A sail is not a wing

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

  1. Sailor Al
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    Sailor Al Senior Member

    Almost without exception, books, magazine articles and web pages that discuss how a sailboat uses the wind to sail upwind, open with an explanation based on the similarity between a sail and an aircraft wing and the fact that both generate LIFT.

    In this article*, I examine the wing analogy and explains how the analogy itself is deeply flawed, in that the fundamental operation of a sail is entirely contrary to that of an aircraft wing.
    I feel a bit like the boy in the story about the emperor's new clothes, and would like to hear your comments on this almost heretical theory.

    [EDIT]
    * I realise that the argument was poorly presented so I have withdrawn it. I have posted a much simpler argument below.
     
    Last edited: Feb 8, 2021
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  2. Alik
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    Alik Senior Member

    Very strange article, who is the author?

    It is not a reasearch paper, the figures are not numbered and I don't think the author is familiar with research paper write style.
    However, the drawing labeled 'Revised Schematic resolving the sail force about the boats centreline' is incorrect, say, Lift direction is wrong.

    There are no Lift and Drag forces. There is aerodymanic force A on a sail, which can be presented in different ways, as:
    - lift LA and drag DA - in coordinate system related to apparent wind;
    - thrust XA and side force YA - in coordinate system related to the boat. They are also called longutudinal and transverse components of aerodynamic force.

    And yes, sail is not always a wing. Downwind, a sail can be high drag body...
     
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  3. jehardiman
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    jehardiman Senior Member

    Sigh...rehashing this again. Your argument appears poorly formed since you only address powered flight thrust, when a SAILplane fall into the energy well is a more correct analogy. You should have read this thread..
    There is no "lift"! https://www.boatdesign.net/threads/there-is-no-lift.47727/
    ...it would have made your argument better.
     
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  4. wet feet
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    wet feet Senior Member

    Curiously enough,and in an unrelated field, until fairly recently the British aerodynamic racecar consultant Simon McBeath had a monthly column in the magazine Racecar Engineering.It usually featured a project car being put into a full size wind tunnel while a series of tests was run to achieve a specific aim.Sometimes it was to give a better balance,sometimes to get a better L/D figure so not too dissimilar to sailing except for the direction in which the forces were acting.In one particular case they were trying for more downforce at one end and the only material available for increasing the wing area was some sheet aluminium.Flap extensions were made and the comment regarding the success of the modification was along the lines of thin sheet devices work surprisingly well.So in summary if they look like a wing and give the same results as a wing are we in the realms of hair splitting and definitions?
     
    Last edited: Feb 8, 2021
  5. Sailor Al
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    Sailor Al Senior Member


    Thank you for taking the time to review the material.
    They are all my own thoughts and words, supported by images unashamedly cherry-picked from from the web, some, but not all attributed with links.
    And no, as you suggest, I don't have any experience writing research papers. This was not intended to be a research paper, but a technical article to present an idea in a clear concise form. I have adopted some of the style of an academic paper to assist me to cover the material in a logical manner.

    Re: "There are no Lift and Drag forces." :
    In the perception of the general community, Lift and Drag are very real forces which are clearly depicted by the vertical and horizontal lines on the myriads of sketches explaining the way an aeroplane wing works. So, to the general public, to suggest there are no Lift and Drag forces is baying at the moon.

    Rw: "Lift direction is wrong.":
    Again, based on those explanatory sketches, the general understanding of Lift is that it is the force that opposes the weight of the plane.
    Discussions about whether that is perpendicular to the chord of the wing, the centreline of the aircraft or the oncoming airstream of an aircraft are quibbles.
    Lift is quite clearly depicted as the component of the total aerodynamic force that is perpendicular to the direction of travel.

    My article refers to the wing analogy when used on upwind sailing.
     
  6. Sailor Al
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    Sailor Al Senior Member

    The latest post on that thread was 8 years ago. Is it really too soon to have another go at it?
    But anyhow, it was clearly aimed at a small community who is comfortable with the operations of matrix algebra and was restricted to the narrow subject of lift, while mine covers the broader field of the wing analogy, so it doesn't appear to be particularly relevant.

    And if you take another look, you will see that I have specifically included unpowered flight in the argument, but anyhow I draw no conclusions that only apply to powered flight.
     
