Either my OS 75AX is a bit small for this airplane or my engine is not developing power. This engine seemed like more than enough power for my Ultra Sport 60. That airplane weighed a pound more than this one. This airplane does not climb vertically with this engine and initial climb is a bit slugish. I did crash the Ultra Sport, but the engine seems OK. I am trying different props now. Started with a Graupner 13.5 X 8. That prop is quite bulky. Next was an APC 13 X 8. I will now try an APC 13 X 7. This engine makes a knocking noise, but it has done that since the very first run. I have heard other peoples OS 75AX engines, and they seem to make that same noise.