What is a significance of a wing thickness

Discussion in 'Hydrodynamics and Aerodynamics' started by markmal, Nov 16, 2012.

  1. markmal
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    markmal Junior Member

    Hi.
    I am a newbie in aerodynamics. I am wandering about what benefits will give a changing a regular soft thin sail with a "thick" wing sail.
    I've downloaded XFLR5 and analyzed two wing sections (mine custom ones):
    1 - an asymmetric section with thickness 10% and camber 3.8%
    2 - From the section #1 I've pulled bottom curves to top ones that produced asymmetric section with thickness reduced to 0 (only 1% in front, where a mast is). camber became ~9%
    3 - From the section #1 I've pulled bottom and top curves to centerline that produced asymmetric section with thickness reduced to 0 (only 1% in front, where a mast is). camber is ~3.8%

    Analysis (for Reynolds number 200000 and AoA from -5 to +30 degrees) gave max Cl 1.4 for #1, 1.6 for #2 at AoA 7.5 degree.
    For #3 Cl was 1.4 at AoA 30 degree (with high Cd of course).

    Does all this mean that just making a "thin" soft sail baggier will give thrust comparable with a "thick" wing sail of similar leeward curvature?
    If so, why advanced Oracle cats use wing sails?
     
  2. Owen423
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    Owen423 Junior Member

    A few pointers.
    1/it's in the rules
    2/ the airfoil generates true lift, not apparent lift
    3/ the airfoil generates significantly less drag.
    4/ the drag reduction translates to less heel in the right circumstances

    For the cup, that translates to bigger sail area on the same size boat. And we all know that bigger is faster ;)

    Fluid testing of soft sails show that the majority of the claimed lift is in fact through deflection of airflow on the inside of the sail, so it is apparent lift... Something that held early aviation back.

    The interaction of head and main together, makes the airflow across the lee of the mainsail detach less, and reduces drag.

    The stay mounted headsail has significantly reduced detachment aerodynamically on the lee of the sail, in comparison to a mast, with a subsequent reduction of drag, because of its fine entry. This makes it more efficient, but not as efficient as a true airfoil, and explains a quest for more aerodynamic mast profiles.

    It also explains the developed cutter sailplan, twin headsails making the system more efficient, again. Not a lot unknown to the old designers, Bourne from sheer experience....


    Owen
     
  3. markmal
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    markmal Junior Member

    >2/ the airfoil generates true lift, not apparent lift
    >3/ the airfoil generates significantly less drag.

    I also read about that.
    But, and it is why I am asking my question here, my analysis shows that the 0 thickness foil generates a little bit more Cl with same Cd!

    Both sections, thick (#1) and thin (#2) sections have max Cl (1.4 and 1.6) at AoA 10.
    Both have same Cd at this angle. But Cl of the thin one is 0.2 greater.

    You can reproduce this if you create "thin" section from a "thick" one same way I described in #2.

    Thick section
    [​IMG]

    Thin section
    [​IMG]

    Analysis. Blue line is for Thick, Green is for Thin
    [​IMG]
    Yellow for #3 (thin to centerline).

    PS/I am researching for a windsurfing sail. So multiple sail rigs are not applicable for my case.
     
  4. tspeer
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    tspeer Senior Member

    First of all, congratulations for actually running some numbers to answer your questions about aerodynamics!

    You are right that at a given operating point, a thin section will out perform a thick section. The reason is simple. When you add thickness to a camber line, you raise the velocity on both sides. Higher velocities mean more skin friction. A higher peak velocity means it is harder to slow the flow down without separation. So, from an aerodynamic point of view, thin is good.

    Thick sections have an advantage when you need to operate over a wide range of operating conditions. The thin section will develop a pronounced leading edge pressure peak on either the leeward or windward side when operating outside of its design operating condition. Whether this is a problem or not depends on the operating range you need and your ability to reconfigure the thin section to match the operating condition.

