are my servo horns too long? Should I use the metal ones that came with the servo instead of the plasic ones?

Thought of additional detail that may or may not have contributed. I had set up the radio so the flight modes (launch, cruise, land) would only operate the prop in launch position. And, when in launch position, the entire trailing edge was a a few millimeters down (both aileron and flap). I was flying around in this mode (launch with flaps and aileron slightly down) because I didn't want to have to start flipping switches if I suddenly felt I needed to power out of trouble. So, when it fluttered coming out of shallow dive, prop not powered, the aileron and flaps were still down a few millimeters neutral. Could this contribute to the flutter I experienced?