RE: prop for a WM Zen 50
Well, I guess no one has a Zen 50 with that engine.
I'd be using a 12x7 if the engine was a YS63 or Saito .72. This would put the max RPM on the ground in the region of 11K or so.
A two stroke .50 probably wants to be at around 14K, depending on how you've piped it. So a pitch of 5.5 would be just right. Then find the right diameter to load the engine to the desired RPM. I'd be trying an APC 11x5 or 11x6 without knowing anything about your engine. I do have a Webra Speed 50, and on the Zen I'd be trying 11x6, or 10x7 to start with.
Dropping out of knife edge probably means you're not going fast enough, or your rudder servo is not strong enough.