This article was last edited for V1.4.14 (29 Mar 2022).
For planes, pitch trim is required whenever the center of mass is not the same as the center of lift. It is normal for the center of mass to be slightly forward of the center of lift to improve stability, but this will result in the plane constantly pitching downwards slightly. Constant control inputs are required to counteract this.
For single-rotor helicopters, the rotor will induce a torque on the vehicle causing it to yaw left or right. A tail rotor is normally used to counteract this. This also requires constant control inputs to counteract.
Trim is a constant control input which can be adjusted to counteract these forces so that aircraft will tend to just fly straight instead of constantly pitching down, yawing, or rolling.
Trim can be set for every axis at pilot seat, pilot seat (HOTAS) or helm. Trim is adjusted from the properties menu of the seat in the vehicle editor, or in-game while in the control seat by holding Left-Alt (Default).