ORIGINAL: carrellh
In the booklet (TT), or sheet of paper (ASP), that came with the engines I own; both TT and ASP list 11x7 as the recommended prop for break-in and general usage on their .61 two strokes.
Is the manufacturer wrong? Maybe yes, maybe no.
I know the 12x6 and 13x4W worked well on my trainer; and the 11x7 didn't work the way it needed to for that particular plane.
Carrell,
Like some other items (... Clint Eastwood - Dirty Harry)), everybody has an opinion...
But, read my explanation thoroughly.
I do agree that an 11x7 prop is great for break-in on this engine and on most other .61s.
For general use, however, it is the wrong prop for most sport .61 engines.
For the prop to be suitable it must fulfill two different tasks simultaneously.
1. To have the engine spinning under full load in the fattest part of its power curve.
2. To have this happen with the particular model at its designed typical flying speed, with the effective pitch.
Vintage pattern models would maneuver at high speeds and their engines would peak around 14,500-15,500 RPM.
That would make an 11x7-11x7.75 suitable, since the engine would spin at its most effective RPM range and the prop blades would pull well at high speeds.
Current sport engines peak at a significantly lower RPM...
So if you were to be flying a vintage pattern model (high speed) with a contemporary, muffler equipped sport engine, the right prop would possibly be an 11x8, 11x9, 11.5x8. This would have both tasks fulfilled on this particular model.
If it is a sport model that flies more slowly, the venerable 12x6, 12.5x6, 12x7 would do great and for a slow bi-plane, or a 3-D model, the 13x4W would do wonders.
But try the 11x9 for the 3-D model...
For general use, the 11x7 is the wrong prop.
It simply cannot load a contemporary, muffler equipped sport engine sufficiently and the engine would just over-run at high speed; like selecting your car's transmission's low ratio on the highway...