I don't see how changing the prop tips would prevent supersonic unless the diameter was also reduced. Supersonic speed would just be a function of prop arc circumference and RPM . RPM X 60 is Revs per hour, this times the circumference in feet is feet per hour . this divided by 5280 is MPH. My guess is if this MPH number exceeded the speed of sound at the altitute you are flying the prop tips go supersonic. Lots of untralights with high reving engines use smaller diameter three blade props for this reason.
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