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.