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Rotor Balancing
I just had a strange one. I have been balancing my rotor system using a DSS
balancer where I would get the ips and phase angle measurements, turn the rotor to the angle with respect to the photo cell described by the phase angle information, then sight over the velocimeter and add weight to the rotor system on the Opposite side of the mast. This Add Opposite has worked time and time again on the main rotor. Recently I did my annual condition inspection and went completely thru the controls, re-adjusting every thing and carefully zeroed out the lead/lag, checked the head shift and then did a hover check. Results 0.96 ips @ 270. Well with the arrangement I have with the velocimeter, the photo cell and the reflective tape on the swash plate this mean that most of the inbalance was chord wise. I added weight to the side Opposite and flew again. Results: 1.96 ips @ 274!!! I couldn't believe the datat. For giggles I reversed the weight to an Add Same condition and got the following results: 0.26 ips @ 184. What little inbalance that was left appeared to be span-wise. Question: What could cause the phase angle information to flip 180 degrees from what I was used to seeing? The only thing that I'm familiar with that causes the 180 flip with all things else being equal is to be operating above a critical speed and the rotation rate hasn't changed and the mass in the rotor mast and head hasn't changed. Further I added some washers to the blade indicated in the 0.26 ips measurement using the Add Opposite technique and it appears that I should have used the Add Same again. So there seems to be some strong consistancy in the mysterious 180 degree phase flip. I expect the Add Same on the tail rotor as I have checked it out and found a critical speed below the operational speed which will produce the velocimeter being on the side opposite of the inbalance weight. |
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