# A sail is not a wing

Discussion in 'Hydrodynamics and Aerodynamics' started by Sailor Al, Feb 7, 2021.

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### GlueandcoffeeJunior Member

I think we are all in agreement now. Does sailor al want to pick apart any of the points made.?

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### HowlandwoodworksMember

We prove what we want to prove, and the real difficulty is to know what we want to prove.
There has been much discussion here about what a sail is but what is the diffusion of a wing.
These open ended statement do generate much discussion though.
We sailors can become a part of a wing if only for a fleeting moment and that is how a small sailboat sailor wins races or just dance across the waves.
May we always be in a modus vivendi within ours lives.
John

Principle of Equivalence
Gravity is the curvature of space time not a force.

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### KeithOSenior Member

"diffusion of a wing " ?????

Do you mean definition ?

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### TANSLSenior Member

Not exactly.

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### DCockeySenior Member

Good summary.

Some vector relationships:
If lift and drag and known along with the apparent wind direction then the thrust and sideways force/heeling force can be easily calculated.
If the thrust and sideways force/heeling force and the apparent wind directon are known then the lift and drag can be easily calculated.
If the thrust and sdeways force/heeling force are known then the total aerodynamic force and its direction relative to the boat can be easily calculated.
If the lift and drag and the apparent wind direction are known then the the total aerodynamic force and its direction relative to the boat can be easily calculated.

Someone who is not familar with vector math and coordinate system transformations may not know these simple relationships. That may be contributing to some of the confusion seen in this thread.

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### GlueandcoffeeJunior Member

Yes that is correct.
Using this point to further the conversation, it really does not matter which way you decide to set the x,y,z axis. It really doesn't matter what you call the force on a wing vs sail or boat vs glider . It really doesn't matter how you resolve the forces because they are all balanced and there is no net force assuming that both vehicles are not accelerating.
The difference between a wing and sail is 90°.

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### Will GilmoreSenior Member

I think the analogy between a wing and a sail is an appropriate analogy, to point. The dynamics of wing Science is touched upon in high school physics class and is relatively simple to understand, given a few accepted laws of physics. The way a sail works to drive a boat has a more complex set of relationships that are easier to grasp from the basic wing analogy. However, when it gets stuck in one's mind that they do exactly the same thing because we are taught that they are the same, then we have to struggle with the differences all the while having been taught that they are the same.

On another forum, we had a long discussion about the legitimacy of the claim that the current foiling AC design could actually sail upstream with no wind on the Amazon River with a five knot current.

I originally held the view that it was impossible because I couldn't see, once the boat caught the five knot wind generated by the current, that it could still maintain lateral resistance after turning upstream. By understanding how a sail, propelled by lift can generate a changing wind vector against the LR of the water, I was eventually able to move into the camp of those who believed while most of those opposed, remained stead fast against such an astounding claim made in the AC video.

There is a point of divergence from the analog. 'Lift' works the same, in both cases, but the way it is used is not the same. Perhaps anyone should be able to come to terms with this, because all the information is there in the analog, but lots of people don't bother going beyond the first confusing problem they have trouble with.

Many thanks, by the way, to Sailor Al for starting this thread and giving me the opportunity to refine my understanding. You all are amazing and I hope I have learned as much as I think I have learned from your posts.

-Will (Dragonfly)

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### tlouth7Senior Member

The trick is to make a distinction between the sail - wing comparison and the boat - aircraft comparison.

An aircraft is typically using its wings to combat the force of gravity which is at right angles to the direction it is trying to move in. A boat is using its sails to generate thrust, or extract energy from a moving air mass, or even transfer momentum from the air to the boat (all different, valid ways of looking at the same effect). I guess this is what SailorAl is getting at.

That said, the way a sail itself works is exactly the same as the way a wing works. This makes it (somewhat) easy to transfer techniques over from the aerodynamics of aircraft wings which of course has a lot more research money being spent on it. I argue that the terms used in that industry (lift and drag) are useful to a sailor who wants to understand the aerodynamics of their rig.

