My theory on lift.

[Arugela]

How close is this to how lift actually works?

My thought is that it is like buoyancy. Particles around the planes act like a solid object. Normally solid objects have enough force or density(magnetic or other forces) to hold or resist another object from passing through. IE it doesn't not displace enough to start going downwards. When a plane starts to fly it moves with particles above and below. If an object were weight less or the same weight/density as the air it would simply float as if on a solid object. As an object moves faster it then moves over particles in a way that it increase the ammount of particles per second it is over in essence increasing the density of the air particles below it. It is in essence on a more solid object as per moment moving forwards makes more particles be underneath it regardless of their desnity in real life or being under the wing/surface at any moment. If moving fast enough this mean a less dense object is then in essence supporting a more dense object as if it were of equal density or greater.(whatever is needed to support an object like a solid.). My guess is that aerodynamic lift is something that happens inbetween as you gain speed. The reason a wing produces lift from having a round top is not that it pulls up but that it is pushed up. If you consider the density on top and bottom as the objects speed increases the relative density of the air surrounding it if it equalizes it has one problem. Weight pulling it down. If you remove or displace air upwards it's not creating equal pressure on top and bottom anymore. This reduces forces around the wing that in essence are pushing up and down simultaneously but at a very steep angle. When you remove the upward air pressure it reduces the top pressure by removing particles in essence creating a vacuum or decreased density. You are reducing the above pressure making the bottom pressure have less pressure on it from the object being pushed towards it from the top pressure. This means the bottom pressure has less difficulty holding the object up and requires less speed/relative particle density. You are in essence standing on air. So, if you go faster over a solid it would also in essence increase relative density and act as a more solid object. I wonder if that could be tested. If you move diamond over a surface really fast will it eventually damage or cut the diamond regardless of the diamond being harder.

I assume this means that it is all about forces creating displacement of other objects/particles. Denser ones can more easily displace another particle from bonds or whatnot. The lesser is moved out of the way.

Edited November 11, 2018 by Arugela

[YNM]

I'll just leave this animation here.

[Arugela]

The angle of attack could be increasing the force of collision of particles creating more upward force. It is all just air particles hitting a solid surface.

What do you mean by the picture? If it has to do with the top going faster it might have to do not with flow but with the force of the wing going naturally downwards against a particle creating upward force. It's collision. The top moving faster is getting out of the way of the bottom faster creating less resistance. Angle of attack and curvature create surfaces to be hit with the forces pushing at the angle of collision in relation to it as a solid making it a lever etc.

The upward air is not hitting the object. It is avoiding the object. The front top edge create force in relation to how it hit the wing. In this case a complex curve. Which should reduce the effect of the top hitting particles. The condensation of particles would then be a product of the impact and displacement of air particles. The lift the impact energy being a combination of the forward motion against the other... but then what makes lift. The density of particles has to have some effect. And hence the motion. It might be creating a more solid object in relation to forward movement.

Note the bottom edge has more density and the top less regardless of speed. This mean more particles per second. Basically part of what I was saying.

Edited November 11, 2018 by Arugela

[YNM]

And now to emphasize that not all collision and acceleration creates lift.

 

[Arugela]

One I can't see the video becuase of mozilla/linux problems. Two it's about the amount of force created and density creating in total force AKA density. I have no idea what you are getting at. Maybe try putting it in words instead of simply linking stuff. It would display your personal point more.

[Reactordrone]

27 minutes ago, Arugela said:

 The reason a wing produces lift from having a round top is not that it pulls up but that it is pushed up. If you consider the density on top and bottom as the objects speed increases the relative density of the air surrounding it if it equalizes it has one problem.

Yes, vacuums (Low pressure areas) don't suck.

[Arugela]

They don't. A vacuum is a lack of pressure. Suction is a lack of pressure from one direction. Hence a force towards it. If two forces are acting in opposite direction and one is reduced the relative force is then increased towards the other direction.

If gravity or some other force is pulling downwards and so is the air on top and bottom of the wing. The density on the bottom decreases or resists the downward force. The reduction of weight or other factors on top of the win also reduces this pressure. Same with particle collision.