    And thick sections provide the structural stiffness needed for the spar. So thick sections can result in less weight.

    The wing sails allow the torsion loads to be taken directly into the spar via the control system cables. A soft sail has to react those loads through leech tension, which acts at a much smaller angle, resulting in far higher loads. So there's an order of magnitude reduction in the loads applied to the boat by the sheet, with corresponding reduction in weight. The lighter loads on the sheet also mean it can be trimmed rapidly, which makes the wing suitable for maneuvering on a short course. And, finally, the slotted flap provides a higher maximum lift, which makes the boat faster downwind.
     
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  5. markmal
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    markmal Junior Member

    Thanks for explanation.
    What I do not understand with thin section is where they get lift power from?
    Thick section has longer upper curve and shorter bottom curve, so velocity of air is higher on the upper line, that (by Bernoulli) creates lower pressure.
    With zero-thin section the length is same. So velocities should be same. Lift should appear from deflecting air down only. Am I right? That should increase drag. But from charts I see same drag on AoA 10. However lift of thin section is higher.
     
  6. philSweet
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    philSweet Senior Member

    The air that is cleaved at the leading edge does not rejoin at the trailing edge. This is a very common misconception. The length of the surfaces between the forward stagnation point and the trailing edge has no direct bearing on the lift produced by the foil. So the airflow at the top IS traveling faster than the flow underneath. The variation is referred to as the circulation induced by the foil. In simple terms, you can view the flow as being composed of two components superimposed. One is a uniform potential flow along the path of travel; and the second is a circulation of flow around the foil. The shape of the camber generates the circulation, and this provides the velocity difference.

    This from a non aero guy. The aeronautic folks here could say it better. The math is usually presented in terms of field equasions. Not something you just pickup in an hour if you're not already familiar with them.
     
  7. markmal
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    markmal Junior Member

    So, the main thing is not a difference in upper and lower lengths of a section, but rather is the camber? I felt this. The thin section (in my analysis) has more camber. This camber makes upper flow go faster?
    Is it like when you bend a thick piece of rubber, outer part stretches and inner part compresses? If we take air instead of rubber, pressure of outer layers should be lower and inner higher. Is this illustration correct?

    If upper and lower flows do not rejoin, what fills the gap? Is it the circulation from lower flow to up and forward against upper flow?
    So they rejoin but somewhere on trailing part of upper side?
     
  8. philSweet
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    philSweet Senior Member

    Not quite what I meant. See below for a visualization. The flow rejoins at the trailing edge if it is sharp. That is a very important point (sorry for the pun). If it is not sharp, a great deal of lift may be lost. But the flow from the upper surface arrives before the flow from the lower surface.


    http://en.wikipedia.org/wiki/File:Karman_trefftz.gif

    it's from the wiki on "lift", which is worth a read.
     
  9. tspeer
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    tspeer Senior Member

    You are right, and you've discovered why the longer-distance-on-the-top explanation is totally wrong in every way. It's wrong because it doesn't account for thin sections or the lift of flat plates. It's wrong because even for flat-bottomed sections it doesn't predict the right value of the lift. It's wrong because the air on the suction side actually gets to the trailing edge before the air on the pressure side. It's wrong because it doesn't account for the influence of angle of attack. It's wrong because it doesn't explain why surfaces experience stall. It's just plain wrong. I don't know why it continues to be spread, when it runs so counter to even basic observations.
     
  10. Petros
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    Petros Senior Member

    This just points to the fact that fluid mechanics is not intuitively obvious, and why it took so long to develop a controllable aircraft, despite materials suitable for a glider being around for several thousand years. It is also the reason why so much sailing terminology is obsolete with regards to the operation of the sail, old ideas still lingers prominently among sailors.

    The lifting force is produced from accelerating a mass of the fluid (in the case of sails the fluid is air). This is the famous F=ma, there is no escaping it. A thin cambered airfoil accelerates more air, with less drag, than a thick one. That is why a thin section will always produce more lift than a thick one. It is as simple as that.

