I am quite happy to report we have finally solved this vibration saga. The culprit appears to have been propeller pitch–i.e., pitch setting and/or blade synchronization.
Determining this was a long (6-week) and costly journey–but a great learning experience! Another plus was that several issues which could have caused serious trouble down the road (er, sky...) got identified and fixed along the way.
This is what happened. About 8 months ago, we retired the airplane’s original Woodcomp SR200 propeller after 912 hours; my mechanic installed a new 3-blade prop of the same make and model. But soon, I began feeling sporadic, spontaneous high-frequency (HF), vibrations through the rudder pedals. These usually manifested shortly after climb-out, as throttle was reduced; they would often go away (or become intermittent) in response to further throttle changes (up or down).
These HF emanations should have been obvious clues, but we did not immediately relate them to the new prop, because a lot of other stuff was also done to the airplane about the same time, including nose-gear repairs, a valve job, exhaust-system repairs, gearbox overhaul, and other stuff.
Anyway, these HF vibrations gradually appeared more frequently. This in turn caused low-frequency (LF) vibrations to emerge due to the resulting wear and tear to the engine mounts and gearbox. Moreover, before too long, the number 3 exhaust pipe started cracking repeatedly (on its fourth weld, now, and holding, thank goodness). Bottom line, I had three troublesome issues: HF vibs; LF vibs; and a repeatedly cracking exhaust (with its attendant sound and vibration issues). I was not a happy camper...
The turning point in the dilemma was when I decided to carefully re-set the prop pitch up a notch and to re-synch the three blades more accurately (using a homemade laser device) than before. This showed that the journey marks on the blades and hub are very coarse indicators; using them alone to set the pitch as outlined in the AMM almost guarantees a set of wobbling blades (i.e., with different “bites”). Also, my mechanic had set the new prop’s pitch to achieve 5250 rpms at WOT on climb-out, whereas my original prop (which was smooth-as-silk for 912 hrs) pulled 4900-4950 rpms. So, I also decided to pitch back up a hair to where the original prop had been; I got close with a WOT climb-out of 4970.
Anyway, to conclude a long story: With those prop changes, I’m now flying smooth and vibration free again. And I have new engine mounts, a shimmed-to-tolerance gearbox, re-built carbs, new throttle and choke cables, at least a half-dozen carb synchs, and several other less costly goodies, to boot! All things that needed to be done anyway.
So now, I think maybe I can finally start to enjoy the airplane’s next 1100 hours–and getting back to making my videos (“Sting Flight”). But, I guarantee one thing: If I get those kind of vibration symptoms again, I’ll be investigating the prop first–not the last–thing! Might save myself a ton of money and frustration... :) :) :)
June 6, 2013 post script: I felt compelled to come back here today and clarify something. I reported above (and maybe in another thread) about using a laser pointer to do a very accurate prop pitch--the idea being to project the laser dot a few feet away, where small changes show up as big movements of the dot. This does work quite nicely--but only with one very important caveat: your prop blades must first be in perfect track. If not, you will be correcting prop track with prop pitch and may end up with a bigger problem (i.e., out-of-synch prop)than you started with. There, I feet better and will be able to sleep now, just knowing that no one will make this critical mistake!
