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Airfoil Musings

Discussion in 'Sail Trim with Don Guillette' started by BobbyFunn, Oct 4, 2018. Add this thread to a FAQ

  1. BobbyFunn

    BobbyFunn

    Joined Apr 16, 2017
    404 posts, 165 likes
    Federation NCC-1701
    US Riverside


    In my quest to understand and appreciate sail trim I've set up a test rig to record telltales in various angles of attack.

    Its hard to find good videos of foils underwater. Most videos are of the smoke streams or are computational cgi.

    I found a broken rudder in the garbage and figured it's size might be perfect for testing. It appears to be from some hobie or small dinghy. It is actually designed for water so tests in water should alleviate concerns about scale.

    Got it home, cleaned it up and then placed telltales on one side of the bottom half. The telltales are a combination of cassette tape and the outer shell of sta-set double braid. The doublebraid fibers were flea combed to spread the fibers out. The mylar is taped on, the fibers are hot glued.

    I choose the Sarasota Hydrodynamic Testing Basin, aka, the community pool. The video was shot with the potato 2000 gopro knockoff in 720p.

    What I observed was interesting and informative with a couple applications.

    1. The rudder cut through the water like nothing, literally a blade. The blade could be directed up or down and the response was instant. While slicing through the water up and down the telltales lay flat everywhere.

    2. If while moving the rudder is angled 20 to 45 degrees the rudder simply changes direction. There isnt any noticable effort to twist the blade, just a quick deflection and the blade is going in the new direction. This is what i would expect a rudder to. If however, I angle the rudder, but do not allow it to go off in the new direction it starts acting more like a lifting wing. This is a common scenerio when the sail plan is not balanced or too much rudder is applied given the hullspeed. The rudder is turned to compensate, but the hull is providing resistance.

    3. With the rudder at about 30 degree angle of attack the leading edge telltales luff first and as the rudder slows down or angles up higher the rest of the telltales all jump up and luff.

    4. Just like a boat coming off a plane, there is a 3D wake behind foils. As the wing slows down abruptly from the drag, the chaos behind the wing catches up and overcomes the telltales, reversing the flow momentarily.

    5. The same experiences were observed with an air mover fan in the garage.

    Lessons learned: Turbulent air is as good as laminar air, if the turbulance is small, big swirls are bad.

    Stalls are first noticable at the leading edge. There is almost no grace period between turblent leading edge and turbulent trailing edge. Its laminar, some leading edge stall or complete stall.

    Seems telltales really only need to be on the immediate area 0-10% from leading edge of sails. If these leeward telltales are luffing, disaster is imminent. Once stalled, the wake of air from behind will rush in. Do everything to keep the boat on course and lighten up the angle of attack to prevent the wake from catching up and making it difficult to retrim and get going again with that big turbulence surrounding the sail.

    I question if leech telltales have any value. If they are luffing the sail was stalling a long time ago.

    Keep rudder and sail movement consistent with speed. If boat is moving slow, let rudder carve a clean line, quick movements that dont allow the hull to respond will result in drag and stalls.
     


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  2. David in Sandusky

    David in Sandusky

    Joined Nov 8, 2007
    1,123 posts, 169 likes
    Hunter 27_75-84
    US Sandusky Harbor Marina, Lake Erie
    Leach telltales have a different purpose. Not to show turbulence, but to show when the sail is over- trimmed. Easing the sail with leach telltales ducking behind it will give more forward power.
     


  3. All U Get

    All U Get

    Joined Oct 2, 2008
    2,728 posts, 402 likes
    Pearson/ 530
    US Strafford, NH
    You need a wave pool like some universities use for research, then you could get a grant to cover all your expenses.
     


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  4. BobbyFunn

    BobbyFunn

    Joined Apr 16, 2017
    404 posts, 165 likes
    Federation NCC-1701
    US Riverside
    Ummm err, hoa fees, new wide angle camera with underwater case.

