I've had several of these as well. Like hoops, I also had flutter, but I was using an OS46SF (precursor to the FX) with a tuned pipe.

These are great planes, but the guy that taught me to fly also had flutter in the tail and like me could never solve it. I checked everything over and over. I even checked the canopy to see if that was vibrating somehow.

Even with all that, I would highly recommend going with a .40 or larger. The plane has a lot of ply so they are not light.

They are a perfect 2nd plane. Similar performance to the Ugly Stick planes, but prettier and a little sleeker in the air.

Sig is always very conservative on their engine recommendations. Almost every Kadet Senior I've ever seen has a .602C up to .904C's and that according to Sig is a .40 size plane. I flew mine with a .40 FP and it was barely enough to fly it. I could almost do a loop if I dove hard enough.