My theory on lift.

[Green Baron]

My problem was with the formulation wings are pushing air down. That raises mental pictures of air molecules colliding with surfaces and thus pushing things up while at the same time are reflected down through drag and resistance. Such a state can be held with riding on or hanging at a powerful engine, but it is not exactly explaining why a plane (and then i am always thinking of glider planes and small propeller planes at low speeds) actually flies.

Of course, in sum all forces must add up to 0 while in flight and the action that keeps a plane up must have a counter action in the mass of the air flowing by. No question.

The original proposal upthread was that an aircraft "stands on thicker air" and that is not the case, in contrary it hangs at low pressure, which can easily be demonstrated with a small model.

6 minutes 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 ?

Yes. And they hang themselves at the propeller and use the propeller air to vent the control surfaces.

Edited November 11, 2018 by Green Baron

[mikegarrison]

28 minutes ago, Green Baron said:

My problem was with the formulation wings are pushing air down. That raises mental pictures of air molecules colliding with surfaces and thus pushing things up while at the same time are reflected down through drag and resistance. Such a state can be held with riding on or hanging at a powerful engine, but it is not exactly explaining why a plane (and then i am always thinking of glider planes and small propeller planes at low speeds) actually flies.

Of course, in sum all forces must add up to 0 while in flight and the action that keeps a plane up must have a counter action in the mass of the air flowing by. No question.

The original proposal upthread was that an aircraft "stands on thicker air" and that is not the case, in contrary it hangs at low pressure, which can easily be demonstrated with a small model.

When considering the pressure around an airfoil, the usual way to look at it is a plot of Cp (coefficient of pressure) v. chord length. What you see is that there is a higher than ambient pressure on both sides of the airfoil! On the bottom side (aka the "pressure side") the Cp is higher than the top side (aka the "suction side"), but there is no actual "suction" going on, in an absolute sense. It is the *net* difference in pressure between the bottom and the top that provides the force.

Sorry, that crossed out bit was wrong. I was thinking of something else. The "suction side" of the airfoil does indeed usually have pressure lower than the free stream ambient. In fact, both sides usually do.

Edited November 11, 2018 by mikegarrison

[Green Baron]

1 minute ago, mikegarrison said:

It is the *net* difference in pressure between the bottom and the top that provides the force.

And that is not called suction ?

[mikegarrison]

1 minute ago, Green Baron said:

And that is not called suction ?

It's called suction informally, but to be clear, it's not actually negative pressure. There is no such thing as negative pressure in fluid dynamics. There are only differences between different levels of positive pressure.

By the time you get close to a true zero pressure, the assumption that the molecules behave like a fluid breaks down and instead they behave like individual molecules.

[mikegarrison]

Here's a Cp diagram. They are usually shown with the y-axis inverted, so the upper surface of the airfoil is also the upper line on the plot.

Cp = (local static pressure - static pressure in the free stream)/freestream dynamic pressure

Since freestream dynamic pressure = freestream total pressure - freestream static pressure, Cp = (local static pressure - freestream static pressure)/(freestream total pressure - freestream static pressure)

When it is 0, that means the static pressure is the same as the freestream ambient static pressure. When it is less than 0 that means the static pressure is less than the free stream static pressure. In inviscid theory, the Cp = 1 at the stagnation point at the nose of the airfoil and also = 1 at the trailing edge.

It should never be able to go above 1. 1 means you have perfectly recovered all of the freestream total pressure.

In real life, viscous losses mean they don't actually equal 1 at the trailing edge. (You don't get perfect pressure recovery.)

You can see that the static pressures are actually lower than free stream as they go around the airfoil (which makes sense, because the velocities are higher than free stream, so Bernoulli). They are more lower than free stream on the top than they are on the bottom, at least in this case. The area between these curves can be integrated to get the pressure difference between the upper and lower surfaces, and therefore the forces on the airfoil (both lift and drag).

The difference in Cp is often greater in the front than in the back, and that's why there is a moment associated with most lifting airfoils (Cm). Usually Cm is defined around the point of 25% chord length.

