From RC Helicopter Wiki
Trim is a small constant offset applied to a control in order to make an aircraft fly correctly.
A helicopter is said to be 'in trim' (or 'trimmed out'; c.f. 'out of trim' when trim is incorrect) when the helicopter has minimal tendency to rotate in any axis (roll, pitch and yaw) when in a stable hover and no control inputs are made. If the helicopter is correctly balanced and properly set up with the swashplate leveled, the amount of trim needed to achieve this should be minimal.
The trim controls can be found as switches (digital trim) or sliders (analog trim) next to the transmitter's two control sticks. The normal 'no trim' position is for the slider to be centered in its travel, except for throttle. Throttle trim should be set to allow a reliable engine idle on internal combustion powered helicopters, or normally pulled all the way down for RTF electric helicopters; the exact setting will depend on how the electronic speed controller is configured.
Digital trims have the advantage that they cannot get accidentally moved when the transmitter is in use, as the transmitter stores their position electronically. Analog trims have the advantage that their position is always visible, and that they can be changed quickly; this is particularly useful for the throttle trim as it makes it very easy to stop a running engine by pulling the trim to the bottom position.
Some advanced transmitters also have a hover throttle trim and pitch trim that allow the throttle setting around mid stick to be changed slightly, and the entire pitch range to be moved up or down slightly, respectively. In general, these should be left centred or disabled; the pitch trim in particular has the potential to wreck hours of set-up work if you do not check it is central before initially setting up your helicopter!
 Trimming tips
The best way to trim your helicopter is not electronically but mechanically, meaning that you adjust the trim through the linkages before you do through the radio. Having too much electronic trim will limit the travel on your servos. First, make sure that your servos are centered and setup your linkage lengths to the factory specifications. This should get you pretty close. If the helicopter still wants to drift to the left, you can either shorten the linkage on the right side of the swashplate going to the servo or lengthen the one on the left side of the swashplate going to the servo.
 Airplane mode trims
Most transmitters in airplane mode have slighlty different behaviour. Rather than just moving the endpoints, they may shift the entire servo range in the same manner that subtrim usually does.
 See also
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