sail equivalent to winglets?

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

  1. jlconger
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    jlconger Junior Member

    Many subsonic aircraft are using wing tip winglets to modify the tip vortex behavior. Does anyone have references to similar experiments with sails? I was thinking more of the lower vortex as shown in this image, but the top of the sail might be game too.

    vortex.png
     
  2. Petros
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    Petros Senior Member

    if you look at the America's Cup boats both the boom and the jib "sweep" the deck to act as an end plate. this reduces the loss from the high pressure side to the low pressure side. Generally the smaller the gap from the lower edge of the jib or main sail to the deck the more efficient it is, but the less convenient for the crew. on racing boats this gap is always minimized.

    for a pleasure craft this is very inconvenient to have passengers or crew get swept overboard every time you tack.

    I have thought about an end plate device at the top of the sail, would work best for cats or tris that do not have a lot of heel. It would add weight aloft, and add more stuff to mess with on a pleasure boat. The purpose is to make the sail more efficient, that is not always a priority on a pleasure boat.

    also consider the winglet on a wing make a given length wing act like an higher aspect ratio one. you would not put winglets on on an airplane where you do not have span limits, such as a sail plane or glider. It is more efficient just to make the wing a bit longer, and you get the same effect without the complication nor the interference at the tip. so it might be helpful in a class of racing that limits mast height, and for designs that do not have a lot of heel.
     
  3. jehardiman
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    jehardiman Senior Member

    Winglets were added to planes so that gate spacing at airports could be kept the same...they are not more efficient.

    Same thing with keel winglets, not more efficient, but rule cheaters.
     
  4. UNCIVILIZED
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    UNCIVILIZED DIY Junkyard MadScientist

    There is/are an equivalent to "winglets" on mainsails as well (the jib bit is covered above, & to varying degrees elsewhere on here, & run of the mill sailing books & periodicals). Albeit, ironically, JUST recently, I saw a writeup & pictures on them with regards to mainsails. Perhaps here, or possibly elsewhere. Just keep digging... it'll turn back up again.
    But also, I have some old, hard copy info, from the www.AYRS.org (Amateur Yacht Research Society) & odds are, if you can find the right POC & or area to look there, you'll find a good bit of info on said topic. Those guys/us, have done a monumental amount of research on all kinds of informative & helpful topics for sailors, including this one. And, I just moved, or I'd know where to look to find the paper & ink version of what I have on the topic.

    I can't say with any certainty as to whether or not, such "sail area" has been "taxed" yet on mainsails by Sailing's various "governing bodies" or not yet, but if you're looking into the topic, such is an issue to keep in mind while you're doing your research.
    As fast & unconventional ideas such as these do tend to at times get legislated out of existence. Kind of like Catamarans were for the first century & a half or so... But, you'll tend to find more published info on the topic in the realms of dinghies, & to some degree multi's, than you will on say, big keelboats. Electronic & otherwise.
     
  5. dinoa
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    dinoa Senior Member

    Winglets also reduce wing bending moments allowing a lighter spar. Some open class sailplanes with no span restrictions use them.

    Dino
     
  6. bjn
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    bjn Senior Member

    There is another thread from 2008 on the topic, which contains some good info:
    Winglets on sails?

    If I understand the posts by user "tspeer" right, winglets aren't the best way to make a sail better (low centre of effort/low drag). That is, if I understand it right, because the winglet increases the wetted area (more drag), and also increases the lift near the top of the sail (higher centre of effort). So it would be better to use that wetted area to increase the length of the sail.

    I'm still not entirely convinced though, and have been trying to find something which says the opposite.

    When searching, I found this site, which has a formula to calculate how endplates changes the effective span.
    http://www.mulsannescorner.com/wingendplates.html

    I also found this:
    "The Boeing 777 has got so called 'raked wingtips' where the wingtips have a larger angle angle of sweep then the rest of the wing. This increases wing span and thus aspect ratio. According to tests from Boeing and NASA this gives a bigger drag reduction compared to winglets."
    http://www.pprune.org/flight-testing/293879-induced-drag-winglets.html

    Has raked wingtips been tried on sails/wingsails?
     
  7. daiquiri
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    daiquiri Engineering and Design

    Generally, one tends to seek solutions in the direction of the KISS principle. You want to use less stuff and complications you can, because you want to have the least possible amount of handling issues and spare parts on board.

    The simplest solution for the jib vortex, as Petros has noted, is to minimize the gap between the foot and the deck, and it is adopted on many hi-end sailboats:

    [​IMG]
    [​IMG]

    As for the head of the mainsail, one way to reduce the vortex drag in that area is to push the vortex upwards by adding the square top. From the aerodynamic point of view, a square top acts like a virtual increase in length of the mainsail (compared to a classic triangular head). It also adds more sail area in the zone of the mast where the apparent wind is more favorable for the production of forward force - hence, a double gain. See again, as an example, the heads of the mainsails in above pics.