    No problems though since im going to be rich with a new patent for telltales. Everyone is going to rip off their existing telltales for my design. By everyone i mean sailors, who use telltales, ok i wont get rich...

    I noticed that the double braid shell fibers had a spring like shape from being wound. This prevented the telltales from sticking.
     


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  5. Will Gilmore

    Will Gilmore

    Joined Oct 19, 2017
    3,480 posts, 1,602 likes
    O'Day 19
    US Littleton, NH
    That is cool stuff and your assessment and conclusions make sense. It would be great to put dual cameras on to see what each side is doing concurrently.

    -Will (Dragonfly)
     


  6. Daveinet

    Daveinet

    Joined Sep 20, 2014
    924 posts, 170 likes
    Rob Legg RL24
    US Chain O'Lakes
    at the same time you need to have a strain gauge to actual measure the lateral force gaaist the rudder at different angles of attack. Ultimately we don't care about what the water flow does, but rather the force applied. Know what the water does only tells us why it is happening, not how much effect it has on the process.
     


  7. Jackdaw

    Jackdaw

    Joined Nov 8, 2010
    9,652 posts, 2,706 likes
    Beneteau First 36.7 & 260
    US Minneapolis MN & Bayfield WI
    Interesting and cool experiment. But I'm wondering how you came to this conclusion? The lack of laminar flow will create drag, which is bad and something your experiment cannot monitor or detect.
     


  8. BobbyFunn

    BobbyFunn

    Joined Apr 16, 2017
    404 posts, 165 likes
    Federation NCC-1701
    US Riverside
    Heres another low budget test. I was inspired by flat plate drawings on Arvel Gentry's site. Im still interesed in overall cause and effect, especially with telltale placement.

    This test demonstrates an airphoil or cellphoil .

    I like this test for a couple reasons. The importance of leading edge telltales is reinforced. Also interesting, the leading edge appears to be more important than surface shape.

    I like the idea of measuring this. I am barnstorming intuitive and inexpensive methods of comparing telltales, streamlines, and lift. For now it mostly looking at telltales lay flat and still versus lifted and wavering tufts.



    I have footage of a hunter 170 rudder in similar tests. The flow over the top is glorious compared to a square leading edge of phone or even the hobie blade. Also part of the test is a comparison of clean edge versus corrupt edge. Ill post that later on.
     


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  9. BobbyFunn

    BobbyFunn

    Joined Apr 16, 2017
    404 posts, 165 likes
    Federation NCC-1701
    US Riverside
    I am convinced there is very little laminar flow in the wild, if by laminar you mean every molecule is evenly spaced and moving the same speed in the same direction. We've seen near perfect laminar flow of water coming out of fountains. The water passes through sponges, then through long straws, then out of a perfect holes. Wind tunnels work the same way. In nature this is rare, like when water goes over a falls or clouds blowing over a ridge.

    Every molecule is on a path determined by the molecules next to it. I realized this floating in the pool during this off season.

    The clouds move across the sky like leaves in a river. They are different and changing as they move. Swirls are visible and rotating. New puffs form in the swirls and others go away. Within the river, there are smaller swirling tributaries, and within those, swirling streams, and within those, swirling drips.

    That is happening everywhere there is gas or liquid. On water we perceive the river as forecasted wind, the tributaries and streams as wind shifts. Gusts are the swirls advancing, calms are the backside of the swirls. Shifts are the swirl quadrants.

    The only way to make them stop swirling is to confront them. Put a foil in a turbulent flow and it has to obey by deflecting under the bottom. The top has to fill in the vacuum so it rushes over the leading edge and fills in the space. The air meets back up (randomly) and continues the turbulence. A good foil doesnt need laminar fliud. It creates it from chaos and leaves it in chaos.

    What i have observed is laminar scale. A swirl a similar diameter of the foil chord is a stall. A swirl many times the size of the chord is "laminar", and a swirl many times smaller than the chord is laminar.

