Question about wingsail foil shape

Discussion in 'Hydrodynamics and Aerodynamics' started by satrams, May 6, 2014.

  1. daiquiri
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    daiquiri Engineering and Design

    You can design a single-section rigid airfoil which can be pretty comparable to a multi-element airfoil in a limited range of wind conditions, but you cannot do it for a wide variety of conditions in which a sailboat will normally operate.
    For example, try to perform the analysis of your foil at Re=500000 and Tom can do the same with his multi-element wing. Then compare the results. You will find out that your airfoil probably won't cope as well in the new condition as the multi-element can. That's because your airfoil doesn't have a possibility of changing the camber and the gap between elements, to delay the boundary layer separation and minimize the wake at various flow speeds and angles of attack.

    And while it is true that for non-competitive sailing it might be more desirable to have a simple and robust solution like a single-element wing, for racing machines like C-class cats (which Jim Caldwell has mentioned) a multi-element wing which can be fine-tuned to varying conditions is a must-have if one wants to be competitive.

    Cheers
     
  2. tspeer
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    tspeer Senior Member

    Simple symmetrical wing sections have been used on landyachts. They have comparable top-end speeds to the flapped sections. The Iron Duck, which was the first yacht to set a World Landsailing Speed Record over 100 mph had such a wing.

    However, their acceleration is much slower. By the time they get up to speed, the flapped wings have already rounded the windward mark and headed down the reach leg.
     
  3. myszek
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    myszek Junior Member

    As you wish :)
    Cl/Cd max 67, Cl max 1.9. Still not bad. I wonder, whether the slotted wing can reach Cl max 2.4 (1.28*1.9) at Re=500 000.

    Anyway, you are right, the slotted wing gives you much more possibilities than the single one. My post was intended to show, that sometimes the simplest solution can also be considered.

    If the max. wing area is restricted, the choice is obvious. But I am even not sure about AC class, where the number of foil sections is restricted to 2. Would it better to use a two-sectioned slotted wing and maintain it manually or a single section one with a tail wing instead of sheet system?
    In the Moth class, with number of sections restricted to 1, the choice becomes obvious again ;)

    Tom, do you have a simple explanation of this? Max. speed of the land yacht is connected with the L/D ratio, so the single foil is comparable. The acceleration is rather caused by max. Cl - is the difference so significant?

    regards

    krzys
     

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  4. daiquiri
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    daiquiri Engineering and Design

    The straightforward answer to the last question is - yes. A two-section wing has a much higher maximum lift than a single-section one. If well-designed, it can also have a better (L/D)max than a single-section wing.

    You need to make a comparative analysis of a single-section vs. double-section wings. Right now you are seeing it from only one point of view. I suggest you to try playing with the JavaFoil applet, available here: http://www.mh-aerotools.de/airfoils/javafoil.htm . It will allow you to analyse both single and multi-section airfoils.
    Do not trust its output blindly, though. All 2-D analysis software, XFOIL (XFLR5) included, still have problems at reliably predicting the Clmax and the Alpha(Clmax). But it is a good tool for comparative analysis and will show you the trends, which is important in this case.

    Also, don't forget to study the theory, if you want to comprehend why do the different airfoil configurations behave the way they behave. In a two-section wing, the aft section essentially acts like a flap (which is not just casually called "a high-lift device"), so if you read a thing or two about the flap theory it will make things much more clear to you. :)

    Cheers
     
  5. redreuben
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    redreuben redreuben

    Disregarding mechanical/build issues, purely from an aerodynamic point of view, would the 2 element benefits be valid if the second element is a soft sail ?
     
  6. Skyak
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    Skyak Senior Member

    A soft second element can help a single symmetric element quite a bit by giving the combination camber. Camber is what you need to power through lower apparent wind.
     
  7. markdrela
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    markdrela Senior Member

    The 2-element airfoil is also much better at Re=500K.
     

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  8. daiquiri
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    daiquiri Engineering and Design

    Well, yes. I can say it with confidence because it is a solution which has been in use for centuries...
    Jib + mainsail, for example. It works pretty much the same way. ;) Or the katch rig - though it doesn't exploit the boundary-layer interaction of the fore and aft sails, just the interaction of their respective bound-vortex systems.
     
