Once a model is built, it is vital that time be taken to correctly trim the model. All too often modellers forget the needs of the model once it is built. The difference between a good and bad model or fast and slow model is often the trim.
Once the new model has been basically trimmed for hands off flying, the first surface to trim (and usually the one least checked by sports pilots) is the rudder.
Fly away from yourself, wing level and into the wind. Pull too a vertical climb (reasonable sharp turn),if the model veers left or right adjust the rudder trim to suit. Re-trim for level flight (it may effect aileron trim) and repeat. When correct try again from inverted. It should also be vertical. If not you have an uneven wing tip weight or warped elevator. Keep adjusting until the model will go vertical form both upright and inverted.
To test for wing tip weight, obtain straight and level flight with correct aileron trim, then roll to inverted. If the model (when inverted) shows a need for the aileron trim to be changed, it is a good indication of a heavy wing tip. Push a large nail into the light tip and try again, and again.
One of the checks for engine side thrust is how the model behaves at the top of a vertical climb. After an initially straight climb the models speed will diminish and the engine thrust will take effect. If at the top of the climb the model starts to go left or right, adjust the engine thrust line to suit.
If at the start of the climb, the model appears to roll slightly, it is often a sign of warp or uneven deflection of the elevators.
Pylon models are usually best with a slight right rudder bias (and right thrust). The test for this is to bank the model vertical and pull elevator to do a sharp turn, as per pylon turn. Adjust the rudder trim until the model will turn flat, (ie, no loss of height). Quite often a pylon model will turn flat to the left but dive slightly to the right. If the thrust line is correct, turns both ways should be flat.
Too much right rudder and the model will fly crooked ... the model may be heading straight but is pointing 5+ degrees off line. Such a trim is obviously wrong, and should be corrected with right thrust.
To test for down thrust trim, fly in straight line at 90 degrees to the wind and suddenly go to idle. The model should continue in a straight line, eventually it will loose height due to gravity. If the model dives when the power is cut, add down thrust. If the model climbs (rare), decrease down thrust.
The most difficult aspect of model flying to trim is knife edge flight especially as very few models except pattern aircraft do it well. Models tend to either continue to roll or roll out of knife edge and or pitch towards the top or bottom of the model. Most pilots can cope with correcting one but not both problems.
The easiest to fix is the rolling tendency. Roll to knife-edge and apply rudder as normal. Make sure you are not holding any extra elevator or aileron. If the model rolls back to the way it came - decrease dihedral. If the model continues to roll, increase dihedral.
To correct pitching towards the top of the model in knife edge, increase wing incidence, decrease tail incidence or move C of G rearward. Do the opposite if the model pitches towards the bottom of the model.
Snap rolls- many models have difficulty snap rolling. The best cure is to increase control throw or move the C of G rearward. If you have difficulty controlling a fast snap, decrease aileron and rudder throw.
The above is only a basic trim guide, quite often the correct fix is to alter the models design, ie, lower or raise tailplane or wing, angle or rudder tailpost, change fuselage areas etc.
Remember, do all tests at least twice before making a change, and for every change there will be a reduction that may also need re-trimming, ie, changing C of G will change many knife edge, thrustline and snap set-ups you have.
It is best to perform most tests in calm weather and flying the model away from the pilot.
Good models are well trimmed models, bad models are often good models badly trimmed.