"Stuart & Kathryn Fields" wrote in message
.. .
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.
If anyone out there is interested in rotor balancing other than me, this is
a follow up to the previous post. The next day I ran another check and this
time I used "Add Opposite" and have the balance down to 0.1ips. I talked to
Murray Sweet of Canadian Home Rotors and he told me to disconnect the
battery of the balancer and re connect it and it often cures the 180 degree
phase flip. On the DSS balancer, I'm going to add a battery disconnect
switch. Hopefully this will eliminate the funny phase data and save me one
run up.