Safety is a concern in very size models. Once a friend was adjusting a OS 91 four stroke in his new Top Flite AT-6, and 5 or 6 fellows came around the working bench just to see the new one. (they were stupidly confident) I back off to my models, when this guy wanted to check the needle settings with the model in vertical position. So he pointed it upwards and push the throttle stick al, theb way quickly. ...the engine rattle once and prop went off and hit the zinc plank that cover the boxes area. If he would not point it up...3 or 4 guys could be hurted badly in the face or wherever!!!