sail equivalent to winglets?

Discussion in 'Hydrodynamics and Aerodynamics' started by jlconger, Aug 5, 2014.

  1. Doug Lord
    Joined: May 2009
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    Doug Lord Flight Ready

    Vertical Endplate/ Wing Tip

    I've used this feature on a couple of multihull models and on my latest Fire Arrow test model. I have no concrete evidence that there is an aerodynamic benefit but I think there may be based on what I've read about wing tips. At any rate, the boat needed masthead floatation and that's what's inside the black thing at the top of the rig. The Wing Tip is attached to the gaff that controls the head of the sail, so it's always at the same angle of attack to the wind as the top of the sail is.
    I forgot to mention another factor in my choice of the "Wing Tip": Tom Speer has mentioned that winglets on aircraft are used because they can't increase span(for many reasons including width restrictions at airports) which,he said, was always preferable to winglets or end plates, if I understood him correctly. So the Wing Tip does effectively increase the span of the rig.
     
  2. jlconger
    Joined: Aug 2014
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    jlconger Junior Member

    clever!

    Needing masthead floatation is a pretty good reason to put on a wing tip!

    The notion of equivalent span for winglets comes in a roundabout way. Air is always trying to go around the wing (sail) rather than over it. The most obvious way to avoid this is to make the wing longer. An infinitely long wing has no bypass at the tip and no induced drag. A properly shaped winglet also helps direct airflow over the wing's surface. To that extent it can be thought of as being equivalent to a longer wing. I think the analogy breaks down pretty fast once you get past this simple comparison.
     
  3. bjn
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    bjn Senior Member

    Nice, that's the "raked wingtip" I asked about in my post. Your model sail, Doug, is the first sail I've seen with that feature. I'm very new to sailing though. I bought my first sailing boat two month's ago. I find sailing fun, relaxing, and rewarding. I'm reading everything I can find about it.
     
  4. Doug Lord
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    Doug Lord Flight Ready

    Well, welcome to sailing! Great way to learn and have fun......
    Here are a few other variations on the Wing Tip main on some earlier models:
     

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  5. Doug Lord
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    Doug Lord Flight Ready

    Here are some winglets used on a C Class wing rig. If I remember correctly they were not viewed as successful:
     

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  6. markdrela
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    markdrela Senior Member

    This is not surprising. Whether winglets are beneficial depends entirely on the constraints of the problem. Two possible situations are:

    1) Lift and span are constrained. Here winglets will definitely reduce induced drag.

    2) Lift and root moment are constrained. Here a wing with winglets will not be significantly better than a straight wing, as long as the straight wing can have a slightly larger span.

    Root moment is the same as heeling moment, which is definitely a constraint on a sailboat. So winglets on a sailboat generally don't make sense. They would make sense only if there is a severe mast-height constraint in the rules, such that case 1) is more relevant.
     
  7. jlconger
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    jlconger Junior Member

    It still looks to me that the winglets are on the wrong end of the sail. The big vortex is at the foot, not the head of the sail. When I get time I'll do a simulation with modifications at boom level and see what happens. Probably not the best place to have big airfoils from a crew safety point of view, but there are not many OSHA certified sailboats out there...
     
  8. Doug Halsey
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    Doug Halsey Senior Member

    Maybe if you're standing on your head?
     

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  9. jlconger
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    jlconger Junior Member

    pictures of top and bottom vortexes

    Here are a couple of views of both vortexes from a CFD simulation. It is clearly going to depend on the boat and sail design, but with a triangular sail shape and a long gap at the bottom, there is a lot of flow bypass near the foot. How much this impacts sail performance is something I do not know. There is not much surface pressure fall off near the foot, so the impact might be pretty small.
     

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  10. johnhazel
    Joined: Jun 2008
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    johnhazel Senior Member

    Not true ref: Horener page 7-9 and 7-10 who ref. others but no mention of himself.

    Pot calling the kettle black here maybe? But it is true, people don't check their "facts" often enough. For example:

    Actually Horener said "The induced drag decreases and/or the effective aspect ratio increases"

    Nasa Dryden web site says "Winglets increase an aircraft's operating efficiency by reducing what is called induced drag" http://www.nasa.gov/centers/dryden/about/Organizations/Technology/Facts/TF-2004-15-DFRC.html

    R. T. Jones of Ames Research Center says "It has been known for many years that vertical fins or end plates at the tips of a wing can significantly reduce vortex drag." http://www.engbrasil.eng.br/index_arquivos/art60.pdf

    So would you call America's Cup sail's aspect ratio, high or low?
     
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