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Linkages |
shop talk
Steve Condon
Making your linkages
strong and slop free
Creating a linkage that's both free from
slop and free from binding can be a challenge. Yet, the investment of time necessary to
make a linkage perfect is worth it in terms of the handling and safety of your model.
Every R/C sailplane has linkages, some
long and some short, they can all have slop in them which at times can adversely affect
the precision of your centering or stick input or worse, can cause high-speed flutter and
possibly catastrophic failure.
Selecting the right pushrod
Make sure you choose the right pushrod for
your application.
FUSELAGE PUSHRODS:
If you are building a HLG, use the small
Sullivan cables, small carbon rods, Spectra pull-pull cables, or something else suitable.
For planes where a lot of load will be exerted on the elevator linkage, such as an F3B
model, slope racer, F3J or duration plane, a strong choice is a must.
WING SERVO PUSHRODS:
Use music wire. The bigger the plane, the
bigger the wire. If you want simplicity and weight savings, buy yourself a z-bending tool
and use z-bends at one or both ends.
STEEL CABLES:
Cables are more flexible and therefore can
provide a smooth, bind-free linkage. Cables in a plastic housing are stiff enough,
provided that the housing is secured along the entire fuselage and is trimmed close as
possible to where the cable attaches to either the servo or the surface. Cables can be
stiffened at the extremities by wiping them with flux, and saturating them with solder.
When the solder is still hot use a paper towel to wipe the excess off leaving you with a
uniform coating that will greatly stiffen the part of the pushrod that travels outside the
end of the housing.
Music Wire:
Music wire is nice and stiff, but must
have a housing with no harsh curves to prevent binding. When installing, it is best to
tack the housing at one end and then let the housing hang down the boom so it follows a
naturally straight or slightly arched path. The only disadvantage with the music wire
pushrods is that the diameter of the music wire must be smaller than the id of the housing
(also to prevent binding) which builds-in an inherent amount of slop. Removing this slop
is difficult since it is caused by the wire flexing in the "free space" within
the housing. It is generally so little, that you may not want to worry about it.
If you're a nut like me that wants to at
least minimize it, you can make he housing "porpoise" it's way down the
fuselage. The arcs must be shallow and you must lightly tack and test the movement before
securely bonding things in place. You must walk the edge of too tight and too loose.
CARBON TUBES:
For some models, like V-tails, a
3/16" carbon tube that is "free floating" in the fuselage works excellent.
Generally one or two guides of an aluminum tube along the way (or a bulkhead with slightly
oversized holes in it) will keep the pushrod from flexing and causing a "soft"
linkage. To attach your clevises at either end, simply glue about 1" of a wood dowel
that fits inside the carbon tube at each end. Drill and tap for 2-56 or 4-40, screw-in a
2" piece of all-thread and attach your clevis.
A trick for gluing-in housings
A trick that I learned from Tim Renaud is
to make two wraps of 1" masking tape around the housing about every four inches.
Then, trim and locate the housing correctly and tack it at the rear of the fuselage. Then,
hold the front taut in its correct position, and so the housing rests on the fuse sides,
and dribble thick CA down the housing (not too much!) until you can see a drip rolling
it's way down the boom. Continue the process (from the nose and or tail) until the housing
is coated along its length. THEN (and only then) mist a little kicker down the boom. Where
it contacts the tape, it will make a very strong bond to both the housing and the
fuselage.
HINT:
To see what you're doing, try wedging a
small, bright flashlight (like a Mini Maglite ) in the nose of the airplane that is
pointing down the boom. If the fuselage is unpainted, you can generally see the bead of CA
easily by looking from the outside. Or, if it's painted, look from the rear.
Removing slop from threaded clevises and z-bends
A trick that Rich Tiltman (from RnR
products) showed me once for taking slop out of clevises seems crazy, but works perfect.
At a slope race once my surfaces seemed to be sloppy and Rich says "here let me see
that." When he started to put thick CA right where the clevis meets the horn, I
panicked and shrieked "what are you doing!" He told me to relax and watch. All
he did was put a small drop of thick CA in there, kick it, let it sit for a minute and
then manually deflect the surface thereby breaking the glue joint free. What you end-up
with is a perfectly solid bearing of CA that removes all the slop from your clevis.
The same trick can be used on metal
threaded clevises (like the Sullivan ones). First, be sure the clevis is where it needs to
be, then simply put a small drop of thick CA at the joint where the threads leave the
clevis and kick it. You can also use the lock nuts that come with the clevis to prevent
the thread slop, but you need to periodically tighten them. If you rarely need to adjust
the clevis or if it's out of reach, just CA it!
This method also works on Z-bends, but you
have to be very careful that you don't over glue it. Just a small drop right where the rod
passes through the horn or servo arm (or both) does the trick. On all of these CA
applications, you may need to repeat the process if the CA gets brittle and crumbles out
of position.
Safety tip
Always inspect your linkages after a hard
landing. Do BOTH a physical and visual inspection. Look closely to see if anything looks
cracked or bent. Then, with the radio on, grab the surface in question (like the stab) and
try to deflect it. If it doesn't move, you're good. If it appears soft, sight down the
housing as you apply pressure and see if it bows away from the surface. If it does,
re-attach it immediately!
Remember: a thorough pre-flight will make
your planes live a long and happy life.
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