Arrrggghhhh...ok why not.

You're all right, and you're all sort of right...by the best I can determine. It reminds me a lot of the anhedral stab question...."why, what does it do". Amazing how long it took to get any form of a semi-clear answer, and it's still just a guess at best. The biggest thing it does is lowers the effective centerline of the stab. Secondary it sort of affects the airflow over the rudder ONLY to the extent that the air is splitting higher on the rudder than the effective centerline of the stab. And to the question of why it seems to help some planes and hurts others? It depends on where the stab should have been in the first place

As to the canalizer. I think you're all close on this one. MOST modern designs with current force arrangements and the way we trim them pitch to the gear in KE, so putting it behind the canopy line (and hence CG) is why it affects all designs the same way. The question is how much...some planes are like night and day, and some...ehh a little maybe. Some just don't really like it all that much.

Jim asked what the design deficiency was: simple answer. Shape, from spinner to the back of the rudder, and everything in between, compounded by a spiral airflow. There is mainly a straightening effect that I can tell so far, and that's about it. But it depends on the shape of the plane as to how much difference it makes. I can tell you that the vertical fin like Jim and Dave are using seem to be the large majority of the benefit. What the horizontal piece does, well that's where the can of worms opens. it is my opinion that these should be used for infinitely fine tuning. Nothing wrong with that except they're ugly (LOL)

I have seen it used to excellent effect by Don Szczur and Cameron Smith. by adjusting the little wing they fine tuned the plane's behavior in uplines, downlines etc. Thier's are adjustable. Perhaps the one Cristophe uses is fixed and designed for a specific airframe? I'm just asking, I don't know.

What I do know is the community has made quantum leaps in design fine tuning over the last few years. A crapload of mix used to be the norm on many designs, now it seems if you have over 5% something is wrong. Obviously they're doing something right, but there will never be a perfect plane. However we as pattern geeks will never stop striving for it, and that's what sets us apart I think. Don't you?

-Mike