Kevin: I sent you an e-mail with an attachment of an article I wrote for
the March issue of our magazine titled Rotor Rigging that goes into more
details and has a graphic showing the relationship between the photo cell,
the velocimeter and the reflective tape. Picture is worth 10,000 of my
words.


"The OTHER Kevin in San Diego" skiddz "AT" adelphia "DOT" net wrote in
message ...
On Wed, 27 Feb 2008 19:14:08 -0800, "Stuart & Kathryn Fields"
wrote:


Ok a little discussion of balancing. If you have a shaft rotating at a
speed below it's "Critical speed" and it is out of balance, and you put a
pencil up near the shaft, it will mark the heavy side and if you are going
to correct for the out of balance condition, you need to put a weight on
the
"Oppisite" side of the shaft. Hence "Add Opposite".

OK, figured it was probably something like that.


When running a balance
check on my helicopter, the balancer reads rpm, ips and phase angle. I
interpret the phase angle measurement to mean the position of the rotor
with
respect to a reflective tape where the max vibration point occurred.

I always understood phase angle to be the relationship between the
feathering axis of a blade and the point at which its control link
attaches to the swash (Advance angle - we use "phasing" in the RC
world to describe the same thing)

How do you determine where to put the reflective tape? Based on the
pencil mark at the heavy side?

I
position the rotor at that angle and look across the shaft from the
velocimeter and Voila I "Add Opposite" and I have a balanced rotor after
about three to 4 runs.

Might be lack of coffee after a whopping 3.5 hours of sleep, but ya
lost me there.



However, the tail rotor is spinning at a speed that
is above the "Critica Speed" and there is a 180 degree phase shift
encountered.

Define "critical speed" and why the 180 degree shift?

Back to the pencil it now marks a spot on the side opposite
the heavy side so that corrective weights must be located by "Add Same".
This techniqu has gotten me balancer readings down below 0.1 ips.
(Military
says anything below 0.4 is ok)

I think you need to redo it until it's 0.0 ips. Job's not worth doin'
unless it's done right!

Note if you run at the "Critical Speed" and it doesn't come apart, the
pencil makes a mark 90 degrees behind the heavy spot. So if you are close
to the "Critical Speed" your pencil is going to mark somewhere and if you
try to "Add Opposite" or " Add Same" you are going to be chasing the
little
dots around your polar chart if you use one.

Ok, I'm off to get coffee 'cuz ya lost me again....

Gawd I love helicopters they are an unending source of questions.

I know we do a LOT of tweaking/balancing on our RC helis, but at least
I understand that. hehehe