"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