airfoil rising

[Zet]

 

 

 

I’m having trouble with the conservation of energy analysis of a rising versus a non-rising airfoil.

 

(The non-rising airfoil is held down in place and prevented from rising.)

 

 

----

 

When an airfoil moves horizontally the surrounding fluid moves more quickly over the top of the airfoil and less quickly under the bottom of the airfoil. This means the downward pressure on the top of the airfoil decreases more and the upward pressure on the bottom of the airfoil decreases less. If the (decreased) upward pressure on the bottom of the airfoil becomes greater than the weight of the airfoil plus the (even greater decreased) downward pressure on the top of the airfoil then it will rise.

 

If it rises then there is an increase in gravitational potential energy.

 

If it is prevented from rising then there is no increase in gravitational potential energy.

 

So, for energy to be conserved, when an airfoil rises there must be a decrease in another form of energy that does not decrease when the same airfoil prevented from rising.

 

And so, when an airfoil rises its motion must slow down so that there is an equal decrease in kinetic energy while if it is prevented from rising its motion must not slow down in this same way.

 

And so (the only way I can think of to end up with this difference), when an airfoil rises there must be an increase in drag that does not occur when the same airfoil is prevented from rising and that increase in drag must slow the airfoil down by the precise amount where the decrease in kinetic energy equals the increase in gravitational potential energy.

 

And if all of this is correct, then the conservation of energy analysis is resolved.

 

But I don’t think I’m right.

 

Two airfoils of equal masses (and so equal increases in gravitational potential energies) will produce two different amounts of drag if they are shaped differently and so will slow down by two different amounts and produce two different decreases in kinetic energies.

 

Unless, it could be that, yes, two differently shaped airfoils will produce two different amounts of drag (in both the rising and non-rising cases), but if they are allowed to rise then there is also an additional amount of drag, and this additional amount of drag is the same no matter the shape of the airfoil (and is precisely what is needed to slow the airfoil down to match the increase in gravitational potential energy).

 

While this is possible it doesn’t seem likely. If two airfoils are shaped differently, and if there is additional drag on them as they rise, common sense suggests that that additional drag will vary depending on the shape of the airfoil (and so energy cannot be conserved in both cases).

 

What am I missing?

 

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I understand that there are other factors that contribute to lift (angle of attack, conservation of momentum, and so on) and a full analysis of lift is complicated and sometimes disputed. But even if all of the other factors were included and the above aspect of lift only contributed a small amount to the overall lift for a particular airfoil, it still is a contributing factor and there still must be some slowing in the rising case that does not occur in the non-rising case for energy to be conserved and this slowing must be the same for airfoils of equal masses but of different shapes.

 

----

 

?

[studiot]

Why do you think energy must be preserved?

 

Isn't a continual input of energy the job of the engine?

[MigL]

Why do you think the only potential energy is gravitational ?

Any force will produce a potential, and a potential will produce kinetic, unless constrained.

A gravitational force has an associated gravitational potential which results in the movement of a test mass, i.e. falling.

A lifting force also has an associated potential, but as your airfoil is constrained, it cannot exchange this potential for kinetic, so it does not rise and potential does not decrease.

 

In effect the net change in potential ( considering only lift and gravitational, not drag ) is zero.

If it were to rise, its gravitational potential would increase but lifting potential would decrease.

 

An analysis of the total energy balance on the other hand, would also need to include thrust and drag to be complete, as these also supply/reduce energy of the system as Studiot has pointed out.

Edited December 11, 2014 by MigL

[Zet]

Why do you think energy must be preserved?

 

Isn't a continual input of energy the job of the engine?

 

Conservation of energy is one of the most fundamental principles of physics.

 

----

 

The engines on an airplane (unlike that of a rocket) are not setting the body into vertical motion but into horizontal motion. They convert chemical potential energy (in the form of jet fuel) into an equal amount of kinetic energy (in the form of horizontal motion).

