For my wife's trainer we had a situationwith the prop for a 90 4-stroke (Magnum) ona 60 size trainer only allowed for a 1/2 clearance on pavement. I decided to go to a 3-Blade prop for two reasons...

1) The 2-bladed prop pulled very well and would cause flutter in straight level flight at full power. Since my wife is learning, throttle management isn'ta natural reflexand the 3-bladed prop slowed the plane down.
2) The other reason was due to prop strikes on the softest landing and using an inch smaller diameter provided another 1/2 clearance...No more problems.

I went to a 3 bladed prop with a 1" smaller diameter and the same pitch (13 x 8 to a 12 x 8). Performance was not as good due to engine RPM being down. I lost a lot of power as I couldn't fly the Intermediate Pattern Sequence with her trainer any more. However, It does have more power than it needs for a trainer and she is more comfortable as it's more forgiving regarding the airspeed with throttle management.

Larry Diamond