A sail is not a wing

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

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Sailor AlSenior Member

Excuse the lesson in physics, but thrust is one of the scalar components of total aerodynamic force resolved around the centreline. As it is a scalar value, it cannot be resolved. You can only resolve a vector.

I think you'll find it is pretty right. Please elaborate.
Again, I think you'll find it is pretty right. Please elaborate.
My whole argument is that I think it is.
Physics: Just like Thrust, Leeway is the other scalar component of total aerodynamic force, resolved perpendicular to the centreline. It is not a vector

Sailing lesson: Heel is the inevitable result of sailing. Extreme heel is bad sailing.

YESSSSS! THATS MY WHOLE POINT!
I AM resolving the force around the useful direction of the vehicle: its centreline. Thrust is parallel, leeway/heel is perpendicular.

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GlueandcoffeeJunior Member

Thrust pushes forewarned. It has direction and magnitude. Vector

You use force and moment interchangeably. They are not the same thing.

Boats don't sail overheated where all lift is perpendicular to centerline and they dont go directly into wind where drag is parallel to centerline. Bare away a sheet out.. now the forces are not resolved only parallel and perpendicular to centerline.

Your whole argument is wrong to assume that. When comparing sails to wings you must only compare sails to wings. Provide them with equal conditions speed ,angle of attack and moments. They will fundamentally behave the same .one particle of fluid moving over a sail or aerfoil is completely unaware of the existence of the plane or boat to which it is attached.. compare a stable wing with a stable sail.

Thrust is a vector with a direction forward and parallel to the centerline and a magnitude equal to the net sum of force in the opposite direction. Leeway is a vector in the direction that the boat is truly moving with a magnitude equal to the true speed of the boat.

The inevitable result of sailing a monohul without balancing the moments. exessive or not this has nothing to do with the actual argument. Wings vs sails

You are only resolving the heeling force perpendicular to centerline and since it is perpendicular to the centerline it has no effect for or aft which inherently means you are not resolving it in a useful direction.

I know the force perpendicular to the centerline is adverse but if we are talking about the sail and nothing else why resolve around the centreline. You resolve around the centreline because there is a boat attached to the sail. The argument you are making is that sails don't work like wings. The evidence you are using for the argument is that boats dont work like planes.

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Will GilmoreSenior Member

You are getting caught up with the arguing, and losing sight of the argument.

The 'lift' force on a sail is not all adverse. It is the force, combined with lateral resistance, that propels the boat forward. It also contributes to heel ('lift' is the only heeling force, for the purposes of this excersize) and it contributes to leeway.

It is the 'lift' force, perpendicular to the cord of the sail, that gives the boat forward movement. This does not happen with an airplane.

By being able to change the angle of attack to something other than parallel to the vessel's heading, the perpendicular force of 'lift' is able to contribute to forward motion. I am not reporting anything new to the people of this forum when I explain how a broad reach accelerates a vessel forward based upon the perpendicular force of lift on a sail set at an angle of somewhere around 90 degrees to the heading. Most of the 'lift' force is contributing to forward motion.

To bring drag into it, for a moment, this is the primary heeling force at this stage. BUT, now the vessel begins to move because the plane of lateral resistance gives the vessel directionality. As it does so, apparent wind (AW) shifts. AW moves forward and changes magnitude. There will be a slight slowing as the AW moves to a beam reach. However, more of the lift force contributes to forward motion as it is more inline with the direction of the boat. For an efficient hull, the vessel continues to accelerate, due to the 'lift' force of the sail, which in turn, moves the AW forward and is accompanied by an increase in wind speed across the deck and over the sail. This has the affect of increasing the 'lift' force even more. However, by this time, the sail has been trimmed in to a close reach and the vector component of the 'lift' force that contributes to drive, is a fraction of the total 'lift' force. Yet, some boats can continue to speed up. This is because the pressure of the 'lift' force against the lateral resistance is like a wedge being squeezed between two wheels that are coming together. Leeway is a consequence of the fluidity of water, but water also has a great deal of resistance to the lateral force against the keel as applied by the sail.

Imagine a sailboat on rails, instead of a keel in the water. It has been said, by sailors I know, that part of a sailboats efficiency is in the aerodynamic lift of the keel in opposition to the sail. They point out that the boat is "sailing" under the water, as well as above, but that is not why the boat moves forward. Put the same vessel on rails, with no fluid dynamics, no 'lift' force below the waterline and thus no leeward give, she will sail more efficiently. That's why iceboats go so fast. Besides far less resistance (the real reason they are so much faster), they also have less leeward movement. They get more force translating to forward motion.

The 'lift' force can not be described as adverse to a sailboat's motion. However, components of the 'lift' force may be.

