For lighter aircraft and aircraft with slow airspeeds, the control surfaces, while being proportionately larger than those of faster aircraft, have less air loads acting on them when the control surface is deflected. This enables the designer to use a direct mechanical linkage between the controls and the control column/rudder pedals.
CABLE TENSION AND TURNBUCKLES
Turnbarrel
Lock Nut Adjuster Thread
Safety Check Hole
Wire Locking
Diagram 5.14 Turnbarrel
Control Cable
To ensure that the pilot’s control inputs are instantaneously transferred to the control surfaces in systems where control cables are used for primary or secondary control operation, the cable system is given a pre-tension. This tension is adjusted by the use of turnbuckles (turnbarrels), which are locked to ensure that they do not self adjust with the cable’s operation. Diagram 5.14 shows this.
If the control cable system loses its tension and becomes slack, the control surfaces can make uncommanded movements and the system suffers from backlash.
Sprocket
Sprockets and Chain Guard Aileron Control Chains
Diagram 5.15 Chains and Cables Control Cables
In a cable system where both inputs and outputs require a rotary motion, the cables attach to a chain passing over a sprocket. Refer to diagram 5.15 showing a control wheel system.
CONTROL CABLES
Diagram 5.16 Cable Operated Controls
Two control cables to the input control column/rudder pedals link each control surface, as cables can only operate in tension as in diagram 5.16.
When the pilot makes a control input, the cable under tension moves the control, and the second cable follows the movement of the control surface. When the reverse input occurs, the function of the cables reverses. Refer to diagram 5.16.
Temperature Compensator
Control Cable Fairlead
Cable Guard
Pulley
Control Rod to Surface
Diagram 5.16A Temperature Compensator, Fairlead and Pulley
For larger aircraft where the expansion and contraction of the fuselage and wings causes them to grow or shrink by several inches, cable systems require a thermal or spring compensator to be fitted. See diagram 5.16A for an illustration. This ensures that the pre-determined cable tension is maintained in the cable run.
To ensure that cables and airframe structures are not damaged by the back and forth movement of the cable, fairleads are fitted where the cable run passes through or by structure, such as frames. The JARs determine that fairleads must be installed, so that they do not cause a change in cable direction of more than 3°. It is normal practice for the cable to be routed through an enlarged aperture where fairleads of a softer material than the cable are fitted. When correctly fitted, the cable does not contact the fairleads, as diagram 5.16A shows.
To ensure that cables change direction smoothly and the system creates the minimum friction possible, pulley wheels are used. Guards are fitted to ensure that the cables and chains do not jump out of the pulley wheels and sprockets, see diagram 5.16A.
The pilot gains mechanical advantage by having the control cables fitted to the base of the control column and by lever horns attached to the rudder pedals. In some aircraft, the control cables directly attach to the control surfaces. For those aircraft where the air loads are slightly higher, the cables attach to bellcranks, and a control rod links the bellcrank to the control surface. See diagram 5.16 and 5.18. This system increases the pilot’s leverage for a slight reduction in movement.
CONTROL RODS
The control rod system makes use of a tube that can withstand both compression and tensile strains, thus allowing a single rod to operate a control in both directions. Again, where rods pass through structure, the fairlead system is used. To support the rods along their length, idler levers can be used. When a control run using rods has to change direction, lever bell cranks are used, as can be seen in diagram 5.17. To compensate for thermal expansion and contraction, a spring strut can be inserted in the run.
CONTROL STOPS
Primary and secondary stops are fitted to prevent the pilot from inputting a control movement that Secondary Control Stops
Push -Pull Rod Bell Crank
Pulleys
Primary Stops
Primary stops are located at the control surface end of the control run. These are adjusted to give the full range of movement for the surface, and then locked, so they do not alter. Diagram 5.18 shows the two primary stops limiting the range of movement for an elevator.
Secondary Stops
Secondary stops are located at the input end of the control system. These are adjusted so that there is a small clearance between them and the control system when the control surface is fully deflected, then locked.
These stops ensure that the pilot has full range of movement of the control. Should the pilot try to force the control column/rudder pedals beyond the control surface’s limit, a physical stop prevents damage to the control linkages. Refer to diagram 5.17 for secondary control stops. These are not normally seen, as they fit below the flight deck floor.