  7. Sailor Al
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    Sailor Al Senior Member

    But that's precisely my point: apart from a couple of superficial similarities, is it really splitting hairs to point out that a wing neither looks like a wing nor works like a wing.

    If it looks like a duck and quacks like a duck, then, sure, it's a duck, but if it only looks vaguely like a duck and quacks like a frog, then it's not hair splitting to suggest it's not a duck.
     
    Last edited: Feb 8, 2021
  8. Alik
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    Alik Senior Member

    Lift and drag is decomposition of aerydynamoc force, in coordinate system related to the flow. They are NOT real forces.

    What is shown in red, is actually the 'side force' or the'transverse force' component. Nothing to do with the lift. Before You approach the problem, You should define the coordinate systems. Usually lift and drag are in flow-related system, and longitudingal and transverse in ship-related system. Try to draw the case of reaching, and You will understand that lift and transverse components of forces are not the same.

    PS On a boat, the weigth is balanced by floatation.
     
  9. Sailor Al
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    Sailor Al Senior Member

    Yes, I have already acknowledged your premise, but you are being unhelpfully pedantic by discounting the fact that my argument is founded on the standpoint of the confused trying to comes to grip with the issue, not someone like you, already well versed in the concepts and theories of aerodynamics.
    Please, if you can, put yourself in their position and see if it makes sense.
     
  10. Alik
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    Alik Senior Member

    I started my study of sailing yacht aerodynamics from reading Marchai and Herreshoff, about 30 years ago. I recommend You do the some reading, before trying to own write papers ;)
     
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  11. Sailor Al
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    Sailor Al Senior Member

    I am comfortable that my length and level of sailing experience, book learning and academic achievements more than qualify me to knock up a reasonable discussion paper on the subject.
    It's a pity that you haven't deigned to respond to my arguments.
     
  12. Alik
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    Alik Senior Member

    This is Your opinion only. The paper You wrote does not reflect the claimed qualification.
     
  13. wet feet
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    wet feet Senior Member

    This thread doesn't appear to be going anywhere useful.
     
  14. Sailor Al
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    Sailor Al Senior Member

    Au contraire!
    The responses have been most revealing and have spurred me to a second wind.
    My errors have been many and egregious.
    My presentation was far too wordy and failed to make the point and I fell into my own trap by using the name LIFT to label the sail's lateral component of aerodynamic force.

    The premise:
    Whilst an aircraft wing and a sail both a) generate a net aerodynamic force and b) exhibit a similarity in surface profile, in the context of explaining how a sail works on a yacht beating to windward, the use of a wing as an analogy is at a minimum confusing and ultimately quite incorrect.
    Here's the simple argument:
    When analysing the net aerodynamic force in the context of the operation of aeroplanes and sailboats:
    • The net aerodynamic force from the aeroplane's wing is resolved around the chord of the wing into useful Lift and adverse Drag
    • The net aerodynamic force from the yacht's sail is resolved around the centreline of the boat into useful Thrust and adverse Leeway/Heel
    The corollary is that, in the context of explaining how a sail works on a yacht beating to windward:
    • Unlike a wing, a sail does not generate adverse Drag
    • Unlike a wing, a sail does not generate useful Lift
    • Unlike a wing, a sail does generate useful Thrust
    • Unlike a wing, a sail does generate adverse Leeway and Heel
    How different does it have to be to be different?

    Regarding the confusion:
    Introducing Lift and Drag into a discussion about yacht's sails results in unnecessary and confusing diagrams like this to distinguish the two frames of reference.

    [​IMG]



    In any explanation following the presentation of such a diagram, the elements P (lift) and R (resistance, drag) are never further referenced. They are just "noise" in the argument.
    If we stop referring to a sail like a wing, the problem evaporates.
    A sail is not a wing.
    We need a simpler term than "Leeway/Heel" to name the lateral component (Fy).
     
    Last edited: Feb 9, 2021
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  15. Sailor Al
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    Sailor Al Senior Member

    In extreme cases, using the wing analogy elements of Lift and Resistance in the context of sailboats can lead to uninstructive diagrams like this
    upload_2021-2-9_8-21-27.png
    where there is no component of Fx providing any driving force on the boat at all!
     
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