Posted for Charlie Kohler <charliekohler@yahoo.com>:
Hi guys,
I was waiting for someone who had some good technical information to step
up--however being as no one has contributed--I'll give you some of my
experiences.
First let me say that there is no real cause to fear disassembling the strut.
The heart of the shimmy damper lies in a rectangular plate that is held in
place by the two bolts that are safetied in the front of the nose strut.
That plate has a slot which holds a straight piece of rubber (O ring). As you
tighten the two front bolts you pull the plate against the shell and make more
clearance underneath this "O-ring". I believe this is the origin of most of
the shimmy. It also depends upon the viscosity of the strut oil. But alas, I
get ahead of myself.
I was in extremely cold weather last winter and was able to taxi an airplane
immediately out of the hangar but when I got it to the runway I could not turn
the nose wheel. The viscosity greatly increased and was exactly like honey.
You could turn the wheel but with great effort. We substituted the 30 weight
fork oil for 5 and managed to get a flight in.
Later, I had a nearly new nose strut which would not turn. It was very
difficult to taxi this airplane consequently we disassembled the nose strut
and were able to make corrections that corrected the problem.
So here is what I consider an unauthorized "shade tree mechanic" approach to
repairing the strut.
For reference: put the nose wheel on a pair of greased plates and attach your
tow bar. Using a fish scale--make a "BEFORE " reference number.
Because the numbers are relatively small --attach the fish scale at the 1
foot lever arm position. Should be in the neighborhood of 5 pounds at 1 foot.
1. Lift the nose wheel off the ground (check to see how freely the nose tire
spins--should be one half/1 turn) and remove the nose wheel and fork. Put a
drain pan underneath the strut.
2. Remove all nitrogen from the nose strut and remove the servicing Schrader
valve.
3. Remove the two safetied bolts on the front of the strut.
4. Remove the three Allen screws in the base of the strut and pull out the
strut. Be careful that the rectangular plate/shimmy damper does not fallout.
Note the position of the O-ring behind it.
You can see that this plate prevents oil from moving one side of the chamber
to the other. This is the key to our problem.
Several modifications may be possible to slow the transfer of fork oil from
one side of this chamber to the other.
If the O-ring "straight piece" has worn -- that could easily be replaced.
Push down on the plate with your thumb and check with a straight edge across
the back of the plate. Too much clearance = shimmy.
Increasing the viscosity of the fork will would also help.
Naturally any looseness or wear would be cause for replacement. But O-rings
can be replaced.
If oil is leaking down the inside of the strut shaft--that requires a further
disassembly which I did not do.
It looked like a roll pin removal and a heat gun to remove the top / or
bottom collar would allow the valve mechanism to slide off the shaft and other
O-rings to be replaced.
Put it all back together Pump cyl up and down many times (refilling with fork
oil each time) to drive air out of the chambers in the strut. When it comes
out clear-- but schrader valve back in and service with nitrogen. Extend to
about three inches.
Re measure turning "fish scale" force.
I'm sure you guys can figure all this out and take it to the next step.
By the way -- tire pressures are 50 nose 60 main
Charlie K.
"""
The new Lancair strut is designed very much like the Columbia strut and uses
5606 fluid which isn't susceptible to viscosity breakdown at higher
temperatures. It also has significantly more damping. It has bent a couple
tow bars.
"""
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