 

However, due to the different decreases in pressures on the top and bottom of the airfoil, this horizontal motion also means there is an increase in the upward buoyant pressure on the airfoil. And this increased upward buoyant pressure (if it is sufficient enough) will cause the airfoil to rise. And as the airfoil rises there is an increase in gravitational potential energy.

 

And if energy is to be conserved this increase in gravitational potential energy must come with a corresponding decrease in another form of energy.

[studiot]

 

Conservation of energy is one of the most fundamental principles of physics.

 

Indeed it is.

Do you know the full principle?

 

In any isolated system total system energy is conserved.

 

So what is your isolated system?

 

You did ask where your thinking was going awry.

Incorrectly specifying the system is a very common error.

Edited December 11, 2014 by studiot

[Zet]

Why do you think the only potential energy is gravitational ?

Any force will produce a potential, and a potential will produce kinetic, unless constrained.

A gravitational force has an associated gravitational potential which results in the movement of a test mass, i.e. falling.

A lifting force also has an associated potential, but as your airfoil is constrained, it cannot exchange this potential for kinetic, so it does not rise and potential does not decrease.

 

In effect the net change in potential ( considering only lift and gravitational, not drag ) is zero.

If it were to rise, its gravitational potential would increase but lifting potential would decrease.

 

An analysis of the total energy balance on the other hand, would also need to include thrust and drag to be complete, as these also supply/reduce energy of the system as Studiot has pointed out.

 

Not all potential energy in the universe in gravitational. Gravitational potential energy is one form of potential energy. And it is part of the conservation of energy analysis of a rising airfoil.

 

----

 

Yes, it will not rise if constrained. But if it is not constrained, and if the upward buoyant pressure (from the different decreases in pressures on the top and bottom) is sufficient enough then it will rise. And this means an increase in gravitational potential energy. And for energy to be conserved this must come with a corresponding decrease in another form of energy.

 

Indeed it is.

Do you know the full principle?

 

In any isolated system total system energy is conserved.

 

So what is your isolated system?

 

You did ask where your thinking was going awry.

Incorrectly specifying the system is a very common error.

 

 

Yes, I did ask you all where I went wrong.

 

I believe I understand the law of conservation of energy fairly well.

 

In this case the isolated system could be the whole universe. (But I don’t see how the rest of the universe, and all of the rest of the various dynamics occurring out there, effects the analysis of the rising versus non-rising airfoils.)

[studiot]

Specifying the whole universe as your isolated system offers more difficulties than it solves.

 

Not least being that we do not know is the universe is finite or infinite.

If infinite standard conservation has no meaning.

 

Incidentally you need to be careful saying you understand The First Law (conservation) and then producing statements like this one.

 

 

They convert chemical potential energy (in the form of jet fuel) into an equal amount of kinetic energy (in the form of horizontal motion).

 

I always thought that the large part of the chemical energy went into heat in an engine of any sort.

[Zet]

Why do you think the only potential energy is gravitational ?

Any force will produce a potential, and a potential will produce kinetic, unless constrained.

A gravitational force has an associated gravitational potential which results in the movement of a test mass, i.e. falling.

A lifting force also has an associated potential, but as your airfoil is constrained, it cannot exchange this potential for kinetic, so it does not rise and potential does not decrease.

 

In effect the net change in potential ( considering only lift and gravitational, not drag ) is zero.

If it were to rise, its gravitational potential would increase but lifting potential would decrease.

 

An analysis of the total energy balance on the other hand, would also need to include thrust and drag to be complete, as these also supply/reduce energy of the system as Studiot has pointed out.

 

 

 

(I missed the edited part of this response.)

 

Are you saying that when the airfoil rises and gravitational potential energy increases, the other form of energy that correspondingly decreases is “lifting potential energy”?

 

And are you saying that as the airfoil moves horizontally and the upward buoyant force on the airfoil increases “lifting potential energy” is created?

 

If so, I disagree. The upward buoyant force on the airfoil is a force and not energy.

 

----

 

Sure, a complete analysis of the conservation of energy of a rising airfoil needs to include an analysis of all aspects contributing to lift.

 

And if you’d like we can discuss the other factors that contribute to lift and how energy is conserved in each aspect.

