rotor balance

Interesting balance job. New Safari and initial balance run. Blades rough
tracked at 300 rpm and a balance run made skids on concrete. Added some
washers using the "Add Opposite" rule but not much change and phase angle
stayed pretty much the same. For an interest, put the helicopter up on
rubber to cancel out any reflected vibrations. What do you think? The same
ips but 180 degree change in phase. Did the skids on concrete create a
reflection that gave the balancer a 180 phase shift?
Got a good set of vibration absorbing rubber pads with 3/4" plywood tops.
Will repeat the test. One thing about helicopters, I don't get bored.

Sti