A sail is not a wing

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

  1. KeithO
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    KeithO Senior Member

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  2. Sailor Al
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    Sailor Al Senior Member

  3. Sailor Al
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    Sailor Al Senior Member

  4. gonzo
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    gonzo Senior Member

    That is not correct. Sailing downwind uses mainly drag on the sails.
    Also, sails can generate lift. For example spinnakers and sailboards where the mast is tilted to windward.
     
  5. gonzo
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    gonzo Senior Member

    It is not erroneous and misleading to use a wing as an analogy to a sail. In fact, an airplane wing makes a very efficient sail; particularly upwind. They are generally called rigid wing sails. The problem is the difficulty in handling and reefing; if possible at all. It usually takes several people and a crane to dismount them on larger boats.
     
  6. Sailor Al
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    Sailor Al Senior Member

    OK, I failed to make the specific point: My issue is with the use of the wing analogy when describing the way a sailboat sails upwind.
     
  7. Sailor Al
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    Sailor Al Senior Member

    That's using an actual wing as a sail, not using the wing as analogy for a sail.
    My issue is with the use of the analogy of a wing for a sail. There is a difference.
    (And anyhow, you'd be hard pressed to use a rigid wing as a jib!)
     
  8. Will Gilmore
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    Will Gilmore Senior Member

    Sailor Al, I take your point with the use of the airplane wing analogy to a sail. I also think your arguement could use some refinement, which, I can see, is the reason for this thread. I would like to give it a go at helping you out.

    The way I see it, 'lift' is as good a word as any to use, and the driving force of a sail comes from the same fluid dynamic forces as the lift on an airplane wing. However, one fundamental difference, I see, between an airplane wing and a sail is in the motive force itself. With an airplane, 'lift' is generated by forcing the wing to move against the air, either through the use of an engine or with gravity pulling on the body. A plane is entirely surrounded by the air, so it must use either of those two methods to force airflow. This is where the flow of wind over the wing ultimately comes from and so, as the article points out, lift is for holding the wing and its attached vehicle up. It does not add to the motive force except, possibly in the case of a very steep dive where lift may also contribute to speed across the ground.

    With a sailboat, it is the lift of the wing that provides the motive force. Movement through the air must necessarily be a very specific limited angle of attack because the sail is what provides the motion in combination with the lateral resistance. An analogous aspect to something like a sail plane would be the keel's LR and the gravity that pulls the sail plain in counter force to the lift of air flow.

    Both the sail and the wing provide lift. That lift is essencialy perpendicular to the cord, not the angle of the wind or AoA (This is where the diagram above is in error). With a plane, that lift does not contribute to forward motion. In fact, forward motion makes the lifting force greater.

    With a sailboat, forward motion is created by the wind moving over the sail at an angle to the desired direction of motion. Once the sail begins moving in relationship to the wind, the 'lift' force is instantly affected, both in angle and magnitude, so is the motive force, such that a sail reaches an inherent limit of performance. But that performance is entirely dependent upon its movement. This is not analogous to the way a wing performs. It is very different than the way a wing preforms.

    Both sail and wing experience stalling angles, both generate lift, that is why boats heel, but the sailboat has an entirely different dynamic when it comes to the driving force. It is completely dependent upon the motion of air over water. A sailboat can't change the water angle or speed, counter to the wind, it is dependent upon the wind entirely for its ability to move. Not at all like a wing.

    Still, it is the same force upon the wing like shape that allows this to happen.
     
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  9. Alik
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    Alik Senior Member

    Write Your research paper, and present at sailboat research conference. Say, at CSYS, HPYD or similar. Then, You can discuss with the experts, to find out whose misconception is correct ;)
    Throwing Your writings in the Internet would not help. There are a lot of such writings, many of them are not science but shamanism.
     
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  10. jehardiman
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    jehardiman Senior Member

    Yep, a better argument would be to show how a sail IS like a wing, then show how they they ARE different.
     
  11. jehardiman
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    jehardiman Senior Member

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  12. Sailor Al
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    Sailor Al Senior Member

    You are right, I have enhanced the premise of the argument, thanks.
     
  13. Sailor Al
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    Sailor Al Senior Member

    Thanks for the encouragement.
    Where "terribly wrong"?
    I deliberately stay away from energy. A force is a vector with value and direction. Energy is scalar. When the (Thrust) force moves the sail forward, it generates the power to do the work to overcome the resistive force of the hull moving through the water(Newtonian terms in italic). But that's a whole other kettle of fish.
     
    Last edited: Feb 9, 2021
  14. jehardiman
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    jehardiman Senior Member

    You are a complete blind man fumbling around in the dark. Yes, mass is a scalar, velocity is a vector, force is a vector, energy is a scalar. If you can't put it all together, why are you even opening your mouth...Answer me this, how are work and energy related? If you can see that, you will understand why you don't understand.
     

  15. Sailor Al
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    Sailor Al Senior Member

    Oi, be nice. I thanked you for your encouragement and asked a polite question: "Where am I terribly wrong?"
    I don't need to. Energy isn't part of my argument. I don't understand your point.
    Yes, I've done high school physics: "energy is the capacity to do work". What's your point?
    What is it I don't understand?
     
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