Transmissions & Control Cables
Gear types and gear ratios; steel-wire control cables, turnbuckles, pulleys and tensioning.
Transmissions & control cables, summary notes
- 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.
- Gear ratio
- ratio = teeth(driven) / teeth(driver)
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.
Gear trains, ratio, torque & direction
Transmissions & cables concept map
Transmissions & Control Cables
Transmissions & control cables quiz
Transmissions & Control Cables, quiz
1. In the gear train shown, a 40 tooth gear drives a 10 tooth gear. The small gear turns:
Four times for each turn of the large gearOnce for each four turns of the large gearAt the same speed as the large gear2. A gearbox has an input speed of 2400 rev/min and a reduction ratio of 3:1. The output speed is:
800 rev/min7200 rev/min2400 rev/min3. A gear with 40 teeth drives a gear with 10 teeth. The gear ratio is:
4:11:4400:14. Gear ratio is calculated as:
Driven teeth ÷ driver teethDriver teeth ÷ driven teeth × speedTeeth added together5. Gearing down (reduction) results in:
Lower speed and higher torqueHigher speed and higher torqueLower speed and lower torque6. A worm gear is notable for being able to be:
Self-locking (drives one way only)Driven from either side equallyUsed only at high speed7. Epicyclic (planetary) gears are chosen for:
A high reduction in a compact space (e.g. prop gearbox)Right-angle drives onlyZero reduction8. 'Backlash' in a gear train is:
The free play between meshing teethThe tooth hardnessThe lubricant type