 

However, one of those aspects is an increase in the upward buoyant force and it is that aspect that I am stuck on (not the others).

[studiot]

 

However, one of those aspects is an increase in the upward buoyant force and it is that aspect that I am stuck on (not the others).

 

 

Another misconception is your use of the term bouyant force.

 

The aerofoil is subject to bouyancy forces, but these are nothing to do with the lift and drag forces generated by airfoil action.

Edited December 11, 2014 by studiot

[Zet]

Specifying the whole universe as your isolated system offers more difficulties than it solves.

 

Not least being that we do not know is the universe is finite or infinite.

If infinite standard conservation has no meaning.

 

Incidentally you need to be careful saying you understand The First Law (conservation) and then producing statements like this one.

 

 

I always thought that the large part of the chemical energy went into heat in an engine of any sort.

 

 

Okay. I’m not exactly sure what the issue is here when it comes to defining the “isolated system.”

 

When I throw a ball up into the air, and as it rises it slows down, energy is conserved because as the kinetic energy decreases there is an equal increase in gravitational potential energy and then when the ball falls back down energy is conserved because as gravitational potential energy decreases there is an equal increase in kinetic energy as the ball speeds up.

 

This, to me, is a perfectly fine analysis of how energy is conserved when a ball is thrown up into the air and then comes back down. I don’t see the need analyze whether the thrown ball is truly in a closed system or not. It can just be stipulated that it is. So, fine, the rising and non-rising airfoils are not in the whole universe but are stipulated to be in isolated systems.

 

----

 

It could be that these hypothetical airfoils have hypothetically perfect engines and so no heat is produced and so all of the chemical potential energy lost becomes an equal amount of kinetic energy.

 

Or, it could be these hypothetical airfoils have realistic engines and so the chemical potential energy lost becomes an equal amount of thermal and kinetic energies.

 

Either way, once the airfoil is set into horizontal motion that is where this question begins.

[studiot]

 

It could be that these hypothetical airfoils have hypothetically perfect engines

 

A hypothetically perfect engine is called a carnot engine, and carnot engines cannot produce motion without also producing heat, unless they violate the Second Law.

 

We are happy to disuss and help you resolve your questions, but since we know more physics than you do, how about letting us suggest things?

 

For instance there is nothing wrong with considering your ball thrown into the air as an isolated system, in fact it is an excellent model for many purposes.

But then your ball does not posses a jet engine and a fuel tank.

 

The important thing is to know where to draw your boundary around your system. A good choice makes the analyis (relatively) simple.

 

 

Either way, once the airfoil is set into horizontal motion that is where this question begins.

 

 

So perhaps you would be kind enough to reiterate your question in plain terms, shorn of all the extra paragraphs you started with.

Edited December 11, 2014 by studiot

[Zet]

 

Another misconception is your use of the term bouyant force.

 

The aerofoil is subject to bouyancy forces, but these are nothing to do with the lift and drag forces generated by airfoil action.

 

 

A hypothetically perfect engine is called a carnot engine, and carnot engines cannot produce motion without also producing heat, unless they violate the Second Law.

 

We are happy to disuss and help you resolve your questions, but since we know more physics than you do, how about letting us suggest things?

 

For instance there is nothing wrong with considering your ball thrown into the air as an isolated system, in fact it is an excellent model for many purposes.

But then your ball does not posses a jet engine and a fuel tank.

 

The important thing is to know where to draw your boundary around your system. A good choice makes the analyis (relatively) simple.

 

 

So perhaps you would be kind enough to reiterate your question in plain terms, shorn of all the extra paragraphs you started with.

 

 

I may in fact be using the term “buoyant force” wrong if it technically only applies to fluid statics and not also to fluid dynamics. If there is a different term I should be using, please let me know.

 

However, the upward and downward pressures from the surrounding fluid on an airfoil do definitely affect lift. How could they not?

 

Like any submerged body, airfoil or not, if the pressure from the fluid pushing down on the top plus the weight of the body is greater than the pressure from the fluid pushing up on the bottom then it will fall, if they are equal it will remain in place, and if it is less it will rise.

