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Control Systems Cable Terminals and Connectors
By Tony Bingelis (originally published in EAA Sport Aviation, October 1981)
FROM THE EARLY days of aviation up to the present aircraft builders have been, quite effectively, transmitting control inputs from the cockpit to the control surfaces with control cables.
A somewhat heavier, more complex and expensive means for doing the same thing was with push-pull control rods and torque tubes. And sometimes a combination of push-pull rods and control cables was used. This eventually led to what is now a very common practice of using push rids between aileron control surfaces and bell cranks mounted on the front spar. Between the bell crank and the control stick, cables are used to complete the system. Likewise, push-pull rods are often used for elevator controls in a similar arrangement in that cables from the elevator run to a walking-beam and the control stick, a push-pull rod is used to complete that type of hook-up.
Rudder controls almost without variation are cable actuated.
As yet, we amateur builders have not gotten into remotely activated control systems and probably never will, so it is most likely that a mix of control cables and push-pull rods will be an essential feature for whatever design you are now building or plan to complete in the near future.
Regardless of the means you choose for transmitting your cockpit control inputs to the control surfaces, selecting the correct type of terminal and connecting hardware for fabricating each assembly to fit your aircraft can be a frustrating experience if your plans do not show your control system in detail. More often plans do not offer any more than a mere suggestion that there are control cables or whatever . . much less a complete Bill of Materials.
The number of terminal connections you will have to fabricate will probably be greater than you would have reason to expect because each cable end must be provided with some sort of a connecting terminal and some means fo adjusting tension.
It may surprise you to learn that it takes 20 to 30 terminal connectors to complete a flight control system in even the simplest of aircraft designs. Add to that requirement a handful of bolts, nuts, washers and cotter keys and you immediately develop the need for a sizeable parts inventory. Since most of us do not have a lot of parts on hand, the next best thing would be to know exactly what is needed and order it all at one time. Is there anything more frustrating than to find at the last minute that you lack a clevis, a thimble or a couple of nuts to complete the control system installation?
The surest way to determine your control system terminal connector and hardware requirement is by roughly sketching each control unit and tallying the items needed at each end. In any cable operated system, for example you will need at least 7 turnbuckles. Do you need turnbuckles with an eye on one end and a fork on the other, or should both ends have an eye terminal? This is the type of detail you should not overlook.
There are two modern ways to fabricate a cable terminal. One way is with a swaged terminal and the other with a Nicopress terminal splice.
Swaged Cable Terminals
Swaged fittings are very nice . . . classy, you might say, but they do require access to a special swaging machine and special terminal fittings designed specifically for that purpose. They cannot be installed on cables with tools normally found in a homebuilder’s workshop. Consequently, cables formed with swaged fittings are not too common in homebuilts. The expense incurred in "farming out" control cables for fabrication is guaranteed to keep many homebuilders from acquiring swaged cables.
You have to admit though that swaged terminals look good and permit a nice streamlined installation. If you feel you have to have them you might consider cutting costs by having only one end of each cable swaged commercially and complete the other end yourself with a Nicopress-formed terminal splice. That way you won’t have to worry about determining the exact length for each cable you send out to be swaged or worry about floating a loan for the whole job.
When you get your swaged cables back, inspect the terminals closely. They should be free of splits and die marks and the swaged fittings should not be out-of-round.
The swaged fittings will probably have been checked with a "Go-No-Go" gauge after swaging to be sure they had been properly squeezed. But you could, in addition, subject the cable and terminal to sufficient proof loading to quell your own anxiety that the cable might slip right out of its swaged fitting. Gradually loading it up to 60% of its rated strength should satisfy even the most devout skeptic. Actually, a properly swaged fitting will develop the full strength of a cable and then some.
Nicopress Sleeve Terminal Splices
A Nicopress-formed terminal splice is not as classy looking as the swaged terminal but it is more popular because the cable terminal can be fabricated right in your own shop using a Nicopress Tool (trade name) to squeeze a Nicopress Sleeve so snugly around a cable that the resulting splice is as strong as the cable. The procedure is simple enough and a good splice (terminal)is virtually assured every time.
Fabricating control cable terminal splices requires but a minimum of hardware and tools. In addition to the cable, a Nicopress Sleeve and a Nicopress Tool make up the essentials. An optional additional tool, a pliers-like cable cutter, is handy to have but you can cut the cable as well with a cold chisel.
Nicopress fittings are less costly and you can even make the cable splice on your aircraft, if necessary, to guarantee a perfect cable fit.
