean to translate force to pressure and back to force. It is about how air pressure interacts with surrounding solid objects (the wing). You explain the inner workings of air pressure. That is one level too into details. We don't need to go so far. Wings push air, which is a force. That air compresses which is pressure. The pressure pushes the wing back up, which is force. Can you see the pattern: acting force -> pressure -> reacting force – Arpi Sz Sep 13 '22 at 03:09
The main problem with that explanation is that it makes conflates lift with profile drag. Lift is a different phenomena and the "lift" in your explanation is less like actual lift and more like the upwards component of profile drag. The explanation contains just enough specifics to be wrong, or at least miss the primary point of discussion (what makes a wing, a wing what it is and not something else). That's why it doesn't work even as a simplified, qualitative explanation.
It would be sort of like saying both propellers and paddle wheels work the same way. Yes, they both push water backwards but they don't do it the same way. An early propeller like a screw propeller might produce thrust using using drag principles like a paddle wheel does, but proper propeller, as we know it, does not and instead operates produces thrust via lift principles. So by implicitly describing it to operate based on drag misses it for what it is.
The simplest explanation I have heard for lift which is vague enough to not be wrong, as far as I know, is that the wing "throws down air" to produce upward momentum but this leaves beginners to ask how the top surface throws down air at all since supposedly it plays an important role.
That leads to the second simplest explanation which is that due to the properties of fluids, the air follows the shape of the wing and is guided downwards which is something that cannot be reproduced with particles such as sand.
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Thanks for the answer. Wings create lift and (a little bit of) Induced Dag as a side effect. Wings cannot create lift without creating Induced Drag. Induced Drag is the reason why Thrust is needed in cruse flight, Thrust compensates for Induced Drag, so that the airplane can keep its speed/momentum, – Arpi Sz Sep 13 '22 at 02:59
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@ArpiSz I very specifically said "profile drag" and not drag. Induced drag is not profile drag. Induced drag is due to the component of the aerodynamic force that is generated by the wing that points backwards. The component that points up is what we denote as lift. However, the lift in your explanation is based almost exclusively off profile drag which is completely different and is just a result of the wing blocking the air. With your explanation you could say that a semi-trailer produces lift as it drives with the front being like the wing bottom and the rear being like the wing top. – DKNguyen Sep 13 '22 at 03:06
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It also suffers from a similar flaw to the equal transit time theory where even a beginner can instantly ask a question that makes it fall apart. With the equal transit time theory this happens when someone asks how come a wing can fly why flat plate or symmetrical airfoil can fly, or why a cambered wing can fly upside down. With your theory it happens when someone asks how a wing can stall at all. It falls apart too quickly to be useful as a lie to children and doesn't pave the way for further understanding. – DKNguyen Sep 13 '22 at 03:11
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How can you say "However, the lift in your explanation is based almost exclusively off profile drag" when I didn't even mention profile drag? You are turning my words inside out and upside down. – Arpi Sz Sep 13 '22 at 03:17
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@ArpiSz Because that's what you are describing. One need not say profile drag to describe profile drag. Good thing too or else we would never be able to define or describe things without circular reasoning. You're describing profile drag but then using names reserved for other things. – DKNguyen Sep 13 '22 at 03:29
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Can you explain how I am describing profile drag, when I am describing induced drag? I don't understand your point. I promise I will study your point seriously. – Arpi Sz Sep 13 '22 at 03:39
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@ArpiSz It's the part where you talk about how the higher pressure forms below the wing and lower pressure forms above the wing. The way this is described is indistinguishable from how you would describe the drag at the front and rear of a semi-truck. You might be able to get away with that for the lower surface but it doesn't work for the upper surface. In fact depending on how you look at it, it could resemble what happens when a wing a stalls. – DKNguyen Sep 13 '22 at 03:49
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StackExchange asks me to stop here. Thank you and Best wishes. ArpiSz. – Arpi Sz Sep 13 '22 at 03:53
The theory of wings pushing air is highly unsatisfactory if you know that the main contribution to the lift comes from suction at the leading edge and on the upper side of the aerodynamic surface.
Even 2D-airfoil generates lift without any need for "air replacement".
Try to have a look to the relation between pressure gradient and curvature of streamlines. When a streamline has curvature, pressure decreases in the direction pointing towards the centre of curvature. Take a uniform pressure at the far field.
The answer to your question is there, if you don't want to take into account circulation and other "advanced" concepts commonly accepted in Aerodynamics.
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Define unsatisfactory! How is a scientifically correct statement unsatisfactory? Can you scientifically explain your rejection of something perfectly true? – Arpi Sz Sep 13 '22 at 17:02
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I can't change your mind. If you think it's true, good for you. I'll keep going rejecting this kind of interpretation for lift generation. I know that this is the classical long and challenging discussion that leads to nowhere – basics Sep 13 '22 at 17:59
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