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Sailor AlSenior Member

Why are you arguing?
You have already agreed with my basic premise: that the aerodynamic force should be resolved around the useful boat direction. All the rest follows
The components are Thrust and Leeway/Heel
Lift and Drag come from removing around the direction of the airstream and are not useful to the discussion.
I'm going sailing.

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Will GilmoreSenior Member

Wow! I just realized I missed over a full page of very impassioned arguement. Great reading, all.

Keep up the great posts.

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GlueandcoffeeJunior Member

Yes I agree with all you have said above. But to compare wings to sails play the scenario of a wing on rails . Given the same wind a angle of attack the wing will act very similar no?.
That is how a glider works.
For a powered plane you assume 0 wind but a greater angle of attack. But to compare apples with apples you push the kart on rails forward so the appartment wind relative to the wing makes the angle of attack similar to the wing or sails on rails. Now all the forces are balanced . So in relation to the airflow and wing(sail) the forces between all scenarios are congruent. A sail is a wing with respect to the air moving around it. Obviously I as an outsider observer can see that a large system of sails,keels water, wind and un balanced moments uses forces differently to a large system of wings, horizontal stabilizers gravity, still air and engines. But the actual argument I'm making is that wings have the same set of forces as sails . Apples with apples. Not apples in a cardboard box carried by a hippie vs apples in plastic bags thrown by city folk. A sail is a wing and Visa versa. But a plane is not a boat.

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GlueandcoffeeJunior Member

I agree with your basic premise but I disagree with "a sail is not a wing" analogy.

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KeithOSenior Member

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Will GilmoreSenior Member

Within the article, the author uses driving force and lift interchangibly. "On a beam reach, the lift and therefore the driving force, is mostly directed forward and there is little drag or heeling force."
What the heck is the arrow marked 'Lift' All about? He has it that way on a number of diagrams. If the driving force is the forward component of the lift force, what's the difference between F and Lift? What is creating the vector force F?

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KeithOSenior Member

If you look at the lower diagram carefully you will note that he deducts the drag of the airfoil from the Lift vector to obtain a smaller vector F from which he then derives the driving force and heeling force.

I think the comparison he is making is that the drag of a conventional sail is greater resulting in an overall smaller propulsion force. Which makes sense since a full cross section airfoil is certainly more efficient than a single skin airfoil.

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Will GilmoreSenior Member

Thanks Keith, that makes perfect sense.

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DCockeySenior Member

Sailor Al, your disagreement appears to be with use of lift and drag in explainations rel
The conventional definition of "lift" in aerodynamics is the component of force perpendicualar to the free stream direction, not perpendicular to the the chord. For boats replace "free stream direction" with "appearent wind direction".
The conventional definition of "drag" in aerodynamics is the component of force parallel to the free stream direction. For boats replace "free stream direction" with "appearent wind direction".

Drag, using the definition above, contributes to the boat moving forward if the apparent wind direction is aft of abeam. Lift always contributes unless the boat is sailing dead downwind.

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DCockeySenior Member

This definition of Lift is the source of the confusion. It is not the standard defintion of lift used in aerodynamics.

The lift of a wing is the component of force perpendicualar to the direction of the freestream.

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Will GilmoreSenior Member

I happily stand corrected. I like that definition better, anyway. It makes more sense to me.

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gonzoSenior Member

Motion does not need external forces. Read Newton's laws. You are having a fundamental misunderstanding of what motion means in physics.
1-Newton’s first law states that, if a body is at rest or moving at a constant speed in a straight line, it will remain at rest or keep moving in a straight line at constant speed unless it is acted upon by a force. So no force is needed for motion
2-Newton’s second law is a quantitative description of the changes that a force can produce on the motion of a body. It states that the time rate of change of the momentum of a body is equal in both magnitude and direction to the force imposed on it. The momentum of a body is equal to the product of its mass and its velocity. Momentum, like velocity, is a vector quantity, having both magnitude and direction. A force applied to a body can change the magnitude of the momentum, or its direction, or both. Newton’s second law is one of the most important in all of physics. For a body whose mass m is constant, it can be written in the form F = ma, where F (force) and a (acceleration) are both vector quantities. If a body has a net force acting on it, it is accelerated in accordance with the equation. Conversely, if a body is not accelerated, there is no net force acting on it.
3-Newton’s third law states that when two bodies interact, they apply forces to one another that are equal in magnitude and opposite in direction. The third law is also known as the law of action and reaction. This law is important in analyzing problems of static equilibrium, where all forces are balanced, but it also applies to bodies in uniform or accelerated motion. The forces it describes are real ones, not mere bookkeeping devices. For example, a book resting on a table applies a downward force equal to its weight on the table. According to the third law, the table applies an equal and opposite force to the book. This force occurs because the weight of the book causes the table to deform slightly so that it pushes back on the book like a coiled spring.

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