Section 6.9 / 6.10

Transmissions & Control Cables

Gear types and gear ratios; steel-wire control cables, turnbuckles, pulleys and tensioning.

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Main ideas

Gears transmit torque and motion and change speed/direction. Spur gears link parallel shafts; helical gears run quieter but create axial thrust; bevel gears turn the drive through an angle; worm gears give a large reduction and can be self-locking; epicyclic (planetary) gears give a high reduction in a compact package (propeller reduction gearboxes).
Gear ratio = driven teeth ÷ driver teeth; speed and torque trade inversely — gearing down multiplies torque and reduces speed. Watch backlash (free play), tooth wear, pitting and correct lubrication; belts and chains need correct tension.
Flight-control cables are flexible steel wire rope (e.g. 7×19 for flexibility) in carbon or corrosion-resistant steel. Ends are swaged, Nicopress or ball/shank terminals; turnbuckles adjust tension and are then safetied (lockwire/clip).
Cable runs are guided by pulleys, fairleads and quadrants. Tension is set with a tensiometer and corrected for temperature; cables are inspected for broken wires (wipe with a cloth), corrosion, wear at pulleys and correct rigging.
Bowden and push-pull (Teleflex) cables transmit control in a sheath; correct routing avoids sharp bends that add friction and wear.
⚠ Exam trap: control-cable tension is set with a tensiometer and CORRECTED FOR TEMPERATURE; a worm gear can be self-locking (drives one way only); and gear ratio trades speed for torque inversely — you cannot gain both.

Key formulas

Solved examples

A rigging check gives the correct cable tension reading on a hot afternoon, but the aircraft was rigged to the same number on a cold morning. Is the rigging correct?
Not necessarily. Cable tension varies with temperature as the airframe and cable expand/contract, so the tensiometer reading must be corrected using the rig chart for the ambient temperature. Setting the same raw number at two different temperatures leaves the cable over- or under-tensioned.

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