Yeah, the 5' radius was "advertising talk", but if you've ever watched someone fly a good control line pattern you'll question the 20' radius as well.

As for "What works really well on controlline stunter has no bearing on what works well on a RC bird" that's not really any more truthful than the 5' radius claim. The discussion was whether or not the OP was a workable idea, to have the thickness remain constant while the chord tapered. Using the CL stunters turning ability as an example most certainly wasn't a claim that everything from CL stunt would cure everything wrong with any RC bird.

If you've ever watched a good flyer (Bob Hunt would be a good choice for sure.) do a pattern then you can ask yourself what radius was required for that square loop you most certainly saw him fly. The top line is 45* from the pilot's shoulders. He's usually flying 60' lines. Figure the top line of that square loop is going to be roughly 33' from the pavement. The bottom line better be around 5' from the pavement (or the judges are going to score you badly). That means the top to the bottom of the square is about 35', and the top line will be a little less than 30' long while the bottom line will probably be a bit more 35' too. (the plane is tracking along a semi-sphere, btw)

If you've ever seen that square loop flown, you'll admit that it looked like the plane flew farther in those 4 sides than it flew in the "square corners". It really does fly a square looking "square loop". And the square figure eight looks just as impossible. So now think about what you saw. There isn't even a hope those maneuvers have room for a plane to get two 20' radius turns bracketing the straight lines you saw flow, is there. Think about what you're seeing.

You're watching a 5.5' guy standing out there. The plane comes straight down at 90* toward the pavement and suddenly is going parallel to the pavement about level with that 5.5' guy's shoulders. You're amazed while you watching, and danged if you didn't see a maneuver that looked square. If he had a plane that couldn't do better than 20' radius turns, his square sides would not have existed. The turn out of the top of the square would not have been finished before the turn to make bottom of the square had better be started. The sides of the square aren't long enough for a 20' radius stunter to get two turns stuffed into the maneuver with room for some vertical.

The 20' radius is bull****. You can't fly square loops or square eights on 60' lines that have square sides with a plane that turns that badly. You probably can't even get round loops squeezed in between your 45* top and a 5' bottom. BTW, if you think I forgot about the plane's room being more than the 39' of AGL the 45* top actually has, I haven't. The distance from the stunter at 45* to where it will be flying 5' AGL is around the semi-sphere and is a bit more than 40'. You can't get two 20' radius turns into that distance with anything approaching 'straight sides to your loop'.