    On a conventional main sail, about half to two thirds of the flow is separated off the surface, the upper third, the area behind the mast, and most of the trailing edge if fully separated and contributes nothing to lift, it just creates drag. I have considered a foil shaped mast for a number of years, just to eliminated these large areas of separated flow. Much like your profile above shows. one of the big challenges is to accurately control the shape of the fabric part of the sail. Battens, down hauls, etc will help, but I think a completely different type of control will have to be developed to keep a thin fabric section in the proper shape to optimize camber and twist on a sail. I have a few ideas but I want to build a test sail for a dingy to try it out to see if it is actually practical.
     
  11. CT249
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    CT249 Senior Member

    Is that actually the case? Personally I don't think that I have ever seen a CFD or wind tunnel pic showing such a high degree of separation, nor do tufts show such a high degree show in a real-life sail that is properly trimmed.

    Mikko from WB Sails has shown with his CFD work that the airflow behind a conventional mast is nothing like as bad as some theories have claimed. I note that Marchaj, for example, ran tests using a round mast section that is vastly over-sized compared to anything seen in racing boats. Since we are dealing with such difficult areas as non-linear flow surely this creates enormous room for error.

    As some of us here have noted, Mikko's work reinforces what many, many years of real life experience with many hundreds of rigs in development classes have shown - that is, "foil shaped" masts really have very little if any advantage in most classes. That's the simple facts of the matter. One way of testing this is to take a popular wing-masted one design out and de-rotate the mast. The loss of performance is very small - so small in fact that for example in one very fast catamaran, de-rotating was a common depowering tool whereas no less a theoretician than Frank Bethwaite would de-rotate his Tasar mast at times.

    I'm not knocking "foil shaped" leading edge rigs, for I own about 12 of them, but there is an enormous amount of experience to show that the advantages are often minor and normally not worth the disadvantages, which is why so many classes have tried and then abandoned them.
     
  12. markmal
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    markmal Junior Member

    I've read article "How Airplanes Fly: A Physical Description of Lift"
    http://www.aviation-history.com/theory/lift.htm

    It says that lower surface does not play much role in lift generation.
    The upper surface diverts flow and the reaction to its mass accelerated down is lift.
    For me it is still not clear a role of lower surface.
    I thought it should "bulldoze" flow under it, slowing it down, that should create positive pressure, and the pressure pushes lower surface up - lift, and back - drag.

    Or, by analogy with "flow bending" explanation from the article, lower surface should also (like the upper surface) deflect flow down, and the reaction to its mass accelerated down is lift. And the reaction to its mass slowed down is drag.

    Is my understanding correct?
     
  13. ancient kayaker
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    ancient kayaker aka Terry Haines

    SailRocket has an asymmetrical foil but it only sails one-way . . .

    Speaking of the influence of the mast, some model aircraft use turbulators to reduce drag, such as a wire ahead of the leading edge of the wing which tolerates a wide range of alpha; I understand these devices function by delaying separation. Perhaps a mast can be designed to reproduce the effect.
     
  14. DCockey
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    DCockey Senior Member

    "Turbulators" on a model airplan wing cause an earlier transition of the boundary layer to turbulent, and a turbulent boundary layer is more resistanct to separtion than a laminar boundary layer. Very similar to the effect of dimples on golf balls. But the benefit is very Reynolds number dependent. At higher Reynolds number the boundarly layer transitions will transition to turbulent sooner and the benefit disappears, and the "turbulators" can cause higher drag.
     

  15. CT 249
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    CT 249 Senior Member

    Turbulators or "vortex generators" were used on the mast of Gretel or Gretel II in the America's Cup 40+ years ago.

    The idea has been used since and IIRC, Frank Bethwaite's "squareback" wing mast follow a similar theory. So that's about 2000+ times that the conventional sailors of the world have been onto this over the past 40 years.
     
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