Rotor Balancing

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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.