PC12 pilot experiences propeller vibrations at 15;000 FT in icing conditions and requests descent. Other problems had been experienced with this propeller; which is a five blade design manufactured by MT Propellers.

Narrative

Upon level off at 15;000 FT MSL for cruise; I encountered propeller vibration while in icing conditions. I immediately asked for a lower altitude and reduced engine power at which point the vibration went away. Center at first would not clear me down to an altitude I desired unless I declared an emergency due to other aircraft. I told them if that was what I needed to do to get lower; I would. They eventually cleared me down to 5;000 FT. During my descent; I encountered VMC conditions with no precipitation present at 11;000 FT. I told them I wanted to maintain that altitude as long as I could. When engine power was returned to cruise setting; the vibration was no longer there. There was no indication of deice boot failure on the prop. The aircraft had experienced prior issues within the last 1-1/2 months with the prop deice; with numerous parts being replaced in the system. The aircraft has a 5 bladed MT Propeller installed on it. The STC installation was completed in January 2011 and the propeller had accumulated around 700 hours on it since installation. The propeller blades were scheduled to be replaced by MT Propeller in December; 2011 due to the owner not being satisfied with erosion wear on what is considered the first generation blade. In addition to erosion wear; 2 blades had experienced bubbling in the composite blade; which upon inspection I was told that it was sap coming out of the wood core. When the propeller blades were replaced to the second generation blades; I asked MT Propeller to have the deice boots on the old propeller blades checked. Three out of the five blades were found to have resistance values lower than what was considered normal and in my opinion; could have been the cause of my propeller vibration in flight.I believe that the installation of this propeller on this aircraft needs to be researched further. For example; with the standard 4 bladed Hartzell propeller installed; when the power is reduced to flight idle the blades would change pitch to a lower value. The 5 bladed MT propeller does not on this aircraft and when power is reduced to flight idle during landing; it seems to still carry too much power. To reduce the amount of power; the condition lever has to be brought out of flight idle to ground idle. This is abnormal and is not something done with the standard 4 bladed installation.