RE: OS 91 VRDF experts wanted
An APC 10-10 isn't meant to spin that fast either - 19k tops according to APC safety warnings (190,000 divided by diameter). Cutting down a wooden 11" prop is a good idea for bench running so you have lots of hub strength. Not ideal for a flight prop but it will get you in the air. Also note that for setting up the pipe is it normal practice to use your intended flight prop cut to 85% diameter to allow the engine to achieve flight rpm on the bench and thus allow you to set up the pipe to optimum length on the bench.
It is standard practice in prop speed applications for 15cc engines to run CF props only of no more than 9" diameter, typically in the 9-10 range, or smaller/higher pitch.
Sources:
RocketRob (CF 9-10 based on trimmed/reshaped APC 10-10 I believe)
These two have props designed for AMA D C/L speed which are pretty much what you want:
Steve Wilk / Eliminator props
Mike Hazel / ZZ Prop
and there are European sources at EU $18+ per prop and shipping [:-]