Originally Posted by
nrpetersen
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.