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Driving a flapping wing

  1. Oct 11, 2006 #1
    I am trying to consider a design for a lightweight flapping winged aircraft, powered by an electric motor for each wing.

    If you took a wing that is attached to the fuselage by some large 'hinge', then if you replaced the pin in the hinge with the rod from an electric motor (geared right down), would that be an inefficient way of driving the wing up and down?

    It is hard for me to explain, but I mean, would it be better to push the wing up and down a few meters out from the hinge, so that there was not so much bad effect from leverage?
     
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  3. Oct 11, 2006 #2

    DaveC426913

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    Digression: On TVO last night, I watched a special on birds. It showed how hummingbirds have a completely different mechanism for flight than other birds. They beat their wings in a figure 8 pattern, but what's really intersting is that on the backstroke, their wings pivot right over and they get as much lift from the backstroke as the forestroke. To accomodate this, their wings are symmetrical in cross-section - i.e. they provide lift whether right-side-up or upside-down.

    How this would apply to your situation is that
    1] you would oscillate your wing horizontally/longitudinally, rather than vertically.
    2] your wing would be free to swing about its pivot point within a less than 180 degree arc (actually: 180 - (2x your angle of attack)). You wouldn't even need to control it, it would flip automatically with each reversal of stroke.
     
    Last edited: Oct 11, 2006
  4. Oct 11, 2006 #3

    DaveC426913

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    In direct answer to your question (and I am no engineer), it would be easier (and thus require a smaller motor) to operate the movement at a point farther away the hinge point. A motor that could directly turn the hinge would need a lot of torque.
     
  5. Oct 11, 2006 #4

    brewnog

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    This has been attempted before, with some miserable failures.


    Yes, it'd be easier to apply your force far out from the pivot. However, moving your wing up and down repeatedly is not going to get you airborne.

    Go away and study the flight of birds to understand why!
     
  6. Oct 11, 2006 #5

    DaveC426913

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    What makes you presume he doesn't know this?
     
  7. Oct 11, 2006 #6

    LURCH

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  8. Oct 11, 2006 #7

    brewnog

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    Perhaps me being partially arrogant today, and partially me doubting whether someone who doesn't understand turning moments is capable of working on the design of an aircraft.

    Sorry, bad day.
     
  9. Oct 11, 2006 #8

    Danger

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    I wonder if one large motor might be more efficient that 2 small ones. If you extend a rod through the hinge point to the interior, it could be linked to the one from the other side and both run by a single crank. That would give you your leverage factor and simplify the mechanism.
     
  10. Oct 11, 2006 #9

    DaveC426913

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    (This occurred to me too, I just didn't have the cahones to say it out loud.)
     
  11. Oct 11, 2006 #10
    I will re-phrase my question:

    As someone who doesn't know much about mechanical engineering, and plans to eventually collaborate with all types of experts, as well as learn all the relevant concepts (in time), could someone point me in the right direction on this issue to save me from researching dead ends.

    I want to be able to quantify just how bad it is to move a wing at it's hinge compared to moving it out from the hinge, further along the wing say. I have briefly read about fulcrums and levers, but I'm still having some trouble working out how much power I would lose with this method (besides loss from very low gearing needed).

    I will look at turning moments in more detail as suggested, thanks.
     
  12. Oct 12, 2006 #11

    Danger

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    I haven't had a chance to check Lurch's links, so forgive me if this has been covered. Your fulcrum length will not give a direct relationship of power to movement, because additional length in that area will result in a higher reciprocating mass. The extent of that increase will depend upon the size and density of the material involved. In that case, you'd be best off using aluminum or composite tubing for your linkages (even cables perhaps). You want the lowest mass attainable that can still provide the physical properties required for the endeavour.
     
  13. Oct 13, 2006 #12
    I think reciprocating mass should not be a problem as the wings will pause after the upstroke and also after the downstroke. I imagine the motor bursting power for the first half of the downstroke and then idling while the wing's intertia reduces.

    At first, I wanted to have an electric motor with its shaft driving the wing at its hinge directly, with the motor's shaft as the hinge pin.

    For that configuration I am trying to calculate the forces involved but having difficulty. If the motor's shaft is 1 cm in radius, then do I use that value when calculating the turning moment? E.g., a motor rated at 200 Nm torque would give a turning force of 200 Nm = F x 1 cm, so F = 200 Nm / 1 cm, equals 20,000 N. So the rod would be turning slowly but at 20,000 N? (all with the proper gearing).

    Please forgive my lack of experience in the subject. This is just a small interest of mine so any replies are appreciated.
     
  14. Oct 14, 2006 #13

    brewnog

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    If your motor's peak torque is 200Nm, and your rod is 10m long, the force you'll be able to apply from the end of the rod will be 20N. Get it? This wouldn't be enough to pull the skin off a decent rice pudding, let alone raise a wing. Think of trying to open a really heavy door by pushing it right next to the hinge.

    However, the motor's peak torque will be at its rated speed, ie when it's spinning. Connecting your motor directly to the wing, and expecting it to oscillate is just not going to happen. This isn't what motors do.
     
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