Stolen Post from another less professional website....
I don't have my manuals at home. A new pilot called me that was flying an OH-6 (MD-500C) at 10,000 feet MSL and had a bad MR vibration at cruise speed of 100 kts. slowed down to 85 kts and it went away. It does not do it at 3000 feet. I told him I thought it was retreating Blade Stall. Anybody else?
My response was the aircraft was exceeding Vne and thus could have been approaching Retreating Blade Stall, some responses had to do with compressibility problems.
The aircraft in question was clean, light, and had a 20R engine vice the -18 that the type normally had. The OAT had to be very cold....event happened a few days ago.
Any ideas?