Presuming that everything is perfect from the
manufacturer could also come back and bite
you. Workers could pinch O rings and the
fault not appear until the unit has cycled a few
times. It’s much easier to check that every-
thing maintains a seal before the units are
installed in the airframe. It also makes later
fault finding easier as you don’t have a three
way doubt about whether it’s the unit, tubes
or valves. During a build I would cycle the
gear at least ten full cycles. Then I pressurise
the system and see how long it holds pressu-
re.
I don’t use on board pressure gauges. Most
air gauges need a T piece to operate and I
look at this as four more joints to leak. If you
want to check for pressure loss, re connect
the pump and see what it reads. Air tanks can
leak, so check these before you install them
permanently too.
Everyone I know uses an air pump with a
gauge, so you know what pressure you put
into the system and will fill the air system up
just before you fly. You do not see the gauge
again until you land, if the gear operated, you
know there was enough pressure!
I use an excess of tube when testing and set-
ting up, you can then leave tube on fittings,
just shortening the tube length on the final
install.
If you have to remove tube from barbed fit-
tings, do not cut off the pipe with a scalpel,
there is a really good chance that you will
score the nipple and it will leak along the
groove you have made!
I used sharp electrical side cutters and “nip”
the side of the tube. Harder tubes ravel,
stretch and can be pulled off because they
stay stretched for a short time and softer
tubes ravel a little and you can snip a section
out, again you can easily pull the tube off.
Always cut the last 5mm off before you rein-
stall.
Cleanliness and attention to detail is so
important for a reliable system. If you have
not finished the building, do not start to install
the air (or fuel) system. Sanding a moulded
airframe with airlines or cylinder nipples
open, you will stand a good chance of getting
highly abrasive material into the seal and it
will fail soon after you finish the model!.
I use silicon model car shock oil to check for
leaks, the thicker weights will stay around the
fitting while you pressurise the system. Leave
it for 20 minutes and re pressurise the
system. It’s worth doing this after several
operations to check the seals don’t have
damage in one place. The O ring seals roll in
use and you might test it with the seal in a
good place.
Leakage through the piston seal is surprisin-gly common, on air up/down systems. This is
often blamed on the valve. This is because
the air travels through the ram and out of the
breather on the valve. It’s not a valve pro-
blem.
You can prove this by clamping either side of
each cylinder, if the air stops coming out of
the breather, it’s the internal cylinder seal.
Wide tolerances on the cylinder bore, debris
not cleaned out during manufacture can
damage the seal after a few operations, and
even a dry seal can allow leaks.
A regular question I get asked is what pressu-
re I should run my undercarriage or brakes
at. It is surprising that the manual rarely men-
tion this.
My fleet all use between 80 and 110psi on the
gear and 60-80psi on brakes. If I needed
much above this I would feel the units nee-
ded larger diameter cylinders.
Obviously long legs with big heavy wheels
mean the units will have to work harder.
Scale models with gear doors mounted on
the legs often end up angled out when down,
and this angle will mean the faster you go,
the more force will be required to pull the legs
up. I see people trying to retract the gear
while climbing steeply, think of the g force
coupled with airspeed acting on the leg/wheel
mass. All of these factors need to be conside-
red. Do not restrict the speed of operation
based on ground operation. Things will slow
down with the airflow moving over the legs.
Other things that can affect the pressure
required is mechanical resistance. I always
remove sharp edges with a fine file and lightly
grease the tracks. The gear mounting area
also should be checked, it needs to be flat
and the two rails level with each other. Any
un-evenness in the two mounting areas willeasily twist even the heaviest frames once
they are screwed down and cause the slider
to become stiff. I unlock the unit and without
pressure operate the leg back and forth. You
will feel any resistance.
I’m really surprised by the lack of understan-
ding on pressure/volume with pneumatic
systems, I have seen people trying to get
high pressure into a system, because they
didn’t want to run out of air or already have a
minor leak and want to ensure they had
enough air!!
What you need is volume, not pressure and
high pressure will obviously make the leak
worse. I generally pressure check any new
cylinders to around 120psi, my feeling is this
is an absolute maximum needed. All my
sports models operate with a pressure
around 100psi. The scale models with bigger,
heavier wheels and legs might need more
pressure. My scale models all require 110psi
for reliable retraction, mainly because they
have gear doors on the legs and big heavy
wheels.
Many of the bigger models have retract units
with huge cylinders that need filling each
cycle. Three of these and it’s obvious that
plenty of air volume is going to be needed.
Sometimes it’s impossible to get one large
tank in the fuselage, so two, three and even
four tanks maybe needed all linked together.
When I assemble an air system I ensure all
the pipe joints are cut square to ensure the
tube butts up against the joiner fully. I also
avoid pressuring the system for as long as
possible after they are first assembled, this
allows the tubes to bed in and shrink fully
around the barbs.
Most model airline like Robart and BVM is
imperial, so larger than 3mm o/d-this won’tAUTHOR & PHOTOGRAPHER:DAVE WILSHERE
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