Understanding Wing Technology

Discussion in 'Hydrodynamics and Aerodynamics' started by Doug Lord, Sep 18, 2010.

  1. Skyak
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    Skyak Senior Member

    The way I see it there is the educated, refined, but verbos explanation you cite -no point in me stating it again, just plenty of opportunity for error.

    Then there is "nozzle". One word, it is as meaningful and effective as it is wrong.

    I can't help thinking that if I can come up with a paragraph explaining why this works, that is not wrong, I have added all the value I can. If you are just being kind not pointing out problems, tell me. If there is nothing 'WRONG' then I will just add a little and call it done.

    What is needed for 'proof' is to go through the design alternatives allowed by the rule and show why they are inferior to this one -even longer and more difficult that the 5 step theory.

    FUN THOUGHT EXPERIMENT

    lets say you were given a chance to race another sailor who is closest to your skill, but better, in wing sail foiling boats. If you win you get to keep the boat. Because he has a minor skill advantage, you get first choice of boats which are identical except one has the tab on the forward element that we are talking about. Windward leeward race, would you choose the boat with the tab and if so why?
     
  2. Skyak
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    Skyak Senior Member

    To answer my own question and help Tom2871 understand;

    "total pressure" in the case of low viscosity like air, is the sum of static pressure and dynamic pressure. In the conditions that sails operate in, the total pressure is constant outside the boundary layer. The static pressure is high and the dynamic pressure is low (and slow) on the windward side, and static pressure is low and dynamic pressure high on the lee side. Inside the boundary layer there is skin friction and turbulence taking energy and changing momentum.

    https://en.wikipedia.org/wiki/Total_pressure
     
  3. tspeer
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    tspeer Senior Member

    The tab does add camber to the main element. However, the flap is affected, too, and the net effect is not necessarily to increase the maximum lift.

    In the attached files, the first plot shows a section from an AC72 wing with 30 deg of flap deflection and tab deflections of 15, 10 (black), and 5 deg. The second plot shows what happens to the pressure distributions at -10 deg angle of attack (measured relative to the main element chord) as the tab deflection is varied. (The arrows show the direction of the trends with decreasing tab deflection.) The change in lift of the main element is offset by the change in lift on the leading edge of the flap, and there is only a very small net change in lift for most angles of attack.

    The lowering of the flap stagnation streamline as the slot is opened shows the increase in mass flow through the slot. There's very little change in the velocity at the main element trailing edge, and most of the effect on the flap takes place behind the main element trailing edge. So, other than the change in mass flow, the conditions in the slot itself are not affected very much by the change in tab deflection. However, the changes on the rest of the section are profound. The lift is changed almost uniformly over the main element. The main element stagnation point varies little, and the mass flow going to the lee of the main element is almost unchanged. There's a big change in the lift on the upper side of the flap where the flow is turned the most. This is due to the change in mass flow through the slot as opposed to flowing by the windward side of the flap. The net effect is the same turning of the flow for this flap deflection and angle of attack, but the details of where the flow is turned, and the development of the main element and flap boundary layers, are strongly affected by the tab.

    The corresponding polars in the third file show the variation with angle of attack for 5 (red), 10 (yellow), and 15 deg (green) tab deflection. The maximum lift is highest for the 5 deg tab deflection. However, the profile drag is higher for most of the angles of attack, making the 5 deg tab deflection less attractive. A tab deflection in the 10 - 15 deg range is probably the best compromise for overall performance for this wing.

    The trends in these results depend a great deal on the boundary layer development for this particular configuration. It's probably fair to say that increasing tab deflection will generally have the effect of increasing the lift on the main element and decreasing the lift on the flap. However, I don't think one can generalize the effects on maximum lift or profile drag. These will depend on how much the characteristics of the main element and flap change in opposite directions as the tab deflection is varied, and at what point flow separation begins.
     
  4. schakel
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    schakel environmental project Msc

    Soft wing sails has the advantage of better handling in the harbor and the prevention of loss in case of capsizing with a hardwing sail. Not sure if these soft wing sails are as powerfull, but for cost reduction a soft wing sail might be an alternative.
    softwing sail.JPG
    https://youtu.be/oNKajpNXb7g

    A very usefull site with aerodynamic research and theory is this one:
    http://www.scoop.it/t/soft-wing-sails
     
  5. CT249
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    CT249 Senior Member

    The strange thing about soft wings is that people spend years on end making claims about performance, and never prove those claims.

    It was something like seven years ago when Omer said they would do some boat-on-boat testing to prove their rig's performance, I think. It seems to have never happened. The Advanced Wing Systems guys were in the same city as a strong sportsboat class, but as far as I can find out they never lined their soft wing rig up against conventional rigs. The Omer or another soft wing rig was trialled against conventional rigs in Seascape 18s, and according to Seascape the wing sail was inferior.

    It's really, really easy to demonstrate superior speed - put the new rig on a known hull and go out and finish in front of the conventionally rigged sisterships. If these rigs are so good why don't the developers just show us instead of talking about it?
     
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  6. schakel
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    schakel environmental project Msc

    On paper it looks alright but inpractice the two parralel sails are pushed to each other is what I can see in the video. So the wing-lift effect doesn't work.
     
  7. Mikko Brummer
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    Mikko Brummer Senior Member

    From the lift point of view, it's only good if the sails are pushed together, lift will increase ;-). Thickness is good from drag point of view, but it will decrease lift, not increase it, as people often think. Filling up the windward side of the sail will decrease the pressure there. But I agree with CT, I'm still waiting for one of these complicated systems to perform.
     
