# Yet another analogy for lift

Discussion in 'Hydrodynamics and Aerodynamics' started by MalSmith, May 15, 2023.

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### MalSmithIgnorant boat designer

I was wading through some of the threads started by Sailor Al, but quickly got bored. However, it did get me thinking about how to describe lift in a non mathematical manner. I came up with the following analogy, bear with me on this :

Imagine a piston in a cylinder. The cylinder has a small diameter open tube connecting one end to the other. We can ignore practical consideration like piston rods.

The piston is analogous to a foil diverting the fluid flow downwards as the fluid moves chordwise across the foil.

As the piston moves downwards, the fluid under the piston is compressed, causing a rise in pressure. Conversely the fluid above the foil is expanded, causing a drop in pressure.

The small diameter tube connecting the low pressure side to the high pressure side is a feedback loop, analogous to tip vortices, tending to equalise the pressure between the underside and the topside of the foil. The tube offers resistance to the equalising flow, the smaller the tube, the greater the resistance, and therefore the greater the pressure pressure differential for a given piston travel rate. The diameter of the tube is analogous to the aspect ratio of the foil. A higher aspect ratio foil provides less opportunity for the pressure to equalise, due to the relatively smaller foil tip length.

The piston travel rate is analogous to the rate of change of downward slope of the moving fluid, the faster the piston moves, the higher the pressure differential. For a foil in a moving fluid, the greatest rate of change occurs at the leading edge of the foil, and hence the center of lift of the foil occurs towards the leading edge of the foil.

Friction losses between the piston and the cylinder are analogous to the efficiency of the foil section.

What I like about this analogy is that it doesn't rely on either Bernoulli or Newton, and it doesn't matter about the compressibility of the fluid.

Anyway, it might be of some interest to some, or perhaps set off another lively debate on the subject.

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### Sailor AlSenior Member

Sorry you got bored, but did you read my paper?

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### rwatsonSenior Member

" What I like about this analogy is that it doesn't rely on either Bernoulli or Newton, and it doesn't matter about the compressibility of the fluid. "
So lets totally make up a story that provides no insight into reality ?
Hollywood makes millions from that, so why not.

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### MalSmithIgnorant boat designer

I have only skimmed through your paper. I will read it in more detail later.

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### MalSmithIgnorant boat designer

Well yes, an analogy is a story of sorts. It's used to convey an abstract idea. If you haven't gained any insight into the idea or hypothesis I'm trying to convey, then I haven't done a very good job. Constructive criticism would be nice.

We know that neither Bernoulli or Newtonian mechanics fully explain the mechanism of lift. The are each based on different principles. So rather than try and mash them together, why not try to think about it some other way.

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### gonzoSenior Member

This explanation ignores all the principles of fluid dynamics. For example, losses due to friction and turbulence. This more analogous to a parachute than a wing or sail.

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### MalSmithIgnorant boat designer

Not entirely. As an explanation for the phenomena of lift, both the Bernoulli model and the Newtonian model ignore these factors. They are factors in the efficiency of the foil, not part of the lift mechanism. In an ideal fluid, the lift mechanism still works despite those factors being non existent. In my analogy, although they can be ignored, these factors are accounted for by the efficiency of the piston. The efficiency is a function of the tightness of the seal and the reduction of friction, both of which affect the rate at which you can move the piston for a given force.

Last edited: May 15, 2023
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### gonzoSenior Member

Fluid dynamics include friction and turbulence in lift and any other flow. If you are referring to an ideal fluid, then there is no problem with Bernoulli or Newton. In your analogy, the model is flawed. It does not take into account differences in pressure along a foil, which is part of the basic mechanism of lift. Also, the flow through a pipe is different than the flow of a compressible fluid in a open system. Further, the efficiency of a piston is in no way equivalent to the efficiency of a foil.

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### MalSmithIgnorant boat designer

Thinking a bit more about this, you could say that friction is accounted for by the piston friction against the cylinder wall and turbulence is accounted for by the seal efficiency. In addition, separation is accounted for as part of the feedback system, because separation allows fluid from the high pressure side of the foil to bleed across to the low pressure side.

To visualise this a bit better, it helps to imagine the cylinder and piston traversing chordwise across the foil, with the cylinder at a fixed height, and the piston following the camber line. I'll do a diagram later.

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### MalSmithIgnorant boat designer

We seem to have crossed posts. No analogy is perfect. A foil is just a mechanism to extract energy from a moving fluid. As such it is subject to the law of entropy like any other mechanism. As I mentioned before, friction, turbulence and separation are what determine the efficiency of the mechanism. When we design a foil section, we are simply trying to improve the efficiency of producing lift. As you know, a flat plate will produce lift, just not as well as a well designed foil.

I guess with this analogy I'm attempting to visualise the role of fluid pressure and the effect of tip vortices etc. on the lift produced by the system. I'm also attempting take into account the effects of aspect ratio I'm not saying that the Bernoulli and Newtonian models are incorrect (in fact they are both correct), but they are often misinterpreted and may not help that much to visualise the system. However, clearly my model is not helping much either.

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### gonzoSenior Member

I don't understand how your model visualizes the effect of tip vortices or aspect ratio. Further, foil sections are not always designed to increase lift. For example, they may be designed to decrease drag.

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### MalSmithIgnorant boat designer

Never mind. It was a silly idea.

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### Aaron FimbresNew Member

I just made this account because sometimes people impress me. I’m looking at you Mal. Courteous mature and creative, three beautiful traits rarely matched for long.

My best ideas are generated when I find parallel (not mirrored) concepts that behave enough alike that I am able to gain fresh angles from the general abstract visualization of both processes simultaneously.
But that’s just me maybe?
Unless the point is to sustain perfect translation between two very unlike models of science magic. I think that is a different beast all together.
Well, to me it is.
Thank you Mal, keep it up.

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### MalSmithIgnorant boat designer

Thank you Aaron, you are very kind.

I would like to say that I had this idea whilst in the bath, and ran into the street yelling eureka! But the truth is, I was sitting in the car, in the Ikea carpark, minding the dog, reading through the forum posts, waiting for my wife and sons to emerge with trolleys full of self assembly furniture (it was Mothers' Day). I explained the idea to the dog. I admit, he seemed a bit perplexed.

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### DogCavalrySoy Soylent Green: I can't believe it's not people

Or to reduce moments about the section. High aspect ratio foils are subject to great torsion strain, and that is more of a challenge than simple bending forces from lift.

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