If you look at other airfoil shapes, you sometimes see very different Cp diagrams.

Edited November 11, 2018 by mikegarrison
edits because I screwed up the definition of CP at first. Pretty sure it is right now.

[Green Baron]

@mikegarrison, what i was arguing against and especially with my "no airfoil pushes air down" was the notion that through collision with air and at high angle of attacks lift is generated, like a fan that blows at an angled sheet of metal. Which will then do funny things if let go but it will not dynamically fly. Two much drag and too little lift is called dynamic stall, which i tried to point out. It is only drag, not flight because the flow around the surface is disrupted. If pushed or drawn by powerful engines, the contraption may still fly, though. But many planes don't even have engines. Which is fun.

It is the profile that keeps the wing in the air and the flow around it, in that we all agree. That happens even at 0 degrees angle of attack with an asymmetric profile, whichever definition one sees fit. In this case the drag vector is very short, as we know, and the plane flies very fast. Or, if it is a very effective design, it glides at ratios of 50-60, which is not rare among glider planes.

@Arugela suggested that the aircraft rests on air that it deflects downwards, if i understood it correctly, but this is not the case, it is the underpressure part that contributes at least 2/3rds to the lift, depending on configuration, design, circumstances. Of course somewhere in the dynamic system, the force that the aircraft borrowed from the flow around its wings must be paid back in form of air moving down or spiraling behind it (induced drag). But this is hardly deflection. A landing craft, for example, though at high angles of attack and with a configuration that produces much drag and high lift, does not cause a storm on the ground because of a downwash or so. You can stand 200m aside from a landing A380 and wont be blown away by the several hundred tons that settle there.

Though behind hit it may get turbulent ;-)

Are we all content with that ?

Edited November 11, 2018 by Green Baron

[mikegarrison]

Fundamentally, drag and lift are the same thing. They are aerodynamic forces on a body. We call the vector component that opposes gravity "lift" and the vector component that opposes the freestream "drag". It's more convenient to analyze this way because we want lift and we don't want drag (usually).

The OP's ideas have some grasp on the situation, but they don't account very well for the ways that moving fluids behave very differently from static fluids. Air doesn't just compress when something moves at it, it also moves out of the way. But many of his basic ideas are correct. Because the air is light and the plane is heavy, you need to sweep over a lot of air in order to generate lift. That's why airplanes fly fast and why wings stretch out from the side so they sweep over more air. But buoyancy isn't a very good model for understanding lift. Lift comes because you get the air moving, not because you compress it and make it "more solid". (Hypersonic re-entry is different, but let's ignore that for now.)

To the extent that you get the air moving down, you generate lift. But to the extent that you pull the air forward with you, you generate drag.

Stall is something different. When the flow mainly follows along the shape of the object moving through it, it is "attached". But when it pulls away from the object and leaves a volume of air that is not following the object, it is "separated". When there is a big amount of separated flow, we call that "stall". It tends to produce very little lift and a lot of drag, so it's generally best to avoid it. (Sometimes we want a lot of drag though, like with airbrakes or a parachute.) I took a whole class in college just on boundary layers -- they are way too complicated to easily discuss here. But generally speaking, the reason an airfoil has a curved upper surface is because you are trying to finesse getting the air to follow along the upper surface as attached flow rather than separating away. It is not because you are "trying to push the air up" -- far from it, you are trying to induce the air to follow the curved shape of the airfoil and flow down. If you can get the airflow to bend down, that means by Newtons law that there must be a force up on the airfoil. When the airfoil goes into stall the airflow stops following the curve of the airfoil and flows straight back. Because it no longer turns to flow down, there is no longer any corresponding force up on the airfoil (from the top, anyway). That's why the sudden loss of lift when an airfoil stalls.

The compression that the OP is envisioning on the bottom of the airfoil is not really a compression. But it does help the airflow to follow the airfoil shape. That's why separation is a big concern on the top of the airfoil but usually not a concern on the bottom.

