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Thread: Hummelbird Engine Vibration Problem - Need Help??

  1. #21

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    Quote Originally Posted by 1600vw View Post
    Washers is not the way to do this, they will do nothing.

    Tony
    Technically that is not a true statement. Adding weights under the prop bolt will have an effect, just not a big one. It is not likely that adding weight under the prop bolts would be enough to get you where you need to be but adding weight will change the force vector and thus the distribution of mass about the crankshaft.

    Just saying.

  2. #22
    Sam Buchanan's Avatar
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    There is confusion due to two different topics being merged together. 1600VW is referring to the crankshaft balance weight that Hummel Engines uses to balance the rotating mass of their large engines. The other topic, the one I was participating in with av-mech, has to do with dynamically balancing the prop using weight (usually bolts and/or washers) added to either the prop spinner or hub.

    The two topics are entirely different and don't overlap--maybe this will clear things up a bit.
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  3. #23

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    Sam with all do respect you are not helping this man. From the pics he posted he is missing a part. That part is the weight I speak of. Until he gets this weight he is chasing ghosts. He will never get this to run like it suppose to.

    Now I do like your post for this could be my problem. I have everything setup just as Hummel states but have a vibration that was driving me nuts without my starter ring mounted to the prop. When I removed this starter gear or ring when I removed my starter then flew my airplane, this vibration although very small drove me nuts. I reinstalled the starter ring and she is smooth as silk, no vibration at all. None.

    I called Scott at Hummel and talked with him about this. He told me his thought was the mass of the ring gear was taking away some torsional vibration I might be having. But I always wondered about an out of balance somewhere. I checked tracking. I would call it perfect. Maybe off by 1/32 tip to tip.I always keep my prop horizontal not sure what else to check.

    Tony

  4. #24
    Sam Buchanan's Avatar
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    Quote Originally Posted by 1600vw View Post
    Sam with all do respect you are not helping this man. From the pics he posted he is missing a part. That part is the weight I speak of. Until he gets this weight he is chasing ghosts. He will never get this to run like it suppose to.............
    Tony
    Tony,

    I have no idea what photo you are referring to unless it is this one of my Legal Eagle that I attached to my earlier post concerning how to balance a prop that has no spinner:

    Name:  engine-22.jpg
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    The crankshaft weight is indeed attached properly in this photo, it is at the 9:00 position but behind the prop so it isn't visible in this photo. Is this the photo you are referring to?

    Here is another image showing the weight on the backside of the prop hub:

    Name:  engine-20.jpg
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    The other photos in the thread are of a complete Hummelbird with a spinner so it would be impossible to tell whether or not the weight is installed.

    I hope this helps to clear your confusion of which threads are referring to the Hummel weight and the others that are referring to dynamically balancing the prop when no spinner is installed (posts #16-19).
    Last edited by Sam Buchanan; 02-18-2014 at 01:34 PM.
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  5. #25

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    Yes, sorry I was referring to those pics. I like how you did that or put the weight on the back side. I might have to try this. Not sure but my cowl might get in the way.

    Tony

  6. #26

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    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.

  7. #27
    Sam Buchanan's Avatar
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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.
    I don't have the answers to all your questions but can tell you the weight is customized for each individual crankshaft. Mine has one corner ground down where the tech fine-tuned the balance and it is marked so it will be installed exactly as it was on the bench.
    Last edited by Sam Buchanan; 02-18-2014 at 10:29 PM.
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  8. #28

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    If I was to redo this or build one myself I would leave the full case or as it is with 4 cylinders. Just remove the back two pistons and rods then block off the jug holes. From my research really no weight penalty in keeping a full case and not cutting it in half. Keeping a full crankshaft should help with the balance.

    Tony

  9. #29
    Sam Buchanan's Avatar
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    Quote Originally Posted by 1600vw View Post
    If I was to redo this or build one myself I would leave the full case or as it is with 4 cylinders. Just remove the back two pistons and rods then block off the jug holes. From my research really no weight penalty in keeping a full case and not cutting it in half. Keeping a full crankshaft should help with the balance.

    Tony
    Not cutting the crank will make the engine impossible to balance. The crank weights are designed to balance in concert with the rods and pistons. Remove the rods and pistons for the two unused cylinders and leave the crank untouched and you have an unusable engine.

    The full-case 1/2 VW is about five lbs heavier than the cut case, but this difference is absorbed to a degree by the need to build an engine mount for the cut-case. Biggest advantage of the full-case version is not needing the extensive machining required by the cut-case. But magneto installation is a bit simpler and lighter on the cut-case.
    Last edited by Sam Buchanan; 02-19-2014 at 08:21 AM.
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  10. #30

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    Quote Originally Posted by Sam Buchanan View Post
    I don't have the answers to all your questions but can tell you the weight is customized for each individual crankshaft. Mine has one corner ground down where the tech fine-tuned the balance and it is marked so it will be installed exactly as it was on the bench.
    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.

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