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Controlling Your Aircraft
By Ron Alexander(originally published in EAA Sport Aviation, May 1998)
In previous articles, I have discussed most of the hardware items needed to build your airplane. The last article in the April issue detailed rivets and their installation, aircraft screws, turnlock fasteners, and small miscellaneous hardware pieces. Again, emphasis needs to be placed on using only quality hardware purchased from a reliable source. In this article I will present information concerning the hardware items comprising control cable assemblies.
The performance and safety of an aircraft can be categorized into three main areas. The first is the airframe structure and design, second is the powerplant and related systems, and the third category is primary control systems. A loss of any one of the three would produce a ride one would not soon forget. This article will focus on the last category. To the operator of any piece of machinery, the turn of the wheel, pull of the handle or push of the knob should produce certain results. Failure to obtain the desired result in an aircraft control system could be catastrophic. I remember experiencing the loss of directional control in a 1950 Plymouth automobile years ago. As anxious as I became I felt assured that the brakes on that car would soon end the carnival ride I was experiencing. Unfortunately, air brakes, if your aircraft is so equipped, are not as effective. With this in mind, I will examine the components and procedures necessary to build, install, and maintain a reliable primary control system.
Cables and their related hardware is the most widely used linkage in primary flight controls on smaller aircraft. Cable type linkage has the advantage of being strong and light along with its flexibility making it easy to route through aircraft structure. Cable systems are mechanically efficient and may be adjusted without leaving backlash. This is important for precise control and to aid in preventing control surface flutter. Cables and their related components, like other aircraft hardware, are manufactured according to AN ( Army-Navy ) or MS ( Military Standard ) specifications.
Certain aircraft designs utilize a system of push-pull tubes for their primary flight controls. These tubes are usually fabricated out of an aluminum alloy tubing with threaded rod ends riveted into the tubing. Bell cranks are then used to change the direction of travel of the controls. This type of control system will often be partially or completely assembled in many kit aircraft. It is a relatively simple and effective way to move the aircraft controls. Even though our discussion will focus on cable assemblies and their fabrication, push-pull tubes offer a very viable alternative for flight controls.
Types of Control Cables
The actual cable used for the primary control of an aircraft is generally manufactured from galvanized steel ( zinc coated ) or corrosion resistant ( stainless ) steel. Its material, diameter of the wire, and type of construction describe the cable. Corrosion resistant steel is usually more expensive than galvanized cable but it has slightly less strength. Stainless steel cable should be used where corrosion will be a problem. Stainless cable also has a glossy appearance that is preferred by many builders.
The diameter of the cable obviously has an affect on its strength. A large number of aircraft use 1/8 inch diameter cable. Using this diameter as an example, its breaking strength is approximately 2,000 pounds. Production aircraft are required to use a minimum of 1/8 inch cable for primary controls. The diameter of cable you are to use should be spelled out in your plans or assembly instructions. It will usually be provided as part of your kit. It may be less than 1/8 inch and, if so, it should have been properly tested for the particular aircraft design.
The type of cable construction is somewhat more involved. One is termed a 7 x 7 control cable. That simply means that the cable is comprised of 7 strands of wire with each strand being made up of 7 individual pieces of wire. The second example shows a 7 x 19 cable. That means it has 7 strands of wire with each strand consisting of 19 individual wires. The 7 x 19 cable is more flexible than the 7 x 7 cable. However, the wires in a 7 x 19 cable are smaller and therefore more easily damaged. 1 x 19 cable is also available but it is non-flexible. This type of cable is often used for bracing purposes, drag wires, etc.. Again, the type of cable should be specified in your plans. The military specification number for aircraft control cable is MIL. W-83420.
Types of Control Cable Assemblies
Once you have selected the type of control cable to use there are obviously other hardware parts that complete the entire assembly. The type of hardware is dependent upon the method of fabrication. Three methods are commonly found and each has a different type of fitting on the end of the cable. The three types of end terminals are swage type, nicopress, and woven splice.
Swaged fittings have a tube like portion that allows the cable to be inserted directly into the fitting. The fitting is then compressed onto the cable using a swaging tool. See Figure 2. This type of fabrication has the advantages of being very strong and attractive in appearance. The big disadvantage is the swaging tool. They are very expensive ($3,000+) requiring most builders to either rent one or have someone else who owns the tool fabricate the cable. This is certainly an alternative that is available at a lot of repair stations or FBO maintenance facilities. You may prefer this type of cable assembly. It certainly has advantages.
The most commonly used method of control cable assembly involves the nicopress system. This method is very simple to use and the cost of tools is minimal. Nicopress fittings are made of copper while swaged fittings are made of steel. Copper fittings are easily compressed using hand tools. The standard nicopress tool costs less than $150. See Figure 3. This tool must be used to compress the fittings. Do not attempt to use pliers or anything other than the nicopress tool. Remember our earlier discussion—we are talking about a primary aircraft component that certainly affects aircraft control and safety. Utmost care should be taken in manufacturing control cable assemblies.
