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Rotor Balancing



 
 
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  #1  
Old February 19th 08, 05:46 PM posted to rec.aviation.rotorcraft
Stuart & Kathryn Fields
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Posts: 328
Default 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.


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  #2  
Old February 27th 08, 05:03 PM posted to rec.aviation.rotorcraft
Stuart & Kathryn Fields
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Posts: 328
Default Rotor Balancing


"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.


  #3  
Old February 28th 08, 03:14 AM posted to rec.aviation.rotorcraft
Stuart & Kathryn Fields
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Posts: 328
Default Rotor Balancing


"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


  #4  
Old February 28th 08, 08:09 PM posted to rec.aviation.rotorcraft
Stuart & Kathryn Fields
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Posts: 328
Default Rotor Balancing

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



  #5  
Old March 3rd 08, 08:59 PM posted to rec.aviation.rotorcraft
Stuart & Kathryn Fields
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Posts: 328
Default Rotor Balancing


"Hawkdoc" wrote in message
...


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!

Stu, not sure where you got the .4 standard from. On US Army rotorcraft
the standard was for .2 IPS in all modes (ground, hover and flight Vne).
Some birds where impossible to get all three so we always went with hover
and flight. Some of the reasons could be structural, mechanical or even
the flight hours on the individual blades.

Kevin, 0.0 sounds nice but, is an impossible quest. To many outside
factors in play to ever achieve (wind, temp, age, even sunlight). I've had
birds down below .1 IPS and couldn't tell the differance with others at
.3. Besides 1 to 1's aren't bad in comparisson to 2 to 1's (tail rotor).
2V1 are the ones that make a lousy flight on a clear smooth day, the ones
that can make you feel sick and listless. 2V1 are the buzz in the controls
that put your body to sleep.

James
Rule #1 Always remember your aircraft was made by the lowest bidder


Oops. James. I was the builder and if problems show up I know exactly who
to blame.
The 0.4 ips was given to me so long ago I don't remember the source. But as
I recall I thought it had to be gospel.
I do all the main rotor balance jobs in hover and the lateral 1/rev stays
good thru forward flight. The vertical only increases slightly at 90 mph.
However, with fuel tank less than 1/2, I get a lateral 2/rev that is above 2
ips. This 2/rev is from the main rotor as the 2/rev with the tail rotor
would be 92hz and very hard to feel. With full tank the main rotor 2/rev
goes away. Haven't been able to get to the cause yet so just keep the fuel
above 1/2. Believe that there is a resonance in the airframe somewhere
that likes the 2/rev frequency of 17hz.
I agree I couldn't tell the difference in feel from 0.1 ips to around 0.3.

BTW you have brought up something that has been confusing to me. I got a
ride in the Hummingbird which is a S-52 derivative and found myself after a
short period in smooth air feeling nauseous. I thought at first that I had
some food poisoning and aborted further flying. Later I thought that it
might have come from the blade flicker coming thru the eybrow windows. I've
never felt nausea in any helicopter before. I thought the Hummingbird was
pretty smooth, but maybe you've hit the cause of my nausea.

Stu
Rule #1 with amateur built ships the source of your problem is very close.


  #6  
Old March 30th 08, 07:03 PM posted to rec.aviation.rotorcraft
Stuart & Kathryn Fields
external usenet poster
 
Posts: 328
Default Rotor Balancing



--
Kathryn Fields
Experimental Helo magazine
P. O. Box 1585
Inyokern, CA 93527
(760) 377-4478 ph
(760) 408-9747 publication cell
"Hawkdoc" wrote in message
...



Oops. James. I was the builder and if problems show up I know exactly
who to blame.
The 0.4 ips was given to me so long ago I don't remember the source. But
as I recall I thought it had to be gospel.
I do all the main rotor balance jobs in hover and the lateral 1/rev stays
good thru forward flight. The vertical only increases slightly at 90
mph. However, with fuel tank less than 1/2, I get a lateral 2/rev that is
above 2 ips. This 2/rev is from the main rotor as the 2/rev with the
tail rotor would be 92hz and very hard to feel. With full tank the main
rotor 2/rev goes away. Haven't been able to get to the cause yet so just
keep the fuel above 1/2. Believe that there is a resonance in the
airframe somewhere that likes the 2/rev frequency of 17hz.
I agree I couldn't tell the difference in feel from 0.1 ips to around
0.3.

