lifting bi-directional foils

Discussion in 'Hydrodynamics and Aerodynamics' started by Anatol, Jun 15, 2015.

  1. Anatol
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    Anatol Senior Member

    Tom Speer has given us some bidirectional foils.
    http://www.basiliscus.com/ProaSections/Paper/ProaSections.htm
    I'm generally engineering-literate, but I'm certainly no hydrodynamicist, so some of his plots are opaque to me.
    I'd be glad to know of report from anyone has tested them.

    My current inquiry is about the possibility of creating a lifting foil, along the lines of an asymmetrical hull of traditional proas.
    If I sliced a Speer Bidirectional foil down the center, would I have a bidirectional foil that would 'lift' or crab to windward? Or am I misundertanding Speers paper, and the use of the term 'hydrofoil' implies it is already a lifting foil. I note that some of Tom Speer's drawings, such as all the "C" sections after C7 - appear asymmetrical - am I correct in reading the drawing this way?

    thanks!
     
  2. tspeer
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    tspeer Senior Member

    So would I!

    I don't expect them to work very well in practice. It's one thing to predict their performance using a two-dimensional computer program, but their characteristics are very sensitive to exactly where the flow separates as it approaches the trailing edge. I think 3D effects, plus small variations in the as-built shape compared to the designed shape, will result in the flow separating earlier than predicted and that would cause a significant loss of lift and an increase in drag.

    Most of the sections were asymmetrical, although there were some symmetrical sections included. They are all lifting sections - the symmetrical ones just require more angle of attack. The fourth digit in the name of the section indicates the amount of camber, in % chord.

    I'm not sure what you mean by the "C" sections. There's an Appendix C, but I didn't use "C" in the designations of the sections.

    An asymmetrical section will produce zero lift at a negative angle of attack, compared to a symmetrical section that produces zero lift at zero angle of attack. But if you take the angle for zero lift as your reference for measuring angle of attack (instead of the chord line), then both symmetrical and asymmetrical sections produce essentially the same lift as the angle of attack is varied from there.

    The notion that an asymmetrical section of any kind will crab to windward isn't really what happens. That's because the boat's heading is not fixed. The boat goes through the water at the angle that is required for the side force from the board, rudder, and hull to exactly balance the side force being applied by the sail rig and windage.

    Since most of the side force is produced by the board, you can think of the board as going through the water at a fixed angle, and the rest of the boat being mounted on top of it. If you angle the board in the boat, as with a gybing centerboard, then what really happens is you rotate hull so it goes through the water with the pointed more away from the wind. So leeway is reduced, but it doesn't come from the boat crabbing to weather. It comes from pointing the bow in the direction of travel.

    The real benefits of asymmetrical sections are somewhat reduced profile drag, and a higher maximum lift for a given area. The profile drag is a small proportion of the total drag, so the performance difference is a modest reduction in a small quantity.
     
  3. Anatol
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    Anatol Senior Member

    Tom
    thanks so much for your reply
    "So would I!
    I don't expect them to work very well in practice."


    oh :(

    "I'm not sure what you mean by the "C" sections. There's an Appendix C, but I didn't use "C" in the designations of the sections."

    sorry, I meant the foils illustrated in figs C7-x in appendix C
    http://www.basiliscus.com/ProaSections/AppendixC/AppendixC.htm

    "The notion that an asymmetrical section of any kind will crab to windward isn't really what happens. "

    uh huh...

    so let me ask this way - if one was to have one foil, say amidships to leeward, like a leeboard, what profile would you recommend to maximise lateral resistance, minimise drag and maximise windward performance (ie pointing angle)?

    Or is hoping a reversible foil will do this overly-optimistic? Is it better to pursue the more technically complex route of a 180 degree pivoting conventional foil? - even with the added complexity and weight of mechanism considered. Your learned opinion is much appreciated.
     
  4. tspeer
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    tspeer Senior Member

    That question can't be answered without a velocity prediction program (VPP). The VPP is needed to establish the operating conditions for the section. The section shape is perhaps the last step in designing the board.

    The VPP will also include a model of the hydrodynamics of the hull (ideally, the hull and board together) so the effects of leeway on the hull are included. Depending on the hull shape, it could be adversely affected by leeway, or it could act as a span extension for the board and reduce the drag.

    There are so many interacting factors in a sailboat, and everything is coupled to everything else. It's not possible to optimize one area in isolation and be confident that one has the right choice for the boat. Invariably, some areas need to operate at less than their maximum potential so that greater savings can be made elsewhere. It's the VPP that combines all these factors to predict the net result.

    Far more important is the depth and the area of the board. The drag due to lift is much more than the profile drag. So you need to get that right first, by sizing the board correctly.

