A sail is not a wing

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

  1. tlouth7
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    tlouth7 Senior Member

    Only if you choose to frame the components of the aerodynamic force in that way. It is perfectly valid to resolve the force on a sail into lift normal to the chord, and drag parallel to it, or the same but normal and parallel to the AWA. This makes it clear why a high lift to drag sailplan is useful when going upwind: at a given sheeting angle higher drag means a smaller component of the overall force in the forwards direction. This is most obvious for very fast vessels such as America's cup yachts and iceboats. The lower the drag of the rig, the smaller the AWA you can have while maintaining an overall forwards component of force, and so the larger the multiple of windspeed you can achieve.

    As has been pointed out, what matters ultimately is the actual aerodynamic force; the same is true for aircraft. However you choose to express it, a sail is a device which sits in an airflow and generates a force in a direction other than directly downstream. The aerodynamic effects are identical to those of an aircraft wing even if the application is different. Given the massive body of research in that field it makes sense to transfer concepts and technology across to sails.
     
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  2. gonzo
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    gonzo Senior Member

    The analogy of an airplane wing and a sail is valid when using a free body diagram. If powered airplanes are what you think makes a fundamental difference, wings on a glider could not use the analogy to a powered plane either. The force from gravity, thrust of a propeller or a keel are just a vector on the free body diagram. What generates the force is irrelevant for the analysis of the foil.
     
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  3. Will Gilmore
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    Will Gilmore Senior Member

    I disagree. Motion is induced by external forces, in the case of an airplane. The changes in motion don't result in any appreciable changes in the direction of the vector forces of lift. Drag, as a distinct element, changes dynamically, but the driving force has no bearing on the lift generated, even with a sail plane.

    For a sailboat, force is both a result of motion and contributes to motion. The vector elements of both magnitude and direction are affected my changing motion of the sail. It is this difference that I would consider a fundamental difference between the way a wing works and a sail foil.

    I would not, however, argue that the forces weren't the same. They are. What I'm saying may be subtle, but I think it is important and where Sailor Al is coming from.

    -Will (Dragonfly)
     
  4. KeithO
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    KeithO Senior Member


    I wasn't able to open your original paper and I seem to have missed its successor.

    In the regime of flight, it is fairly obvious that the goal is to get the airplane off the ground and move in a certain direction, with control of altitude above ground. Thus "Lift" is logical in that circumstance. When one looks at the subject of aerobatic maneuvers and aerial combat, those boundaries are distorted rather grossly...

    However, in sailing, it is understood that the surface of the ocean is a constant and that movement at some compass heading is desired, requiring sails etc to be arranged accordingly. Thus in sailing it is probably more accurate to use the word thrust vs lift since the boat is floating on the water and it is just a matter of what direction and how fast you can go. The aerodynamic principles of how the thrust is generated is basically the same as with a wing, except for the fact that the sail geometry is not defined by rigid ribs and spars. Thus one has to steer in such a way that one has an appropriate pressure distribution on either side of your sail so that it fills out properly and maintains the most appropriate shape and angle of attack relative to the apparent wind. Assuming that this basic premise is met, and substituting thrust for lift, what else do you think is wrong with the theory ?
     
  5. Sailor Al
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    Sailor Al Senior Member

    It's in post #14, with an expansion #15 (on Page 1 in my browser). It probably would have been better to start a new thread.

    And there's the very nub of the problem.
    Thrust and Lift are orthogonal.
    Thrust is the component parallel to the centreline (like Drag). Lift is the component perpendicular to the centreline (or chord if we're being pedantic), like Leeway/Heel.
    When you use the wing analogy and talk about Lift (the USEFUL component of a wing) , you are talking about Leeway/Heel, which is the ADVERSE component of the sail.
     
  6. Sailor Al
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    Sailor Al Senior Member

    But I'm not using a "free body diagram" (whatever that is), so your argument really doesn't relate to the premise of the thread.
     
  7. Sailor Al
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    Sailor Al Senior Member

    The flaw in this argument is that we aim for LOW L/D ratio in a sail to produce a LARGER force in the forward direction, not a SMALLER one!
    That's precisely where the wing analogy is so inappropriate.
    You have to resort to pedantic logic to correct me by saying "But lower positive drag is the same as higher negative drag"
    In any language of planes or boats, Drag is an ADVERSE component, Thrust is a USEFUL component.
     
