I offer this information primarily as a data point. But, I welcome advice and advisories!
Occasionally, my 6 will shudder during the downwind leg of the landing pattern. In smooth air, I think the shudder would continue indefinitely if I did not reconfigure the aircraft. I've let it go for 10-15 seconds, in the past, in order to investigate. It's a steady, consistent, stable sort of a vibration. Nothing is loose. It's like a mild/moderate pre-stall buffet but it seems to me to be at a higher frequency.
So far, this has only ocurred in the landing pattern. I only remember it ocurring with full flaps--but I'm almost always at full flaps in the pattern. 80 IAS seems more likely to provoke the shudder than does 90 IAS--but I'm almost always at 90. Typically, I run 1700 rpm on downwind.
Although the whole aircraft shakes, I've always had the impression that the cause is closer to the nose than the tail--here, I could easily be wrong. I've tried to isolate the problem by altering engine speed. But, something else always seems to ruin the test--rough air, increased air speed, a pitch change, etc. It doesn't take much to end the shuddering.
The 6 has Van's cowling, fat pants, good fitting intersection fairings from Fairings Etc, and average quality wing root fairings per plans. The underside of the horizontal tail has average to good quality fairings per plans. The empennage fairing fit is average where it meet the vertical tail. It is average where it meets the horizontal tail except at the leading edge--there the fit is pretty poor.
At cruise, the elevator counter-weights stick up above the horizontal tail by perhaps 1/4 inch or less. I've not noticed the problem when practicing stalls. I've done a number of stall series with all combinations of flaps and rpms varying between idle and 1700. The stall really is not preceded by a noticeable buffet. The aircraft has always dropped the left wing 10-20 degrees and pitched down steeply. Recovery has been normal.
The engine/prop combination is as follows: GAMI injected parallel 360, Lightspeed III plus magneto, dynamically balanced Sensenich 72FM (and spinner) pitched at 85 inches, Vetterman exhaust, 9.2: 1 pistons. This has run about 200 hours so far.
I've tentatively accepted this vibration as normal. Should I be afraid?
Monte