Flexible Control Systems

Normal aircraft cables are only capable of performing a pulling action, due to their lack of rigidity. So, where a two-directional movement (push/pull) is required, it would be necessary either to employ the use of rods with the attendant weight penalty or to use flexible control systems. The two most common are Bowden Cables and Teleflex Control Systems.

Bowden Cables

The Bowden system of control consists of a stainless steel wire, housed in a flexible sleeve or conduit (refer to figure below). The control is intended for pull operation only, with the cable being returned on release of the control lever by a return spring. The transmitting end of the cable is attached to the actuating lever, whilst at the receiving end, the cable is secured to the component to be operated.

The flexible cable is made up of several strands of stainless steel wire with nipples soldered onto the end of the wire. The nipples are of different shapes, depending on their use. The flexible conduit consists of close-coiled wire, covered with cotton braiding and a waterproof coating. For long runs or runs not requiring flexibility, the Bowden cable is fed through rigid metal tubing, which can be bent over large radius curves if required.

FIGURE

The control fittings are used at each end of the cable to transmit and actuate the movement required. These fittings are the hand levers and adjustable stops (refer to figure below). The illustration shows a simplified set-up of a Bowden cable control with an operating lever and an adjustable stop. The double-ended stop is used if the component does not permit access to the stop at that end of the cable.

At points along the conduit, connectors may be found which allow the conduits to be separated for maintenance. Junction boxes are also used to permit either more than one input, to actuate a single operating lever or one input to operate a number of operating mechanisms.

FIGURE

Teleflex Control Systems

The Teleflex control system differs from most other cable control systems in that, rather than have a pair of cables. The Teleflex system allows a single, flexible cable to operate in both push and pull mode, without the need for a return spring. Examples of the types of systems, operated by Teleflex controls are engine and propeller controls, trimming controls and fuel valves. Teleflex controls can also be used to transmit movement from one place to another, such as in a mechanical Flap Position indicator or as interlocks between controls and throttles during control lock operation.

Like the Bowden system, the Teleflex system consists of a flexible transmitting cable operating inside a rigid or flexible metal conduit. The main advantages are that it provides a more accurate and positive control throughout the range of movement and the controlled component can be temporarily locked in any desired position.

The control cable is a unique design of a helically-wound high-tensile steel wire (left/right-handed coil). The ‘pitch’ of the wire coil is designed to engage with gear teeth of the control units and the end-fittings.

The conduits operate in a similar manner to the Bowden system and are made from copper, aluminium or steel. The linings are of PTFE on most conduits except in high temperature areas like engine bays. To operate the system, the cable and conduit are connected to control units at each end of the control run and in between to other units and fittings, which are used to direct the run. In many locations, the cables are attached to lever-operated wheel units or to push-pull handles. At the receiving end of the run, another wheel unit or sliding end-fitting is used to actuate the mechanism.

The Teleflex system allows a variety of controls to operate a wide selection of end-fittings (refer to figure below).

FIGURE