Graupner Speed Props
I know guys are spinning these up in the 30's on NovaRossi .15's, .21's etc. in the speedcup circuit overseas. I have few in the prop drawer, the 6x5.5 being a particular favorite for perky .15's.
But doing the math on the safety limits in the Graupner instructions - 3438/(dia in meters) = max rpm, for this prop at 152.4mm dia, I get 22,560rpm and m=0.53 at the tip which seems pretty conservative. Hardly a "speed prop" if the 6" caps out under 23k.The large hub 6" APC's are 31.6k.