Rear Wing Idea

[variante]

Winglets, which are airfoils operating just like a sailboat tacking upwind, produce a forward thrust inside the circulation field of the vortices

It's not the first time i read something similar, but i fail to see how that is achieved (especially considering that winglet's AoA is pretty much 0°)

[trinidefender]

Winglets, which are airfoils operating just like a sailboat tacking upwind, produce a forward thrust inside the circulation field of the vortices

It's not the first time i read something similar, but i fail to see how that is achieved (especially considering that winglet's AoA is pretty much 0°)

That bit seems a bit vague and not very well explained. However the part I was mainly focussing on is the fact that the vertical winglets do two main things.
1. Reduce the size and strength of the vortex created by placing it at '0' angle of attack. Although there is extra frontal area and surface area of the winglet itself, this is generally offset by the reduction in vortex size.
2. It moves the already weakened vortex away from the lift developing portion of the wing meaning that for a given angle of attack the wing is generating more lift.

[variante]

Yes, the explanation you gave about winglet's main funcion is obviously the right one.

However some more words about this point are needed:

1. Reduce the size and strength of the vortex created by placing it at '0' angle of attack. Although there is extra frontal area and surface area of the winglet itself, this is generally offset by the reduction in vortex size.

Winglets are useful tools to play with when dealing with high and low pressure fields of a wing. They increase the lift generated by a wing at a fixed wingspan. Their extra frontal area is balanced by the extra performances of the airplane (for example: an airplane with winglets with the same lift output of an airplane without them will take advantage of an inferior wingspan, therefore: same lift, same drag, but better mechanical characteristics. And so on..)

BUT their aerodynamic efficiency is lower than the efficiency of a wing with increased wingspan. The reason why winglets are favored over extra long wings are two: 1=less mechanical stresses, and 2=wingspan limited by airports dimensions (as already mentioned by someone else).



Still, my original question remains unsolved: how does a winglet decrease drag??

[trinidefender]

Yes, the explanation you gave about winglet's main funcion is obviously the right one.

However some more words about this point are needed:

1. Reduce the size and strength of the vortex created by placing it at '0' angle of attack. Although there is extra frontal area and surface area of the winglet itself, this is generally offset by the reduction in vortex size.

Winglets are useful tools to play with when dealing with high and low pressure fields of a wing. They increase the lift generated by a wing at a fixed wingspan. Their extra frontal area is balanced by the extra performances of the airplane (for example: an airplane with winglets with the same lift output of an airplane without them will take advantage of an inferior wingspan, therefore: same lift, same drag, but better mechanical characteristics. And so on..)

BUT their aerodynamic efficiency is lower than the efficiency of a wing with increased wingspan. The reason why winglets are favored over extra long wings are two: 1=less mechanical stresses, and 2=wingspan limited by airports dimensions (as already mentioned by someone else).



Still, my original question remains unsolved: how does a winglet decrease drag??

It was me who mentioned about the wingspan limited by airport dimensions in a previous post and yes I know about longer wingspans. Another method that we will probably start seeing are raked wingtips. They look like the wing is extending further out and racked backwards but that is another topic entirely.

Ok well let's look at how it reduces drag.

1. It takes energy to create a vortex. Is energy is in the form of drag. Reduce the size and strength of the vortex and you reduce the drag. You can reduce the size and strength of this vortex by reducing the pressure differential between the two sides of a wing. The vertical winglet essentially creates a "fence" between the lower and upper portion of a wing helping the separate the two. With the resulting lower pressure differential, a smaller vortex is created.

2. You said it yourself. A certain wing at a certain angle of attack (AoA) produces a certain amount of lift and drag. A vertical wingtip increases the amount of lift created by the wing at that AoA. Therefore to achieve the same lifting force as without the vertical wingtips you reduce the AoA. When you reduce the AoA of a wing you reduce the drag.

Understand now?

[variante]

I don't agree with the first part of your point 1.

