RCU Forums - Checking Trim in Flight

Here is something I copied from a previous post. Some of this is not that basic....

<div><font face="Arial" size="2">(Basic) trimming 101

Starting with a model that has been built and balanced as per the kit instructions then it should fly quite well. Some simple trimming will help remove any minor building errors and generally take the model to a relaxing hands off type of flyer.

The following is a basic trimming process and as long as it is done in the order below it does not need to be an iterative and long drawn out procedure. Generally a couple of evenings will take you through the process.

All checking will be started with the model trimmed to fly straight and level (hands off)at a low to medium power setting. All checking will be done flying into the wind.

A couple of control facts need to be understood.
Control surfaces are more effective at higher speeds and less so at low or take off/landing speeds.
Engine thrust lines (sometimes known as the effect from prop wash) are very predominant at low speeds( landing/take-off) and are negligible at flying speeds.

Step one: Set Balance Point.

The starting balance point should be marked on the model.

Trim out to fly hands off straight and level into the wind at a low to middle throttle setting.

For a low wing aerobatic model:

Roll inverted, release the sticks and observe.
Severe dive ..... nose heavy.
Slight dive..looks good.
Straight and level..perfect.
Climb..bit tail heavy.

For a high wing or non aerobatic model.

Go rapidly to full power and observe the flight reaction.

Climb..nose heavy.
Very slight climb to no change..ok.
Dive..tail heavy

Add or remove weight as required and then repeat the test until you get the right balance point is found.
The plan balance point is usually quite conservative as a nose heavy model is sluggishly stable. As the true balance point is approached the model will become more lively and sensitive to elevator input and may require less elevator throw for normal flight. (low rate on the transmitter).

When the balance point is found mark it on the fuse as a future reference point.

Step two: Correct Thrust line.

This step is quite sensitive to the wind direction and extra effort should be made to always fly straight into the wind.
The objective of this test is to fly a shortish vertical line and observe what direction the nose of the plane falls to as the speed falls off and the plane comes to a halt.

Before taking off set the rudder at the neutral position and do not touch the rudder trim.
Trim out to fly hands off straight and level into the wind at a middle throttle setting.
Pull to the vertical and fly a half throttle vertical line with no corrections and observe.
Two things can happen:
if the model immediately starts to pull to one side then it is likely that the rudder is not straight. Go back to the straight and level flight and add a couple of clicks of opposite rudder trim (ie model pulled left...add right rudder trim) then repeat the test.
The model climbs reasonably straight and then pulls left/right etc as the speed falls to zero. This pull to the left or right is caused by the thrust line of the engine being incorrect.

To confirm this the test should be repeated a couple of times just to make sure its not the wind causing the problem. Typically the model will pull to the left as it comes to a stop. The model may also try to “fall on its back” or fall to its landing gear in combination with going left or right. The first step in correcting the thrust line is to shim the engine in the opposite direction to the way it falls. For example; pulls left then add some right thrust. It is usually best to correct the side thrust first before correcting vertical thrust lines.

With the side thrust corrected you should see a noticeable improvement during take-offs and landing overshoots. The quick application of power should now have the model tracking straight dwom the runway with no tendency to pull/roll left.

The test can then be repeated to check for down thrust correctness. If the model “falls on its back” then you need some down thrust. If it falls to the landing gear side then you need less down thrust. Unless you are planning to do hovering type maneuvers or some advanced aerobatics it is probably not worth worrying about getting the down thrust angle correct.

Step three: Aileron Roll correction.

This is a very easy test to do and corrections can be made at the field.

Climb high and throttle to idle speed.
Push down to a vertical dive give full right aileron and observe.
If it rolls axially then no corrections are needed.
If it rolls as if in a barrel then you need less 'up' aileron throw on the right side.

Repeat the exercise for the left roll.

If you have a computer radio and one servo per aileron per channel, ie left servo on ch2 and right on ch6 with 2 and 6 mixed then you can adjust the aileron travel individually on the radio. If the servo's are joined with a Y harness or a single servo is used then you will have to adjust the throws mechanically.

I have found that mid wing type planes usually do not need any aileron differential. High wing and low wing models typically need some differential so that the down aileron travel is about half of the up aileron travel.

Step four: Remove rudder bad habits.

This is not worth doing if you are flying a trainer model as the rudder on a trainer is not very effective.
Also if you do not have a radio with mixing capabilities then this step can not be done.

Fly straight and level and give full right(or left) rudder and watch.

If the plane dives then mix some up elevator trim into the rudder.
If the plane rolls rapidly then correct correct with some aileron mixing.

Remember to check and correct for both left and right rudder.

Step five: Loop trim.

Straight and level, medium power.
Pull up and do three reasonable tight loops in succession. Do not add any corrections.
If the plane tracks well but drops a wing then it is heavy on that side and you need to add a balance weight to the other wing tip.
If the plane tries to corkscrew ie each loop is progressively further left (or right) than the other one then you have a aileron that is not in alignment to the wing. If the plane tracks left then the leading edge of the left aileron is either high or low in relation to the trailing edge of the wing and is causing the plane to skew to that side. You need to correct the aileron alignment.
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