This will be the most challenging of your trim steps, mainly because you are dealing with very subtle deviations.
There are two ways to adjust engine thrust. The first, and preferred method, is to add shims under the engine mount to
change the engine thrust angle as required. The second is to simply use the radio to mix the throttle into the rudder and/or
elevator to accomplish the same thing. Unless you make a prop change, thrust angle will remain constant. That is why I always
take the time to properly shim the engine to get it right. Personally, I would only use the mix technique if I change props
frequently, or I am in a hurry and do not have time to “pop” the cowl before or during a competition.
To shim the engine, I prepare some plywood discs of varying thicknesses. I like to have 1/64th, 1/32nd, 1/16th,
1/8th, and ¼”. This allows me to set any angle necessary. I prefer plywood because it is lighter than using
washers. Seal each with thin CA on both sides to protect it from oil and keep it from crushing when the mounting
bolts are tightened. Now, let’s get started.
Engine Side Thrust Trim
Start by flying several full power vertical up-lines to trim in the rudder. Make sure your trim steps are set to 1 unit.
Fly the up lines in both directions to negate the affects of wind. A plane will lean towards a crosswind. In no wind, the
plane should track perfectly straight going up. If there is a crosswind, the plane should gradually bend towards the wind…and
when you change directions, it will bend the exact same amount in the opposite direction.
Once you are satisfied with the rudder trim on up-lines, reduce the power to ½ throttle and fly level. With the wings level,
the aircraft heading should remain constant. To check, fly towards or away from yourself, and either directly into or away
from the wind. Do not confuse heading change with crabbing. Crabbing is when the wind pushes the plane sideways, but the heading
remains the same. Turning is when the plane heading changes, so it is trying to fly in a circle.
Which ever way the plane is turning is the direction the engine thrust needs to be adjusted. For example, if it is turning
to the right, shim the two left engine mount studs to tilt the engine to the right. If it was turning left, shim the right
side of the engine. Once you have shimmed the engine, go back and re-check again. You will have to retrim the rudder every
time the engine is adjusted.
Engine Up Thrust Trim
You will once again fly vertical up lines. If your plane is under-powered (slows under full power) then use full power.
If your plane does not decelerate during up-lines, then fly the lines at about ¾ throttle, so the plane WILL slow as it climbs.
Pull the lines absolutely vertical and hold the line while the plane slows to zero. If you find yourself holding up elevator
to keep the line (nose is trying to tuck to the gear), then you need up-thrust…shim the bottom of the engine mount.
If your find yourself pushing on the stick to hold the line (plane tries to pull to the canopy), then shim the top of the
engine to lower the thrust angle. Be sure to check in both directions to negate the effects of wind, once again.