BTW you have brought up something that has been confusing to me. I got a
ride in the Hummingbird which is a S-52 derivative and found myself after
a short period in smooth air feeling nauseous. I thought at first that I
had some food poisoning and aborted further flying. Later I thought that
it might have come from the blade flicker coming thru the eybrow windows.
I've never felt nausea in any helicopter before. I thought the
Hummingbird was pretty smooth, but maybe you've hit the cause of my
nausea.

Stu
Rule #1 with amateur built ships the source of your problem is very
close.

Yup, forgot that one, about you being the builder LOL no aspirations
intended.

As to the increase in speed this would be probably controlled by your tabs
if you have one. Not sure if you have access to a storbe but if you need a
climbing blade would be the first suspect to look at.

In regards to the 2/rev I'd doubt the M/R being the problem. As I was
taught and experienced, the problem was always secondary drive related.
You've got the Hz now you need to figure out what resonates at that
frequency. I suspect a bearing could be your culprit based on your
sysmptoms. Have you worked up the different CG's based on when it occurs
versus full? A CG change could cause a bind in the air frame loading a
bearing.

Lastly, vibrations, fumes, air flow versus temp or as you suggest flicker
vertigo from the eyebrow windows or you could have had an inner ear
infection that day. One time I had an irate Lt. Colonel complain his
aircraft was out of balance. I went for a test flight and found nothing
wrong with it. This went on for a month with the LTC wanting my head
because he insisted there was a problem. We finally figured out that they
had installed the wrong window and it was really noticeable at his eye
level. Good luck and let us know what you find out.

James
Rule # 7 A plan never survives the first 30 seconds of combat



James: When I set up to measure the 2/rev, I put two reflective tapes to be
read by the photo cell. This, thru the filter in the balancer guarantees
that I'm reading 2/rev frequencies. Also I can show mathematically, and I
think with a wet thumb, why any two bladed helicopter will generate a 2/rev
vibration at the rotor head level. 1st. In a hover the air velocity that
generates the is being generated only by the rotation of the blades. You
have pretty much a constant lift and drag force no matter where the blade is
with respect to the main axis of the helicopter. However once you begin
forward flight, you now have a lift and drag force caused by the rotation
plus both lift and drag forces caused by the velocity of the air caused by
forward flight. It is not hard to see how the lift and drag forces due to
forward flight would be higher on the advancing blade than on the retreating
blade. The math does verfy that there would be a 2/rev force generated.
Further if the vibration was being caused in the drive train it should be
there also when hovering. My 2/rev only shows in forward flight. As soon
as I enter the hover I can feel the ship smooth up and the data recorded in
my balancer confirms this. Further I can't imagine a bearing making 16.7Hz
vibrations. Especially at the energy level needed to shake the instruments.
Another Safari owner reported 2/rev readings at reduced fuel levels of
3.7ips!! and vibrations that rendered his instruments unreadable.
We recently hung the helicopter from the overhead beam and used a laser
vibrometer to record frequency response readings on the helicopter. On the
frame just below the fuel tank we recorded 20Hz. This was taken with low
fuel and just by tapping on the frame with a rubber mallet. Now the true
resonant frequency of just that frame tube without the rest of the frame
should be significantly higher than 20Hz. It seems the thing that is
responding to the 16.7Hz from the main rotor is more involved than just a
couple of frame tubes. Witnesses of the test included several professional
vibration whizzes. Their opinion is that it is going to require a more in
depth look to identify possible fixes.