    There is no way that fan i use for testing is generating parallel laser laminar flow. Thing is the fan provides the same result as the pool water which would be much cleaner. The turbulence is probably massive with the fan, yet a foil with a good shape and angle of attack will temporarly line the air up until its off the surface.

    In other words a good foil consumes turbulance or laminar air, redirects and channels it to laminar over the surface, then ejects it back into the wild as turbulence again.

    Check out this picture of Jupiter. There is nothing solid to create turbulence, yet there it is. Hardly any laminar looking lines.

    I suspect a wing the size of the red spot, flying through the red spot would stall, while a tiny wing, like a hurricane hunter would have no problem flying through it.
     

    Attached Files:



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  10. Will Gilmore

    Will Gilmore

    Joined Oct 19, 2017
    3,480 posts, 1,602 likes
    O'Day 19
    US Littleton, NH
    I think, since we are sailing in a 3D medium, laminar, in the case of a foil like a sail, means following parallel to the cross sectional curve of the foil. Moving from leading edge to trailing edge. The lines might twist and eddy up and down, in directions X and Y, but not in Z. The overall direction of flow is along X, but we are looking for as close to 0 change in Z as possible. There is nothing to provide for uniform parallel flow along Y.
    I have been taught that velocity of flow decreases pressure, but this has to be relative to the relationship and direction of force upon the surface the fluid is flowing against. If I turn on my garden hose, the water is being pushed into the hose and stretches the walls outward, increasing pressure. When I turn my hose off and the remaining water flows out because the hose is running down a hill, the walls collapse because now the fluid is being pulled through. Velocity of flow is unimportant in this case.
    This is well stated, the vacuum has to exist first. Lift is increased by increasing the difference (delta) between the pressure in front of the curve and the vacuum the fluid is rushing to fill on the backside while also increasing the difference in pressure from the underside.
    A Newtonian perspective would suggest the bending of airflow by the foil creates the vacuum by forcing a deflection of the vector momentum of the air passing over the top surface. The air wants to follow straight, but the curve won't let it without forcing a vacuum to form. A stall, detachment of air, happens when the air on the underside can rush around the trailing edge to help fill that vacuum. This would reduce the overall pressure on the underside of the foil and interfere with the flow on the upperside, loss of laminar flow. Turbulence along the upper surface would have a similar effect by slowing airflow and allowing air from underneath to come up over the back edge to help equalize the difference in pressure. Not efficient for lift.
    The leading edge determines how that flow is divided and, ultimately, the difference in pressure between both the top and bottom and the front and back of the foil. It will introduce turbulence or help reduce it.

    What a great discussion this is.

    -Will (Dragonfly)
     


  11. BobbyFunn

    BobbyFunn

    Joined Apr 16, 2017
    404 posts, 165 likes
    Federation NCC-1701
    US Riverside
    That's pretty technical Will. I had to process each sentence and draw it out. Wasnt sure on the xyz system, but i think you were visioning y as mast head, x as outhual, z as draft?
     


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  12. Will Gilmore

    Will Gilmore

    Joined Oct 19, 2017
    3,480 posts, 1,602 likes
    O'Day 19
    US Littleton, NH
    Exactly. Considering the foil's surface isn't a flat plane, it is non-Eclidian, but the axis perpendicular to the foil is Z. A strict definition of laminar doesn't apply because the lines of compression change across the surface, but it should be a smooth, non crossing movement. As the air speeds up on the forward belly of the curved surface, those lines are closer together with greater vector force (moving with direction and magnitude). Lines moving together equal increasing pressure, lines parallel represent a null (constant) pressure change, lines moving away from each other represent decreasing pressure. These dynamics tend to happen in reverse on the concave side of the foil bur the overall effect is an increase in pressure versus the convex side of the foil.

    -Will (Dragonfly)
     



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