 

And when an airfoil moves, the pressure from the fluid on the top and on the bottom decreases (Bernoulli’s principle). And it has been experimentally shown that the air moves faster over the top of the airfoil than it does under the bottom, which means (again, Bernoulli’s principle) the pressure pushing down on it decreases more than the pressure pushing up on it decreases. And so, a horizontally moving airfoil has more upward “buoyant force” on it from the surrounding fluid than when it is at rest. And this is definitely a factor in lift.

 

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I agree that simple is better. In my original post there is nothing to do with engines or fuel. And when engines were first mentioned I should have simply responded by saying “There are no engines. The airfoil is simply stipulated to have been set into motion.” In trying to address it so we could then move on, it became a distraction.

 

----

 

I don’t know how to state the issue more simply that I did in the original post. It’s not a super complicated question, but there is enough to it that it takes a couple of paragraphs to state it. I wish I could reduce it to a couple of sentences, but I can’t.

 

Perhaps there is something specific that I didn’t make clear to you, and you can let me know and I’ll be happy to try to be clearer.

[studiot]

 

Perhaps there is something specific that I didn’t make clear to you, and you can let me know and I’ll be happy to try to be clearer.

 

 

Sometimes doing an energy balance make things easier, sometimes it makes them more difficult.

In the case of flight, energy balances are not as helpful as direct consideration of the forces involved.

 

Much of what you say is correct, but your description of the held down airfoil is lacking.

 

What holds it down?

 

You need to include any hold down forces acting in your analysis.

 

You have shown the 'non-rising' airfoil as moving forwards. How is it driven forwards at the same time as being held down?

 

Address this and you will find the resting place of your misplaced energy.

 

 

 

Bouyancy forces act on immersed bodies whether they are moving or not, but they are additional to any lift forces that act due to the motional interaction of the fluid and the body. For most practical airfoils bouyancy forces are small to negligable compared to the weight of the foil, but in hydrofoils they can be significant.

[Zet]

 

Sometimes doing an energy balance make things easier, sometimes it makes them more difficult.

In the case of flight, energy balances are not as helpful as direct consideration of the forces involved.

 

Much of what you say is correct, but your description of the held down airfoil is lacking.

 

What holds it down?

 

You need to include any hold down forces acting in your analysis.

 

You have shown the 'non-rising' airfoil as moving forwards. How is it driven forwards at the same time as being held down?

 

Address this and you will find the resting place of your misplaced energy.

 

 

 

Bouyancy forces act on immersed bodies whether they are moving or not, but they are additional to any lift forces that act due to the motional interaction of the fluid and the body. For most practical airfoils bouyancy forces are small to negligable compared to the weight of the foil, but in hydrofoils they can be significant.

 

 

 

I left out what holds it down, for the same reason I left out what set it in motion, I wanted to keep things simple.

 

The wheels in the non-rising case are in a frictionless track that prevents the airfoil from rising.

 

(If I’m not allowed to stipulate that something is “frictionless” for the purpose of analysis, then the analysis becomes a little bit more complicated but overall it doesn’t really change. In the throwing the ball up and then letting it fall analysis presented above the implied stipulation is that there is no friction.)

 

----

 

Whether the change in the upward “buoyant force” on the moving airfoil is enough to make it rise by itself, or if it is only a part of all the factors contributing to lift (and even if it is only a tiny, tiny part) it is still a factor in lift and there must be a decrease in another form of energy (even if it only a tiny, tiny decrease in another form of energy) for energy to be conserved.

 

And the point of my post here, and my issue, is that I cannot find that decrease in another form of energy.

[MigL]

Called in to work as its snowing real bad, so I'm back again.

 

Whether an airfoil is moving through the air or the air is moving past an airfoil makes no difference.

A certain amount of air has to change direction for the resultant pressure difference to generate lift.