The problem comes in your sentence right here: "[The OP claims] the aircraft rests on air that it deflects downwards, if i understood it correctly, but this is not the case, it is the underpressure part that contributes at least 2/3rds to the lift". The thing is, that "underpressure" and the downward deflection are physically linked. It's not a case of lift being one or the other. They are two ways of looking at the same thing.

Edited November 11, 2018 by mikegarrison

[Green Baron]

1 hour ago, mikegarrison said:

Fundamentally, drag and lift are the same thing. [rest snipped because common knowledge]

 

Dude, some of the drag is induced by the wing depending on lift (heavily depending on design, very little from a highly effective glider), but apart from that drag and lift are fundamentally different things, actually two of the fundamental forces on a flying apparatus. But lift induced drag plays little role at low aoa. That is (again) why i proposed that experiment.

Again, i don't move air down nor does a wing. If it did, you'd have heavy winds at the side of a landing or starting aircraft, which is not the case. The equalizing of the pressures mostly takes place behind it, in the wake turbulence. Which probably is the induced drag you meant. But that is an outcome of the generated lift (or the pressure difference), it is not the cause of it.

With that, I quit. It doesn't move the world at all.

Have a nice one everybody :-)

Edited November 11, 2018 by Green Baron

[mikegarrison]

6 minutes ago, Green Baron said:

Again, i don't move air down nor does a wing. If it did, you'd have heavy winds at the side of a landing or starting aircraft, which is not the case.

What do you think trailing vortices are?

[Green Baron]

42 minutes ago, mikegarrison said:

What do you think trailing vortices are?

I wanted to quit, but you don't let me :-)

I have actually described how they form and so have you, right ? It is the pressure equalization between under- and upper wing surface. And they are behind the wing up to several kilometers, not at the side or under it. There are a plethora of photographs all over the web visualizing them. An airliner has heavy ones, a glider almost none with its high aspect.

Were the wing shoving air down, the air would be squeezed to the side when it lands or takes of, causing heavy gusts to nearby thing like waiting aircraft, people at the fences, etc. But there is no such thing. Behind the craft, though, things can get funny. But you surely know that - horizontal spacing separation and all that.

Quitting one more time :-)

Edited November 11, 2018 by Green Baron

[Arugela]

9 hours ago, Green Baron said:

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:

It would have to be that the total forces of the impact don't make it stop. Hence the net force. Propulsion from a fan is always angled. Hence it must be hitting molecules in a way that the net force hits the molecules and the density(which should be a produce of impact) collect and the force/energy directs in a forward motion from the propeller/ultimately jet engine. This is machine transference of energy including some that use total force changing it's directional forces. Hence inefficiency in flight. But enough to produce the end result. After the plane is propelled it has enough forward energy(also net result of other mechanical forces) and isn't stopped. Hence flight/floating(and pressure etc) are all net results of mechanical forces. leverages is also a net result. Everything is a net result because that means it is physical an real. IE it's the result of the things causing it. It's a description of full cause and effect. AKA describable. The foundation of reason and science. The thing that is is fully the consequence of the things cuasing it. This is the only reason our brain can think it out potentially because our sensory data(mechanical procces, even if you don't consider it physical by some definitions. it is still logically mechanical in the true definition as it is consequential. Hence it is physical. Just on a smaller realm than we are used to seeing etc.) gets info into our brain and can ultimately think something out. Which is an abundance of nodes being used to form a thought.

Basically it's all leverage etc.Which is als a description of net forces going in a specific way.

Most of this stuff is just logical descriptions that something is happening to cause something else. You just have to deeper to figure out the specifics. What we do know if that something must be causing it. The rest is just identifying parts of what it is. Which could be a single thing or talked as such, or multiple things even including things we aren't considering. It's a logical representation of things from a limited perspective using the necessities of logic I already stated. But most of these terms are just purposeful descriptions of net results and not meant to describe the specifics of how it does it. Like pressure. It's just something easy to put in a simplified/non completely detailed calculation.
 