  8. oldsailor7
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    oldsailor7 Senior Member

    Originally Posted by CT249.
    "The strange thing about soft wings is that people spend years on end making claims about performance, and never prove those claims".

    Chris is right on about that.
    The 1973 A class nationals were expected to be swept by the reigning class champion from CA, with his top performing soft sail A cat.

    Ian Turner, an australian, living in Canada at the time, trailered his Crowther
    Typhoon A class wing masted Cat down to Florida and entered the competition.

    He won all the races on both elapsed and corrected times, much to the concern of the other top class racers.

    It was an early precept of things to come. :cool:
     
  9. CT249
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    CT249 Senior Member

    Yes, what I find interesting is the clear performance gain and acceptance of wingmasts in some classes, compared to the repeated failed attempts (often by leading sailors) to get them to perform well in other classes. There seems to be lessons in that.

    The soft wing sails, on the other hand, don't seem to perform in any class. It's also interesting that the wingsails used in the As, like Halls, don't perform all that well.

    I have a bunch of wingmasts and similar leading edge devices so I'm not anti-wing; it's just a case of trying to learn by seeing where they perform rather than ignoring it when they don't.
     
  10. Erwan
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    Erwan Senior Member

    Well, comparing soft sail wing slotted sails and soft wingsail is a difficult exercice, especially as you are in a turbulent environment, and there is no hope for your solid wing sail to achieve a laminar run on your wing section.

    Ben Hall's wing was outperforming soft sails in the middle range winds, while soft sails outperforme Ben Hall wing sail in low and strong winds (ref: Glenn Ashby & Jimmy Spithill interview @ Erquy/France during F18 World 2010).

    Probably these informations are consistent with flat water conditions.
    With significant chop "The tail moves the dog" according to Ben Hall experience.

    In other words an heavy aerodynamic solution can have its outperformance's potential more than offseted by boat pitch and moment of inertia.

    Compared to a soft sail+ teardrop mast solution, there is a point which is not often addressed while it can be a driver of performance windward.
    Everyone can see that AC wing boat pull the bottom rear corner of the wing (the clew)to windward of the middle mark of the boat, but it is not a negative AoA.

    The reason is the leading edge of the wing moves windward as you "camber" the wing, so if the clew of your wing remains at the center of the boat like for a soft sail + teardrop mast, you just loose a few ° in your windward performance.

    And in most of wing experiences, this point is not always addressed. That is why comparisons with soft sail is not always relevant.

    Now considering a windsurf-like rig for an A-Cat, the diameter of the mast will create the same problem (around 90 to 100 mm to achieve the appropriate inertia, without spreader & diamonds wires), so you have to choose a 60 mm diameter solution with spreader and diamonds wire which makes the "clean wing concept" much less clean.

    In addition at these Reynolds regimes with turbulent BL everywhere, the profil drag is thickness dependent, so the thinner the better as aboved explained by Mikko.

    The points which remain difficult to appraise are :

    -How much drag improvment to expect if windward separation behind the tear drop mast
    is addressed with a luff pocket or another similar concept.

    -How the turbulent BL would be less draggy with a very smooth surface compared to a classic wind surf or soft sail surface smoothness.

    Cheers

    Erwan
     
  11. Mikko Brummer
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    Mikko Brummer Senior Member

    I was referring to lift, not drag. Adding thickness on the windward side will reduce lift.

    Your windward sheeting explanation feels a little shaky to me, too ;-)
     
  12. Erwan
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    Erwan Senior Member

    Yes, you are right, and I quoted you a bit quickly, confusing the 2 different issues:
    1-adding thickness to the windward side decreases the camber of the section hence the lift, everything else equal.
    2- adding thickness increases the drag too,
    any soft wing project should consider these points.

    I agree I am not that clear with my windward sheeting. Another approach would suggest to look at the bottom leading edge of GroupamaC wing is another illustration of what I mean.
    If your leading edge is in front of the rotation line of the mast, the more camber in the section, the more the leading edge moves windward.
    If you want to address the apparent wind twist correctly, either you decrease the "leading edge area" going down the wing, either you increase the chord going down.
    other wise your botttom area will not be as useful as the upper parts of the wing, as it will not address correctly the wind twist in the bottom

    That is why I mentionned the windward sheeting, to compensate for some leading edge windward move, when leading edge is not designed like GroupamaC.

    All of this might look a bit candid too you, because I am not a professional, I never designed sails sucessful at Olympics, and I am not proficient with system like OpenFOAM.
    Also I never investigated the fine structure of the wind with engineers at ENV/Saint-Pierre Quiberon.
    But I hope my candid posts will not discourage you to post your uncredible CFD animations.

    Best regards and happy Sunday

    Erwan
     
  13. Mikko Brummer
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    Mikko Brummer Senior Member

    Yes, I agree that the approach of Groupama C with the cut-off down in the main element is very clever, and I actually did understand what you were aiming at with your explanation.

    We did recently a webinar about sail shape optimization, I will post the link as soon as it becomes available. Thank you for your nice words,
     
  14. schakel
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    schakel environmental project Msc

    test platform wing luna rossa.png
    Luna Rossa is testing their softwing. Might happen..
     

  15. Erwan
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    Erwan Senior Member

    Thanks for the info: Should we understand that Luna Rossa is testing a soft wing on a catamaran bed test??
    It is an opportunity to say Hello to Mikko, of course I am a bit interested in saying Hello as I would love to get the proceeding of the sail optimization webminar as above- mentionned

    Thanks & Cheers

    Erwan
     
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