OS 1.20 Surpass III w/pump question?
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Prop the engine so that full throttle when on the ground is in the mid-to-high-9,000's. Any propeller that will give you that RPM range and fly your airplane is the "correct" propeller. Don't worry about "scale" size or blade count. You can try an APC 15 x 8 prop, which is just about the best all-around prop for the FS-120 Surpass III. A Master Airscrew 15 x 8 may be OK, too. That will turn nearly 10K due to its blade profile and airfoil, and may work out better. You just have to try different props and see.