Many EAA Chapters have purchased a Nicopress Tool and make it available to the membership. It usually gets frequent and heavy use by the members and is a worthwhile Chapter asset. The biggest problem is in keeping track of the tool as it often changes hands several times before being checked back into the Chapter custodian of such things.
The standard aircraft control cable used is 1/8" in diameter and is identified as MIL-C-15511A (galvanized) and MIL-C-5424 (stainless). I’ve also seen other numbers but just about everyone refers to it as 7x19 extra flexible aircraft control cable.
The corrosion-resistant steel cable, although fractionally weaker than the galvanized steel cable (1,760 vs. 2,000 psi), is highly resistant to corrosion and will maintain its strength indefinitely. However, you will have to pay the price. As a result, the high priced variety is usually a second choice with homebuilders.
Aircraft with light control loads, such as ultralights, use 3/32" extra flexible 7x19 cable for the controls. The strength of the 3/323" cable is virtually identical for both the corrosion resistant variety and the galvanized type (about 920 psi).
Before cutting off a length of cable wrap a bit of masking tape around it and make your cut through the middle of the wrapping. Both cut ends will be protected from fraying and unraveling allowing easier assembly later.
When measuring and fitting your cables to an exact length, be sure to screw in both ends of your turnbuckles so that a few threads show. When the completed cable is installed and the turnbuckle tightened to obtain the proper cable tension, no more than 3 threads should remain protruding from either end of the turnbuckle.
Figure 1 shows the detailed sequence for making a Nicopress Sleeve terminal splice in a cable. Notice that the terminal loop is formed around a Thimble (AN100) to form a better bearing surface for the connecting pin or bolt. The Thimble also serves to protect the cable, and allows the development of the full 100% rated load in the cable. Should the splice be formed without a protective thimble, reliability would be compromised and cable failure may occur well below the cable’s rated strength.
Although Thimbles are available in cadmium plated and stainless steel, the recommendation is to use the stainless variety. When ordering your Thimbles, the AN number to use is AN100-C4 for both the 3/32" and 1/8" cable sizes.
Nicopress Oval Sleeves
For 1/8" cable you will use the 18-3-M Nicopress Oval Sleeves. I prefer the plain copper type . . . most supply houses carry them in stock as it is the most popular variety. The 3/32" cable requires a smaller 18-2-G sleeve.
The letter "M" or "G" in the specification number also identifies the correct opening to be used in the cable swaging tool for compressing the Nicopress Sleeve.
The 18-4-M Nicopress Sleeve requires 3 compressions in the sequence. The sleeve will stretch somewhat during the compression process increasing in length from approximately 9/16" to about 3/4 " after it has been squeezed.
Heat Shrink Tubing
A short piece of heat shrink tubing (Radio Shack and Avionics shops), if slipped onto the control cable before forming the Nicopress terminal, will allow it to be shrunk in place later with an electric soldering gun or other heat source.
Making The Connections
Your control system components will generally have one of two basic types of terminal ends, and eye-end or a fork-end. It is often a challenge to match the control cable with the proper terminal to accept the rudder horn, elevator horn and similar components. The listing is by no means exhaustive and other combinations may be equally functional.
Generally speaking, drilled bolts and clevis pins are used in connecting control system elements and these are securing with cotter pins and castle nuts.
Turnbuckles, of course, must be safety-wired to keep them from loosening. The correct way to do this is illustrated in a number of EAA references and in CAM 18.
Self-locking nuts should never be used where there is movement between the nut or bolt head and the underlying fitting or part. It is O.K., however, to use self-locking nuts in most rod end bearing installations as the movement would take place in the rod end bearing and not under the bolt head or nut.
Reminders . . .
All connections in a control system should be made accessible for inspection and perhaps lubrication.
Avoid locating connections closer than 2 inches from any pulley bulkhead or fairlead as jamming may occur during extreme limits of control travel.
Select the proper dash number for each hardware part and avoid mixing sizes.
A number of the terminals are accepted industry builders since the early days of aviation. Others are unique and may be sued to solve some particular problem.
They say a chain is as strong as its weakest link, and so it is with a control cable assembly. No fitting or part used in the control cable assembly should be weaker than the primary cable. As always, the recommendation is to use whatever guidance has been provided you by the designer of the aircraft you are building. If no guidance is in evidence, you are, of course, on your own and just maybe all of your controls can be installed using a combination of terminal connections illustrated.