Edited November 11, 2018 by Arugela

[YNM]

42 minutes ago, Arugela said:

Maybe try putting it in words instead of simply linking stuff. It would display your personal point more.

Okay.

Here it is :

You might have heard that "aerodynamic lift (to differ from buoyancy) is caused by pressure difference". Another time you might have heard "aerodynamic lift is created by deflection of air downwards".

The truth ? It's somewhere in the middle.

- Planes can demostrably be created using an airfoil that's symmetrical with it's longitudinal axis. On these planes, they primarily operate by deflection of the air. Hence these planes generally requires a larger angle of attack compared to planes designed to utilize more pressure lift - if they are flown with 0 AOA then they'll start falling. Examples are planes designed for aerobatics (hence they can comfortably be flown upside down with fairly OK-looking attitude. If you try this on a jetliner it'll require a much large angle of attack as their wings are designed for the upright attitude, not the other or neutral).

- Fluids tend to follow the surface they're going over, called Coandă effect. Combined with the acceleration due to air getting out of the way of the wings and a slight downward deflection to reduce the acceleration on lower side, they will help create a faster flow on the upper side - slightly lower pressure - and a slightly slower flow down the lower side - slightly higher pressure. Applied correctly they will help generate lift that's otherwise rely on the amount of air that has been deflected.

The catch with the 2nd mechanism is that if the length of the flow on the top side is too long, the flow will start to separate; this will destroy the lift and creates a stall. Although in some cases they are very helpful to help a plane lands using flap extension - the (highly) extended flap destroys residual lift, allowing the plane to be firmly landed on the ground and not bounce off the runway or slightly hovering above it. (there's a really good image of how this is done but I forgot where I've seen them.)

Both of these are manifested in the animation I posted above - the flow is indeed faster above the wing, but the flow of the air is deflected downward as a whole.

Edited November 11, 2018 by YNM

[Arugela]

Yes, but force can't magically happen in the opposite direction. If something is going in one direction something made it go in that direction. So the force making the lift has to be from the same direction(or the opposite technically. The force the object is pushed is the same as the net direction of the forces applied.) it's being pushed in essence. It's not a vacuum sucking. It's a vacuum as a result of other forces being removed out of the way of another force and that force finally pushing through the lack of resistance.  Everything happens in a mechanical way. There is no pressure. It's just describing and end result. The angle of attack exposes the wing to more direct impact from air particles creating energy or force in the opposite direction. If the force is greater overall than the weight falling down it holds up. All of that flow is the just end consequence of the other forces and factors. It's what it is left to do after all other considerations. Hence it is the consequence of the forces. It is not the cause.

The coanda effect is just an equilibrium. The bigger question is what is gravity. If it's just a condensation of matter pushing other matter then the question is how did space get there.

Edited November 11, 2018 by Arugela

[YNM]

2 minutes ago, Arugela said:

If something is going in one direction something made it go in that direction.

Indeed there is - there's a higher pressure down on the ground compared to if there was no airplane above it.

Also, there are lift mechanism that purely relies on pressure differential such as compression lift.

Aerodynamics and fluid mechanics is f***ing complicated. Thankfully I only deal with non-compressible liquids IRL.

[Arugela]

Compression lift is just particles condensing like I said. I don't see what you are arguing against. That is why it's called compression. If can float on it because of an abundance of particles. Literally a more solid surface. Not to mention any movement and displacement of a non solid surface that is fluid and can move from the forces applied more easily. AKA fluid motion. Everything is a direct consequences of the thing causing it or it is not logical or mechanical and didn't happen. In other words it's incorrect. All mechanical motion is the result of all things causing it. Nothing happens without cause.

Air particles are not bouncing from the ground to hit an aircraft to produce forces against the bottom of the wing unless they are close to the ground. So it's not relevant. Air is less dense so it's not pressure. It's impact. Pressure is a description of net results.

Unless air particles are less dense because of a stronger magnetic or other field keeping them apart.

Edited November 11, 2018 by Arugela

[YNM]

11 minutes ago, Arugela said:

Literally a more solid surface.

Or, "higher pressure".

The reverse is seen on Concorde : vortices creates "lower pressure" on top of the wings.

And we haven't even talked the full 3D lift capability, on which I have nothing that I can say on it apart from "it's bloody complicated".

