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Cable System - Description

This module prevents an overview for cable systems. The module only deals with aspects of cable systems and does not address a complete flight control system. A cable system is considered a functional element within a flight control system and as such is addressed as an individual element in the series of Cable System modules. A simple elevator control system is shown in Figure 1. The elevator system shown in Figure 1 shows a linkage connecting the pilot to a sector and a sector to a flight surface. In figure 1, the cable system consists of both sectors and the cable. Flight control systems are addressed in the Flight Control modules (see Flight Controls Systems – Description).

Figure 1 Simple Elevator Control System

In general, a cable system is defined as two cables connecting into a cable sector (quadrant), T-lever, capstan or bellcrank arrangement, plus any pulleys and fairleads. There must be two cables since the cables can only be pulled, i.e., they can only take tension loads. This leads to a cable system as illustrated in Figure 2 and Figure 3. When sectors are used they will usually have equal pitch diameters. Therefore, no mechanical advantage is gained in the cable system. When T-Levers or bellcranks are used the radius may change from one end of the cables to the other to obtain a non-unity gearing ratio. Selection and features of sectors and T-Levers are discussed in Cable System – Sectors and Cable System – T-Levers. The main difference between a system with a sector and a system with a T-lever is that the torque applied to a sector for a given cable tension is the same for all rotation angles of the sector. For a T-Lever the torque is impacted by the angle between the cable and T-lever arm so cable tension will vary with rotation angle for a fixed input torque.

Figure 2 Basic Cable System with Sectors

Figure 3 Basic Cable System with T-Levers

Cables (wire rope) are constructed of individual wires that are wrapped together to form a strand and then weaving several strands together. For example, a 7 x 19 cable consists of seven strands woven together with 19 wires woven together to from a strand. This makeup of cables gives cables good strength characteristics while maintaining a good flexibility. However, as cable is stretched or wrapped around a sector or pulley, the wires rub on each creating friction and wear. Therefore, cables do not have infinite life. Cables should be periodically inspected in airplanes for wear (flattened or cold worked wires, broken wires and excessive wear in a pulley/sector groove). Cables are discussed in more detail in Cable System – Cables.

The main advantage of cable systems is that a force (or torque) can be transferred the length of an airplane with relative simplicity and high efficiency. Cable systems are also fairly lightweight, easy to maintain and low cost. The disadvantage is that the systems are not rigid since a cable will stretch under load. Also, in cable systems with small angle pulley cable wraps there may be undesirable wear at these pulleys, leading to increased maintenance and shorter inspection intervals.

Cables are routed through an aircraft fuselage and wing using pulleys. A pulley is used whenever a change in direction of the cable is required. Pulleys can also be used on long cable runs to prevent excessive droop or to ensure a slack cable will not catch on some other component or structure. An illustration of a system with pulleys is shown in Figure 4. Each pulley must lie in the plane defined by the cable runs entering and leaving the pulley (two lines define a plane) so that the cable does not rub against the pulley flange. Pulleys add friction in the system and introduce a cable wear location. Thus pulleys (direction changes) should be kept to a minimum. Selection and features of pulleys are discussed in Cable System – Pulleys.

Figure 4 Cable System With Pulleys

When the cables are installed a turnbuckle is used to preload the cables. Preload tensions are in the range of 75 – 150 pounds. A typical reversible flight control system will have cable pre-tension in the 70-90 lb range, while an irreversible system may have cable pre-tension in the 130-150 lbs range. Cable pre-tension are set such that the cable won’t go slack under maximum expected pilot forces. Also, cable pre-tension may be set higher to increase cable stiffness (i.e., to achieve less cable stretch for a given load), since stiffness increases with increased cable tension. More information on cables can be found in Cable System – Cables and more information on turnbuckles can be found in Cable System - Turnbuckle. An example of determining the cable pre-tension level can be found in Cable System – Design Example.

To understand the dynamics of a cable system, refer to Figure 5. Under the applied torque shown in Figure 5, the tension in top cable will increase while the tension in the bottom cable will decrease. When cable tension increases, the cable stretches. When cable tension decreases, the cable stretch (cable length) is reduced. For maximum expected pilot input, a design goal is to not have a cable go slack. Slack cables are undesirable because slack cables are at risk for catching on surrounding structure, fasteners or possibly other equipment. The cable preload should be set such that the bottom cable will not go slack when the top cable is stretched to maximum expected pilot input torque. For commercial aircraft, the maximum expected pilot input is defined by 14 CFR 25.143(d).

Figure 5 Cable System With Loads

For forces acting on a sector, the tension acts equally and opposite in direction on both sectors as shown in Figure 6. A torque balance on the left sector includes the pilot applied torque, torque due to top cable (tension top x pitch diameter), torque due to bottom cable (tension bottom x pitch diameter), and friction due to pulleys/sectors (includes friction due to cable wrap and bearing friction). Cable tension arises from the initial pre-tension and then any difference between θ1 and θ2, which implies additional cable stretch that occurs when a pilot force is applied against a reacting surface hinge moment.

Figure 6 Cable System Showing Free Body Diagram

More detail on cable system equations and modeling can be found in Cable System – Equations. Design consideration for design and layout of cable systems can be found in Cable System – Design Considerations. A design example for a cable system is presented in Cable System – Design Example.

Cable systems are used for other functions beyond primary flight control. Cable systems are also used to connect an autopilot servo to a sector or T-Lever, control of secondary trim systems (such as trim wheel connection to a pitch trim surface) and other functions such as landing gear freefall cables. Autopilot servos often utilize a cable capstan to create greater cable travel from a small diameter sector (see Cable System – Capstan). The fundamentals covered in the Cable System modules apply equally well to these other cable system applications.