Quote

 

I wanted to quit, but you don't let me :-)

I have actually described how they form and so have you, right ? It is the pressure equalization between under- and upper wing surface. And they are behind the wing up to several kilometers, not at the side or under it. There are a plethora of photographs all over the web visualizing them. An airliner has heavy ones, a glider almost none with its high aspect.

Were the wing shoving air down, the air would be squeezed to the side when it lands or takes of, causing heavy gusts to nearby thing like waiting aircraft, people at the fences, etc. But there is no such thing. Behind the craft, though, things can get funny. But you surely know that - horizontal spacing separation and all that.

Quitting one more time :-)

 

When you say it shoves air down think, "how it does it do this." What you are using is an analogy based on your physiology. Normally your arm applies leverage other mechanical forces to push something away. We are machines. The wing does not, for the most part, physically push down. So something else is mechanically causing a net result. But it is something actual and detailed. AKA nothing happens magically from nothing.

The other way to look at it is most of these terms are like a placeholder in a video game. They are partial and can be described in more depth.

I think the other part of this is that you're not just describing single forces being applied. It's because you are looking at multiple forces simultaneously being applied creating different logic. Differentials like tug of war can do more things logically than a single force. In realistic case I would say tug of war with a mostly unknown number of directions and forces above 1(for the most part) all being applies to mechanical objects creating leverage and other forces creating machines that change force and direction or similar ultimately.

My original analogy of more particles under the wing over time would be very steep impacts or mechanical energy transferences from multiple angles ultimately stopping downward movement. An object rolling over an object is just a way of describing some level of mechanical impact or transference of energy in some direction or so ultimately creating a net result. In this case not down.

Also, compression must be a net result of other things. It just means when the air particle hit/colided something it was stopped/ held on it's surface(from the reality of energy transference. Whatever that is.) or moves less quickly or something along it to be displaced and accumulates temporarily. So, it is never the cause. It is the result.

I think the basis of this argument has to do with that last part. That is an end result. The rest is describing the entirety of the details of the forces at play. It has to be a net result of impact/collision. That is the description of mechanical forces and energy transference. without it there is no means of mechanical interaction hence nothing happened. That is what impact is meant to describe. The bigger question is what is impact and energy which are also net descriptions. Something had to do something to produce something. It's just a matter of the full array of directions and impacts and other things. The rest is seeing the result of said impacts and what did or did not have the energy/momentum/force to move/displace the other and what happened as the result.

Impact/collision is the description of mechanical realities as the result of movement. If it involves movement it involves impact or similar as logically that is the point of the word(s). This is all argument over the purpose and limits of terminology.

Vertices are also the net result of the impact and change in energy or whatever from impact. It does not overcome the wing so it accumulates and the shape of the wing and the net trailing and forces of the surrounding air make that shape. All net forces. Just more complex ones. It creates an area of less density of air particles or whatever. The atmosphere is just a lack of empty space. Whatever cause them to accumulate over the earth is just an abundance of particles to impact as opposed to space where there are less of them. Unless there is a fundamental lack of understanding, which there can be as we don't know what causes stuff in detail to accumulate like gravity. For all we know it's not an attraction but a repulsion or something more complicated and we are using the wrong basis to understand things. But that changes the answer a lot more and in much more complex and unpredictable ways that would be too hard to get into detail to figure out without knowing more details of the cause and comparing it. Technically it seems vacuum is the norm. So thinking things accumulate from attraction could be incorect somehow. It could be they move in that direction because a force is pushing them in that way. Gravity is a net result afterall. It is normal to get things backwards in theories like this. It makes more sense if the force being applied goes in the direction. Although if it's more complicated it could be the result of more forces hence not that simple. Just like the lift argument. It is one of the forces in lift like mass afterall. Presumably gravity is basically the same as what makes matter come together in a general or specific sense. It's an attractant force. which again is a net result. Just what is causing it. Maybe space is spinning or something and that is what allows physical objects to float in space. A more complex reality of flow pushing net result of bigger particles together. We might just be living in a cosmic snow globe and spinning around. That or the spinning in a vortices in some intergalactic/dimensional beings toilet... 8d Even different dimension wouldn't' truly have different rules. Just different circumstances and instances (aka common occurrences) or "objects"(another net result) as the words we use are just describing logical things and the condition for things happening are the same. It's still the result of all thing causing something. Hence physical/mechanical.

