Flight Trimming Guide


Article courtesy of G.S. Air.

There are two stages in trimming your plane for the optimum flight characteristics. The first involves building a straight airframe, and setting up the static trim on the workbench. This means setting all linkages to be uniform and linear as appropriate, contain any differential as required, and have all the surfaces tracking properly in the case of multiple servos/multiple control surfaces such as dual elevators. There are some simple gauges on the market to help you set your tracking and control surface throws. These gauges verify that both surfaces are tracking the same, such as with dual elevator servos, or ailerons. Once this is done, and the maiden flights have been performed, its not time to trim your plane dynamically.

Dynamically trimming your plane involves performing multiple flights, watching its behavior in various situations, and making the corresponding adjustments in an attempt to tune the plane for neutral flight. By neutral flight, I mean that when you apply left rudder while flying straight and level, the plane will yaw left, but will not show any tendency to climb or descent, or roll right or left. While this is the ideal, it is usually not obtainable. However, it is quite reasonable to expect to minimize the effects considerably through changes in CG, control linkage adjustments, radio mixing, and so on. 

With repeated flights, and adjustments based on the recommendations on the chart below, you can dramatically improve the flight handling characteristics of any plane towards that elusive goal of a totally 'neutral' flying plane.

I have seen this chart published in many articles, magazines, and web sites. The 
author is unknown, and I as such, I can't give proper credit where credit is due.

To Test For Test Procedure Observations Adjustment
1. Control neutrals Fly model straight and level Adjust the transmitter trims for hands off straight and level flight Adjust clevises to center transmitter trims
2. Control throws Fly model and apply full deflection of each control in turn Check the response of each control *Aileron Hi-rate: 3 rolls in 4 seconds.
*Lo-rate: 3 rolls in 6 seconds.
*Elevator Hi-rate: to give smooth square corner.
*Lo-rate: to give a loop of approx. 130' dia..
*Rudder Hi-rate: approx. 30-35 degrees for stall turns.
*Lo-rate to maintain knife edge flight.
3. Decalage Power off vertical dive. Release controls when model is vertical (elevator must be neutral). A. Does the model continue straight down?
B. Does the model start to pull out (nose up) ?
C. Does the model start to tuck in (nose down)?		 A. No adjustment
B. Reduce incidince
C. Increase incidence
4. Center of gravity Method 1: Roll model into near vertically banked turn.
Method 2: Roll model inverted.		 A. Nose drops
B. Tail drops
C. Lots of down elevator required to maintain level flight
D. No down elevator required to maintain level flight, or model climbs		 A. Add weight to tail
B. Add weight to nose
C. Add weight to tail
D. Add weight to nose
Tip Weight (course adjustment) Fly model straight and level upright. Check that aileron trim maintains wings level. Roll model inverted, wings level. Release aileron stick A. Model does not drop a wing
B. Left wing drops
C. Right wing drops		 A. No adjustment required
B. Add weight to right tip
C. Add weight to left tip
Side Thrust Fly model away from you into any wind. Pull it into a vertical climb (watch for deviations as it slows down). A. Model continues straight up
B. Model veers left
C. Model veers right		 A. No adjustment needed
B. Add right thrust
C. Reduce right thrust (move thrust line left)
Up/Down Thrust Fly model on a normal path into any wind. Parallel to strip, at a distance of around 100m from you (elevator trim should be neutral as per test No.3). Pull into a vertical climb & neutralize elevator. A. Model continues straight up
B. Model pitches up (goes towards top of model)
C. Model pitches down (goes towards bottom of model)		 A. No adjustment needed
B. Add down thrust
C. Reduce down thrust
Tip Weight (fine adjustment) Method 1: Fly model as per test #6 and pull it into a reasonably small dia. inside loop (1 loop only).
Method 2: Fly the model as per test #6 and push it down into an outside loop (1 loop only & fairly tight).		 A. Model comes out with wings level
B. Model comes out right wing low
C. Model comes out left wing low		 A. No adjustment needed
B. Add weight to left tip
C. Add weight to right tip
Aileron Differential Method 1: Fly the model towards you, before it reaches you, pull it up into a vertical climb. Neutralize controls, then half roll the model
Method 2: Fly the model on a normal pass and do 3 or more rolls		 A. No heading changes
B. Heading change opposite to direction of roll commands (ie. heading veers to models & your left after right roll).
C. Heading changes in direction of roll command

A. Roll axis on model center line
B. Roll axis off to same side as roll command (ie. right roll, roll axis off right wing tip)
C. Roll axis off to opposite side of model as roll command		 A. Differential OK
B. Increase differential
C. Reduce differential

A. Differential OK
B. Increase differential
C. Reduce differential
Dihedral Fly model on normal pass and roll into knife-edge flight, maintain altitude with top rudder (do this test in both left & right knife-edge flight) A. Model has no tendency to roll out of knife-edge flight
B. Model rolls in direction of applied rudder
C. Model rolls in opposite direction in both tests		 A. Dihedral OK
B. Reduce dihedral
C. Increase dihedral
Elevator alignment (for models with independent elevator halves) Fly model as in test #6 and pull it up into an inside loop. Roll inverted and repeat the above by pushing it up into an outside loop A. No rolling tendency when elevator applied
B. Model rolls in same direction in both tests
C. Model rolls in opposite direction in both tests		 A. Elevators are in correct alignment
B. Elevator halves misaligned. Either raise one half or lower the other half
C. One elevator half has more throw than the other (model rolls to the side with the most throw). Reduce throw on one side or increase throw on the other
Pitching in knife-edge flight Fly model as per test no. 10 A. There is no pitching up or down
B. The nose pitches up (the model climbs laterally)
C. Nose pitches down (model dives laterally)		 A. No adjustment needed
B. Alternate cures:
1. Move the CG aft
2. Increase wing incidence
3. Add down trim to ailerons
C. Reverse the above


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