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Sub Trim and Linkage Setup |
Richard Lindberg
Sub Trims are intended for minor adjustments to servo linkages. Using excessive sub trim values (percentages) can cause servos to be over-driven where they try to move past their internal stops. This can cause servo damage.
The diagram below illustrates an ideal servo/linkage setup when the servo is at neutral. Notice that at the servo arm is positioned at 90 degrees or perpendicular to the servo. Also note that the linkage or rod is attached at 90 degrees to both the servo arm and the control surface horn. This setup will result in the same amount of throw in both directions (0 differential throw).
Follow these steps to help ensure the proper use of Sub Trims and to achieve an optimum servo/linkage setup.
Access the Sub Trim function on your radio and make sure Sub Trim settings are set to zero (0).
Access the Trim Offset function (another name for Trim Memory) and clear any offsets. Also make sure that the mechanical trim levers on the transmitter (TX) are centered in their center-detents.
Plug the servo into the appropriate channel of the receiver (RX). Turn on both the TX and RX. The servo should now be at its electronic center position.
Test fit the servo arm to the servo, trying to get the servo arm to be at 90 degrees to the servo as shown in the illustration. Try different positions if necessary, removing the arm, rotating it 90 degrees, and inserting it back onto the servo output shaft. Use the position that is closest to 90 degrees. (See "Servos" for an explanation of this.)
If the servo arm is not at 90 degrees or perpendicular to the servo, use the Sub Trim function to adjust the arm so that it is at 90 degrees to the servo.
Position the control surface so that it is in its neutral position. Now make and adjust the linkage. Adjust the linkage so that the control surface is at neutral when the servo is in its neutral position.
If the mechanical linkage cannot be adjusted precisely enough, get it as close as you can and then use the Sub Trim function to make the final adjustments.
Now use the Travel function (or ATV, or EPA, depending on your radio) to adjust total travel in both directions.
Fly the aircraft and use the trim levers for trimming. Now use the Trim Offset function (or Trim Memory – your radio should have a similar feature) to store the trims, allowing the trim levers to be returned to their center positions. If excessive amounts of trim are required, it is best to mechanically adjust the linkages and try to keep the servo as close to its electronic center as possible. (See "Servos" for more information.)
