ORIGINAL: carrellh
I've seen several guys using 11x4 props on .46 engines with good results. My brother's TT46Pro on a Big Stik 40 performed better with APC 11x4 than anything else he tried. A lot of people like 12x4 or 12.25x3.75 props on their .46 engines but my brother never got good results with them on his. Maybe his engine was "weak" but it didn't like the heavier prop loads.
An 11-4 on a powerfull BB .46 like an OS FX, requires carefull throttle management. At full throttle in level flight it will permit the engine to wind way past its torque peak, which is doing nothing very usefull & is abusing the engine. On the other hand -- when going straight up it is doing what it should !!!!!!
A 10-6 is a break-in prop for a 46 FX & it is just like an 11-4 in engine load, but it will produce less thrust, is not great for vertical performance & is less suited to a trainer than the 11" props. It will let a 46 FX wind its guts out without particularly usefull results.
Prop the engine so that its revs fall in the band between its torque & power peaks (closer to the torque peak is better for a trainer airframe). With a 46 FX, that means an 11-5, 11-6, or 12-4. If you don't have adequate ground clearance for a flat pitch 12" prop, & the 11" prop you need for landing braking won't keep the engine adequately loaded, try an appropriate 11" 3-blade, such as an 11-4 3B.