Lets look at the hardware parts that constitute a nicopress cable. A thimble is the first item. Thimbles are used to spread out the load of a cable to protect it and to allow the cable to reach its full rated strength capacity. Thimbles ( see Figure 4 ) are designated as AN100. They are made in different sizes according to cable diameter. They also are zinc plated or stainless. A typical designation is AN100-4—defining a zinc plated thimble used with 1/8 inch cable. An AN100-C4 is a stainless thimble for a 1/8 inch cable. A round cable bushing is also occasionally used in certain applications. These are similar to thimbles and their designation is AN111. The next piece of the assembly is a sleeve. The sleeve ( see Figure 5 ) is slipped over each end of the cable and then compressed using the nicopress tool. Sleeves are either plain copper that is used with galvanized cable or zinc that is used with stainless cable.
The last type of cable end fitting is the hand-woven splice. This method of cable assembly was used on a number of older aircraft but with the advent of the other two methods of cable fabrication it is seldom used today. The hand-woven splice requires a lot of time and patience and is no longer necessary because of the newer alternatives. Unless you are a real fanatic for originality of an antique I would not consider this as an option.
Fabrication of Control Cable Assemblies
For purposes of this discussion, I will assume that you want to use the nicopress method of assembly. With that in mind, lets step through the actual process of building up a control cable.
- Determine the length of cable necessary. This should be stated in your assembly manual. If not, you can use a stretchless cord the same diameter as the cable to be installed. Route this cord as the cable will be routed then mark it for cutting.
- Cut the cable. Invest in a pair of cable cutters. They are inexpensive and they will save you a lot of time and effort. After marking the cable, place a piece of masking tape around the cable where the cut will be made. This will leave a nice even cut and will keep the strands of wire in place during assembly. Do not cut with a torch or subject the wire to excessive heat in any way.
- Take the thimble to be used and cut off the 4 tips you will see on the thimble. This will allow the sleeve to fit more snugly.
- Route the cable through the sleeve and then through the turnbuckle end fitting or the fitting that is being used. Then route it around the thimble. ( Remember, copper sleeves for galvanized cable and zinc sleeves for stainless cable ).
- Place the taped end of the cable through the other portion of the sleeve to complete the initial assembly. Be sure to leave a sufficient amount of cable extending past the sleeve—usually an inch or so.
- The compression process will be easier if you slide a cable clamp over the loose end of the cable and push it securely against the sleeve.
- Make the necessary compressions of the sleeve as shown in Figure 6. Cables larger than 3/32inch diameter must have 3 compressions done in the sequence presented. The first compression is made in the center followed by a compression next to the sleeve. Then the final compression is made. Before making the final compression remove the cable clamp. Also, prior to beginning the first compression, insure that a minimum of 1/8 inch of cable will protrude out of the sleeve after the assembly is complete. The compression is better performed with a helper. If no one is available then clamp one arm of the nicopress tool in a vise. That will allow you to properly perform the compression without assistance.
- When all three compressions have been completed, use the "go-no-go" gauge that comes with the nicopress tool to check the width of the crimps. The widest part of the compression should slide into the appropriate slot of the gauge. If it does not, it means the sleeve has not been properly compressed.
- Carefully cut off any excess cable and leave at 1/8" inch protruding beyond the sleeve. Do not nick the working cable in any way.
- Mark the portion of cable that protrudes with a red paint. This will allow you to see if any slippage has occurred during subsequent inspections. You may want to slip a length of clear heat shrink tubing over the cable before you begin the nicopress crimping. When complete, you then slide the tubing over the loose end of the cable until it is against the sleeve and shrink it down. This prevents snagging a hand on the sharp strand ends of the cable and still allows you to view the red mark for slippage.
- A final recommended step is to test the cable by doing a pull test up to 60% of the rated strength of the cable. Using the torque-arm formula, a weight suspended from a beam on a fulcrum may be easily constructed to perform this test. The rated strength of aircraft cable may be found in Advisory Circular 43-13. Even if you purchase cables that are already assembled or have someone swage the fittings the pull test is desirable.
Installation of Cable Assemblies
After we have fabricated the control assemblies we now must install them in the airplane. Cables must be routed throughout the structure using fairleads and pulleys. These also insure proper movement without cable damage. Fairleads have no moveable parts. See Figure 7. Fairleads are used to prevent a cable from sagging and to allow a very slight change in cable direction—usually less than 3 degrees. Fairleads are commonly made of a plastic material that is softer than the cable to prevent cable damage. When a change of direction of more than 3 degrees is required in the cable a pulley is used. Pulleys are made of phenolic or aluminum material and have a ball-bearing center. Pulleys must also have some type of guard to prevent the cable from slipping out. Common designations of pulleys are AN210, AN220, and MS24566. All of these are phenolic pulleys. It is also very important that the installed cable be aligned with the pulley. This will prevent the cable from riding on the flanges of the pulley and chafing against any adjacent structure or the pulley guard. Pulleys are manufactured in different sizes according to the size cable used and its application.
