I agree with Matt .. you're plane is not trimmed .. you need to adjust the Clevises. center your trim controls then adjust the clevises to put the elevator and rudder back to center.
being a owner of a PZ Super Decathlon i'm fully aware or its short comings and would never recommend it to a beginner.
its basicly under powered in its stock form ..
if you have a flying field with long ( one to two feet tall ) Grass i would recommend taking it there and removing the landing gear to fly it .. it will fly a bit better with out the landing gear with the stock motor ( less weight to pack).
and crashes will result in at worst a broken prop rather than a plane that will fit loosely in a small bucket
