Dynamic Prop Balancing.

[Paul462]

Amigos,

Last Tuesday we spent five hours (and $400) dynamically balancing the front and rear engine/propeller installations of our 1968 T337C. Prior to this exercise the plane ran fairly smoothly, without any excessive vibrations. We used a Mircovib II, state-of-the-art digital balancer, and obtained print-outs of the results. The engines/props were balanced at cruise power setting of 25 in/2400 RPM.

Both engines/prop installations started out around .25 inches/sec; the front installation was reduced to .018 in/sec; the rear to .012 in/sec (in improvement of greater than a factor of 20!). There should have been a noticeable reduction in cabin vibration.

During test flight yesterday the airplane shuddered during the take-off run, kind of a low-freq vibration (we never felt that before), and then during climb-out at 28 in/2600 RPM there was a new, sharp vibration at about the engine frequency. During cruise vibration levels appeared somewhat greater than usual - the 496 mounted atop my glare-shield vibrated slightly, which hasn't hapened before.

Any thoughts, everybody? Has anybody dynamically-balanced their planes, and what success did you have? Why would a dynamically-balanced plane vibrate more AFTER than BEFORE !?