  9. redreuben
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    redreuben redreuben

    Slot.

    Thanks daiquiri,
    I figured it was similar interaction yes, so tighten the slot upwind, open it up down wind ?
    Further; I have been wondering if a tall and narrow blade type jib like a leading edge flap set to windward to create more leading edge camber would be worth consideration or is this getting over complex ?
     
  10. myszek
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    myszek Junior Member

    Not very much - slightly better than at Re=1.4M. Anyway, thank you, Mark, for these plots.

    Daiquiri, I am not such a newbie at aerodynamics :rolleyes: and I know that high lift devices increase the max. lift. Sometimes they are necessary, sometimes they are not. Sometimes different approaches to the wing loading and high-lift devices can lead to very similar results. The famous example: Spitfire vs. Bf 109

    Many airplanes don't have any flaps.
    Many have rather simple devices, like plain flaps, split flaps or single slotted flaps.
    Few have more complicated systems.
    None use slotted flaps of 1/2 wing chord.

    When sailing on the water, even a fast multihull rarely exceeds 2x wind speed. This gives a speed range about 2:1 (that's a rough approximation, but useful). In aviation, for such a speed range you usually don't use flaps.

    On contrary, the land yachts can reach 10x wind speed. This leads to the speed range about 10:1, like jet fighters. At max. speed you need really small lift, and obviously as small drag as possible. At small speed you need as much Cl as possible, for the good acceleration. That's exactly the same issue as the fast airplane with a given landing speed. The additional condition is the symmetric airfoil - but it's less serious restriction than it appears.

    The best solution known in aviation is a laminar flow airfoil with effective flaps. Fowler flaps are the best, split flaps are not much worse. Other high lift devices (except blowed flaps etc.) will spoil the laminar flow.

    It took me another hour to draw a symmetric section of Cd=0.0036 at Cl<0.2 for Re=4M, and Cl_max with split flaps 1.95 for Re=400K. This means Cl_max/Cd_min>500. I guess Fowler flaps (you need 2 sets of flaps on both surfaces of the wing) could give you Cl_max about 2.5, which could increase Cl_max/Cd_min to 700.

    The only problem can be connected with turbulence. The performance of the laminar foils fall down in turbulent air, which can be expected near the ground. For ncrit=5 I can obtain Cd_min=0.0035, but for ncrit=1 it grows to 0.01. Then, it's not much less than for the slotted wing, which is better at higher Cl.

    For the slotted wing you can expect Cl_max/Cd_min (at approproiate Reynolds numbers) about 200. For a simple, high lift symmetric foil it was 180. For good old NACA0016 it can be 160 (for ncrit=1) to 180 (for ncrit=5). The 10% difference can be important for very high performance landyacht, otherwise the simplest solution is a reasonable choice.

    That's exactly my point: sometimes one solution is better, sometimes another, sometimes they are equally good. In turbulent air, all you can do is to increase max. Cl. When you can maintain the laminar flow, you can minimize Cd instead.

    regards

    krzys
     

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  11. daiquiri
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    daiquiri Engineering and Design

    Myszek, it is now clear that you quite informed about these aerodynamic issues. But that was not easy to infer from your initial posts though. ;)
    Airplanes should not be used for a straightforward comparison of technical and aerodynamic solutions, because they operate under different airflow conditions, and different operational, safety and structural requirements.

    For example, one fundamental difference between an aeronautical wing and a wingsail is that the latter one needs to work equally well on port and starboard tacks. Hence, it needs a symmetry. On the other side, an airplane has a cambered-airfoil wing right to begin with, because it needs an upwards-directed lift only. That is a very important difference, which guides the configuration choice and the relative size of the two wing elements. Think about it - a wing with a built-in camber vs. a symmetrical wingsail which has to work as if it had a camber (if it is made for high performance).
    Hence, you can intuitively see that the size of the 2nd element on a wingsail will not be comparable to the common size of an aeronautic flap, though they both work through same physical principles. It has to be much bigger.

    Can you share the offsets of the airfoil you have created? Let's try to see what comes out of a comparison to a 2-section wing based on the same airfoil. :)

    Cheers
     
  12. redreuben
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    redreuben redreuben

    Slot Mechanism

    Here's a mechanism I found on a land yacht site, sorry lost the page.
     

