RE: 4 blades prop
I run APC 13.8x10 four-blade props on the YS110 engines that are on my P-61. They turn 8900 RPM on the ground and unload nicely in the air giving excellent performance. The plane weighs in at 23 lbs. I have also tried this combo on my 8 lb Extra and it was very fast and unlimited.
This is the only engine/prop combo that I have seen work well on a 60-size airplane. The extra blades really take a lot of power to spin. The YS110 gives similar performance to a Saito 150 in a much smaller package and it will fit in most 60-size cowls.
I have pics and video posted of the 'Widow flying with the four-blade props. Click on the link and select the video taken at Warbirds Over Valley: [link=http://www.flytrca.org/video1.html]http://www.flytrca.org/video1.html[/link]
Walt