This means you cannot simply consider the vertically acting forces in your energy balance, you must also consider the thrust moving the airfoil forward or the fluid past the airfoil, and the various forms of drag, such as form drag ( due to airfoil shape ) and drag due to lift ( vertical lift produces horizontal drag because the fluid is accelerated vertically around the airfoil ).

 

Every 1st year Aeronautical Engineering text dealing with airfoils, uses the four vectors of weight, drag, lift and thrust in any analysis.

You need to do the same for your energy balance, as that is your ( nearly ) isolated system.

[studiot]

 

I left out what holds it down, for the same reason I left out what set it in motion, I wanted to keep things simple.

 

The wheels in the non-rising case are in a frictionless track that prevents the airfoil from rising.

 

(If I’m not allowed to stipulate that something is “frictionless” for the purpose of analysis, then the analysis becomes a little bit more complicated but overall it doesn’t really change. In the throwing the ball up and then letting it fall analysis presented above the implied stipulation is that there is no friction.)

 

 

No problem with the frictionless model.

 

I wondered if you were thinking of an airfoil mounted on some sort of jet or rocket sled.

But remember MigL's four horsemen:

 

Horseman 1

The sled's weight has to be added to the foil's weight.

That immediately makes the rising foil different from the non rising one.

 

Horseman 2

You cannot ignore thrust

 

Horseman3

Yes some effort is lost in drag.

This energy loss heats the air

 

Horseman4

Whatever is left over becomes lift.

HiHo Silver!

 

I'm trying to concentrate on the major parts and keep out the difficult bits.

Edited December 11, 2014 by studiot

[Zet]

Called in to work as its snowing real bad, so I'm back again.

 

Whether an airfoil is moving through the air or the air is moving past an airfoil makes no difference.

A certain amount of air has to change direction for the resultant pressure difference to generate lift.

This means you cannot simply consider the vertically acting forces in your energy balance, you must also consider the thrust moving the airfoil forward or the fluid past the airfoil, and the various forms of drag, such as form drag ( due to airfoil shape ) and drag due to lift ( vertical lift produces horizontal drag because the fluid is accelerated vertically around the airfoil ).

 

Every 1st year Aeronautical Engineering text dealing with airfoils, uses the four vectors of weight, drag, lift and thrust in any analysis.

You need to do the same for your energy balance, as that is your ( nearly ) isolated system.

 

 

I understand that a complete analysis of lift requires looking at every factor that contributes to lift.

 

And I understand much of how energy is conserved when it comes to most aspects of lift (such as when there is an angle of attack and there is a redirection of airflow downwards as it collides with the airfoil and moves it upwards).

 

But, regardless of all the other factors involved, there is a greater decrease in pressure pushing down on the top of the airfoil than there is a decrease in pressure pushing up on the bottom of the airfoil and so there is a greater upward “buoyant force” on the airfoil and this is one of the factors contributing to lift. And if this force leads to an increase in gravitational potential energy (the rising airfoil) then there needs to be a decrease in another form of energy that does not also decrease when there is no increase in gravitational potential energy (the non-rising airfoil).

 

What is it?

 

----

 

If it is within the realm of possibilities for something to be within an isolated system, then it is so stipulated that the airfoils are in an isolated system.

 

No problem with the frictionless model.

 

I wondered if you were thinking of an airfoil mounted on some sort of jet or rocket sled.

But remember MigL's four horsemen:

 

Horseman 1

The sled's weight has to be added to the foil's weight.

That immediately makes the rising foil different from the non rising one.

 

Horseman 2

You cannot ignore thrust

 

Horseman3

Yes some effort is lost in drag.

This energy loss heats the air

 

Horseman4

Whatever is left over becomes lift.

HiHo Silver!

 

I'm trying to concentrate on the major parts and keep out the difficult bits.

 

 

 

1

 

In both the rising and non-rising cases the airfoils, their shape and weight, are exactly the same. It’s just that in the non-rising case there is something (a frictionless track) holding it down.

 

2

 

In both the rising and non-rising cases the airfoils start out in motion and in the exact same positions and with the same velocities. There is no additional thrust keeping them going.

