Just as a point of order, the motor setup depicted in Paul's last post is a "Contra-Rotating" unit (as used, for instance, on the Fairey Gannet). On twin engine aircraft (where there is one engine/motor on the right wing and one on the left wing...) if the rotation of the props is opposite each other, that is called "Counter-Rotating." I did a lot of research on this when I was building my twin.
Of interest here might be the fact that on Counter-Rotating twins, the direction of the counter rotation is not always the same depending on the aircraft design. For instance the P-38 Lightning was set up with counter-rotating props that had the tops of the props turning away from the canopy (which is just opposite of most counter-rotating twins...). I noticed this on an episode of "Wings" where a P-38 had landed on a tropical island strip and parked. When the engines were shut down you could clearly see the counter rotation of the props with the tops of the props turning away from the canopy.
I did a bit of experimentation with the prop rotation direction on my twin at the suggestion of both Dean Pappas and Howard Rush (Howard even supplied a flow chart for this test program...). Hey, I was going to do that anyway... The results were interesting. At first I had the tops of the props spinning towards the canopy (okay, the "painted-on" canopy...). This worked very well. At the Nats in 2015 I opted to try it with the top of the outboard prop turning turning towards the canopy (reverse rotation to what we normally have on a single glow engine model), and the inboard prop turning in the same manner. There wasn't too much difference, but I did perceive a bit more vertical tension (it was very slight). I flew the Nats with the rotation in that manner. After the Nats I did an adjustment to the area of the inboard flap (adding more area near the tip of the flap with a long tapered piece of balsa) and that cured the vertical tension problem I had been having through the top of the vertical eight. I flew it that way with the prop rotation as per at the Nats, and then with the counter rotation variation with the tops of the props again turning towards the canopy. That proved the best of the permutations tried. I'm sorry to say that I did not continue the test program to include the other two variations (the tops of the props turning away from the canopy, and both props spinning in the normal glow direction.) I recently sold the airplane, so further testing will have to wait until I test fly Frank Imbriaco's new Turning Point twin that I designed. Hey, better to test on someone else's plane anyway!
Dean Pappas may chime in here with some information about the Contra-Rotating setups that are being used in RC these days with great success. I asked him about using the MPI counter-rotating setup and he nixed it, but I don't remember the exact reason why. Hey, I'm getting old, weird,
and forgetful...
Either way - Counter-Rotating or Contra-Rotating - the advent of the modern electric systems and the availability of "pusher" props has provided a whole new world of experimentation possibilities.
Bob Hunt