    A reversible foil might be the best solution. I think in practice, a sharp-edged section will prove to perform better than the rounded-edged sections. That brings with it the well-known problems of sharp leading edges, though, which form separation bubbles.

    I don't have any experience sailing a proa, so I don't know if it's better to raise and lower daggerboards/rudders or to rotate through 180 deg. If you only have a single board, then you have to go the reversible route. Rob Denny has a lot of proa experience and has settled on the rotating board approach. I think the way he mounts his boards off the inboard (windward) side of the leeward hull has a lot of merit, as it looks easier to rotate the foils than to pull boards up and down.
     
  5. Anatol
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    Anatol Senior Member

    Tom
    thanks again. Point taken about the functionality of the system as a whole.

    rudders and foils on a proa are quite a conundrum. There seem several possibilities, each with its own merit and drawbacks.

    Rudders at the ends of the vaka are more effective in terms of distance from axis of rotation, but the bow one must be lifted on shunting.
    Rudders in cassettes capitalise on hull-as-endplate, with the added complexity of internal cavities etc.
    Quarter hung rudders are convenient but lose on both endplate and distance from center.
    Some avoid rudders altogether and steer by weight and sail trim. And some go with the traditional steering oar.

    Ease of shunting is a design goal for me, so one, or two, rotating foils, quarter hung off crossbeams seems parsimonius. Conventional foil shape, pivoted at about 1/3 of chord length, like a balanced rudder, presumably, would automatically pivot as boat changed direction.

    hmmm...
     
  6. rwatson
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    rwatson Senior Member

    And of course, their is the 'new science' based on whale flippers to take in to account. :p

    "The advantage of the humpback-whale flipper seems to be the angle of attack it’s capable of–the angle between the flow of water and the face of the flipper. When the angle of attack of a whale flipper–or an airplane wing–becomes too steep, the result is something called stall. In aviation, stall means that there isn’t enough air flowing over the top surface of the wing. This causes a combination of increased drag and lost lift, a potentially dangerous situation that can result in a sudden loss of altitude. Previous experiments have shown, however, that the angle of attack of a humpback-whale flipper can be up to 40 percent steeper than that of a smooth flipper before stall occurs."

    http://www.technologyreview.com/news/409710/whale-inspired-wind-turbines/

    "The scientists found that the bumps reduce drag and increase a flipper's lift so it behaves more like an airplane wing. This extra lift and reduced drag lets a humpback whale make sharper turns than other whales can make.

    Someday, engineers designing flippers or fins to drive boats and submarines might add bumps or scallops, too."

    https://student.societyforscience.org/article/are-propellers-fin-ished
     

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  7. Anatol
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    Anatol Senior Member

    RW

    "the 'new science' based on whale flippers to take in to account"

    yes, I'd read a bit, here and elsewhere. Keen to know when someone does some actual physical tests. (tank etc)
     
  8. Erwan
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    Erwan Senior Member

    I guess Most of the foils operate with a Cl (Lift coef around 0.4) but for take-off

    I'm afraid whale-bump on the leading edge are most useful to delay transition and/or avoid LE separation @ high AoA .

    Some efficiency tests have been conducted with 2 similar wind-mills, (normal blades vs whale-bump blades) and the whale-bump blades seemed to deliver more electricity, but I don't remember details, I guess it was an average, with significant portion of low wind, where whale-bump blades have more lift.

    Also, I remember that for "cruise" at hight speed and straight line, Whale bump foils are a little more draggy.To be check

    Best regards
     
  9. DrawnOnward
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    DrawnOnward Junior Member

    This approach to rotating the board is changing. A new rudder mechanism has been developed by Etamax: http://etamax.com.au/

    The concept can be seen here (select the text if you find it hard to read):

    http://proalucaantara.blogspot.com.au/2015/01/the-kick-up-concept.html

    http://proalucaantara.blogspot.com.au/2015/02/waiting-on-delivery-from-oz.html

    As I understand it, this concept is going to be applied to the 3 18m+ proas being built. I'm building a 16.8m proa and plan to trial bi-directional lifting foils. To this end, I tweaked the boat design CAD software I wrote in order to draw foils. Given Tom's doubts re blunt trailing edges, I've developed much finer edged foils that appear to perform better - at least the computer says so. Rather than cross posting I'll direct you to the relevant discussion in which I posted a 5 foil comparison: http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/foil-performance-discussion-53742-2.html#post746105
     
  10. Anatol
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    Anatol Senior Member

    Drawn Onward
    thankyou for this little goldmine of sophisticated thinking on the subject. I look forward to more news on this. I am about to begin my Vaka (main hull) build, but am deferring my decision about (1st generation) foils to capitalise on up to the minute data and texts :). I should reiterate I'm buiding an Atlantic, iunfashionable as that is :) - and am toying with (Bruce type) foils in the Ama.