  #7  
Old April 15th 08, 08:32 PM posted to rec.aviation.rotorcraft
Stuart & Kathryn Fields
external usenet poster
 
Posts: 328
Default Rotor Balancing


"Hawkdoc" wrote in message
...


James: When I set up to measure the 2/rev, I put two reflective tapes to
be read by the photo cell. This, thru the filter in the balancer
guarantees that I'm reading 2/rev frequencies. Also I can show
mathematically, and I think with a wet thumb, why any two bladed
helicopter will generate a 2/rev vibration at the rotor head level. 1st.
In a hover the air velocity that generates the is being generated only
by the rotation of the blades. You have pretty much a constant lift and
drag force no matter where the blade is with respect to the main axis of
the helicopter. However once you begin forward flight, you now have a
lift and drag force caused by the rotation plus both lift and drag forces
caused by the velocity of the air caused by forward flight. It is not
hard to see how the lift and drag forces due to forward flight would be
higher on the advancing blade than on the retreating blade. The math
does verfy that there would be a 2/rev force generated. Further if the
vibration was being caused in the drive train it should be there also
when hovering. My 2/rev only shows in forward flight. As soon as I
enter the hover I can feel the ship smooth up and the data recorded in my
balancer confirms this. Further I can't imagine a bearing making 16.7Hz
vibrations. Especially at the energy level needed to shake the
instruments. Another Safari owner reported 2/rev readings at reduced fuel
levels of 3.7ips!! and vibrations that rendered his instruments
unreadable.
We recently hung the helicopter from the overhead beam and used a laser
vibrometer to record frequency response readings on the helicopter. On
the frame just below the fuel tank we recorded 20Hz. This was taken with
low fuel and just by tapping on the frame with a rubber mallet. Now the
true resonant frequency of just that frame tube without the rest of the
frame should be significantly higher than 20Hz. It seems the thing that
is responding to the 16.7Hz from the main rotor is more involved than
just a couple of frame tubes. Witnesses of the test included several
professional vibration whizzes. Their opinion is that it is going to
require a more in depth look to identify possible fixes.


Stu,
It's been quite a while since I was taught most of this. (20 plus years, I
probably needed to go through my boxes of junk and refresh my memory)
so I'm a little confused. I've balanced most Army helicopters excluding
the Chinook (thank god!). This has included the pole and tape method, to
the Vibrex and the last that I can't remember.
Maybe we're not talking the same thing. 1 /rev problems are decribed as
shaking or bounce. 2 per/rev problems are hums or better described as
buzzing feelings. Is this what your talking about?
I'm not sure I'm understanding your comment about putting 2 pieces of tape
to get the 2 per rev info. 2 pieces on one blade? a piece on each? How
does the system know the master blade? This was the track portion of track
and balance. Not alot to be done with this except to ensure both blade fly
through the same space. If one blade was higher than the other we'd make a
P/C link adjustment. If one blade climbed as our speed increased we'd make
a trim tab adjustment. These were usually the last things done.
On the newer system we'd put a piece of tape on one blade but only to give
the computer a target 0 degree referance angle. All the vibration analysis
was done by accelrometers. One on the swashplate and one in the cockpit.
We'd then dial in the M/R speed and get a reout in ips at and angle
coresponding to target 0. Again, I'd have to dig out the books but as I
remember 1/rev such as the problem your friend describes are M/R balance
issues. Shaking so violent you can't see the instruments is M/R. Think
large mass and pendulum. We'd get that on some ships depending on load and
configuration going in and out of ETL due to changes in the air flow over
the M/R.
Let me know if we're on the same page and I wish you all the best. Sorry
to hear about your ruined flight the other day at least the gas was cheap.
I just hope the repair didn't destroy that price advantage and great
feeling of the day.