 

Edited November 11, 2018 by YNM

[Green Baron]

If it was impact, then why does the wing move forward ?

 

Experiment 1:

Build a balsa wing with an asymmetric profile, underside flat, upper side arched, exact profile doesn't matter. Hold the end of one wing and drag it around you, holding the wing level to the airstream. You will realise how the outer part is sucked upwards with 0 aoa(*). If you have managed a nice profile and very light balsa it will do so long before any compression takes place. Observe the outer edge.

Experiment Stage 2:

If you glue a light balsa stick with a paper control surface for pitch to the wing, put some weight in the front before the wing, you can experiment with the aoa until you find a nice configuration for a stable glide. Then let it fly, it now knows how to do it :-)

Try it ;-)

 

If the wing was really just pushed by compression, bad things would happen because that an unstable condition aka stall.

(*) Let's say you have a 4-6mm thick, 5cm wide and 30cm long balsa leaf, have managed a somewhat 1/3 to 2/3 profile depth on the upper half, the leaf will actually lift itself at walking speed. Impossible to explain with compression.

 

Edit: visibly underpressure, extreme case:

Edited November 11, 2018 by Green Baron

[mikegarrison]

The easiest way to understand lift is with Newton's laws. In order to get a force up (lift), you need to accelerate some mass down (air). Wings exist to push air down, which therefore creates lift up.

[mrfox]

mental experiment

does this following wing work under your assumption? Why or why not?

 

 

 

[Green Baron]

 

9 minutes ago, mikegarrison said:

The easiest way to understand lift is with Newton's laws. In order to get a force up (lift), you need to accelerate some mass down (air). Wings exist to push air down, which therefore creates lift up.

That is a stall or short before. Do the experiment. 0(!) aoa. The wing will lift up and try to accelerate forward against the airstream. BECAUSE the suction on top of it lifts it. The force vector over the wing, in the first 3rd over the profile depth, shows up and in front.

Just do it ;-)

Edited November 11, 2018 by Green Baron

[mikegarrison]

34 minutes ago, Green Baron said:

 

That is a stall or short before. Do the experiment. 0(!) aoa. The wing will lift up and try to accelerate forward against the airstream. BECAUSE the suction on top of it lifts it. The force vector over the wing, in the first 3rd over the profile depth, shows up and in front.

Just do it ;-)

In a symmetric airfoil at 0 AOA, there is no difference in pressure on either side of the airfoil. There is no circulation, therefore no air being pushed down, therefore no net lift. It's got nothing to do with stall. A symmetric airfoil at 0 AOA can still stall, and if it does (symmetrically) it produces a lot of drag but no lift. (IRL stall is a complex and time-dependent phenomenon and can produce lift transients through vortex shedding -- that's why knuckleballs move around.)

If you add camber to the airfoil, or you give it some AOA, or both, that ends up pushing some of the air down. You get lift. Mathematically we say that the flow around the wing now has circulation. If you look at the pressure on the wing you see higher pressures on the bottom.

Now you can think of lift as higher pressure on the bottom of the airfoil than the top of it. That's valid. You can think of it as the wing pushing the air down. That's valid. You can think of it as net circulation around the wing. That's valid (but more complicated math). They are all valid ways of thinking about lift, and crucially they all calculate the same exact amount of lift no matter which one you try (if you do the calculations correctly).

Edited November 11, 2018 by mikegarrison

[Green Baron]

23 minutes ago, mikegarrison said:

In a symmetric airfoil at 0 AOA, there is no difference in pressure on either side of the airfoil. There is no circulation, therefore no air being pushed down, therefore no net lift. It's got nothing to do with stall.

We were talking about asymmetric profile. No airfoil pushes air down. I do not understand how this comes up over and again. Do the bloody experiment.

Here is one of a million more or less equal explanations. Believe it or not ;-)

https://web.mit.edu/2.972/www/reports/airfoil/airfoil.html

 

Quote

If you add camber to the airfoil, or you give it some AOA, or both, that ends up pushing some of the air down.

And that part works against an airfoil flying. It is called form drag (or so). And if it gets too high and the flow around the airfoil ceases, it stalls. The higher the aoa, the closer the stall.