So let's to take it from another point of view:
A- the vortex is not the cause of the drag, but just a symptom of a waste of energy given by an unwanted interaction between the pressure fields. Reducing its stregth does not necessarily mean reducing drag.
B- if my only target is drag reduction without any other performance change (=same lift), how do i achieve that only with a winglet?

[trinidefender]

I don't agree with the first part of your point 1.

So let's to take it from another point of view:
A- the vortex is not the cause of the drag, but just a symptom of a waste of energy given by an unwanted interaction between the pressure fields. Reducing its stregth does not necessarily mean reducing drag.
B- if my only target is drag reduction without any other performance change (=same lift), how do i achieve that only with a winglet?

It doesn't matter if you agree with my first point. It is a FACT, facts are facts. To create a vortex you have to expend energy. Energy cannot be created nor destroyed, only transferred. That is also a fact. Therefore we have to get this energy from somewhere. That vortex is created by turning the energy from the planes forward motion into energy in the form of a twisting air mass known as a vortex. When you reduce the energy used to create a vortex then you reduce the size of the vortex itself. Therefore more energy = bigger vortex = more drag created by vortex.

For your "B" part. When you increase the effective span of a wing then you increase the lift that the wing will create at a certain AoA. That means to reduce the lift to original levels need for the plane to fly you reduce the AoA of the wing. When you reduce the AoA of the wing, it reduces the drag created by the wing.

[variante]

It doesn't matter if you agree with my first point. It is a FACT, facts are facts. To create a vortex you have to expend energy. Energy cannot be created nor destroyed, only transferred. That is also a fact. Therefore we have to get this energy from somewhere. That vortex is created by turning the energy from the planes forward motion into energy in the form of a twisting air mass known as a vortex. When you reduce the energy used to create a vortex then you reduce the size of the vortex itself. Therefore more energy = bigger vortex = more drag created by vortex.

I'm sorry you're taking it that way... Let me show how you are wrong with some more elegance than yours...

Mental experiment: a flat wing with virtually no drag creates pressure differential thanks to a pump moved by a combustion engine. That wing has 0 drag but X lift; the interaction between pressure fields will generate a vortex anyway.
The "fan cars" such as the Brabham BT46 apply that principle in real life.

Another mental experiment: let's imagine a winglet which is infinitely long; that winglet will generate an infinitely small vortex. By your logic that would mean no drag... By physics logic that would mean infinite drag.

EDIT: About point "B": i totally agree. Yet many airplanes have been retrofitted with those winglets (meaning that AoA remained unchanged), and the consequent claims were basically "more lift, less drag". Now, are the claims wrong? Or there's actually a way to make winglets work as "sails" taking advantage of the interactions with vortices (as briefly mentioned by NASA's article)

[trinidefender]

It doesn't matter if you agree with my first point. It is a FACT, facts are facts. To create a vortex you have to expend energy. Energy cannot be created nor destroyed, only transferred. That is also a fact. Therefore we have to get this energy from somewhere. That vortex is created by turning the energy from the planes forward motion into energy in the form of a twisting air mass known as a vortex. When you reduce the energy used to create a vortex then you reduce the size of the vortex itself. Therefore more energy = bigger vortex = more drag created by vortex.

I'm sorry you're taking it that way... Let me show how you are wrong with some more elegance than yours...

Mental experiment: a flat wing with virtually no drag creates pressure differential thanks to a pump moved by a combustion engine. That wing has 0 drag but X lift; the interaction between pressure fields will generate a vortex anyway.
The "fan cars" such as the Brabham BT46 apply that principle in real life.

Another mental experiment: let's imagine a winglet which is infinitely long; that winglet will generate an infinitely small vortex. By your logic that would mean no drag... By physics logic that would mean infinite drag.

