wiggle drive propulsion application

Discussion in 'Propulsion' started by SudorracMechEng, May 7, 2012.

  1. Boston

    Boston Previous Member

    Easy there Frosty ole been, lest we each end up where we started again.

    lets back up to this fin drive thing again. If I remember someone mentioned an efficiency somewhere in the 85%+ range for the fins, yet props are at best in the 70% and as I recall thats for some really large slow spinning props. So what about it Sudo, your kinda a new voice here, whats your take, and can you give us some examples or comparisons between similar sized drive systems but using the two forms of drive, IE fin vs prop ?

    also what of the spring steal idea, if pitch is important for acceleration then wouldn't a simple spring steal fin tend to want to "follow" the correct pitch as long as it was "sprung" to the correct pressure for the given application, work without any additional mechanical losses ?
     
  2. SudorracMechEng
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    SudorracMechEng Junior Member

    Mr Efficiency: In terms of further development into the field of propulsion, there is much to benefit from oscillating foils. Ofcourse, conventional propellers are more practicable given the current state of how thing stand. However, that is universally known - my argument was never that oscillating foils is more practicable - since it is not commercially available. My argument is that there are many efficiency advantages.
    Also in terms of this thread, its application is for a surfboard. Thus a conventional propeller is clearly not practicable - oscillating foils can utilize the inherent fin shape that already exists. Modify this, and allow it to propel a surfboard.
     
  3. SudorracMechEng
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    SudorracMechEng Junior Member

    Boston - i have a number of summarized research reports i could PM you.
    They basically outline the experimental procedures, along with the result summaries and graphical representations of comparisons to conventional propellers.

    Would you like the reports FROSTY? You havent presented facts yourself, just insults.

    In regards to the spring steal idea: Research has suggested that for efficiency gains in oscillating foils, flexibility has a key role - with flexibility being only a small percentage of chord length. There were many studies conducted using standard NACA foils, however they used homogeneous materials. For optimal efficiency, heteregenous materials are needed. Current studies are underway developing a stiffness profile for optimal efficiency.
    So, i do think spring steal could be a plausible idea - however more complex fins would be optimal. :)
     
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  4. Mr Efficiency
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    Mr Efficiency Senior Member

    What about the pulse effect ?
     
  5. SudorracMechEng
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    SudorracMechEng Junior Member

    If pitching and rolling kinematics are correct, there will be no pulse effect.
     
  6. spork

    spork Previous Member

    Not so much.

    The prop on our downwind cart is 90% or better. The prop on Drela's human powered boat will certainly be better than 70%

    http://lancet.mit.edu/decavitator/
     
  7. Boston

    Boston Previous Member

    on the spring steal idea, what if the verying thickness of the foil could serve to moderate the flex as needed, IE doesn't need to be a consistent taper, does it ? Just enough so the trailing edge makes a better "push" Kinda like a wing mast and sail, but a lot stiffer.
     
  8. swashdrive
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    swashdrive Junior Member

    On the subject of sprung fin
    I have found from experiments and in the evolution of our swashdrive unit that a rigid fin works best with a swashplate. The loading and unloading of the fin is taken care of within the swasplate cyclic motion.
    I also found that flexing fins tended to fatigue and finally bust, the lateral forces are a pulsing force on the fins !
    The foil shape is more critical, it needs to equal the turning change of direction, if that makes any sense ?
     
  9. Boston

    Boston Previous Member

    it does but I'm not sure what that translates to in terms of how you modify the foil to match the angle of attack.

    One thing that I find really appealing about this thing is that the fin can be rotated up from the vertical position in shallow water, unlike a standard prop. Although the forces wouldn't offset as nicely and you'd probably get a bit of porposing to the ride, that could probably be managed simply by going slower, which is likely more advantageous in shallow water anyway. I'm considering a cat and don't want to give up the advantage of shallow draft.

    So whats up with the flex fin idea, unless I've misunderstood you guys, its pretty much up in the air. I got one vote for it and one against.

    I guess I'd go with a rigid fin then since its less complex. But its a great idea as a larger fin/prop makes for greater efficiency, but with a prop your stuck with the draft, the fins could rotate up under the bridge deck and paddle along adequately in shallow waters. Of course there's a leverage consideration and thats where the question about the bearing came in. A props leverage on the individual blades is offset by the symmetrical arrangement but with a fin, its all on the bearing. Kinda critical to get that bearing right or with all that moment its a goner.
     
  10. SudorracMechEng
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    SudorracMechEng Junior Member

    Swashdrive - can you elaborate on "it needs to equal the turning change of direction"

    Rigid fins are good for thrust (if they hold together haha). In nature, animals have variability in the stiffness of fins. They will be flexible, to efficiently cope with the static thrust - once at higher speeds they will be rigid.
    Basically, efficiency (static starts) -- flexibility; thrust (rigid) --> animals integrate the two.
     
  11. Boston

    Boston Previous Member

    foil section vs stall angle

    fatter foil = higher stall angle

    I think, might be the other way around, depends on what you consider higher

    either way this is a great idea, someone really should try and build a full size test bed ;-)

    someone mentioned a 30° swash plate angle/arch so if I understand that measurement correctly its the angle between the peak and the valley on the plate, the fins "leg" would follow this path and at its maximum pitch be at something like this 30° figure, depends on how long the contact area is between the contact end of the fin and the diameter of the plate.

    Great idea though, really liked that model in the duck pond.
     
  12. swashdrive
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    swashdrive Junior Member

    im not saying rigid is the rule, just that is my conclusion for the observation, given some good materials, modelling, experimenting etc would probably find the flex better. I'm no hydrodynamisist just a toolmaker, it more about the mechanics of it for me !
     
  13. swashdrive
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    swashdrive Junior Member

    What size do you consider full size ?
    Tiny size or tanker size, it is defiantly scalable !
     
  14. Boston

    Boston Previous Member

    well I'm considering a 37' cat, so for me, I tend to think anything between 30 and 40 as full sized. ;-) I'd like to see something under a 5' fin length at that size. Top speed of something under hull speed, I'm not in a hurry, and able to rotate up under the bridge deck, which seems very doable. Who knows but I keep thinking that I know how hard it is to manufacture a descent prop, but a fin is going to be much easier. The foil section is the pain in the but part, other than that it would be a simple flat piece of metal. Which leaves me wondering what the section looks like ? The prop needs to be balanced, but a fin, not so much.
     

  15. swashdrive
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    swashdrive Junior Member

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