BTW, to get a correct answer you must completely describe something in full detail. So, that is why these arguments and subjects go in these directions. Just like mechanical force. It's all cause/effect to put it simply.

Edited November 11, 2018 by Arugela

[mikegarrison]

Again there is something of a disconnect here. *Some* of the trailing vortex is due to 3D effects (non-infinite wingspan) at the wingtips. But much of it is due to the circulation around the wing. I mentioned before that one equivalent way to calculate lift is "circulation". The difference in velocity of the airflow over the top of the wing and under the bottom of the wing can be reformulated into a steady freestream component that is the same on both sides and a circulation component that circles around the airfoil. https://en.wikipedia.org/wiki/Circulation_(fluid_dynamics) This circulation has vorticity, but in inviscid potential flow vorticity must be constant (it can't start or stop). What it does is come off the end of the wing and sort of turn from being oriented along the wing to being oriented along the direction of flight.

Anyway, the point is that any lifting wing sheds a vortex in proportion to the lift that is being generated. This is the primary mechanism for lift-induced drag. So the trailing vortices are directly induced by the generation of lift itself. They are not some sort of ancillary phenomenon.

Edited November 11, 2018 by mikegarrison

[YNM]

2 hours ago, mikegarrison said:

*Some* of the trailing vortex is due to 3D effects (non-infinite wingspan) at the wingtips. But much of it is due to the circulation around the wing.

I could see that it's basically near-impossible to generate lift without any of the higher-pressure area 'flowing over' into the lower pressure area.

Or maybe by vorticity you're referring to the bending of the flow itself ?

[mikegarrison]

2 hours ago, YNM said:

I could see that it's basically near-impossible to generate lift without any of the higher-pressure area 'flowing over' into the lower pressure area.

Or maybe by vorticity you're referring to the bending of the flow itself ?

This is where math and physics collide in aerodynamics. Explanations about pressure and turning the airflow down are basically physical explanations of lift. Circulation and vorticity is more a mathematical explanation of lift. It's taught to aero engineers and aerodynamic scientists, but not usually to pilots or the general public because the math gets really difficult and it's nowhere near as physically intuitive.

https://en.wikipedia.org/wiki/Horseshoe_vortex

https://en.wikipedia.org/wiki/Lifting-line_theory

https://en.wikipedia.org/wiki/Vortex_lattice_method

https://en.wikipedia.org/wiki/Airfoil#Thin_airfoil_theory

https://en.wikipedia.org/wiki/Kutta–Joukowski_theorem

Edited November 12, 2018 by mikegarrison

[wizzlebippi]

I had to take a grad level aero class to get answers to some of the questions posed here.

The equation is:

Lift = density * Forward Velocity * Circulation.

Hold density constant (constant altitude), and the result is circulation is inversely proportional to forward velocity, assuming the aircraft is airborne.

2 hours ago, YNM said:

I could see that it's basically near-impossible to generate lift without any of the higher-pressure area 'flowing over' into the lower pressure area.

Or maybe by vorticity you're referring to the bending of the flow itself ?

If the leading edge of the wing (usually a separate piece for repairability) isn't sealed to the rest of the wing, stall speeds are significantly increased due to air being pulled through the gaps.

Because the wing isn't infinite, air flow below the wing moves increasingly outboard as you approach the wing tip. It attempts to come around the wing tip and forces air above the wing to move increasingly inboard as you approach the wing tip. This produces a theoretical (and mind meltingly complex) structure called a vortex sheet. Since it's inherently unstable, it balls up into vortices on any discontinuity, like control surface gaps, and static wicks. If the aircraft happens to be in an extremely humid environment, these vortices are visible, and makes Facebook as "the pilot left the Chem trail sprayers on."

