I have a super decathalon with 48 mds and I'm having trouble finding a prop that works well. I'm hoping for some advice. I've tried 10X7,11X5,11X6,12X6. It seems like the moter revs but the plane does'nt go anywhere. Thanks in advance for any help.

I think you got something else as a problem, you have tried all the 'recommended' sizes for a 40s size engine and how can you have all that RPM but no oooomph? Have you used a tacho?