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.

"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.

"The OTHER Kevin in San Diego" skiddz "AT" adelphia "DOT" net wrote in
message ...
On Wed, 27 Feb 2008 09:03:34 -0800, "Stuart & Kathryn Fields"
wrote:


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.

I'd be interested if I knew WTF "Add Opposite" etc was.


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

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.

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

Stu Fields

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