Many aircraft have vortex generators near the ailerons. They can be strips attached to the leading edge, or even finger like protrusions from the bottom of the wing. This is because near stall, high pressure air from under the wing is either coming around the wing tip or trailing edge and reducing roll authority. The vortex generators keep air moving chord wise and maintain roll authority deeper into the stall. If you see vortex generators ahead of the aileron on top of the wing, that's a fix for something else.

[Arugela]

https://www.youtube.com/watch?v=zUehWUoiPHQ

I came up with another idea. Same thing but explains it more. It's basically a description of what could create pressure. And how it is the result of impacts.

1. Presume the air molecules are in an energy state of psueod equalibriam. This is the reality of the impact.

2. The force of the wing is impacting the molecules because the wing is moving faster. You get a net energy from impact. This is movement and energy of both molecules and wing.

3. The low pressure has to do with molecules in essence skipping and their distance. Number of impacts! In this case less impacts from farther/higher skipping. Assuming reimpact. Basically available energy.

4. Impacts are the cause of mechanical power and cannot happen or happen with a severity in relationship to them and the total energy.

5. Bottom of wing hits molecules and cause build up. This cause more tiny mass impacts etc. Net energy gain. Molucuels don't get pushed downwards completley but skip along teh surface and/or compress. More impacts.

6. The top is a long skip. The vacuum/low pressure is a lack of molecules as the move faster causing them to take long to drop and hit the wing again. Assuming the do. It's direction of molecules up in totality. Less potential energy. Less mechanical force from impact. Pressure over a flap causes stalls or loss of energy as raised flap(angles upwards) relies on top air to impact the surface and with a low pressure there is  loss of molecules destroying said mechanical advantage removing power of flaps or surfaces.

Part of this is direction of impact others from the basic start of the molecule considered in the impact. It take longer to float back down. This is why it is floating to start.

Basically the surfaces and directions of movement are causing larger skips and less impacts on top and more on the bottom. AKA less density of air molecules.

I also found this: https://www.youtube.com/watch?v=RoT2upDbdUg

Edited November 13, 2018 by Arugela

[mikegarrison]

You still don't seem to understand the difference between fluid dynamics and the static properties of gases.

Edited November 13, 2018 by mikegarrison

[Arugela]

I'm assuming there is no difference. One is how one acts in a particular situation. Fluid is the ability to move in an impact situation. Or some other event involving motion.

Edited November 13, 2018 by Arugela

[wumpus]

Anybody know what the Einstein wing looked like?  https://wrightstories.com/einsteins-wing-flops/

Back when I believed the "classic" "wings work because aerofoils produce lift" I assumed that something like a Concorde or Delta-wing would be ideal, with long thin wings to maximize the area pushed on and minimize cross-sectional area causing drag.  But for whatever reason, long thin wings produce more lift to drag than anything else.

[mikegarrison]

21 minutes ago, wumpus said:

Anybody know what the Einstein wing looked like?  https://wrightstories.com/einsteins-wing-flops/

Back when I believed the "classic" "wings work because aerofoils produce lift" I assumed that something like a Concorde or Delta-wing would be ideal, with long thin wings to maximize the area pushed on and minimize cross-sectional area causing drag.  But for whatever reason, long thin wings produce more lift to drag than anything else.

I guess by "cross-sectional area causing drag" you are talking about wing thickness?

Drag is usually broken down into friction drag, form drag, lift-induced drag, wave drag, interference drag, etc. The reason high aspect ratio wings have good lift-to-drag is because they reduce lift-induced drag.

[YNM]

On 11/14/2018 at 2:49 AM, wumpus said:

Einstein wing

Well, that's like hoping a modern wing with the flaps fully extended flies better than no flaps.

It's also about flow separation - Coandă effect is still an effect, in any case.

Bernoulli's principle still holds that a properly designed wing can indeed bend more air than what might be expected off a colision-thing.