Hey guys, I tried a 13-4 apc today on my K&B 61 and was NOT impressed. It turns the same rpm as the 12-6, but has no pulling power. Most noticeable was the lack of climbout from a hover. It only went up half as fast as before. Top speed on the model was way down, too, but that was to be expected. To me, it does better with the 12-6 in every respect, as long as you are prudent with the throttle it won't overspeed. I may try a 13-6 next to see what effect that may have. Not all props are the same, and this 13-4 is not a wide blade, and in fact the tips are very skinny. A master airscrew 13-4 might do better. I only had enough time for one flight, so no other testing was done.