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  13. daiquiri
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

  14. tspeer
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    tspeer Senior Member

    For the wing on the Lydia experimental landyacht, we used hinges made by flattening metal tubes and bolting them together. The tubes were inserted into chordwise holes in the flap and wing structures. Pins through the flap and wing held them in place. The tubes had several holes in them for the pins. By sliding them in and out, we could vary the location of the hinge line.

    It was important to be able to vary the hinge linkage on both the flap side and the wing side. This allows you to vary the hinge point while still keeping the leading edge of the flap just clearing the trailing edge of the wing when tacking. You don't want a big gap between the flap and wing when the flap is at neutral deflection, because you want a little overlap when the flap is deflected.

    The picture above doesn't look like a practical hinge mechanism. It appears to be something to hold two templates together in a way that makes it possible to visualize and measure the slot.
     

  15. fusioneng
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    fusioneng Junior Member

    I have been reading everything possible on wing sails for 3-4 yrs now and have a different take on the concept of wing sails verses conventional sails.
    I have made several wing sails now, and they are all of the soft parafoil type variety (basically they have air slots in them that fill the inside with air making the wing rigid).

    My boat is a Hobie Tandem Island pedalboat. I use a combination of all the power sources to propel my hybrid boat. The 33 sq ft wing jib I have mounted on the front of the boat serves two purposes, first purpose being it works as a giant amplifier (just like an air conditioner works) that takes your existing forward motion and amplifies it. Secondly the wing jib organizes the airflow over the main conventional sail to make it more efficient (the main sail will be replaced with a furlable wing sail as soon as I finish it, it's designed, and I have the materials, just a matter of making it at this point).

    By designing everything around this the actual wind (if any) is not a factor in the propulsion of the boat, as it basically creates it's own wind via the forward motion and apparent wind.
    My boat works no differently than a piper cub airplane, on the piper cub the propeller drives the plane forward, which in turn creates lift on the wings, as long as you keep applying power, the plane continues to fly, if the engine fails you fall out of the sky, with my setup if I stop pedaling, or stop my hybrid engines the boat slows down and eventually stalls out, or reverts back to conventional sailing.
    The boat has two sets of mirage pedal drives, if I have two strong peddlers, we can get everything to operate, but unfortunately we run out of energy and get pooped out after a mile or so, thus the reason for the gas engines which provide enough supplemental power to make the chain reactions work.
    The engines are twin Honda 2.3 4 cycle outboards with custom designed super high pitch props installed. A normal Honda 2.3 will propel the boat to 6 mph at wide open throttle (WOT) and the 1 liter of fuel will last one hour at WOT (Honda's published fuel economy, which I confirmed). This works out to around 20-25 mpg with no sails of any kind and not pedaling the boat.
    Operating the engines at just above idle (high enough rpm to engage the centrifugal clutches in the engines) 1 liter of fuel lasts 3 hours of runtime at this throttle setting (per engine).
    In the video below the winds were around 5 mph for the day, my average speed for the day was 8 mph with top speeds around ten mph. I consistently get in excess of 100 mpg with this setup, and at times have achieved over 150 mpg. Because the boat works from apparent wind created by your forward motion, the actual wind direction really doesn't matter, in fact the best performance is almost directly upwind (creating max airflow over the wing).
    The wing is totally automatic, and if not engaged acts like a weathervane with almost no drag of any kind, actually to test it out I went out in 25-30 mph winds and pedaled in figure 8's around the bay, just letting the wing weathervane on the front of the boat 360 degrees.
    The jib in the video is a prototype and has been in service a couple years now with quite a few thousand miles on it, and is pretty near end of life (in other words it saw it's better days quite a while ago LOL)
    Here is a quick video of my setup:

    https://www.youtube.com/watch?v=AaaBSQtlcA8

    My question to the group is has anyone else thought about using wings for any other purpose besides trying to emulate what regular sails and sailboats the exact same way they have been doing it with no changes for 2000 yrs. Thinking outside the box is what I do, Wings work on different principles from regular sails, so why does everyone try to design wing sails to conform to the standard, 'just change the standard' figure out a way to propel our boats taking advantage of wings strong design points rather that try to conform to 2000 yr old stuffy sailing standards and ideas.
    Bob
     
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