In a previous post I had problems balancing my tail rotor on my Baby Belle
helicopter and received a lot of feedback on possible causes. I've since
got the bugger balanced and discovered a few things about both mechanical
vibrations and some characteristics of my ship. In the process I found a
paragraph in a textbook on mechanical vibrations that went something like:
If you have a shaft rotating at less than the "critical speed" (quotes
added) and hold a piece of chalk up close, the chalk will mark the heavy
side of the shaft. Note when using a balancer, the protocol of "Add
Opposite" would hold. That is if the shaft was positioned with the chalk
mark facing you, the correct procedure would be to add a corrective weight
on the opposite side of the shaft from the chalk mark. However, the
interesting paragraph continued and said: If the shaft is rotating at a
speed higher than the "critical speed" then the chalk would mark the light
spot. Meaning the corrective weight should be added on the same side of the
shaft as the chalk mark. Or "Add Same". The paragraph further stated that
if the shaft was turning exactly at the "critical speed" the chalk mark
would mark a spot 90degrees behind the heavy spot. When this idea finally
sunk in and the balancer gave an indication of Add Opposite at 1600 rpm and
an indication of Add Same(the phase angle changed some 180 degrees from that
at 1600rpm) at 2700 rpm, I knew that I had a "Critical Speed" somewhere in
between. Further, balance attempts with the ship setting on the concrete
pad gave indications that the nearness of the "Critical Speed" was
introducing some vibration thru the skids and back into the tail boom. The
light came on: If you are operating close to the "Critical Speed" the phase
angle of the balancer is very sensitive to rpm. And it doesn't have to be
very close. I moved the helo to grass and was able to balance the tail
rotor rather well and the balancer readings were much more repeatable.
Thanks to all who provided ideas. They helped me stay with the project.

--
Stuart Fields
Experimental Helo magazine
P. O. Box 1585
Inyokern, CA 93527
(760) 377-4478
(760) 408-9747 general and layout cell
(760) 608-1299 technical and advertising cell

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