Advanced Pilot Training: Howto Trim

Because the loading of an airplane is never uniform-the load. changing with the number and placing of the crew, the placing of the cargo, the power used, and the continually changing weight of the gasoline in the tanks-some means must be employed to keep the ship flying naturally in a straight line with its wings level and its nose neither up nor down. This aerodynamic counterbalancing for load change is accomplished by offsetting the natural flight positions of the control surfaces through the use of "trim tabs." The number of trim tabs varies with the size of the airplane, ranging from a single tab on the elevator of a small airplane to tabs on the ailerons and rudder as well as the elevator of larger airplanes.

For Take-Off

MAKE SURE THAT ALL TRIM-TAB ADJUSTMENTS ARE IN THE TAKE-OFF POSITION.

In Flight

When trimming a small airplane fitted only with a "nose up" and "nose down" trim-tab adjustment:

FIRST-climb to the desired altitude. If there is too much forward pressure on the control column during a normal climb, adjust the elevator trim tab to bring the nose up.

SECOND-adjust the engine to its rated cruising power,

THIRD-move the wheel or control stick back and forth and determine by feel if the ship is "nose heavy" or "tail heavy." If continual back pressure is needed to keep the rate-of-climb indicator at zero, the airplane is "nose heavy." If forward pressure is required, it is "tail heavy."

FOURTH-adjust the trim-tab adjustment control to counteract this tendency, being careful to follow the direction of turn indicated to bring the nose "down" or "up."

To trim a large airplane equipped with f ailerons and rudder as well as elevator:

FIRST-climb to desired altitude and set engine to its rated cruising power.

SECOND-adjust for "nose up" or "nose down" as indicated for small ships as above.

THIRD-adjust for low left or low right wing adjusting aileron trim-tab control.

FOURTH-adjust for right or left yaw (turn) by adjusting the rudder trim-tab control.

FIFTH-recheck the "nose up-nose down" low-wing trim-tab adjustments.

Landing

Most airplanes must be trimmed for the gliding speed when coming down for a landing

Warning

The placing of the trim-tab adjustment controls and the direction in which the tab controls are turned for a desired trim-tab reaction varies with the airplane, Check the indicated directions before taking off.

Trim-Tab Don'ts

Don't adjust the trim tabs too rapidly. Cases have been reported where a too rapid adjustment of the tabs has thrown the airplane into a snap roll.

Don't forget that as your gasoline supply decreases the balance of the airplane may change and further tab adjustments may be necessary.

REMEMBER-the main function of the trim, is to ease the pressure on the controls and save the pilot work. If that is not accomplished if the adjustments are not correct.

Reference_ PIF-6-5-1, May 1, 1943