The flight control systems of most aircraft comprise a series of rods and bellcranks that connect the flight control surfaces, such as the aileron, rudder, or elevator, to a series of cables that run through the wings and fuselage to the yoke or stick. When motion is applied by the pilot to the stick or yoke, these cables transmit the motion to the control surface. Similar to the way an automobile needs regular alignments and adjustments to help it steer properly and track straight, the aircraft control system must also be regularly adjusted to ensure the aircraft flies straight and level.
When checking the rigging and tension in a flight control system, the first step is to put the stick/yoke in a neutral position or another position if recommended by the manufacturer. The ailerons are commonly the most time-consuming system to rig. During the process, a long, straight piece of metal is placed across the top of the control wheels and secured in place to keep the wheels together. Next, with the flaps in their retracted position, look at the trailing edge of both ailerons in relation to the trailing edges of the flaps. If properly adjusted, there will be a certain amount of drooping where the trailing edges of both ailerons hang slightly below the trailing edges of the flaps.
Prior to any adjustments, the next step is to check the cable tensions and determine baseline readings. Aileron systems have three cables - two connecting the left and right bellcranks to the control wheels and a third that connects the bell cranks. To check the cable’s tension, a tensiometer is used. This will give the most accurate reading for a cable tension when the tension is measured along a free section of the cable, away from pulleys or turnbuckles. Turnbuckles are internally threaded such that one side has a left-hand thread and the other has a right-hand thread.
When making tension adjustments, the cables must be held in place so they do not twist as the turnbuckle rotates. This can be done by inserting a bent welding rod or coat hanger through the holes in each cable end. As the tension on one cable is adjusted, the tension on the other cables changes and alters the control surface position. Once the tensions are set, the droop can be altered by lengthening or shortening the length of the rod connecting the aileron to the bellcrank. The rod ends are threaded and feature small holes to ensure the rod end is not backed off too far.
At this point, check the ailerons to ensure they have a full range of motion. The control surface should be able to move freely. Once everything is in place, all turbnuckes should be secured and all hardware installed. Additionally, all pulleys should be lubricated and inspected as they are moved to ensure they are not frozen. The elevator and rudder systems are as critical as the ailerons, but typically much easier to rig as they only have a single surface. Much like a properly aligned car is easier to drive, a properly rigged aircraft will fly better.
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