The following is a list of the most common trim problems and their effects on the model:-

Wing Incidences set too positive.
1.   The model tends to climb through right and left turns.
2.   When pulling out of a dive, the model tends to zoom upward instead of returning to level flight.
3.   When full throttle is applied the model tends to zoom upward.

Wing Incidences set too negative.
1.   The model tends to dive throughout the complete flight.
2.   The model will react opposite to 1,2,3, above.

Centre of Gravity too far forward.
1.   The nose tends to drop in steep left and right turns.
2.   Rolls will be of the barrel type rather than axial.

Centre of Gravity too far rearward.
1.   The tail tends to drop during turns.
2.   The controls will be too sensitive.
3.   Poor wind penetration.
4.   The model will tend to balloon when turned into the wind.
5.   The model lacks stability during flight.

Too much engine down thrust.
1.   The model jumps up from straight and level flight when the engine is suddenly cut.
2.   The elevator requires excessive up-trim to maintain level flight.

Not enough engine down thrust or slight engine up thrust
1.   The model goes into a dive from level when the engine is slightly cut.
2.   The engine requires down trim for level flight.

Not enough engine right thrust
1.    During a steep climb, the model tends to pull up to the left as it loses speed.
2.    Model tends to pull to the left near the top of a loop.

Too much engine right thrust
1.   During a steep climb, the model tends to pull up to the right as it loses speed.
2.   Model tends to pull up to the right near the top of a loop.

One wing panel is too heavy.
1.   The same wing panel will tend to pull outward during inside and outside loops.
2.   A heavy wing panel will cause the ailerons to trim with one aileron up, and the other down.

Too much dihedral. (Pattern aircraft)
1.   The model will roll in direction of the applied rudder.
2.   Rolls will be barrel rather than axial.
3.   The model will want to roll out of knife edge flight.
4.   Inverted manoeuvres will be difficult.

Too little dihedral.
1.   The model will roll in the opposite direction of the applied rudder.
2.   The wing wants to tuck under during knife edge flight.
3.   Poor tracking through inside loops.
4.   The model tends to lack stability during upright flight.

If the model cannot be trimmed properly when the probable cause has been established,
1.   Check all surfaces for warps.
2.   Check model alignment.
3.   Check for aileron and elevator gap seals.
4.   Check for play in control linkage and servos.
5.   Check for poor servos that don't centre properly.
6.   Ensure that there is no problem with the radio.

Trimming a model may, at first seem like a very tedious task. In order to realize the full potential of any model, it must be trimmed properly. Many models have been labelled as dogs because they were never trimmed properly. With time and patience, trimming will become easier and enjoyable.

TRIMMING NOTES
These checks may be used to enhance your plane's rolling performance:
 

Test	Procedure	Observations	Adjustments
Centre of gravity	1.  Roll model into near       vertical banked turn  2.  Roll model inverted	a.  Nose drops  b.  Tail drops  a.  Lots of down       elevator needed to       maintain level flight  b.  Little or no elevator       needed to maintain       level flight	a.  Add weight to tail  b.  Add weight to nose  a.  Add weight to tail    b.  Add weight to nose
Aileron differential	Fly the model towards  you, pull into a vertical  climb. Neutralise the  controls then half roll  the model (ie. vertical  half roll)	a.  No heading change  b.  Heading changes       opposite to direction       of  roll  c.  Heading changes in       the direction of roll	a.  Differential OK  b.  Increase differential   c.  Reduce differential
Dihedral	Roll model into knife-  edge flight. Maintain  height with top rudder	a.  Model has no       tendency to roll out  b.  Model rolls in       direction of applied       rudder  c.  Model rolls in       opposite direction       of  applied rudder	a.  Dihedral OK  b.  Reduce dihedral    c.  Increase dihedral
Wing incidence	Fly model as for  "Dihedral"	a.  There is no pitch up       or down  b.  Nose pitches       toward canopy(up)  c.  Nose pitches away       from canopy (down)	a.  No adjustment  b.  Reduce incidence  c.  Increase incidence

NB As with all checks of a trimming nature, repeat the tests to confirm your observations.