Problems caused by overpropping a 4-stroke
I had a Graupner 14x7 three blade on two different airplanes; a CG Ultimate with an OS 120, and Super AeroMaster with a YS 120. Both airplanes had excellent performance, with the AeroMaster/YS combination being simply awesome. If your engine will swing it, a Graupner 3-blade will perform.
Magnum four-strokes , at least from what I have seen, are like Thunder Tiger helicopter engines; if you get a good one, it runs fine. If you don't, you're in for loads of misery.
Steve