MODEL TRIM PROBLEMS

Copyright 1997 © Mike Briese

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

• Wing incidence 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 incidence set too negative.
  1. The model tends to dive throughout the complete flight.
  2. The model will react opposite to a, b, and c above.
• Center 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.
• Center 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 flight 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 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 the 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 maneuvers will be difficult.
• Too little dihedral. (Pattern aircraft)
  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 center properly.
  6. Ensure that there is no problem with the radio.

Trimming a model may, at first may 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 labeled as dogs because they were never trimmed properly. With time and patience, trimming will become easier and enjoyable.

Reprint with permission of Howard Sullivan - Arab, AL, USA

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