FROM HORN to HORN
Our radio's servos move the airplanes control surfaces in accordance with our desires. In almost every case, though, there is some linkage between the servo and the control surface; specifically, between the servo's output arm and the surfaces control horn. We usually call this linkage the control rod.There are several types of rods used for this purpose, each with its own good and not-so-good features. Exactly what type connection you use will depend on what the designer calls for, what you are comfortable installing, or maybe what you happen to have on hand at the time. I'll go through the general types and explain them.
Dowel/Wood Rod:
Wooden dowels and balsa square rods are still common on smaller
planes and gliders, where there is not much force involved, lightness
is critical, and where little sideways forces are expected from
aircraft manoeuvring. Short wire rods are used at the ends,
held in place by thread and glue.
Nyrods:(snakes)
These are available in several grades of stiffness and work
beautifully; they're easy to install, the end fittings just screw
in, and they will handle curvatures nicely. If curves are made
smoothly, there is very little friction. But snakes do have
disadvantages - they must be supported every few inches along
their length, they expand and contract with temperature changes,
and can have a tiny bit of slop as the nyrod squiggles around in
its tube. Great for sport planes, but need to be used with care
on competition or larger planes.
Wire:
Unsupported wire is often used for short links, such as the links
between aileron horns and their servo. Also used at the ends of
other types of rods to provide for connections to servo and
control horns.
Supported Wire:
A special method, using piano wire for its strength, and snakes
as the support and for bearings, is sometimes used where positive
control is needed along with the snakes ability to bend. The wire
is encased in an outer nyrod shell, using short lengths of inner
snake as support and bearing surface. This system will tolerate
slight smooth bending, but does not have the expansion problems
that a pure snake would. It does require the same periodic
supports along its length that snakes do.
Housed Cable:(bowden
cable) Metal cable, running in a plastic housing, is commonly
used for throttle control or other uses where loads are light and
there will be bends to go around. A throttle cable can go through
some amazing contortions and still have little friction. The main
disadvantage - cable can stand very little compressive load. If
cable is used for nose gear steering, you may have good positive
steering when the cable pulls, and mushy, unreliable turns when
the cable is pushing. End fittings for clevises must be soldered
on.
Closed loop Cable:
Under the right circumstances and with the right hardware, pull-pull
is a neat way to go. Most commonly used for rudder and sometimes
for elevator control, pull cables are light, strong, and positive.
Used heavily throughout the years on full size aircraft, from
flying fleas to B-17s. For model use closed loop require a ball
bearing servo, and special fittings for adjustment. They also
must run reasonably straight.
Arrowshafts: The
modern version of wood dowels, these are used where strength and
precision are paramount, and can be either aluminium or carbon
fibre tubing. They require a straight, unobstructed run, and
special end fittings must be made or bought to attach wire for
clevis attachment. Arrowshaft control rods are often used for
pattern and larger planes; they are strong, light, do not expand
and contract much if at all, and are very precise in operation.
LINKS
Having shown you various types of control rods with some of their associated advantages or disadvantages, of course these rods need to be connected to the servo at one end & the control horn at the other. There are several types of links used for this purpose, each with its own good and not-so-good features. Exactly what type connection you use will depend on what the designer calls for, what you are comfortable installing, or maybe what you happen to have on hand at the time. I'll go through the general types and explain them.
The Z-Bend: Used
right from the inception of escapements and servos, the z-bended
piece of music wire WORKS, and has negligible chance of
disconnecting. They're easy, cheap, totally reliable. So why not
use them everywhere? First, there is no way to adjust a z-bend;
you're stuck with it once installed. They're relatively hard to
connect and disconnect, they usually require drilling out the
holes on standard servo and control horns, and have a tendency to
self-enlarge those holes over time, causing slop. But they're
still very useful in some installations - an example would be the
connections on an aileron bellcrank hidden in the wing, where you
can't see to inspect or get in to adjust anyway. Here the
reliability of the z-bend overshadows any other considerations.
The Snap Link:
These wonderful little devices are heavily used, and rightfully
so. They come in metal solder-on, and nylon and metal screw-on
forms, and can very easily be connected or taken off. The screw-on
types, of course, provide for easy linkage adjustment. So where's
the problem? For one thing, the lead-in from the control rod must
stay pretty close to the plane of rotation of the control or
servo arm; while they rotate nicely IN their little holes, any
bending in other directions tend to make them self-disconnect.
And they can also cause binding if they are overrotated up
against the servo or control horns. The nylon types can break,
the metal screw-on types can vibrate on the threads, causing
radio glitches or even thread failure, although after final
adjustment (trimming) a small drop of thin cyano in the threads
will cure the jingling on the threads.
The Snap Link with
Keeper: There are several ways to keep the snap link from
being wrenched open and off, ranging from a piece of fuel tube
slipped over it to some neat little springs that do the same
thing. Perhaps the best, though, is the "golden link"
which has a metal clip that will keep the link positively closed
until YOU want to open it. For positive peace of mind, these are
worth the money!
Swing Keepers:
These are nylon clips that will keep an L-bended wire onto its
control or servo arm. They snap on the wire on both sides of the
arm, these make for a very cheap & reliable linkage and are
great for small to medium size models.
The Quick Link:
Perhaps the easiest of all to install, these are indeed quick -
just insert into a hole in the servo or control arm, snap on a
nylon keeper, and thread through the wire or cable, and tighten
the screw down where you want it. But these links have some
pitfalls which may cost you a plane if used on flight controls.
First, the nylon keeper "button" that holds the unit on
the control or servo arm may come off , allowing the whole thing
to disconnect - this is especially true if there is any wrenching
or twisting involved. And under those conditions, they may not
want to rotate properly, causing binding. Also, especially when
used with music wire, the screw can slip. Quick Links are quite
often included as kit hardware, but a lot of careful flyers will
only use them on throttle cables, if at all.
The Ball Joint: These cost
a bit more than other links, and are quite often worth it. A ball
joint has it all -
positive no slop control, no binding with moderate wrenching or
twisting, easily adjusted, and hardly any way they will
disconnect themselves. In any situation where the control arm
must be free to swing in more than one direction, this is the way
to go.
Having just gone through some of the most popular types of control rods & links a few important points to remember when connecting up linkages are,
Just because the links & rods came with the kit it does not mean that they are the best parts for the job, you can upgrade these parts to improve your setup.
Control runs need to be as straight as possible, as short as possible, & as slop free as possible without any binding or stiffness. They also need to be made up from quality components that are up to the task.
(e.g. not a small plastic snap link used on a quarter scale Extra 300 elevator control)
NOTE: Almost all the above hardware comes in two popular sizes - 2mm or 2-56unf and 3mm or 4-40unf. These figures refer to the wire and thread sizes involved, the 3 mm or 4-40 hardware being much heavier and stronger than the 2mm or 2-56 versions.
The heavier gear should be used in any plane above .60 size, and should even be considered for a fast .60, especially on aileron and elevator control systems.
Dave P
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This Site is maintained by David Probert, Committee Member for Alport Model Flying Club.