    My major concern about the Denney style foil is its lack of endplates, top and bottom. I'm a bit persuaded by endplates. My intention is to minimise induced drag where-ever possible, this includes foils and sails. Any foil solution on my boat will have end plates.
     
  11. rob denney
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    rob denney Senior Member

    The 18m harry in Melbourne has end plates on it's rudders. Reckons they work well in a draft constrained application.
    Rudders have 2 main purposes. Steering and stopping leeway. However, reality imposes certain requirements. To be safe, and as light as possible, they should kick up in a collision. They should also be liftable for balance, storm survival and shallow water/beaching. Ideally, they are visible and easily reached to remove weed and plastic bags. End plated rudders are not ideal for any of these. Hence I don't use them.

    It is easy to get carried away with foil shapes. We currently have simple ogives on the kite proa which work remarkably (and surprisingly, given they were not designed for the job) well as leeway reducers and lifting foils, in both directions. However, because of the sharp leading edges they are lousy as rudders. Not a problem on the kite proa as we steer by moving the kite lines. Only one foil in the water is a big step forward in reducing cost and complexity. As is no mast.

    The Luca Antara solution has been superceded. The Cruiser 60 has simpler, lighter and considerably cheaper rudders mounted on the lee hull. The Custom 65 in Norway will be lighter and cheaper again, and are mounted on the lee end of the beams. Bucket List (12m racing charter harry) has beam mounted 0012 rudders, currently being trialled on the kite proa and including a much simpler, cheaper construction method.

    There is no single solution for all boats, but they all have in common that they kick up, lift up and have no holes below the waterline to leak and cause drag.

    Atlantic proas are cool. Are there any pictures of yours on line?
     
  12. Anatol
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    Anatol Senior Member

    Rob
    thankyou for chiming in. I'm a great admirer of your work.
    My remark about foil endplates on your big foils was as much about plating the top end, having just read a report of air being sucked halfway down the foil on a similar kind of free hanging foil.

    "We currently have simple ogives on the kite proa"
    by ogive you mean a kind of bullet point curve (?). This is in profile, right (not cross section)?

    OK, I'm looking forward to seeing images, and reading about tests.


    you and I and a couple of other guys think that :)

    Thanks for your interest. At present I have lots of drawings and a rough 10:1 scale model. Getting ready to build a sailing model.

    In general the plan for my boat (Orthogonal) is 2 mast cat schooner rig, possibly a balanced lug to get the CE forward. I'm really concerned about CE and about ease of shunting. Rig sits on a 10:1 tortured ply hull, 30' long, vertical bows, with as much flare as possible. A windward 'pod' is cockpit and safety Ama.

    I've just been given a pair of solcat hulls which I will probably join end to end for the (first prototype) Ama. I'm thinking of using its centerboard slots to run foils on the ama.

    Crossbeams at present will be ply/glass box, about 12"x8" cross section. These socket into the major Vaka bulkheads (allowing demountability). The main bulkheads (at roughly 1/3 and 2/3 of length) also support the masts, and cockpit, keeping forces aligned with major strength.

    I'm postponing foil/rudder decisions. (Waiting on your new results :) I was planning on bow rudders but am currently considering quarter hung rudders on the windward cockpit side of the Vaka, for ease of use/flipping in a shunt. It seems to that rudders on proas have two different functions -trimming during sailing and major maneuvers at slow speed in harbor. Seems like decoupling the functions and having a steering oar and a pair of trim tabs might be smart.
     

  13. DrawnOnward
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    DrawnOnward Junior Member

    Is this due to the sharp leading edge or the fact that the rudder section was highly cambered? I have my concerns re sharp leading edges for rudders, but so much is unknown re bi-derectional rudders. I am inclined to have more confidence in the bidirectional approach when relatively narrow angles of attack are required, i.e. lifting foils.

    I raised the Luca Antara solution because detail is available. The rudder set up on the other two craft looks similar, i.e. too close to hull to be rotated through 180 degrees. If this is the case, as I said I understood it to be, then then the rudders are still going to be bidirectional, which was the intended message. Is this indeed the case? Presumably then, the rudders aren't going to have sharp leading edges?

    True. This also applies to foiling configs also. Full foiling has more critical pitch, heave and directional stability requirements than your application. So the foil section and configuration requirements become more critical.

    At the moment I'm working on both rotating and bi-directional solutions. It'd be great if some of these empirical questions were answered before I have to commit. It is for this reason that I'm disseminating the theoretical information I acquire.
     
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