Even though I understand and am a big fan of dynamic power plant balancing, it never ceases to amaze how much difference it can make in an airplane that you have gotten used to before it is balanced - or rebalanced. Generally speaking, once you've done a good job of dynamic balancing, the airplane should stay smooth until something changes - a prop removal or overhaul, significant engine work, or a prop leading edge cleanup. But things wear and change gradually over time, and the slight change in balance goes un-noticed as the airplane just reverts to what we'd call "factory roughness" - the amount of imbalance that is acceptable for a mass production job, but that just isn't quite as good as it could be.
We recently had the prop overhauled on our RV-3, and that included new blades and some work on the hub - so I expected that we'd need to re-balance it. It had last been hooked up the machine about 400 hours and three years ago, so we borrowed a Dynavibe Classic from a friend, and hooked things up. Sure enough, we were off by 0.40 IPS on the first try - nor horrible, shake your teeth rough, but certainly not where we'd like it. A couple of runs brought us back inside the generic target of 0.10, and since it was cold and starting to rain, we quit at 0.050 IPS. It was noticeable smoother on the ground, but the rain was turning to snow, so we buttoned her up and put her away instead of flying.
I took her out this morning for the first time since the weather had cleared, and the difference was once again remarkable! It's the old (probably apocryphal) problem of boiling the frog - if you gradually turn up the heat, it doesn't know that the temperature is rising. With a slight degradation over time, we don't realize we are losing balance. Then we reset, and all the smooth magic is back!
Watch Kitplanes for an article in the next year on the process of dynamic balancing, and see how easy it is to do it yourself (if you have access to the machine), or better understand what is being done if you watch someone else perform the process. Either way - its well worth the effort to have that Smooooth feeling back!