From RC Helicopter Wiki
Correct cyclic trim is essential in making a helicopter fly well. Cyclic trim sets the neutral position of the swashplate; incorrect trim will require the pilot to expend much more effort keeping the aircraft under control than is necessary.
The expectations of correct trim vary depending on the type of helicopter. As wind will appear to upset the trim of any helicopter, initial trimming should be done in still air.
With coaxial helicopters, the trim can be set so that the helicopter will hover with no control inputs almost indefinitely, although it will still be blown around by air currents.
First, insert the battery into the helicopter, and suspend the helicopter from the upper flybar. If the body of the helicopter leans forwards or backwards, try to move the battery to counter the lean until the helicopter body is horizontal. Weight balance is not too critical for coaxial helicopters, but they fly better when well balanced.
Next, put the helicopter on a smooth floor (or on a piece of paper if there's carpet) in a large room, and slowly increase the throttle control on the transmitter until the helicopter becomes light on the skids. It may begin to yaw, in which case use the rudder control or rudder trim to counter this. Smoothly increase the throttle sufficiently to bring the helicopter 1-2 feet in the air, then smoothly reduce the throttle to zero, stopping the main rotors.
Note which direction the helicopter has drifted in. Move the cyclic trim controls on the transmitter a few notches away from the direction the heli drifted in, and try again. Eventually the helicopter should be able to hover out of ground effect with virtually no cyclic or rudder input at all.
If you end up with the trim pushed all the way over and still need more, look at the helicopter and the direction the trim tilts the swashplate. Remove the appropriate ball link to the swashplate, and turn the link to unscrew (length) on screw on (shorten) the linkage so that the swashplate keeps the current angle when the trim is returned to center. This mechanical trim is coarser but generally preferred to using the trim controls on the transmitter.
Trimming a fixed pitch helicopter is much more difficult, as they will never maintain a hover for more than a few seconds without pilot input. Additionally, the helicopter's translating tendency will always make the helicopter drift off to one side (usually the left) when it initially takes off. Using training gear while doing this will help reduce the risk of damaging your helicopter.
Just as with a coaxial helicopter, we should first check the helicopter's weight balance. Often with small electric helicopters, it is difficult to get the weight sufficiently forward. While just hovering, this can be fixed with trim, but as you progress into forward flight the flight performance will suffer if the weight balance is poor.
Next using a large room with smooth floors, place the helicopter on the ground and increase the throttle until the helicopter becomes light on the skids. Control the tail using the rudder if necessary, then increase the throttle to bring the helicopter up 1-2 feet. It will drift sideways, but watch the rotor disk for any tendency to incline one way or the other. Cut the throttle before the helicopter tilts too far over, or drifts into something. Adjust the trim controls on the transmitter to counter any lean that the rotor disk acquired in its brief flight, and then try again.
Just as with the coaxial, if you run out of trim travel, then lengthen or shorten the appropriate linkage to the swashplate, center the trim and start again.
As the rotor disk becomes more neutrally stable (having no tendency to tilt without a command from you to do so), you can begin making brief hops trying to hover, by making small brief inputs against the direction the helicopter is drifting---not too much or too often, though, or you'll get pilot induced oscillation. As you get the hang of hovering, you'll probably notice that you put more corrections in in one direction than another, an extra click or two of trim should help stop that.
Trimming collective pitch helicopters is very similar to trimming fixed pitch helicopters. If you are reading this, you'll benefit hugely from a sturdy set of training gear.
Weight balance is especially important in collective pitch helicopters; it makes the setup easier, and is essential if you progress into inverted flight. If your weight balance is good, you can use a swashplate leveller to set the initial swashplate position.
As with the fixed pitch helicopter, begin by making brief hops to see any tendency of the rotor disk to tilt one way or the other, and then progress into hovering hops.
One complication is the use of CCPM in many modern helicopters. If you run out of trim, there no longer one link to adjust that single control. I suggest that forward/ backwards trim be adjusted by changing the length of the link at the direct front or back of the swashplate, and sideways trim be adjusted by adjusting the links on each side of the swashplate, alternately shortening one and lengthening the other as needed.
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