Also, apart from the mathematical point of view, in the real world the speed in the turn and better climb of the 109 will let him do a hi yoyo to get a shot even if his turn rate and radius doesn’t allow him to ‘catch your tail’.
If you are at stall speed on the deck and he is outside your turn but faster he can use speed and climb, pointing the nose up over the circle, rudder into the turn and drop back into the circle. This will let him cut across the circle in the vertical plane using gravity assist to tighten the turn in the horizontal plane. He is still flying a longer way from his point of view, but in the plane of your circle he has decreased his turning radius a lot and cut inside you and pulled lead for the shot.
A way to defeat him is with scissors instead of a sustained turn, the hard part is to keep him down and turning hard instead of doing what he should do and use his climb to extend over your nose.
Turn radius is a very effective tool to deny the attacker the shot by keeping inside his turn, but doing just this will never provide you with an offensive opportunity unless the attacker mess up and pull to hard, trying to force the shot, which seems to happen a lot.
(I think everyone who posted to this thread are well aware of all this, so this is just for the benefit of others)
-Jinx