After routing the cables through the aircraft structure, the cable tension must be addressed. Without proper tension the cable cannot do its job. We also need a way to remove a cable for a possible repair. The hardware item used to solve these problems is called a turnbuckle. Turnbuckles also solve the problem of making small errors in cutting and fabricating cables.
A typical turnbuckle assembly consists of a brass barrel and two steel ends. The two steel ends have different threads—one left hand and one right hand—to allow tightening and loosening. A few of the different pieces that comprise a turnbuckle assembly are as follows:
- AN155 - turnbuckle barrel
- AN161 - fork open end piece that fits over a bellcrank or tang ( piece of metal attached to structure)
- AN165 - end fitting in the shape of a pin eye designed to fit inside a fork
- AN170 - cable eye end fitting that matches a cable and thimble
Placing these end fittings and barrels together produces a complete turnbuckle assembly each of which has another set of AN numbers. An AN130 turnbuckle assembly is an example. See Figure 8. The entire assembly consists of an AN155 barrel, an AN170 cable eye end fitting, and an AN161 fork end fitting. Gets confusing doesn’t it. Again, your plans should specify what you need and if not, the aircraft supply catalogs will spell it out for you. An additional note, the turnbuckle barrel is available in long and short lengths. The turnbuckle end fittings are manufactured with both right-hand and left-hand threads. Virtually any necessary combination is available to construct your cable assembly.
Once a turnbuckle is installed in our control system it must be properly secured to prevent the ends from vibrating loose and releasing from the barrel. This is accomplished by using .040 safety wire. One method of safetying is shown in Figure 9. This is termed a single wrap method of safetying. Other methods are depicted in AC43-13. As an alternative to safety wiring the turnbuckle, you may want to purchase a special type of turnbuckle that uses a clip to lock the turnbuckle in place. The designation for this type of turnbuckle barrel is MS21251. Using this type of assembly precludes the use of safety wire and replaces it with simple clips ( MS21256 ).
As mentioned earlier, one of the functions of a turnbuckle is to provide proper tension on the cable itself. Once the cable assembly is in place, tighten the turnbuckle to achieve the proper cable tension. The tension value is dependent upon the ambient temperature and varies with the cable diameter. A typical value for a 1/8 inch cable at a 70 degree temperature would be about a 60 pound load. Several types of cable tensiometers are available to measure the proper tension. These tools vary considerably in price depending upon their quality. An alternative to purchase may be to borrow or rent this tool as you will usually only work with it one time.
Turnbuckle Installation Tips
Use a tensiometer to measure the proper amount of cable tension.
A handy tool for tightening a turnbuckle can be made using a piece of welding rod. This tool, pictured in Figure 10, can be placed in each end fitting and the barrel assembly rotated without moving the end pieces.
When the proper cable tension is achieved, no more than 3 threads of each end fitting should be exposed.
Do not lubricate turnbuckles. Lubricating only helps them move which we want to prevent.
Safety the turnbuckle using .040 safety wire and the proper techniques. Do not reuse old safety wire. Clip-type turnbuckles are available that do not need safety wire.
Inspection and Repair of Control Cables
AC43-13 provides a very comprehensive discussion on proper control cable inspection techniques. Several pictures are presented showing wear areas and patterns. I would recommend reviewing these pages. Basically, you will want to regularly inspect your control cable system. A few of the basic items to inspect are:
- Security of turnbuckles and hardware.
- Incorrect routing of cables.
- Fraying, binding, or twisting of cables. Fraying is easily detected by passing a rag over each cable. The rag will snag on a broken strand of wire.
- Check cables very closely in areas subject to high corrosion such as battery compartments or wheel well areas.
- Rotate pulleys to check for freedom of movement and to insure that the pulley does not wear in the same place.
- Be sure cables are properly aligned with pulleys and are not chafing against their guard.
- Do not use solvents to clean cables. Doing so will remove internal cable lubricants.
- Lubricate galvanized cable using Par-Al-Ketone, or a low temperature oil.
Splicing of control cables is permitted in accordance with AC43-13. Splices shown in AC43-13 are pictured in Figure 11. Obviously, a splice is not permitted where it would pass over a pulley or through a fairlead.
Remember to follow your kit manufacturer’s recommendations as you fabricate your control system. After it is installed be sure to periodically inspect the cables and supporting hardware. You are dealing with one of the most important systems found on the aircraft. Prior to test flying your airplane, insure that the control systems move the flight controls in the proper direction and are moving the proper amount of travel. Also, as part of the rigging procedure you must be sure that the precise number of degrees of travel of each control is being achieved. A properly installed, well-maintained control system will result in reliable and safe control of your aircraft. Take your time and be very meticulous during this phase of construction.