About Pushrod
Linkages for Model Aircraft
A linkage is a component that hooks one moving part to another.
That would include the entire
pushrod system. On this page, I am
changing the meaning to include only those parts that are attached to the
ends of pushrods to connect the pushrod to a horn (servo arm,
control
horn, etc.)
There are a variety of linkages available for an equal variety of
installations. The linkage you choose is often a matter
of personal preference.
The three most important properties of any control linkage
- Must align properly with the arm it connects to.
- Must be strong enough to transfer and withstand the maximum load
requirement for the entire pushrod/control surface system. A
$125.00 digital
servo is worth precisely squat if the $1.25 linkage
connected to it fails.
- Must fit properly to avoid binding or introducing excessive play.
We do not have data on how much load is on a linkage or even how much
load a linkage is supposed to withstand. Instead, we are again
making selections based on experience and examining the system for obvious
faults.
Always keep in mind what the purpose of it is and try to imagine ways
in which it might fail in your intended application. If you think of
something, don't use the linkage. Again, I said, 'reasonable,' not
'any possible.' Anything can fail with the right circumstances.
No matter what type of linkage you use, it should fit properly.
Over-sized holes in horns are asking for slop which can result in
trim problems or devastating
flutter. Do not wallow out holes to
make things fit. If a hole needs to be enlarged, use the right size
drill bit.
The "right" size equals the same size as the wire or pin going through
the hole in the horn. Most horns are made of nylon which is a slippery material.
If the fit is a little on
the snug side, it will not cause problems. If the fit is so tight that
things
bind then the hole should be enlarged.
Every Modeler should have a set or two of Number Drill Bits: If
you remember that you acquired them due to my recommendation then you'll
thank me for this advice over and over. I'll become a God!
Caution! Never use linkages on both
ends of a pushrod that will allow the pushrod to rotate such that one of
the linkages becomes unthreaded. For example, having threaded metal
clevises at both ends — even with locking nuts — is a bad idea.
Generally speaking, only one end of a pushrod needs to be adjustable.
Most people prefer to put the adjustable end on the outside so that it is
convenient for making trim adjustments without having to open the
airplane.
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