F24 tailwheel strut rebuild story (long)

[danMichael]

'Fellow Fairchilders:

My 1946 F24R has finally returned to flight after a long annual. One
of the major tasks was a complete rebuilding of the tailwheel strut.
Thanks to everyone who provided assistance with this task. I thought I
would pass on my experience in case it could help someone else. I am
not an A&P, although as owner of an antique airplane I sometimes have
to play one.

Background

My 24 had a history of tailwheel shimmy when landing on hard surface
runways. Over time, it became worse. One of the drag link tabs that
had cracked and been welded prior to my ownership cracked again.
Rather than simply re-weld this, my IA fabricated a steel bracket to
connect the drag link to the fuselage longeron cross tube. He assured
me that this would never break. A year (and 1000 miles from home base)
later, it did, and had to be re-welded. The shimmy became apparent on
the takeoff roll. The tailwheel tire had been worn bald in record
time. Crosswind landings were a little too exciting. Something had to
be done.

Disassembly

When the plane went in for its annual inspection this summer, I knew I
had to investigate and repair the source of the shimmy. [I discovered
that lifting the tail by an engine hoist works much better than jacking
from below: after removing the stabilizer fairing, I wrapped a nylon
webbing strap at the right end of the diagonal cross tube adjacent to
the cluster.] After removing the strut, I discovered that the inner
and outer strut tubes had significant radial play indicating that the
splines were worn. I had some difficulty removing the cam / attachment
fitting from the piston ram tube because the tube had been peened over.
I eventually realized that this was abnormal and I relieved it with a
file. After putting the assembly in the refrigerator for an hour and
heating the fork with a hot air gun for a while, I pressed the strut
out from the fork with an arbor press.

In addition to play between the splines and keyways, the splines were
loosely attached to the outer strut. It had obviously been rebuilt
before, but the splines had been attached with small countersunk head
machine screws that had worn in their holes; one had sheared off
completely. On removal, I discovered that the spring was broken in 3
pieces. The leather seal was badly worn and a small O-ring had been
inserted in an attempt to compensate for this.

Reconstruction

Fortunately I know a semi-retired machinist and he was kind enough to
spend many shop hours machining it back to health. I was able to
purchase the correct 4130 tubing from Aircraft Spruce, $5 worth
provides enough material to make 8 new splines (only 2 are required).
The existing keyways were carefully milled out as required to make
their sides clean and parallel. They were not equal in size; the
splines that were replaced were of different sizes. New splines were
milled to fit the keyways as an interference fit (approx .001"
oversize). After locating them in the keyways in the proper position,
the assembly was carefully inserted into the outer tube and it was
tightly clamped to keep everything in alignment (the 4130 tubing
springs back after the clamps are removed). The holes in the outer
tubing were carefully transferred to the splines, the depth adjusted to
avoid penetrating the inner tube. Since these holes had been worn out
of round, they were drilled to fit a 5/32" rivet rather than the 1/8"
called for in the factory drawings. After disassembling the tubes, the
holes in the splines were drilled all the way through and countersunk.
The countersink must be slightly deeper than normal due to the curve of
the splines — you want the middle of the rivet head to be flush with
the inside of the spline. The splines were now ground to clearance
fit. The drawings called for .001" clearance but we found that this
was insufficient for smooth operation even after lubrication. Trial
and error yielded the correct fit, carefully grinding a small bit each
time. This is very time consuming because every time we mounted a
spline in the mill fixture it required considerable time for setup at
the measuring table to ensure the edges will be square and parallel
(remember that the splines are from a tubing section, not flat, so
clamping them is not trivial). After machining was complete, the
splines were riveted to the outer tubing using 5/32" countersunk head
steel rivets after cutting the rivets to the correct length and
squaring the ends. Locating these rivets was a task in itself!
Riveting was a bit tricky since the heads are inside the tube. My
friend was able to back-set them by using a large piece of round rod
stock inserted into the tube and hammering the rivet end. Afterwards,
he hand-filed the splines until the fit was perfect.

After assembly, the tubes telescoped smoothly with no radial play at
all! My friend was a life saver.