Quote

You get lift.

You get much drag and in the beginning a little more lift, that abruptly rips off when the aoa gets too high. We have an asymmetric one here, for ease of understanding.

Quote

Mathematically we say that the flow around the wing now has circulation. If you look at the pressure on the wing you see higher pressures on the bottom.

0-30% percent contribution to the overall lift, depending on many things.

Quote

Now you can think of lift as higher pressure on the bottom of the airfoil than the top of it. That's valid.

The pressure, if it exists, contributes only a very small part. The airfoil hangs at the unerpressure. Do the experiment.

Quote

You can think of it as the wing pushing the air down. That's valid.

Nope.
No (properly flying) wing pushes air down. That is pure drag and the definition of a dynamic stall. A wing moving only by its form drag.

Even wikipedia has it right: https://en.wikipedia.org/wiki/Stall_(fluid_mechanics)#Formal_definition

;-)

 

Edited November 11, 2018 by Green Baron

[mikegarrison]

Yes, airfoils do push air down. It is a NECESSARY requirement for lift that they do. Draw a control volume around an airfoil and the net velocity of the air going in and out of the volume *must* push down if there is to be an upward force on the airfoil inside. It's Newton's law. All forces and accelerations must balance out. If there is a force up on the airfoil, there must be a force down on the air. And if there is a force down on the air, by F=MA that must result in an an acceleration down on the air. Just think about it a little.

[YNM]

27 minutes ago, mikegarrison said:

Yes, airfoils do push air down. It is a NECESSARY requirement for lift that they do.

However, you can change the design that will change how much air you're actually bending.

Which isn't all just like straight hit-then-bounce thing.

[Green Baron]

9 minutes ago, YNM said:

However, you can change the design that will change how much air you're actually bending.

If you have powerful engines you can push anything against the drag and it'll generate some lift somehow.

An asymmetric airfoil at 0 degrees aoa will still produce lift, without deflecting anything on its underside (sucking air down if you see it that way). That's why i suggested the experiment.

Edited November 11, 2018 by Green Baron

[mikegarrison]

9 minutes ago, YNM said:

However, you can change the design that will change how much air you're actually bending.

Which isn't all just like straight hit-then-bounce thing.

Yes, of course. There is actually no "bouncing" at all in fluid dynamics. (At the actual molecule level of analysis it is different, but at the fluid dynamics level the fluid follows streamlines that, by their definition, can not "bounce" off of a surface. They bend around it.)

[mikegarrison]

Here's an analysis of a simple cambered airfoil.

http://airfoiltools.com/airfoil/details?airfoil=naca2412-il

Notice that for most Reynolds Numbers, CL=0 happens around alpha (AOA) = -2.5 degrees. At alpha = 0, the airfoil is producing lift.

[YNM]

14 minutes ago, Green Baron said:

If you have powerful engines you can push anything against the drag and it'll generate some lift somehow.

That's called a rocket. Or supermaneuverability.

16 minutes ago, Green Baron said:

An asymmetric airfoil at 0 degrees aoa will still produce lift, without deflecting anything on its underside (sucking air down if you see it that way).

The convergent lines at the rear will still make the deflection as the angles won't "average" out to the same as the entry. But they way that was achieved is not through the air hitting things then bending, rather the air was "sucked" downwards. Hence why it's an "effect" (Coandă effect).

2 minutes ago, mikegarrison said:

cambered airfoil

Speaking of which, is it true that aerobatic airplanes (like, say, Extra 300S) have a symmetric airfoil, that they just always have to fly with an AOA ?

[mikegarrison]

Supersonic flow complicates things. In subsonic flow, the streamlines can start to bend around the airfoil far in advance of the actual airfoil. But in supersonic flow the streamlines in front of the airfoil don't "know" it is there until they hit a shockwave. A large part of the bending happens all at once, which is basically what a shockwave is.

1 minute ago, YNM said:

Speaking of which, is it true that aerobatic airplanes (like, say, Extra 300S) have a symmetric airfoil, that they just always have to fly with an AOA ?

That sounds right, but I'm not an expert on aerobatic airplanes so I can't confirm this from personal knowledge.