One thing that people forget to mention, is clearance. I saw someone flying a Nitro Cessna 337, and the size of the props made it difficult to taxi and for landings and takeoffs because of the tendancy for the prop on the rear engine to hit the ground. The front prop demonstrated the same problem. He considering 3-blades on both engines because they can be downsized by 1 inch. Some efficiency is lost, but the power will still be sufficient to make the craft do what's intended. I think a 3-blade would work well for an Avistar if a rough grass runway is used.

NorfolkSouthern