The next task was to locate a replacement spring. No one seemed to
have a spare, not surprisingly, and it doesnt seem to be common with
Aeroncas, Swifts, etc. Fortunately the factory drawings from the CD
give the spring specifications. Unfortunately my research reveals that
there are thousands of spring manufacturers and that most springs are
made to order. The second spring is quite reasonable in cost, but the
first one is not. Since I dont know anyone else who wants a F24
tailwheel spring, I attempt to locate a stock spring in the correct
spec. This is not easy since most springs of that diameter and force
are much shorter. Eventually I located a spring that is as close to
the original as I expect to find. Its free length is 1/4 inch too
long, so the preload will be about 30 pounds higher, but I doubt that
will be significant given the weight on the tail. It is made from
chrome-silicon steel rather than chrome-vanadium, but this material is
used on off-road vehicles so I assume this will be ok. It is wound
from rectangular wire rather than round so the inside diameter is
smaller than the original. A friend of my IA used a lathe to turn down
the brass spring seat to fit.

I bought a package of leather washers from McMaster-Carr to replace the
rod seal. I thought of using neoprene or another modern material, but
my IA tells me that leather will be more durable on the worn, unchromed
rod. They are the correct inside diameter and thickness, but I must
punch the outside diameter to 1". It takes a lot of hammering with a 2
pound mallet to cut through it. I soak it in oil for a couple days to
help soften it before assembly. If anyone needs a replacement seal,
let me know and I can send one — I have about 97 left!

The neoprene bump stop is cracked and badly worn, so I decide to
replace it, too (the aircraft manual states that it helps seals the
strut and mine was leaking). I cant find a stock washer of the correct
dimension, so I order a neoprene tube of the correct inside diameter.
A cutoff saw yields the correct height and a few minutes with the belt
and disc sanders yield the correct outside diameter.

I clean and prime the strut parts, fork and link. The link is stripped
and inspected for cracks. It seems slightly bent, but it is assumed
that it was that was from new. The fork and link get repainted, too.
The threads of one of the trunnion bolt holes was stripped and had to
be Helicoiled. After thoroughly coating the splines and inner tube
with graphite grease, reassembly was straightforward except for
replacing the fork. Freezing the strut, heating the fork, and lots of
lubricant made this just possible with a couple of little hammer taps.
It is far more difficult than disassembly because I couldnt use the
press (due to the geometry) and you must precisely align the fork to
allow insertion of the through bolt.

The tailwheel strut was reinstalled in the plane and reattached after
greasing all the bushings. The grease fittings in the trunnion
shoulder nuts were purged to ensure that they were not plugged and that
no dirty grease was in them. These fittings dont seem to have a check
ball in them so I made sure they each received a grease fitting cap
when I was done. The strut was filled with oil. Due to the design,
the overfill will run down the inner tube and contaminate the spline
grease, so I tried to fill slowly to minimize this. I added a neoprene
dust cover (intended for motorcycle forks) to help prevent dirt from
getting into the exposed splines; it also covers the level hole access
port. I very much want to keep the splines clean because I dont want
to have to remove the fork to regrease them for a long time.

I had intended to increase the spring tension on the steering breakout
cam, but I couldnt remove the threaded cover due to staking. I was
afraid I would wind up destroying it before removing it, so I let it
be. In retrospect, I do not think this is required, since release is a
function of steering angle and not force.

I also adjusted the rudder cable tension, which was too loose. Since
the rudder cable turnbuckles are in front of the linkage to the
steering cables, this also increases tension in the steering cables. I
made sure that the steering springs were not overly compressed.

After lowering the tail, it sat much higher than it did previously —
the broken spring badly affected the tailwheel geometry.

Conclusions

The final results are not yet in. After finally finishing the annual,
and a long delay waiting for a good day with minimal crosswind, I
finally flew it last week. Two out of three hard surface landings were
completely free of shimmy, but unfortunately one still had shimmy.
Steering on the ground feels more precise. I didnt notice any leakage.
It is certainly a big improvement, but I cannot yet say that its
cured. I will have to see after additional landings, but that probably
wont be for a while — it took half an hour to get the Ranger oil
temperature up to min takeoff and that was after coming out of a heated
hangar!

:Dan
:NC81323'