Twin flapping tail propulsion HPB

Discussion in 'Boat Design' started by jakeeeef, Dec 6, 2016.

  1. Horton HCCI
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    Horton HCCI Junior Member

    Well, maybe, but a few considerations are that a) I'd like to try flexible rather than rigid foils, so two "reversible" leading edges wouldn't seem practical, and b) at speed, my expectation is that the range of foil rotation will be pretty restricted ("sheeted in," if you think of the foils as watersails).

    If I understand falling leaf sculling correctly, the technique is typically used where the "effective" blade moves through a broad arc--say 120 degrees or so--when it reverses direction. So it's a good plan when one is, for instance, using a yuloh to move a junk, very slowly. In that case, the blade is moving largely transverse to the boat's direction of travel. With DaggerRo's configuration, however, I'd expect, at speed, an arc of something more like 45 degrees or even less on the "tack." As the vector of the oncoming water flow moves more and more forward due to the boat's forward motion, the foils need to be pulled more parallel to the centerline of the boat to continue to have an angle of attack sufficient for generating lift. I think Jakeeeef describes this well at the top of this thread. If you looked at it head-on, you'd see foils nearly broadside at slow speed, but as speed increases, the apparent width of the foils as they shuttle back and forth would become narrower and narrower.

    So in this case, a falling leaf action would seem to actually require more rotation of the foil than using a fishtail-type, single leading edge setup. No? Seems like it to me when I play around with a pen.
     
  2. DogCavalry
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    DogCavalry Soy Soylent Green: I can't believe it's not people

    Good points. Falling leaf certainly seems more suitable to slow advance rates.
     
  3. Andrew Kirk
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    Andrew Kirk Pedal boater.

  4. Horton HCCI
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    Horton HCCI Junior Member

    Nice-looking boat! I can see the advantage of a paddlewheel over a single foil flapping on a horizontal axis, or a vertical. I'm convinced two vertically-oriented foils moving transversely in opposition to each other (not "flapping," per se) holds more promise over the single-foil model, but I want to start modeling to test that out. Paddlewheels are a pretty different approach, although proven.

    Curious--why the decision to use rods to turn the paddlewheel, rather than chain and sprocket?
     
  5. Andrew Kirk
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    Andrew Kirk Pedal boater.

    The answer is simple. I didn't want 4 meters of greasy, flailing chain in the boat! It has the levers on the pedal mechanism and paddlewheel at 90 degrees to each other so one rod will always push or pull when the other is in a dead spot where it can't turn the wheel. The downside of this is that I really need to "gear it down". My pedalling cadence is half or less what I'd be using on a bike and it stresses the muscles but never makes me breathless. I might build a mark 2 version with the rods rotating a crankshaft behind the pilot then a short chain so I can use appropriate sized sprockets.
     
  6. Andrew Kirk
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    Andrew Kirk Pedal boater.

    Horton HCCI. You may like to know that my first, vertical tail moved on a parallelogram linkage to keep the angle of attack constant through the majority of the stroke. I then had adjustable bolts to set the angle of attack for up and down independently and the fairly rigid foil flipped automatically at the top and bottom of the stroke. This was the only brief point when the angle of attack wasn't firmly fixed. I didn't do this with my attempt at a horizontal opposing 2 tail version. It would have been too heavy and cumbersome. This system, however, provided very little forwards motion. The varying angle of attack mid stroke could have been the reason for this but since it was so poor I didn't do much testing. Looking at videos of yellow fin tuna (it's possible I need to get out more) the tails look rather stiff so flexible foils aren't certain to be vastly better than rigid. [​IMG] https://mountainbikerhome.files.wordpress.com/2021/03/3a42da5c-fbff-4ac1-b94c-5c3050345862.jpeg?resize=668,668 This was the first time I took it out. The foil was way too small and I later extended the mechanism to give a bigger range of movement. For flapping foils you need to learn about Strouhal numbers and keep within the efficient range of 0.2 to 0.4. I managed this with my first version. It would need an essay to go into details about Strouhal numbers but if you don't know already it's easy to find information via the web.
     
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  7. Horton HCCI
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    Horton HCCI Junior Member

    Good point. :eek:). I like the shaft idea--but why a paddlewheel and not a prop? Not that I love props--I don't--but paddlewheels are pretty crude, no?
     
  8. Horton HCCI
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    Horton HCCI Junior Member

    Well, that's discouraging. I see how the transition at the top and bottom of stroke would be unlikely to provide much useful lift/thrust. What is your length of stroke? My thought on this is that fish have short strokes of pretty fast periodicity, but this isn't a natural tempo for humans. We like relatively long, slow strokes. So if I take the fish as the model, what I'm doing is taking two of them, ripping their bodies off, and then crabbing their tailfins 5 (or 6?) feet sideways, in opposite directions, in about a second, taking a competitive rower's stroke length and a rate of 30/min for a guide. At the end of the run, there's a dead spot of coasting/feathering before the return stroke, which to my mind can't be helped (and maybe shouldn't--humans seem to be designed to pulse, then relax and recover, if rowing motion is an indicator).