 

3

 

In a real friction-filled world there will be a loss of kinetic energy to thermal energy. But for energy to be conserved in the rising case versus the non-rising case via thermal energy, this means in the rising case there must be a decrease in thermal energy that does not occur in the non-rising case or this means less of an increase in thermal energy in the rising case than in the non-rising case and these dynamics are not imaginable.

 

4

 

Having a greater “buoyant force” means greater lift, and this force is allowed to lead to an increase in gravitational potential energy in the one case and not in the other.

 

Where is the offsetting decrease in another form of energy in the rising case?

[studiot]

I'm sorry but you cannot legitmately pick and choose what energies you will include or not include in your energy balance.

 

Work is done on a rising body by the force causing it to rise.

 

The force causing it to rise is not a single force but the resultant of the vector addition of several forces.

 

Some of this work goes into the kinetic energy of vertical motion. (did you remember that?)

Some goes into the increased gravitational energy.

 

If the body is not rising there is another force acting, which is the equilibrant to the lifting resultant.

 

Similarly if the body is in level flight the drag will be doing work on the air, leaking kinetic energy away to the air.

This KE is maintained by the power of the engine supplying the thrust, which does work on the body.

 

So the engine is continually supplying energy to the body.

This energy supply is enough to also power any vertical gain in PE or KE that happens, but the exchange is via the kinetic energy of the body.

 

So there is no energy balance crisis.

 

If the thrust ceases then the body will eventually fall, not rise.

It may be possible to temporarily take advantage of the combined lift and drag vectors and upwelling air to glide for a considerable distance.

But the glider will eventually return to earth.

[MigL]

Ok lets simplify.

We consider two stationary airfoils with the fluid moving past both, except that one is tied down and one isn't.

The fluid has a certain amount of energy and it imparts some of it to the airfoils ( heating, lift, etc. ).

In the case where the airfoil is allowed to rise, an even smaller amount is imparted to the airfoil as a rise in gravitational potential.

In the case where the airfoil is constrained, the fluid is 'deflected' more strongly. In effect the fluid keeps more of its energy

 

This is more easily seen with a flat plate. If you introduce a flat plate orthogonally to a fluid flow, it will tend to fly away in the direction of the fluid flow. The plate gains some energy from the fluid which loses some. The only way to keep it stationary is to hold it, in effect, by giving it energy in the way of thrust.

[Zet]

I was wrong when I wrote:

 

“If so, I disagree. The upward buoyant force on the airfoil is a force and not energy.”

 

There is also energy involved in this situation.

 

When a ball is submerged in a column of water, there is an upward buoyant force on it, and if let go it will rise. Energy is the ability to do work. For the ball to be moved (increase in kinetic energy) there must be a decrease in another form of energy. And there is. The more dense fluid displaced upwards by the less dense ball is an amount of gravitational potential energy. And as the ball rises and speeds up there is an equal decrease in gravitational potential energy in the form of the falling water (ignoring friction and losses to thermal energy and the movement of the fluid itself).

 

And so for the airfoil to rise (in part) due to the different decreases in pressure pushing down on top of it and in pressure pushing up on the bottom of it creating a greater upward buoyant force on the airfoil (which, if the airfoil is light enough could lift the airfoil on its own regardless of the other factors contributing to lift) must come from a decrease in another form of energy to offset the increase in kinetic energy, as with the rising submerged ball.

 

But, with the rising submerged ball that came from a decrease in the form of the more dense fluid falling and thus a decrease in gravitational potential energy, but here the airfoil is more dense than the surrounding fluid and so as kinetic energy increases in the form of the now also vertically moving airfoil there is not a corresponding decrease in gravitational potential energy, but rather the opposite.

 

I have a better understanding of this analysis now thanks to this conversation, but I’m still missing something.

 

?

[studiot]

 

but I’m still missing something.

 

 

Progress, so try reading my post#18 again and then ask questions about it.

 

I can expand the bits you need further detail on.

