Articulating Foil Tail

Discussion in 'Hydrodynamics and Aerodynamics' started by viking north, Apr 3, 2013.

  1. viking north
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    viking north VINLAND

    Since i'm housebound with a sinus cold that is about to blow the top of my head off, not enough energy to spit, and hopped up on over the counter cold remidies, what better time to ask this crazy question. If for some reason the appendage could not be extended in order to taper it's trailing edge within acceptable specs,but there was opportunity to fasten a fixed hard rubber extension, would this improve exit flow and reduce drag. My gut feeling says yes but considering the tail piece will flex transversley with the flow possibly it will create more problems than it solves.
    Note this for sure has been covered on the forum in the past but I am at a loss as what to search under.
     

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  2. gonzo
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    gonzo Senior Member

    It would fair the aft end. However, it is hard to envision a rudder where you have space for a rubber edge and not for a hard one.
     
  3. viking north
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    viking north VINLAND

    Not a rudder Gonzo -- an appendage such as a low aspect keel or a skeg and not even necessarly a foil shape but slab sided with some shaping on the leading edge but for some reason or other not enough tapering on the trailing edge was possible. Seems I have read an engineering study on this somewhere but where ????
     
  4. viking north
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    viking north VINLAND

    Oops --maybe in my foggy state "articulating" was not a good choice of words -- lets reword that heading " Fixed but Flexiable" hard rubber tail piece.
     
  5. PAR
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    PAR Yacht Designer/Builder

    This would improve the efficiency of the foil to some degree, as the rubber would articulate with pressures, but it wouldn't make huge gains on your low aspect appendages. If this is going on the back of your keel, where you need to taper it down, but have other issues preventing it, it's an option, but I don't see how a stiff rubber keel extension would be effectively attached or faired. If you do, make it as stiff as you can (maybe foam filled). Under power, it would help in front of the prop, under sail it would bend a little to the low pressure side, which isn't a bad thing at all.
     
  6. viking north
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    viking north VINLAND

    So it has more positives than negatives --interesting -- I have no specific intended use right now but It's another tid bit I will jot down in the build book. It's actually an idea I've been thinking about since i got hooked on foil shapes and flow patterns. My math. level limits a total understanding but i've always wondered if we mimic living flesh would it not better influence a cleaner exit for the flow. I.E. Have the flow influence the shape rather than the other way around. I have no doubt rubber density would also play role. Possibly it might be a way to fool the flow into controlling it's own cleaner exit. Attachment might be tricky but if one wanted to experiment a simple setup as shown on the doodle below should keep the transition from solid to flexiable clean while at the same time facilitating easy changes for trials on different rubber densities. Oh if I were only a younger more educated man the fun I could have with rubber :D
     

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  7. PAR
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    PAR Yacht Designer/Builder

    I fail to see the need. If you can mount this, why not a laminate as a portion of the appendage?
     
  8. Sketchy
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    Sketchy Junior Member

    blunt trailing edges do have some advantages sometimes, unless you have a very sharp trailing edge, the flow from the pressure side of the aerofoil can creep round to the suction side and thicken the boundary layer leading to an earlier stall. most aircraft even transonic aircraft actually have a blunt trailing edge although some finer than others, some even have the thinnest section forward of the trailing edge before thickening and a blunt trailing edge. This gives a double recirculating vortex behind the trailing edge keeping the flow off the wing smooth. Basically the same reason why pick up trucks are more aerodynamic with the tail gate up than down.

    http://en.wikipedia.org/wiki/Schilling_rudder
    [​IMG]
     
  9. viking north
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    viking north VINLAND

    PAR i have no planned use for it --it is just something i've wondered about weather it would actually be more effective than an actual solid tapered tail section with a sharp cutoff. Can't seem to find any info on it so I thought i'd throw it out there. Possibly after a long cold winter and now this dam head cold i'm going shack wackey :D
    Sketchy i was aware of the blunt (sharp cutoff) an early forum education when I started to play around with foil shapes. A rule of thumb I think (Mr. Marchaj) states it's dimension can be up to 10% of the cord length.
     
  10. gonzo
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    gonzo Senior Member

    A flexible edge may flutter creating a lot of turbulence. However, there would be a minimum amount of pressure for that to happen.
     
  11. sottorf
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    sottorf member

    Adding a fairing to a blunt trailing edge will theoretically reduce the drag IF the rubber fairing that is added is flush with the surface of the profile. The sharp leading edge will allow full pressure recovery along the foil reducing the profile drag. Secondly the sharp trailing edge will lead to a smooth separation of flow from the trailing edge without the generation of a so-called "vortex street."

    I do see a potential vibration problem with adding a flexible rubber trailing edge. Even on sharp trailing edges there is vortex shedding where the vortices shed alternatively from the upper and lower surface of the trailing edge. The frequency of shedding depends on the geometry as well as the flow velocity. If the shedding frequency corresponds with natural frequency of the trailing edge, the foil/rudder/skeg will vibrate and if the vibration is in the acoustic frequency band it is usually called "singing."

    Given your trailing edge is made of a plastic/rubber, the stiffness of the material will be quite low hence the natural frequency will be low too. You could therefore pick up a problem at relatively low speeds. You could delve into complicate maths and try and calculate it all out to determine if there will be a problem or not but much easier to just go and try it out and see what happens.

    If it does vibrate, it can lead to fatigue cracking so i would not recommend leaving it like that even if it is not in the audible range.
     
  12. sottorf
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    sottorf member

    thanks for the information. Does anybody on this forum have further information on this rudder. Any technical papers?
     
  13. viking north
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    viking north VINLAND

    Sottorf yes I can see this happening and tuning out one natural resonance could simply lead to another. What a fun project for some up and coming engineer to see if such a trailing edge could actually be developed to the point where it would be superior across a band of hull speeds. A sort of self adjusting flow enhanser. I can see the headlines now --rubber keeled boat wins the Americas Cup.
     
  14. P Flados
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    P Flados Senior Member

    Over at

    http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/what-significance-wing-thickness-45383-6.html

    Starting on page 5, is a string of posts where I describe foil sections I am considering for wing. However, the results can also be applicable under the water to some extent (stay away from cavitation speeds). On page 6 post 85 is pretty much a summary.

    It shows that a thin pointy flat surface rear half behind a big fat blunt forward portion can be very low drag. If you use the rear half as a big flap, you can get very good L/D values. If you make it a "double flap" arrangement you can also get very respectable lift values.

    As shown on some of my earlier posts in the referenced thread, I was looking at blunt rear edges for a while early on. I was getting pretty good lift and decent L/D. Eventually I recognized that they just do not get down to the really low drag values and you really do want the low drag number for the low lift conditions that are a reality at times.

    FYI, I probably like XFLR5 more but must admit that http://www.mh-aerotools.de/airfoils/javafoil.htm was a lot easier to just jump in and start using. His web page has decent "just getting started" info and Javafoil can be run as a web application with no downloading. I recommend giving it a try.
     

  15. viking north
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    viking north VINLAND

    Thank you sir, while my math skills are limited I discovered a long time ago there are many keys to unlock the understanding and measuring of concepts. For some reasom I have become fastinated with foils and their dynamics in liquids. I have always been attracted to early aerodynamic shapes of automobiles of which happily modern automobiles are returning to. Thanks again for the references to your postings and the application program. ---Geo.
    P.S. Been in your neck of the woods many times --beautiful country
     
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