ORIGINAL: RCPAUL
There at least used to be a rule somewhere in the AMA rule book that a spinner had to have a minimum radius - at least that is my memory in the two brain cells that I have left. As i recall (here I go again) it was stated to be so many millimeters - minimum - in radius. I think this was there way back in the good old days when people flew Frome spinners, which were very pointy.
Paul
The minimum radius rule is in the Pylon racing regulations... I beleve also in the RC combat regs. It may also be in some of the CL competition regs... There is such a thing as an "AMA Safety nut (which is essentially a large "acorn" nut) But its only required for certain types of competition.
RC Pattern Aerobatics rules have a minimum radius for the LE of the wing... but no spinner required.