  8. Glueandcoffee
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    Glueandcoffee Junior Member

    Excuse me if I come across as new to the forum (I am). Still trying to figure out how to reply here. But anyway, the way I understand the problem is that the forces on a free body airfoil, whether it is a plane , glider or sailboat, are all essentially the same. The only thing that changes between the 3 scenarios is which way you decide to use those forces.
    We must make a few assumptions first.
    A. The vehicle is already moving and is in a steady state where all forces have an equal and opposite force so as it is not accelerating in any direction (one of newtons laws).
    B. The aerodynamic forces, lift and drag, must be resolved parallel and perpendicular to the flow of fluid, independently of the chord or boom.
    C. There is a greater than 0 angle of attack with respect to the flowing fluid , but less than than the stall angle.
    This set of assumptions ensures we are talking about a plane or glider in stable flight and a sailboat going close hauled up wind.
    So here it goes.
    The vehicle moves in a direction to provide a (.1)fluid flow (apparent wind) which creates an(.2)opposing force parallel to the (.1)fluid flow ,which acts upon the airfoil (sail). This force (.2) is aerodynamic drag.

    Perpendicular to (.1)fluid flow and (.2)aerodynamic drag is the (.3)aerodynamic lift force.
    The (.3)aerodynamic lift force can be broken into two parts, a (.4)thrust force parallel to the chord and a (.6) useful lift force perpendicular to the chord.
    The (.4)thrust force is opposed by the (.5)skin drag parallel to the chord.
    The (.6)useful lift is opposed by (.7)gravity or hydrodynamic forces.

    The analogy breaks down because the righting moment of the boat is brought into it. The wing vs sail argument is about the forces... not the moments. Its apples and oranges. If a glider had only a left wing then lift would become a huge problem as is the lift on a sail . Remove the righting moment of a boat and hey presto lift is a boats best friend again and the efficiency goes way up . Take a look at sailrocket. It's a boat but built like an aircraft to perform like both. Correct me about anything I've said wrong. I know that what you call any of the forces might have a different name than what I used but the principle is same. I would be in opposition to the argument that OP is making however I agree that not all wings are sails and not all sails are wings. It just depends on how you resolve the forces .
     

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  9. Sailor Al
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    Sailor Al Senior Member

    So am I, but the forum platform is really good, easy to use, easy to embed images, easy to quote. I like it..
    Here's where analogy breaks down: On a yacht the force perpendicular to the chord is mostly adverse Heel and Leeway, (not "useful lift").
     
  10. gonzo
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    gonzo Senior Member

    You should learn what a free body diagram is, so your statement that my argument doesn't relate makes sense. It is the most basic representation of forces in physics, which you insist is the basis for your position.
     
  11. gonzo
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    gonzo Senior Member

    You need to take the time to learn definitions so your statements are clear. There is no such thing as an adverse force. Also, heel is a rotation and leeway a translation. They are the result of forces. This is where a free body diagram excels at explaining the behavior of a body. Lastly, a force perpendicular to the chord of a foil will act in the same way whether it is on a yacht, a plane or the spoiler of a race car.
     
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  12. Sailor Al
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    Sailor Al Senior Member

    My argument is based on Newtonian physics, and I'm sure Newton never heard of a "free body diagram".
     
  13. gonzo
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    gonzo Senior Member

    He discover that each force has an equal and opposite reaction. That is what the free body diagram. Back to school son.
     
  14. Sailor Al
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    Sailor Al Senior Member

    I'm sorry you're unhappy with the my describing of thrust as being useful and drag being adverse, but I'm pretty sure the rest of the world would be content.
    And yes, heel is a rotation, but it is caused by a heeling force. You're being pedantic.
    But no, it doesn't act the same way. On a plane it provides a USEFUL component, on a boat it provides and ADVERSE component.
     

  15. Sailor Al
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    Sailor Al Senior Member

    Let's not descend into abuse. It's the rhetoric of a losing argument.
     
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