RE: Goldberg Sukhoi engine
I had the similar Goldberg Extra 300 and the original design did have flutter problems. I've seen 2 lost to flutter, including mine. The problem was with the suggested split elevator pushrod. It was split in the back, "pickle fork" style and just wasn't stiff enough. After the rebuild I went to three servos in the tail with short straight pushrods and eliminated all the problems. Separate pushrods or a split pushrod that is split in the front with good straight runs would also work.
I had an OS 1.20 fs in it and while it flew the plane well, it definitely could have used a little more power. I wished I had put my 1.08 in it.
Dave