The problem with the staic thrust IMHO anyway. The 10x9 is going to unload in the air. Not that the 10x9 should ever be run on a trainer but in the air it would build more speed with time than say a 12.25x3.75 apc. Thats what it's built for is speed. The longer lower pitch prop is a lot better for a trainer. While it wont have the speed the throttle response will be much better. This is what gets you out of trouble with a trainer. When I buddy box someone I want instant response when I let go of the trainer switch.

David