[Zet]

I read post 18 several times and worked on a couple of different replies. But I kept finding myself writing way too lengthy replies trying to return this conversation away from what I believe are tangential points (valid in their own right, but tangential to the one simple aspect of lift I am having an issue with when it comes to the conservation of energy analysis). And all my attempts just ended up sounding like I’m not taking the other people’s points here seriously and just reiterating something already shown to have been incorrect over and over again. So, I’m not sure how to reply.

 

But, once before you asked me to shorten my original question/issue. And I said I couldn’t. But maybe I can in the form of some simple questions.

 

1. If the force from the fluid pressure pushing up on the bottom of a submerged object is greater than the weight of the object plus the force from the fluid pressure pushing down on the top of it, will it rise (if it is free to rise)?

 

2. When a less dense solid object (overall density) is submerged in a more dense fluid is the force from the fluid pressure pushing up on the bottom of it is greater than the weight of the object plus the force from the fluid pressure pushing down on the top of it (due to fluid statics)?

 

3. When a more dense solid object is submerged in a less dense fluid is the force from the fluid pressure pushing up on the bottom of it is less than the weight of the object plus the force from the fluid pressure pushing down on the top of it (due to fluid statics)?

 

4. If a less dense submerged object rises in a more dense surrounding fluid is the increase in kinetic energy of the rising object and falling fluid balanced out by an equal decrease in gravitational potential energy (ignoring losses to thermal energy)?

 

5. If, due to the different decreases in pressures on the top and bottom of a horizontally moving airfoil, the force from the fluid pressure pushing up on the bottom of it is more than the weight of the object plus the force from the fluid pressure pushing down on the top of it, will the more dense airfoil rise (due to this aspect of fluid dynamics (Bernoulli’s principle))?

 

Did I make the issue/problem any clearer? I know it may be kind of rude to ask someone to answer a bunch of questions, but they are only yes/no questions, and I hope they help put a finer point on my problem.

 

----

 

In regards to post 18.

 

Perhaps you and I disagree, but I believe that to understand all of lift then each part of lift must be understood individually and not in some gestalt way. And I’m not bothered by the other aspects of lift. With each of the other dynamics that contribute to lift I can reason my way to how energy is conserved. And if you’d like to talk about those aspects and how energy is conserved, we can.

 

But when it comes to the buoyancy contribution to lift and the fluid dynamic changes in buoyancy with a horizontally moving submerged body in the shape of an airfoil, I’m stuck.

 

----

 

And we can talk about thrust but it is really beyond the scope of the issue.

 

1. If it is within the realm of possibilities for a body to have been set in motion while the source of that motion is unknown, then it is so stipulated here.

2. If it is within the realm of possibilities for an airfoil to have been set into motion beyond that necessary for lift while also being held down and then later allowed to rise, while another airfoil is in the same situation but never allowed to rise, then it is so stipulated here.

 

There is no need to talk about thrust, I don’t think, but we can.

 

---

 

The question as laid out comes to an end before the rising airfoil starts to fall.

 

----

 

And if you saying that the lift of an airfoil comes from the same lift in a rocket, in that it is the engines themselves pushing the body vertical and not horizontal, then we disagree.

[studiot]

 

The question as laid out comes to an end before the rising airfoil starts to fall.

 

 

The airfoil, as with any material object, starts to fall the instant the force keeping it up there is removed.

There is no honeymoon period before Newton and gravity kick in.

 

 

And if you saying that the lift of an airfoil comes from the same lift in a rocket, in that it is the engines themselves pushing the body vertical and not horizontal, then we disagree.

 

 

Rockets work on an entirely different principle, which is how and why they will work in space, where airfoils will not.

 

 

1. If the force from the fluid pressure pushing up on the bottom of a submerged object is greater than the weight of the object plus the force from the fluid pressure pushing down on the top of it, will it rise (if it is free to rise)?

 

Probably, but not necessarily.

 

I can think of several situations where this does not happen.

 

 

2. When a less dense solid object (overall density) is submerged in a more dense fluid is the force from the fluid pressure pushing up on the bottom of it is greater than the weight of the object plus the force from the fluid pressure pushing down on the top of it (due to fluid statics)?

