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Thread: Hummelbird Engine Vibration Problem - Need Help??

  1. #31

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    Quote Originally Posted by nrpetersen View Post
    I'm surprised the Hummel 1/2 VW engine can be balanced with only only a single predetermined offset weight. Given the amount of engine modification (i. e. cutting away 1/2 of it!) theoretically I would think there would have to be a second offsetting weight on the rear of the engine to get reasonable dynamic balance as well as static balance.

    There are two components to every balance situation, in-plane (static) balance and - moment (dynamic) balance. (In addition, there can be 1/ rev propeller blade pitch errors, which would have to be handled by installing tapered shims behind the prop.)

    True dynamic balance is not obtained with simply rotating everything and measuring at a single point. It cannot be achieved by having the correcting weights in a single plane. Dynamic balance involved having two motion sensing transducers, one near the prop and one near the rear of the engine so that yawing and pitching of the engine can be sensed.

    Also something is funny about an unbalance situation in which adding a starter ring gear (which should be balanced by itself) makes it run smoother. And pure torsional vibration affects the cyclic stress in the crankshaft, and may even be acoustically sensed, but is generally is not reflected in the pilot's seat.

    This doesn't address 2/rev vibration, which is a big contributor to torsional vibration. However, there isn't much that can be done about that.
    I believe you need to speak with this man..

    http://www.betterhalfvw.com/engine.htm

  2. #32
    Sam Buchanan's Avatar
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    Quote Originally Posted by nrpetersen View Post
    OK that passes the theoretical test. A single offset weight will work for both static and dynamic balance - providing it is properly located along the axis of rotation. I'm still surprised (amazed?) that just happens to be immediately behind the propeller.

    IOW there may still be a first order unbalance.
    Field history has proven this method to be reliable and yields an acceptably smooth engine. Keep in mind we are talking about a very short crankshaft.

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    The Hummel version of the 1200cc engine uses an additional bearing on the prop hub (not shown in the image above, that is a different crank) that beefs up the front bearing considerably.
    Last edited by Sam Buchanan; 02-20-2014 at 08:18 AM.
    Sam Buchanan
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  3. #33

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    Yes my engine is a Hummel. But nothing special about that crank. That is a standard crank cut in half. If you look at the end without the taper it should be machined to except a impulse coupler but nothing special about this crank.

    Tony

  4. #34
    Sam Buchanan's Avatar
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    Quote Originally Posted by 1600vw View Post
    Yes my engine is a Hummel. But nothing special about that crank. That is a standard crank cut in half. If you look at the end without the taper it should be machined to except a impulse coupler but nothing special about this crank.

    Tony
    Tony,

    Reread my post, I specifically stated the image was NOT a Hummel crankshaft. I used the image only to show the length of a 1/2 VW crank.
    Sam Buchanan
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  5. #35

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    That is the same crank. Nothing special about a Hummel crank. Its a standard crank cut in half. Here is a pic of the bearing you speak of. Along with the other end machined for a Impulse Coupler

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  6. #36
    Sam Buchanan's Avatar
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    Sigh......I hate to belabor the point but the image I used is a 78mm Scat crank:

    http://www.vermilyea.com/engine.htm

    The 1200cc Hummel/Casler engine (45 hp) has an 86mm crank, and I'm pretty sure it uses Chevy journals in order to achieve that huge stroke within the confines of the VW case.

    Once again I apologize to readers for continuing this thought, but having incorrect conclusions drawn from my posts is a pet peeve. I hope this clears up the confusion.....again....
    Last edited by Sam Buchanan; 02-20-2014 at 09:55 AM.
    Sam Buchanan
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  7. #37

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    If you really want the scoop on these cranks and any other part of this engine do not ask here. But do what I did..Call Scott at Hummel, he will set you straight. No guess work. As to the crank, Scott did not tell me any of the things Sam mentioned. Does it mean he is wrong. No. Just that Scott did not tell me any of this when I spoke with him about these cranks.

    I would send you more pics but Scott has my engine.

    Tony

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