I have an Impulse which is a 40 size pattern type model made by phoenix models. The problem IÆm having is if I push hard on inverted flight, to go vertical or do a tight inside loop, without any other control input it does a near perfect half right snap roll. Upright flight, it behaves fine. I only need slight down elevator for inverted flight. I have a small weight on the left wingtip to obtain lateral balance for the engine / silencer offset. IÆve also checked for elevator differential movement, and as far as I can see there is no warp in the wing. What could be the cause?