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### GlueandcoffeeJunior Member

Bravo. I literally couldn't have put it better myself (No matter how hard I clearly tried.). Good learning experience, brilliant exchange of knowledge and unbelievable dialogue.

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### tlouth7Senior Member

I thought I would have a go at correcting some of this, as it is rather odd. Firstly you have misunderstood what a gradient means and why it is relevant to a sailor (think meteorology). A glider on the ground experiences just as much gravity as it does in the air; the difference is that it can't move downwards. A boat can sail quite happily in a place with no atmospheric pressure gradient, see the final paragraph of this comment.

Secondly the situations of a sailing boat and a glider are much more different than just being 90 degrees out. A boat works at the boundary between two media (air and water), whereas a glider is only in one medium (the air). Just as it is impossible in thermodynamics to extract work from a single reservoir of heat, so it is impossible for a vessel to extract energy from just one medium. A glider must always move downwards relative to the air mass it is in so that it can extract gravitational potential energy to offset energy lost to drag. It can move upwards only briefly by trading kinetic energy for GPE.

In contrast a sailing vessel can extract energy from the difference in velocity between the two media it interacts with. This is why it can continuously move "uphill" into the wind. This way of thinking also makes it clear why a boat can travel faster than the wind - if it is very good at extracting energy from the air/water interface, then it will keep getting faster until the work done against drag balances that. There is no reason that this balance should occur with boatspeed at or below the windspeed. It also explains why we care about wind speed and direction with respect to the water rather than the land. And finally it explains why a boat cannot sail if the wind is blowing in exactly the same direction and speed as the water is moving.

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### GlueandcoffeeJunior Member

The media in which they travel has no bearing on the dynamic interactions of the vehicle. Its gravity vs air or water vs wind.I understand that if the wind is at equal direction and speed of water no work can be extracted by a boat. But similarly if the air was accelerating down at 9.81m/s^2 a glider could also do no work. It would be in free fall. It would never reach terminal velocity. As if it was in a vacuum or a place with no gradient in atmospheric pressure .

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### Will GilmoreSenior Member

Only a sailboat gains forward motion from lift on the sail. With a sailboat, because the airfoil angle can be adjusted to the direction of motion and AW, it can convert the energy to drive the craft. All the other vessels need forward motion to gain lift, perpendicular to motion. They do not convert the energy taken from lift on their wings, to forward motion.

This is a very big and fundamental difference between how sailboats work and every other craft with lift generating foils.
(please note that under the term 'sailboat', I include ice boats, windsurfers, kite boards, land yachts, anything that uses a lift generating foil for drive force. Sailplanes are not on that list because they basically are driven forward by gravity, not lift. Lift just keeps them aloft. To change the angle of attack for a sail plane, the entire plane must change its angle, just as any other plane).

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### tlouth7Senior Member

I'm afraid you misunderstand how a glider works if you think it can somehow extract energy from still air to keep itself going. Ignoring thermals a glider gains all the energy for its flight from its initial launch. From then on it is constantly losing GPE to do work against drag. There is one scenario when a glider can extract energy from the boundary between two media and that is dynamic soaring where there are two air masses moving at different speeds.

In contrast a boat is always at such a boundary, and that is an important distinction. Remember in this situation I am highlighting the limits of the boat-'plane analogy, without saying anything about the sail-wing analogy.

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### GlueandcoffeeJunior Member

No. A glider gains forward speed from the forewarned component of lift. It stays aloft because of the upward component of lift. Gravity does not pull gliders forward. Where did that last point about angle of attack come in?

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### GlueandcoffeeJunior Member

Don't fear. I know perfectly well how they work as a paraglider and rc glider pilot. Then you should also know that a sailboat cant go either in still air. Ignoring tides a sailboat gets all of its energy from the difference in pressure from one place to another. From the moment it sets sailors losing positional potential energy against the atmosphere around it.
Thanks quite familiar with DSing.
Yeah I am long after mentioning the plane vs boat not wing vs sail analogy.

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