5.trim-systems. Trim Systems
Trim systems relieve the pilot of the need to maintain constant pressure on the flight controls and reduce pilot fatigue, particularly during long flights or while flying at a constant attitude, airspeed, and power setting. After establishing a desired flight condition, the pilot adjusts the trim to balance any aerodynamic forces acting on the control surface so that the airplane will maintain that condition with little or no control input. The most common trim systems found on light aircraft are trim tabs, balance tabs, antiservo tabs, ground-adjustable tabs, and an adjustable stabilizer.
Trim Tabs
The most common trim device is a small, hinged surface — a trim tab — located on the trailing edge of the elevator (and sometimes on the rudder or ailerons). The tab is moved by a control wheel, crank, or electric switch in the cockpit. When the trim tab is deflected into the relative wind opposite the desired direction of elevator travel, aerodynamic force on the tab pushes the elevator in the opposite direction. For example, to trim the airplane nose-down, the pilot rolls the trim wheel forward; this deflects the trim tab upward into the slipstream, which forces the elevator down and lowers the nose. The result is that the elevator is held in the trimmed position without any pressure on the yoke.
Proper use of trim is a fundamental airmanship skill. The recommended sequence is:
- Establish the desired pitch attitude with elevator pressure.
- Set the desired power.
- Trim off the control pressure.
- Re-trim whenever airspeed, power, or configuration changes.
Never use the trim to establish an attitude — use trim only to relieve pressure once the attitude is established.
Balance Tabs
On larger aircraft, the control forces required to deflect a primary control may be too high for the pilot to overcome with muscle alone. A balance tab is linked to the airframe such that when the primary control surface is deflected, the tab automatically deflects in the opposite direction. The aerodynamic load on the tab assists the pilot in moving the control surface, reducing stick forces. When the linkage is also adjustable from the cockpit, the same surface can serve as both a balance tab and a trim tab.
Antiservo Tabs
Aircraft equipped with a stabilator (a one-piece, all-moving horizontal tail) — such as the Piper Cherokee and Tomahawk — use an antiservo tab. Because a stabilator is highly effective and easy to overcontrol, the antiservo tab moves in the same direction as the trailing edge of the stabilator. This produces an aerodynamic force opposing pilot input, increasing stick force per g and giving the controls a stable, well-balanced feel. The antiservo tab is also adjustable in pitch and serves as the trim tab for the stabilator.
Ground-Adjustable Tabs
Many small airplanes have a ground-adjustable tab on the rudder. This is a simple metal tab that the pilot or mechanic bends by hand on the ground to correct for a yaw tendency in cruise (often caused by torque, P-factor, or slipstream effects at the design cruise power). The tab cannot be adjusted in flight; refinement requires several test flights and progressive bending until the airplane flies straight at cruise.
Adjustable Stabilizer
Instead of using an elevator trim tab, some airplanes — and most jet transports — use an adjustable stabilizer. The entire horizontal stabilizer pivots about its rear spar; a jackscrew driven by a trim wheel or electric motor changes the leading-edge incidence. Moving the leading edge down lowers the tail's angle of attack and trims the airplane nose-up; moving it up trims nose-down. Because the entire stabilizer changes incidence, this system produces very large pitch trim authority with little drag — important at the wide CG and speed range of transport aircraft. Runaway trim on such systems is a serious emergency, which is why pilots are trained to identify the trim disconnect or stabilizer cutout switch.
Electric and Autopilot Trim
Many modern light aircraft also feature electric pitch trim, with a switch on the yoke and an autopilot interface. The pilot must know how to disable runaway electric trim — typically through a yoke-mounted disconnect, a circuit breaker, or pulling the autopilot/trim breaker — and how to verify proper trim operation during the preflight runup (full travel both directions, autopilot disconnect functional).
Effect on Stability and Performance
A properly trimmed airplane is easier to fly accurately on instruments, holds altitude in turbulence with less workload, and recovers naturally from minor disturbances because the pilot is not fighting residual control pressures. Conversely, mistrim can mask abnormal conditions: heavy back-pressure required to maintain altitude may indicate an aft CG problem, ice accretion, or improper flap setting. As a rule, if the airplane requires unusual trim to fly straight and level, the pilot should investigate the cause rather than simply trimming the pressure away.