    I really can't justify the preference for flexible foils over rigid, other than all fish I know of have at least some degree of flex, even the bony ones like yellowfin tuna and sailfish and lamnid sharks. Swim fins work dandy. Also, flexible foils are pretty easy to make, so something to start with.

    Never heard of Strouhal numbers. Let's see.....ugh. Nope. Just not gonna happen. I'm a mediocre poet, not even a bad engineer or mathematician. I claim I like to feel things rather than calculate them. Not a choice, in my case.

    I have run across this, which I found useful and think I understand:
    https://www.americanscientist.org/sites/americanscientist.org/files/200510384919_866.jpg
    What I do not buy is their claim of thrust. How so? That's not what an airplane wing does, I don't believe. Using any old thing--this, for example--
    https://www.researchgate.net/figure...re-represented-in-terms-of-two_fig1_333045321
    the "resultant force," which is what we're interested in, is behind the lift, not in front of it. It's the resultant vector of lift plus drag, obviously. So I think the fish diagram is baloney, but it does seem to tell me that an airfoil (vertical propulsive hydrofoil, in this case) may want to get as close to pure transverse motion as it can reasonably get, in order to orient the force vector as close as possible to the direction of travel. Flexing should let it do that. I hope.

    A simple test might be to stand in chest deep water and wave a swim fin back and forth, vs. a cookie sheet or just a board. See which pushes you backward harder. Pretty sure the swim fin will, for an equivalent area. Suppose one could try a symmetrical foil of some sort--small dinghy rudder, say. Still think the swim fin will win, but I should try it. And will, in that I'm gonna first try my model with a flexi-fin and then a rigid one. If the rigid one shows more promise, I can go about making a wing out of it, with smooth leading and trailing edges and whatnot. It won't. It will lose, badly. I can feel it. ;o)
     
  9. Andrew Kirk
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    Andrew Kirk Pedal boater.

    Actually, Horton HCCI, Strouhal numbers are quite simple. They are dimensionless because the units cancel out, so it's just a number. It will allow you to know the best stroke length and frequency for a target speed.
    Supposing your desired speed is 2 meters per second, which is 4.4 mph. You've decided on a stroke length of one meter. To know the efficient frequency range you put it into a simple equation.
    St=Stroke length X frequency / speed
    At a frequency of 1 per second St will be 0.5, so too high for efficiency. You'll therefore need to reduce your stroke length or frequency. Simple! A frequency of one in 2 seconds will give St of 0.25, right on the money. Of course if you find in testing that your speed is higher than 2 m/s then St will reduce to the right area.
    It is proven theory so if you want efficiency you'll need to pay it some attention.
    Andrew.
     
    Last edited: Sep 16, 2021
  10. Horton HCCI
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    Horton HCCI Junior Member

    Um, this I don't get. Whose stroke? Conventional oar? Rigid fin? What sort of foil shape? Flexible fin? How flexible? Are you trying to plane, or not? Sorry, but this is just not the way I approach the world. Moreover, that simple equation has nothing to do with what I see on Wikipedia:
    The Strouhal number is often given as {\displaystyle {\text{St}}={\frac {fL}{U}},}[​IMG] where f is the frequency of vortex shedding, L is the characteristic length (for example, hydraulic diameter or the airfoil thickness) and U is the flow velocity. In certain cases, like heaving (plunging) flight, this characteristic length is the amplitude of oscillation. This selection of characteristic length can be used to present a distinction between Strouhal number and reduced frequency:{\displaystyle {\text{St}}={\frac {kA}{\pi c}},}[​IMG]

    Say what? This just for starters, and is way, way way in the weeds given the measurement tools and facilities I have at my disposal. Plus, regardless of which equation may or may not be "proven," why calculate frequency and stroke length, when you can just do it? My proposed design should have a functionally limitless range of stroke lengths, and frequency can be anything the rower feels like or is capable of sustaining. I plan to try it real-world in various configurations and scenarios, starting with many iterations of a crude-ish model(s) to try to get some general observations first, and go from there. I was asking what stroke length you were using out of curiosity, not how I should calculate mine, which in any event I am not going to do, ever, sorry. I find equations and calculations misleading and frankly a waste of time that should be spent prototyping. You take a different approach, and that's fine, but this is my project, and equations are not my gig. Call it stubborn ignorance if you like--I'll live with it. ;o).
     
  11. Andrew Kirk
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    Andrew Kirk Pedal boater.

    If a fish, like a tuna, moves it's fin in a 10 cm sweep it will reach a speed and frequency which places St in the range of 0.2 to 0.4. This way it will conserve it's energy. Why would you build a propulsion system which doesn't take this simple maths into account? It will ensure your system is efficient.
     

  12. Horton HCCI
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    Horton HCCI Junior Member

    Ah, but a tuna does not move its fin in a 2-meter sweep without a body and without a fixed pivot, which is more like what I'm going for. I am going to build a propulsion system which makes no reference to simple maths because maths are a mental construct that attempts to describe known phenomena and make predictions about future performance. I am introducing phenomena that are not known. Maths will not "ensure" anything, in this case. But if people want to make up equations to describe what I discover empirically after experimenting with unprecedented configurations, I won't stop 'em.
     
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