EDIT: About point "B": i totally agree. Yet many airplanes have been retrofitted with those winglets (meaning that AoA remained unchanged), and the consequent claims were basically "more lift, less drag". Now, are the claims wrong? Or there's actually a way to make winglets work as "sails" taking advantage of the interactions with vortices (as briefly mentioned by NASA's article)

Read this: http://www.grc.nasa.gov/WWW/k-12/airplane/induced.html

It is about induced drag. Induced drag is created by the inteaction of the two different pressure zones at the end of a wing. All wings have it to some degree.

Let's explore the 3 types of drag present on an airplane:
(I) There is parasite drag. Parasite drag is any drag created by non-lifting components of an aircraft; the fuselage, tail plane, landing gear and so on.
(II) Profile drag. Profile drag is made up of form drag and skin friction:
- Form drag is created by the shape of the airfoil (at sub-sonic speeds)
- Surface friction just that, drag created by the roughness of the surface moving through a fluid
(III) Induced drag. Induced drag is the drag created as a result of lathe creation of lift. I.e. The pressure differential on the two sides of a wing.

Now onto your thought experiment:
In your first experiment your wing does not exist in actuality. A wing cannot have zero drag and produce lift. If you read the NASA article you will see how induced drag is created at the ends of the wing when lift is created.

On to your next thought experiment. A wing with an infinitly long vertical wingtip will, IN THEORY, have zero induced drag as a result of interacting pressure differentials. However the longer you make the vertical wingtip the more form drag and surface friction you create because there is simply more that has to be pushed through the air. That is why designers try to get the best trade off by minimising induced drag through the use of the winglet and making the winglet as small as small as possible to reduce form drag and skin friction. At some point the benefits of a decrease in induced drag become less than the downsides of the increase in form drag and skin friction.

Now onto your point 'B' I never said the claims of more lift and less drag are wrong. You are misunderstanding me. They can produce more lift, hence planes fitted with vertical winglets have a greater rate of climb. They can also produce less drag, hence higher cruising speed for less power or a lower fuel burn for the same cruising speed. Or lastly which is what they claim, a combination of both more lift and less drag.

On to your very very last point (this is getting long). The angle of attack of airflow on a wing on an airplane is not static. When the aircraft pitches it's nose up, itswings AoA increases and it's wings produce both more lift AND more drag and the plane climbs. When an aircraft pitches it's nose down its wings AoA decreases and it's wings produce both less lift and less drag and the plane descends. Now do you see my point about a plane lowering its angle of attack to produce the same lift with lower drag (cruise performance) or increasing its angle of attack to produce more lift with the same drag (climb performance).

*edit* in this post originally I accidentally typed "A wing with an infinitly long vortex will, IN THEORY, have zero induced drag" it should read "A wing with an infinitly long vertical WINGTIP will, IN THEORY, have zero induced drag" it has been corrected.

[trinidefender]

It doesn't matter if you agree with my first point. It is a FACT, facts are facts. To create a vortex you have to expend energy. Energy cannot be created nor destroyed, only transferred. That is also a fact. Therefore we have to get this energy from somewhere. That vortex is created by turning the energy from the planes forward motion into energy in the form of a twisting air mass known as a vortex. When you reduce the energy used to create a vortex then you reduce the size of the vortex itself. Therefore more energy = bigger vortex = more drag created by vortex.

I'm sorry you're taking it that way... Let me show how you are wrong with some more elegance than yours...

Mental experiment: a flat wing with virtually no drag creates pressure differential thanks to a pump moved by a combustion engine. That wing has 0 drag but X lift; the interaction between pressure fields will generate a vortex anyway.
The "fan cars" such as the Brabham BT46 apply that principle in real life.

Another mental experiment: let's imagine a winglet which is infinitely long; that winglet will generate an infinitely small vortex. By your logic that would mean no drag... By physics logic that would mean infinite drag.