James


James: The 2/rev I'm referring to is the vibration incurred in any two
bladed rotor system that is encountered only in forward flight. To get my
balancer to read the 2/rev vibration levels, I have to fool the filter that
is set with just one piece of reflective tape which reads the vibrations
occurring at the rotor speed by putting two pieces of reflective tape, one
180 degrees from the other, and the filter will now be enabled to read
vibrations at twice the normal rotore speed hence now 2/rev. The Master
blade idea is only meaningful in the 1/rev tracking and balancing
measurments. With the 2 pieces of reflective tape the Master blade is
ambiguous and the phase angle is not of interest; only the amplitudes.
I typically set the track at full chat with the skids just light on the
ground using the pole and tape, or in our case a paper towel inner roll on
the end of a pvc pipe and then tracking lights installed in the tips of the
blades looking back at the pilot. The tracking is adjusted with the pitch
links and if a climbing blade is encountered during forward flight, trim tab
adjustment is used.
The Safari helicopter has a resonance that I believe occurs in the airframe
when the fuel level gets below 1/2 tank. This vibration frequency measures
to be 16.6Hz whis is 2/rev. Similar measurements have been made on other
two bladed helicopters. Some show no wiggle in the pilot's seat but have
some above the transmission mount. Other's like the Safari show both
vertical and later 2/rev vibrations in the cockpit. My ship exhibits very
little vertical 2/rev but I have moved the battery out in front of the
instrument panel ala the Bell 47. I still however can measure 2.2ips of
lateral measuring just below the swashplate. Having to perform these
measurments on a non-certified ship that has no instructions in a
maintenance manual has forced me to dig in deeper than a lot of guys who had
the advantage of a factory supplied manual discussing such things. It is
both a plus and a minus.

BTW just back from Bensen Days and Sun'n Fun. At Bensen days I hover
taxiied a 254# helicopter. Kinda nervy checking your self out in a single
seat, ultra lite helicopter. I expected it to have a strong tendency to
travel to several different zip codes simultaneously. Not so. Pretty
darned stable little bird. See the Mosquito Air.

Stu


  #8  
Old April 16th 08, 10:49 PM posted to rec.aviation.rotorcraft
JohnO
external usenet poster
 
Posts: 120
Default Rotor Balancing

On Apr 17, 6:51 am, The OTHER Kevin in San Diego skiddz "AT" adelphia
"DOT" net wrote:
On Tue, 15 Apr 2008 12:32:41 -0700, "Stuart & Kathryn Fields"

wrote:
BTW just back from Bensen Days and Sun'n Fun. At Bensen days I hover
taxiied a 254# helicopter. Kinda nervy checking your self out in a single
seat, ultra lite helicopter. I expected it to have a strong tendency to
travel to several different zip codes simultaneously. Not so. Pretty
darned stable little bird. See the Mosquito Air.


254lbs? Wow. I almost weigh as much as that...

Was wathing video of the Mosquito XET last night. Cool little ship...


Yes, much nicer than the Hirth powered option. That lawnmower engine
sound drives me nuts!

But these T62-32 units are getting harder and harder to find nowadays.
  #9  
Old April 16th 08, 10:52 PM posted to rec.aviation.rotorcraft
JohnO
external usenet poster
 
Posts: 120
Default Rotor Balancing

On Apr 17, 9:49 am, JohnO wrote:
On Apr 17, 6:51 am, The OTHER Kevin in San Diego skiddz "AT" adelphia

"DOT" net wrote:
On Tue, 15 Apr 2008 12:32:41 -0700, "Stuart & Kathryn Fields"


wrote:
BTW just back from Bensen Days and Sun'n Fun. At Bensen days I hover
taxiied a 254# helicopter. Kinda nervy checking your self out in a single
seat, ultra lite helicopter. I expected it to have a strong tendency to
travel to several different zip codes simultaneously. Not so. Pretty
darned stable little bird. See the Mosquito Air.


254lbs? Wow. I almost weigh as much as that...


Was wathing video of the Mosquito XET last night. Cool little ship...


Yes, much nicer than the Hirth powered option. That lawnmower engine
sound drives me nuts!

But these T62-32 units are getting harder and harder to find nowadays.


Oh, I see it is actually a T62-2A1, not a -32. Is the price/
availability any better?
 




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