Hi Donald,
Some have to look at the gear on the plane to perceive how the action
of the oleos and the rubber "taxi" donuts is sequential, and not
simultaneous.
Fornaire Service Bulletin No. 113 states: "Lack of the fluid in the
oleo may cause severe damage to your airplane". Not really. The plane
can sit forever with no oleo fluid without damage. Only when it is
flown do the excess gear forces empty oleos make possible cause such
damage, although one will surely follow the other. ;<) I'll try to
explain as this applies to the main gear on Serials 813 and up.
On takeoff, as the weight of the plane is taken up by the wings,
properly functioning gear will extend downward approximately a foot (as
measured "up-down" at the bottom of the tire). In that process, the
fluid in the oleo piston should exit from the orifice in the bottom
into the cylinder at a rate controlled by the diameter of said orifice
by some combination of suction and gravity. There is a rubber bumper
assembly mounted on an ear cast into the main gear upper leg (see Fig.
12 in the Parts Manual, items 2 and 24 through 26). When the circular
support (flange braised around the cylinder containing the oleo filler
cap) reaches the top of this bumper assembly, the relatively slow
downward travel of the oleo is thus limited.
On takeoff, if the absence of proper fluid and/or piston-cylinder
sealing allows the gear to drop freely and rapidly it will subject the
rubber bumper assembly with force far in excess of design intent. If
the rubber is old and hard (these are seldom changed), it will
disintegrate.
The gear fairings conceal the oleo and bumper in the "taxi position"
(oleo fully compressed), so the problem will not be discovered during
the next preflight inspection. Upon the next takeoff the gear can then
drop the additional 3/4" or so the missing rubber bumper took up. The
oleo flange accelerates downward until it slams into the unyielding ear
on the gear leg. There have been instances where said ear has been
sheared or broken off by such abuse, since the ear is an aluminum
casting and the oleo flange is steel. This was possible the "slam to a
stop" to which I referred.
There is approximately 4-1/2" oleo travel physically possible of the
piston in the oleo cylinder between the filler opening and the
projection of the lower end attaching fitting braised into the cylinder
tube. I have never measured the full up-down oleo movement available
of an installed oleo, and would be interested in what that is if you
would do so. I would estimate it to be about 4-1/4". In any event,
the oleo and piston reach a point of travel at the upper travel of the
trailing arm where essentially all fluid has been forced back up
through the orifice out of the cylinder and into the piston through the
orifice.
Only at that time (when the main landing gear is in the uppermost
position supporting the weight of the plane and the oleo fully
compressed) does the function of the rubber donut assemblies come "into
play". The maximum thickness of the rubber donut assemblies on the
oleo is fixed at the bottom by the circular supporting flange on the
cylinder and fixed at the upper end by the snap ring inside the sleeve
assembly (Item # 12-21), and is, therefore constant. Compression of
the rubber donuts over time such as results in "play" allows the
cylinder to traveling further upward. I see nothing to prevent the
oleo piston from "bottoming out" in the cylinder if sufficient "play"
is present in the "donut stack".
Regards,
William R. Bayne
.____|-(o)-|____.
(Copyright 2010)
--
On May 24, 2010, at 18:48, Donald wrote:
I don't think one could ever make an oleo "slam to a stop" Bill. I
was measuring mine today, and it would be impossible to compress the
washers or donuts enough to bottom out the piston, there is like 3 or
4 inches free at the bottom of the piston travel..
--- In [email protected], William R. Bayne <ercog...@...>
wrote:
Hi Jim,
You're absolutely correct in that the donuts absorb bumps from taxiing
on irregular surfaces since the weight of the airplane will fully
deplete all possible oleo travel/action after the oleos absorb any
"shock" incurred in the actual process of landing and the movement of
the fully extended trailing arm-oleo combination up into the normal
taxi configuration.
Obviously there is SOME give in rubber donuts that aren't ten years
old
and hard. If one's oleos don't extend upon takeoff, the majority of
crosswind landings by a proficient pilot (or one that does not fly in
crosswinds exceeding 20 mph or so) might well seem "normal" at the
moment of touchdown. More damage is likely from the movement of free
but dry oleos slamming to a stop at the end of undampened travel than
touching down with frozen oleos.
No one would go out and practice crosswind landings in a brisk, gusty
breeze if reasonable proficiency and decent landings under such
conditions were not possible for most. If there is the slightest
doubt
as to being in full control at the time of touchdown, DO NOT LAND but
go around until an opportunity for a "good" touchhdown is perceived
and
timely accepted. The process should not be one of "shut your eyes and
pray".
More than a few Ercoupes have changed hands with marginally functional
oleos and been flown without apparent damage until the next annual by
a
competent mechanic.
The trailing arm gear works fine without oleo action. Excess stress
to
the spar and landing gear imparted by a poorly controlled "arrival" is
less likely to result in permanent damage when the generous amount of
shock absorption provided in the design is maintained and available.
Regards,
WRB
--
On May 24, 2010, at 16:20, Jim Gall wrote:
I may mistaken, but I though the donuts were for taxiing, and the
hydraulic piston with the small hole in the button that was forcing
the hydraulic fluid up into the piston was for landing.. I can't see
how anyone could land in a crosswind with the piston stuck in the
cylinder?
JIM GALL 94020
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