    Cheers
     
  8. tom28571
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    tom28571 Senior Member

    Even if you are able to build an aerodynamically efficient winglet at the top of the mainsail, it will only be effective in a stable regime. The tip of a mainsail is anything but stable, what with pitching, heeling, surging and other erratic movements that an aircraft does not have to deal with. If the winglet is not dynamically aligned parallel to the apparent wind direction during all these movements, it will be a pretty effective brake and heeling enhancer.
     
  9. jlconger
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    jlconger Junior Member

    thanks for feedback

    Thank you for the replies. A few notes:

    1) Glider wing tip decorations have another motivation which does not apply to sails. The winglets help maintain aileron effectiveness when near stall speed, which is a flight regime gliders use frequently (when climbing in lift.)

    2) Even with the boom as low as possible, the gap between the bottom of the sail and the deck is large. I suspect that the lower vortex is significantly larger than the tip vortex for any case with conventional sails.

    I'll probably modify some of my existing CFD simulations with various shapes at the boom level to see what happens. I'm not clear in my own mind what 'better' would look like.
     
  10. daiquiri
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    daiquiri Engineering and Design

    This is a good point.
    It reminded me of these dynamic CFD simulations made by Mikko Brummer, which are very eye-opening on that regard: http://www.boatdesign.net/forums/hy...s/ultimate-validation-44809-2.html#post586342 :)
    Cheers
     
  11. tom28571
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    tom28571 Senior Member

    I knew that such disturbances must be happening but have never seen any simulations like that.

    Thanks for the links.
     
  12. philSweet
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    philSweet Senior Member

    I think we are at the point where it would be usefull to hang some numbers on the phenomonen.

    Case 1. A unirig with a lower gap between the hull and boom. TSpeer has already published an analysis of RM and span loading for this case on his website, but the way he treated the gap analysis has been challenged, if I remember correctly.

    Case 2. A sloop rig. Typical numbers for a decent sloop working to windward are Jib = 85% of drive, Hull = 10% of drive, and main = 5% of drive. Given this sort of breakdown, and a jib foot at least equal to the boom length, How bad is the vortex under the boom really? Particularly if you have a lapping jib. Slam the jib down and hang leecloths on the stanchions. Get the jib loaded up and reduce the load on the main. If you regard the mainsail as a flap to adjust the camber of the entire sailplan, I think the span of the mainsail is less of an issue than treating it separately would suggest.

    A bimini or hardtop can endplate a mainsail on the right sized boat - say 35 - 60 feet.
     
  13. jlconger
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    jlconger Junior Member

    Dynamic animations and CFD calibration

    Great animations of a pitching sailboat - thank you.

    My approach to CFD calibration has been to check the overall results by comparing the predicted boat speed and boat heel to that measured under the same wind/sail conditions with a real boat. I interpolate between CFD runs to match each measured point. So far I have only been able to do this for upwind simulations, and I restrict the data to relatively smooth wave conditions. Here are some details:

    https://sites.google.com/site/sailcfd/home/boat-speed-calibration

    I'm getting tired of filling out a clipboard with numbers while sailing, so I have been putting together a little data acquisition system to at least partially automate the process. Hopefully data averaging will help remove some of the scatter, or point out more problems with the model...
     
  14. CT 249
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    CT 249 Senior Member

    A bit OT, but there are many myths about how many advances in design were prohibited; for example here we've had people claim that rotating masts were banned under rating rules like the RORC rule, when the text of the rule shows that they were specifically permitted. A lot of so-called "advances" just languished for practical or performance reasons.

    Similarly, modern research tools allow us to prove categorically that catamarans were NOT legislated out of existence; it's a complete myth primarily picked up from secondary sources half a century after the mythical ban.

    Of course that's not to say that things haven't been banned, just that the claims about such bans are often exaggerated or fabricated.
     

  15. jehardiman
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    jehardiman Senior Member

    Yep, and in that post he references Hoerner who did the basic research (documented in Fluid Dynamic Lift) on winglets. Too many people never read the original basic research and assumptions/test methods. Hoerner never said winglets increase lift to drag ratio, only that they increase the lift to span area ratio...which is a whole different item. Winglets actualy increase the induced drag for the unit lift.

    Daiquiri's reference to Mikko's post is a good example to how basic data is applied wrongly. Yes, in low variable, perfect wind tunnel data flow, high aspect works better; but in the real world with pitch and yaw, high aspect stalls so much more as to make it unviable.
     
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