EDIT: About point "B": i totally agree. Yet many airplanes have been retrofitted with those winglets (meaning that AoA remained unchanged), and the consequent claims were basically "more lift, less drag". Now, are the claims wrong? Or there's actually a way to make winglets work as "sails" taking advantage of the interactions with vortices (as briefly mentioned by NASA's article)

In your first thought experiment you are talking about something like a hovercraft. That isn't a conventional wing. The lift is not created by the action of airflow moving offer the top and bottom of the wing. The lift is created purely by a fan blowing air downwards. On a planes wing the lift is created by the air flowing above and below the wing and the resultant pressure differential.

[variante]

Now i've got nothing to disagree with. My original complain was simply about the arrangement of words you used, where it looked like the sole presence of a vortex means drag (whatever the geometry the vortex is interacting with...therefore my first, extreme, example where induced drag is always 0 despite the the presence of a vortex).

The second example was oviously about the tradeoff between parasitic and induced drag. I hadn't seen you actually talked about it (correctly), so that was actually my fault for accusing you. Mea culpa.

Lastly there's always that unsolved question about the winglet as a thrust generating device...

[trinidefender]

Now i've got nothing to disagree with. My original complain was simply about the arrangement of words you used, where it looked like the sole presence of a vortex means drag (whatever the geometry the vortex is interacting with...therefore my first, extreme, example where induced drag is always 0 despite the the presence of a vortex).

The second example was oviously about the tradeoff between parasitic and induced drag. I hadn't seen you actually talked about it (correctly), so that was actually my fault for accusing you. Mea culpa.

Lastly there's always that unsolved question about the winglet as a thrust generating device...

Read it again. Also read the links I have posted previously because that is what I am claiming.

On a wing that produces lift through the action of moving through a fluid (air in this case), induced drag will always be present as a result of the pressure differentials between the high and low pressure sides of a wing. At the same time a vortex will be formed. So yes, a vortex does create induced drag.

Your first example does not make sense. It is not a wing that generates lift through moving through the action of moving through the air therefore the same concepts cannot be applied. Therefore you cannot bring the "induced drag is 0" argument to this table. It does not exist.

[variante]

I previously complained about the lack of precision of your previous statement, and that is a mistake you've just repeated:

On a wing that produces lift through the action of moving through a fluid (air in this case), induced drag will always be present as a result of the pressure differentials between the high and low pressure sides of a wing.

It should be: “On a wing that produces lift through the action of moving through a fluid (air in this case), A vortex will always be present as a result of the pressure differentials between the high and low pressure sides of a wing. As a consequence, induced drag may be present.

That's because there are certain airfoils that do not suffer from induced drag. Here is an example:

If a first look isn't enough to understand it, i tested with CFD...just a simple test...qualitative but not necessarily quantitative. Well, guess what, it actually showed a decrase of drag when subjected to a different relative AoA (the cause of induced drag).

0°
Drag: 289 N
Lift: 77 N

1°
Drag: 201 N
Lift: 56 N

5°
Drag: 46 N
Lift: -64 N

See now where the relation "bigger vortex=more drag" fails?

[trinidefender]

I previously complained about the lack of precision of your previous statement, and that is a mistake you've just repeated:

On a wing that produces lift through the action of moving through a fluid (air in this case), induced drag will always be present as a result of the pressure differentials between the high and low pressure sides of a wing.

It should be: “On a wing that produces lift through the action of moving through a fluid (air in this case), A vortex will always be present as a result of the pressure differentials between the high and low pressure sides of a wing. As a consequence, induced drag may be present.

That's because there are certain airfoils that do not suffer from induced drag. Here is an example:
http://i1372.photobucket.com/albums/ag3 ... 75cbf2.png
If a first look isn't enough to understand it, i tested with CFD...just a simple test...qualitative but not necessarily quantitative. Well, guess what, it actually showed a decrase of drag when subjected to a different relative AoA (the cause of induced drag).

0°
Drag: 289 N
Lift: 77 N

1°
Drag: 201 N
Lift: 56 N

5°
Drag: 46 N
Lift: -64 N

See now where the relation "bigger vortex=more drag" fails?

