Rudder trim is a secondary flight control that adjusts the aerodynamic forces on the rudder surface so the aircraft maintains the desired yaw attitude without any input from the pilot's feet. By relieving the steady pressure otherwise required on the rudder pedals, it alleviates control forces related to yaw and keeps the aircraft in coordinated flight.
Single-engine aeroplanes with strong propellers commonly need rudder trim to offset the left-turning tendency created by P-factor and propwash striking the vertical fin. Like elevator trim, but acting around the vertical rather than the lateral axis, rudder trim is typically provided by a small servo-tab attached to the rudder itself.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is rudder trim in an aircraft?
Rudder trim helps reduce the amount of control pressure needed to keep the rudder in a certain position. Rudder trim reduces forces on the rudder pedals related to yaw, and many general aviation aircraft have ground-adjustable trim tabs on the rudder.
Rudder trim is a secondary flight control system that adjusts the aerodynamic force on the rudder so the aircraft maintains the set yaw attitude without any pedal input. To trim an aircraft is to adjust the aerodynamic forces on the control surfaces so that the aircraft maintains the set attitude without any control input. A rudder trim fulfills this purpose on the yaw axis, very similar to elevator trim on the pitch axis. By relieving pedal pressure, rudder trim reduces the amount of manual input required from the pilot and is used to maintain coordinated flight, to counter asymmetric thrust from the engines, and during climbs, descents, and engine-out emergencies.
What is the purpose of a rudder trim tab on an aircraft?
The purpose of a rudder trim tab is to trim or counteract the aerodynamic force on the rudder and thus stabilize the aircraft in the axis of rotation associated with yaw. By creating an opposing air deflection, the tab builds a low pressure that pulls the rudder around, relieving the control pressures required to maintain the desired flight path. This reduces or eliminates the need for the pilot to place continuous pressure on the rudder pedals.
Some aircraft designers build in a permanent anti-torque rudder trim tab that is bent to a specific shape and ground-adjusted to overcome unwanted yaw tendencies. Such fixed tabs apply a steady force on the rudder in flight and reduce pilot and rudder trim workload at all speeds. On training airplanes, a ground-adjustable tab is commonly found on the rudder, while other types are adjusted in flight using a manual trim wheel, electric switch, or crank. Whether fixed or adjustable, the trim tab enables the airplane to fly straight and level with minimal pilot input, even when asymmetric thrust or p-factor induces yaw.
