I generally have pretty poor aim, but I've had some success flyin' my 'ol P-39D and I even occasionally hit something with that darned cannon. First off, get close, really close. I try not to fire at much over 200 yards with the 37mm. Secondly at first you're probably not leading enough, the shell trajectory is fairly flat but the round is very slow, I'd guesstimate that you're looking at about double what you would do with a .50 cal round. The final thing is to really try to fire down the fuselage rather than at the wings. If you can fire down the center of the target it will give you a much larger margin for error.