Was very interested by the discussion, Bryan and Dave. (Two people I hjave a lot of respect for) And I, being a tech type, immediately searched the internet for spiral slipstream references. Saw a good video with smoke.

http://www.youtube.com/watch?v=H8nKz...eature=related

This shows the smoke in flight, I don't see the spiral. Full scale pilots reported many times the oil and soot patterns on the fuse, indicating spiral, but it occurs equally and in the same direction for multi-engine planes with props counter rotating. Also interestingly, there is a shortage of actual data. Meaning, none. There was ONE report "Stick and Rudder" which simply made the assumption, without quantifying or proving the effect of spiral slip stream. Saw plenty of props in water, didn't seem relevant to me since there was no wing between the prop and spiral. Nor does slip stream theory hold up since similar effects are seen on pusher prop applications.

"P" effect, however is greatly quantified. I don't know, me. Would like to see some actual data. Seems interesting, would justify the large expense for contra setups if proven. If not, then it's either "P" effect, wing tip deflection or just a fad. It would be nice to know for sure.....

Brian Clemmons