 

For some objects yes certainly, but not for all such objects. In particular balloons do not meet this requirement.

It also depends what you mean by 'weight'.

 

 

When a more dense solid object is submerged in a less dense fluid is the force from the fluid pressure pushing up on the bottom of it is less than the weight of the object plus the force from the fluid pressure pushing down on the top of it (due to fluid statics)?

 

Yes but again it also depends what you mean by 'weight'.

 

 

5. If, due to the different decreases in pressures on the top and bottom of a horizontally moving airfoil, the force from the fluid pressure pushing up on the bottom of it is more than the weight of the object plus the force from the fluid pressure pushing down on the top of it, will the more dense airfoil rise (due to this aspect of fluid dynamics (Bernoulli’s principle))?

 

That is a meaningless question since you haven't stated what 'pressures' you are talking about.

Since you invoke Bernoulli and pressure, we could argue all night where you say the 'pressure' decreases and I say it increases simply because we are talking about different Bernoulli pressures.

If you truly understand bernoulli, you will understand what I am referring to.

 

 

Did I make the issue/problem any clearer? I know it may be kind of rude to ask someone to answer a bunch of questions, but they are only yes/no questions, and I hope they help put a finer point on my problem.

 

The problem has never lacked clarity for me, but I think you are beginning to confuse yourself again, through lack of detailed knowledge I exhort you to aquire.

 

 

In regards to post 18.

 

Perhaps you and I disagree, but I believe that to understand all of lift then each part of lift must be understood individually and not in some gestalt way. And I’m not bothered by the other aspects of lift. With each of the other dynamics that contribute to lift I can reason my way to how energy is conserved. And if you’d like to talk about those aspects and how energy is conserved, we can.

 

But when it comes to the buoyancy contribution to lift and the fluid dynamic changes in buoyancy with a horizontally moving submerged body in the shape of an airfoil, I’m stuck.

 

----

 

And we can talk about thrust but it is really beyond the scope of the issue.

 

1. If it is within the realm of possibilities for a body to have been set in motion while the source of that motion is unknown, then it is so stipulated here.

2. If it is within the realm of possibilities for an airfoil to have been set into motion beyond that necessary for lift while also being held down and then later allowed to rise, while another airfoil is in the same situation but never allowed to rise, then it is so stipulated here.

 

There is no need to talk about thrust, I don’t think, but we can.

 

I have already indicated that I am not prepared to suspend the laws of Physics for anyones' convenience.

 

I is a shame if you choose not to aquire the knowledge necessary to provide satisfactory answers to your questions, but you must follow the laws of Physics, like everyone else.

[I-try]

Zet.

 

With regards the manner in which you have drawn that you call an aerofoil, then of course the leading section would generate a down force and the trailing section would generate an upwards generated force. Such a shaped wing would attempt to twist if set in horizontal motion.

 

To generate lift, the raised leading section must be much shorter than the relatively longer tapered trailing section, and the angle of attack must be slightly raised to counter balance the down pressure on the leading section. Strange as it may seem, it is gravity that enables an aircraft to maintain lift due to atmospheric pressure.

[Zet]

Zet.

 

With regards the manner in which you have drawn that you call an aerofoil, then of course the leading section would generate a down force and the trailing section would generate an upwards generated force. Such a shaped wing would attempt to twist if set in horizontal motion.

 

To generate lift, the raised leading section must be much shorter than the relatively longer tapered trailing section, and the angle of attack must be slightly raised to counter balance the down pressure on the leading section. Strange as it may seem, it is gravity that enables an aircraft to maintain lift due to atmospheric pressure.

 

 

 

Yes, airfoils are complicated things. And I probably should stay away from them.

 

I know most of you don’t think I’m listening and learning from this discussion. But I have. I still don’t know how to reconcile the conservation of energy analysis, but I am further along than when I started this discussion.

 

I have another conservation of energy analysis question. And I believe I could learn from a discussion of that. It’s about a different part of our physical world, so I’ll not put it here but rather in a new thread.

 

And any additional help on that would be greatly appreciated.