I have had my 68" prop up to 6600 Engine RPM (2.17 PSRU) at 17k' (about 15 deg OAT - which makes the speed of sound slower..  i.e. increases you mach #) and didn't notice anything unusual.  Then again, I wouldn't know the sound of supersonic tips if I heard them.  I regularly cruise at 6000 RPM.

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Dave Leonard
Turbo Rotary RV-6 N4VY
http://members.aol.com/_ht_a/rotaryroster/index.html
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