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1. 12:27 PM - Yak 55M - Change in vibration when swapping props (Royden Heays)
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| Subject: | Yak 55M - Change in vibration when swapping props |
I changed from the Russian 2 blade prop to the 3 blade MTV 250 diameter
prop.
With the 2 blade, general engine to airframe vibration as perceived in
flight from the cockpit was proportional to engine RPM as expected. The
prop was dynamically balanced. Albeit, vibration is a lot more than sitting
behind a Lycoming in a Decathlon. I have never flown any other Yak 55M so
have nothing to gauge. Engine mounts were new.
With the 3 blade and stock M14P governor, vibration was very different from
the 2 blade in the 70% to 85% RPM range. The vibration becomes harmonic in
that range akin to a twin with RPM out of sync. The beat frequency is, I
guess, around 3 or 4 cycles per second (Hz) and increases as PRM
approaches 85% at which point it becomes imperceptible. After a dynamic
balance there was no change.
I wonder if it is the governor? I have read from Richard's website that the
stock Russian governor is somehow underpowered for the MTV 3 blade. But I
don't know the symptoms.
Any comments. Have others had this problem?? Maybe this is just a feature
of changing to an MTV, whether on a Sukhoi or Yak 18T?
Royden Heays
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