It is like the other replies ,Your either tail heavy,or you might have to put a few degrees of down thrust on your motor mounts. You never want to fly tail heavy. When you build,remember that for every 1oz.in tail,you have to add about 3oz. in the nose. If I remember right. Please correct me if I'm wrong. One more thing, If your tail heavy, you will probably have to land under power,or allitle hot.