The purpose of a control linkage is to take the motion generated by the radio control servos and transfer it to the airplane’s control surfaces and other controll devices. Since this motion is mechanical, there are considerations for choosing one technique versus another. So, in it’s simplest terms, a control linkage will include a servo control arm, push rod, control horn, a way to attach the push rod to the servo control arm and control horn, some way to adjust the position and distance of movement and the controlled device itself. This is obvious to those of us who have been around the R/C circuit for a while, but to the newcomer, this is a challenging topic.

Always plan ahead and avoid mechanical interference between the moving parts. Engine vibration and G-forces will cause our control linkages to behave erratically. These introduce stress and must be considered, even in a docile trainer.

Cost

The real cost of the control linkage is the price of the entire model if it were to fail doing its job! If we take into consideration the initial cost of the hardware, the time it takes to install, adjust, and lock, special tools ,as well as any maintenance during the life of a model, we might want to consider using the higher initial price of carbon fiber push rods (titanium ends give you special bragging rights!), nylon bushed control horns, ball/stud clevises, etc. The old adage “You Get What You Pay For” comes into play here, especially for the Giant Scale and speed models. Many times we use parts because they are part of a kit. We forget that the kit manufacturer makes choices based on their costs – many times providing parts that “will do”, but may not be the best for the application. Some don’t even provide these parts, leaving the choice to the preference of the model builder.

Precision and strength

The important measurement for the control surface is will it provide the proper movement, with no slop, exact mechanical repeatability, no wear, and no maintenance.  It must tolerate the stress placed on it during normal reasonable flight. It should tolerate changes in temperature and wear slowly. Parts that have been problematic over time:

Threaded metal clevises that can split apart and/or become stripped by vibration. Sullivan provides an interlocking design that is good.

Nylon parts that are too soft or brittle

Wooden dowels that twist and warp from moisture

Incorrect application or number of supports

Incorrect application (ie. Braided wire for elevators yikes!??)

Size and space

These seem obvious, until considering that each model has many moving parts and these may interfere with each other as they move. Some planning for the elevator

and rudder push rods is required even on ARFs or problems will occur.  Some of these problems occur with the aileron movement, noticed only once the wing is mounted to the fuselage (parts hit items mounted in the fuselage). Sometimes the needed supports cannot be installed because the construction has already progressed past the point for making this easy (like an ARF fuselage).

Mechanical Gain and Differential

Many times the control horn and servo arm have different locations for installing the push rod. If the push rod (or pull-pull cables) are installed at the same distance from the pivot center, the travel and torque are linear. Some modelers will install the push rods so they are in a mounting hole further from the pivot center in the servo and closer to the pivot center at the control surface. This will increase the travel and decrease the torque. For precision and higher torque, moving the push rod to the inner most hole on the servo end and the farthest from the pivot point in the controlled surface provides the highest torque and the greatest precision (but the lowest possible movement). Some vendors provide longer servo arms to help in getting the amount of travel a control surface needs. Just keep in mind that the torque rating for a given servo is given at 1-inch from the servo pivot point. At one-half inch the effective torque doubles, while the travel is reduced.

Wear

Providing free movement for our control linkages is one of the goals. Checking that wear has not created slop is one of the routine inspections we should make. Those nylon parts will wear oval holes where they were once round. This introduces a great amount of slop. Check and replace these as needed. Make sure the parts aren’t too tight. This speeds up the wear and causes repeatability problems.

Weight

Although not usually a primary factor, weight in some of the lighter models is a BIG thing.  Building with components that add unnecessary weight is poor practice. Using composite materials like carbon fiber rather than wooden dowels or threaded steel rods make a difference in both weight and precision. Usually the choice of materials is dependent on several of the other factors already mentioned. A good scale (digital or otherwise) is a good investment for the builder. Choosing parts that perform identically based on their weight is the right way to build. If a model needs additional weight for balance, why not choose the parts that will help balance the model, rather than installing dead weight (i.e.

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