Hi!
My experience from flying R/C for nearly 40 years is that the OS FP .40 or LA .40 is very capable of swinging 11x6 props. That is flying from sea level. Especially so if used in a high wing trainer like the above mentioned.
Whats importat is that you use the correct prop brand. I have said it before and say it again...there is a vast difference in performance between prop brands and this is a fact many newcomers don't know.
If you want good performance there is no better props than British made RAM, German Graupner G-Sonic or American APC props.
If you use MA white tipped props I can understand your troubble taking off. MA "Scimitar" props are somewhat better though.

It's also important to use the correct fuel,glow plug and not using a large plastic spinner (Balancing problems) and also important...use a Alu spinner nut and suitable plastic mount (Dave Brown makes one of the best). Screwing down an engine to a warped plastic mount t (most are) is not the best way to treat an engine as the crank case easily gets warped that way.

Fuel for your engine should contain 5-15% nitro and around 20% oil (some percent Castor oil).
Glow plug should be OS 8 or Enya 3.
And most important....to set the engine you should rely on your ears!!!!! Hold the nose of the plane straight up and set the high speed needle so the engine rews max rpm...then back off some rpm. How much?...Well you learn that by experience or by getting help from more experienced flyiers in a club.

The small hole up front in the carb is the idle air intake hole and should be set half open . If the engine hessitates when you give full power either open the idle needle some or close it. The OS .FP or LA .40 carb is not that critical on idle needle setting and it runs just fine most of the time with the factory setting (idle hole half open) , at least at sea level. The full speed needle on the other hand must be set
every time you go flying!