I'm really late getting in on this thread - someone told me about it a couple days ago. I have been flying a Push / Pull style plane since 1997 and have come to some conclusions about the sizes of props and their composition. My plane uses two .25 motors swinging 9x6 props. Here's what I found in my experimentations with different props. Please visit: www.dawnaviation.com for more information and photos.

I use the same size props on both ends of the plane. The pitch of these props is also the same. I found that using wooden props (especailly on the rear engine) is not the way to go. The tips of a wooden prop when mounted on the rear engine will dance around fairly violently. I didn't like what I was seeing plus it caused some unwanted vibration. Using a plastic or composite prop STOPPED this problem completely. The tips of the prop stayed in a single arc and the vibration was eliminated. For this reason, the props I use are APC 9x6's on each motor. I will add that I tried a couple different brands of wooden props and they all had the odd tip movement on the rear engine. I also tried using props of different diameters between the two motors but didn't find any advantage to doing this.

Another thing I found interesting is that with both engines running together, the RPM of each engine increases about 500 RPM. For example: If I take a Tach reading on the front engine running by itself, it might read 12,000. I then run only the rear engine and it also runs at 12,000 RPM. When I start both engines and run them up, each engine is now running at 12,500 RPM.

Finally, I see some discussion on the location of the CG. On my plane, the CG is located right at 25%. The plane rotates easily and will fly vertically for as long as I point the nose up.