I previously wrote about two different types of control cables: threaded end vs. solid wire end, and some issues associated with each. In this article, I will highlight typical installation techniques and dive into several categories to assess each style of control cable from an engineering perspective.
There are many options for fastening control cables. For the outer jacket, we have Adel clamps (or P-clamps), eyeball fittings, and collet-type fittings. Eyeball and collet fittings are perfect for firewall pass-throughs. Both of these options work well when sized correctly. Adel clamps are slightly less robust and may allow the cable jacket to move around a little due to the rubber insulator. I would recommend them for everything but a throttle or mixture cable. The other two styles perform double duty when passing through the firewall by securing the cable and sealing up the hole.
For threaded end cables, the inner wire attachment is as simple as threading on a rod end or clevis and securing with jam nuts. A familiar AN3 or AN4 bolt or pin attaches this to the bellcrank.
The inner wire attachment for solid wire controls is where things can get sketchy. Most options involve some variety of squishing the solid wire, much like a set screw. I have a lot of gripes with set screws that I won’t go into here, but here are a few examples. The cable “B nut” (not to be confused with the AN tubing nut) uses a set screw to retain the solid wire end and secures to the bellcrank with a cotter pin and a stack of washers to take up any gap. The wire grip is a really tiny collet that clamps on the outside diameter of the solid wire and leaves a threaded end to work with. A wire clamp bolt, often seen on certified aircraft, uses jam nuts to squish the solid wire in a shearing fashion.
There is no list of approved methods for cable attachment, though there are some certified fittings. There is no reference in AC 43.13-1B regarding hardware to use or not to use. So, for an experimental builder, there is no clear path to the best option like there is with AN bolts, grades of spruce lumber, or solid rivets. Though I haven’t found a reference, I would wager that these methods previously used on certified aircraft underwent some level of pull or strength testing.
Now let’s look at a range of categories to weigh the pros and cons of each style of cable.
Cost
Threaded end cables are usually a little more expensive due to additional fittings.
Length Precision
Since solid wire controls can be cut to length, the fit is usually perfect. Threaded ends come in standard increments or custom lengths, which add more cost. Getting the perfect length plays a huge role in minimizing friction in the system.
Routing
Solid wire cable typically has a smaller jacket diameter and has less bulk at the end fitting attachment, which makes routing much easier. Threaded ends have a rigid straight section of 4 to 6 inches at the attachment end, which can take up a lot of space when routing.
Ease of Attachment
Bolts and nuts are much easier to install and don’t damage the cable in the process. Set screws and clamp-type fittings on solid wires can distort the wire and make things difficult to remove.
Strength
The inner wire used in most control cables is an extremely high-strength steel similar to music wire (ASTM 228). This grade of steel typically has a tensile strength of over double that of a high-quality grade 8 bolt. This is due to the intense cold-work drawing process that progressively forms the round wire shape. For reference, a 1/16-inch-diameter music wire can support around 800 pounds in tension.
In theory, the strength of each cable style is the same given the same inner wire diameter. However, set screws in solid wire attachments become the strength limit compared to pure tension in the inner wire.
Stress Concentration
Stress concentrations are usually described in two forms: a change in cross-section, or surface imperfections, such as a scratch or gouge. It is important to note that with a very high-strength material like music wire, stress concentrations play a much larger role in failure compared to weaker, ductile materials. They cause sudden and complete failures rather than gradual, detectable ones.
Both cable end styles contain stress concentrations as seen in the images below. Any region of swaging induces a cross-section change, and anything clamping on the surface induces surface imperfections.
Vibration Resistance
Bolts, nuts, and swaging resist vibration better than set screws.
Failure Mode
The resulting failure we would expect from a threaded end would be a tensile break at the swage location (stress concentrator), where the strength has been slightly reduced. For the solid wire end, we expect the inner wire to slip in the fitting.
It is clear that neither option is perfect. I have a strong preference for solid wire controls due to their simplicity, smaller size, flexibility, and ease of routing as a result. The negatives, however, are too great to ignore.
Look out for Part Three of the series where I explain what I believe is the best of both worlds: the rolled wire end.
