ORIGINAL: Woody218-RCU
No wonder, an 11x7 is too much prop for a ST 51, they like to be loaded a little less than that.
Woody,
The .51 is supposed to be the bored and stroked engine of the .40-.51 ST series.
In short; a torque engine!
But it is not.
For other engines of similar size, like MVVS .49, Irvine .53 and even the OS.46FX/AX, props as large as 12x7 are recommended.
It seems this engine is like the Webra .50GT, which when used with a Bolly 10.5x7, bogs down to unspeakably low RPMs and when the load is reduced to a 10x7, it is freed to over 15K...
These engine have their torque peak around 13,000 RPM.
When a slightly heavy prop does not allow them to get to this level, the RPM goes down even more.
Using a prop which is a bit too large is dangerous for such an engine, just like using a tuned pipe, which is a bit too short... The engine unloads dramatically and needs more fuel, as flight speed increases. But no extra fuel is available, after the engine was peaked on the ground....