Firstly is that a cross section view of the wing or the plan view of the wing? Secondly is your CFD 2D or 3D? There is a big difference in wing design between 2D and 3D CFD concerning wing tips.

Read this link as I'm sure last time you didn't: http://www.grc.nasa.gov/WWW/k-12/airplane/induced.html

"The wing tip vortices produce a swirling flow of air behind the wing which is very strong near the wing tips and decreases toward the wing root. The effective angle of attack of the wing is decreased by the induced flow of the vortices and varies from wing tip to wing root. The induced flow produces an additional, downstream-facing, component of aerodynamic force of the wing. This additional force is called induced drag because it faces downstream and has been "induced" by the action of the tip vortices. It is also called "drag due to lift" because it only occurs on finite, lifting wings and the magnitude of the drag depends on the lift of the wing."

Right there it clearly states the relationship between; an induced flow, vortices, drag and lift. Or are you telling me that the NASA article is wrong?

An additional note. If you are so adamant that the creation of a vortex doesn't create drag then explain where the energy to create this vortex comes from? Energy has to be put in to create this vortex. Therefore the energy has to come from somewhere. I'm challenging you to explain where this energy comes from, that's what I have done and you have failed to do.

[variante]

Firstly is that a cross section view of the wing or the plan view of the wing? Secondly is your CFD 2D or 3D? There is a big difference in wing design between 2D and 3D CFD concerning wing tips.

Cross section. Left is the front end.
I simulated infinite wingspan. Induced drag flow was simulated changing angulation of the airflow (0°, 1°, 5°)
3D solver. Decent mesh accuracy.

Or are you telling me that the NASA article is wrong?

I'm saying that it is not totally precise, as often happens with articles trying to generalize something to facilitate comprehension.
After all, another NASA article claims that winglets work like sails as well...

If you are so adamant that the creation of a vortex doesn't create drag then explain where the energy to create this vortex comes from? Energy has to be put in to create this vortex. Therefore the energy has to come from somewhere. I'm challenging you to explain where this energy comes from, that's what I have done and you have failed to do.

I'm not saying that either. It should be: the creation of a vortex doesn't always produce induced drag. As simple as that.

[trinidefender]

Firstly is that a cross section view of the wing or the plan view of the wing? Secondly is your CFD 2D or 3D? There is a big difference in wing design between 2D and 3D CFD concerning wing tips.

Cross section. Left is the front end.
I simulated infinite wingspan. Induced drag flow was simulated changing angulation of the airflow (0°, 1°, 5°)
3D solver. Decent mesh accuracy.

Or are you telling me that the NASA article is wrong?

I'm saying that it is not totally precise, as often happens with articles trying to generalize something to facilitate comprehension.
After all, another NASA article claims that winglets work like sails as well...

If you are so adamant that the creation of a vortex doesn't create drag then explain where the energy to create this vortex comes from? Energy has to be put in to create this vortex. Therefore the energy has to come from somewhere. I'm challenging you to explain where this energy comes from, that's what I have done and you have failed to do.

I'm not saying that either. It should be: the creation of a vortex doesn't always produce induced drag. As simple as that.

There is your problem. You cannot simulate infinite wingspan and expect results to be realistic. That is because in your simulation there is no induced flow around the wingtip because there IS NO WINGTIP in your simulation. If you put wingtips on with a fixed wingspan then your simulator would show that airflow goes around the wingtip from the high pressure side to the low pressure side. This creates a vortex and manifests itself as drag.

Try running the same simulation with a wingtip and I can guarantee that you will never be able to produce results showing zero induced drag with any decent 3D CFD program.

I have to ask, where are you getting your information from? I get a lot of my information from a variety of sources and mainly a few aerospace engineers I know personally. I asked 2 today and they both told me that what you were saying is totally false.

Btw you still haven't been able to state where the energy to create the vortex comes from if it isn't manifested in the form of drag. Or are you just avoiding my question?