I have a semi-rigid rotor two bladed helicopter that has a 2/rev vibration that is very low with full fuel. However as the fuel level nears 1/2, the vibration level increases to an uncomfortable level. At a hover, the 2/rev is essentially gone. The 1/rev.balance readings in hover both vertical and lateral are below .2 ips. There is no increase in 1/rev vertical ips with forward speed. I'm sure that the blade c.g. is aft of the pitch axis and I believe that the center of pressure is in front of the pitch axis. Any hints as to what to try to eliminate/reduce the level of vibration?
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