Trim it!


Any full scale pilot will tell you that the secret to smooth precision flying is to have the plane trimmed up, always. This is especially important to formation work, instrument flying, bombing runs, and so on.

Keeping our RC aircraft in trim is also important to our flying, although there are some important differences.

But, first, what do we mean by "trimmed up"? We generally trim for straight and level flight conditions -- such that if we take our thumbs off the sticks, the plane will continue flying straight and level, within the scope of the stability of the plane. A couple of facts to ponder: 1) A plane can only be trimmed for ONE flying speed; 2) The more stable the plane, the more trim change occurs when the speed is altered. Note that we're speaking of mostly PITCH (elevator) trim here -- lateral and directional (aileron and rudder) trim will generally not change as speed increases -- if they do, you've got other problems, such as warped wings, crooked tailfin, or thrust line problems.

In a full size airplane, the trims are located where it's easy to do the trimming, without having to go "hands off" the stick or wheel. Unfortunately, the RC transmitter is not so convenient, and it's just not very easy to constantly be messing with the trim. So most pilots trim for the fastest condition they normally fly, usually full throttle, and full speed. That means that in any slower condition, some back stick is required for level flight. For most of us, this is best -- because pushing on the stick seems to give less precise control than pulling on it. And if your normal flight involves flying around at 1/2 throttle, then by all means trim for that condition, keeping in mind that the addition of more throttle (and speed) will result in a nose up climb.

Now, a bit about rudder and aileron trim. Assuming that you don't have warped wings, a crooked tailfin, or bad thrust alignment problems, the aileron trim should, once you get it set, remain set. That's because the linkage is short, and even if the links shrink or expand, they will both raise or lower the ailerons the same amount. But the rudder link, unless you have a pull-pull system, will deflect the rudder with shrinkage or expansion of the pushrod. And that pushrod is usually pretty long, meaning significant expansion in the heat of the day. If your rudder is offset to one side, your plane will fly oddly, because it will be in a continuous slip, or skid, and may also want to roll to one side.

The point here is that you are wise to check the rudder trim (by looking at the rudder) frequently, if not every flight. And if you get airborne and the plane flies weird and wants to roll, it's more likely to be rudder trim than aileron trim that is at fault. This is especially true of trainer type planes because of the high wing dihedral, and is most true of planes using "nyrod" type pushrods -- because the nyrods have a rather large expansion rate compared to other type pushrods.

Those of us who have flight instructed in full size planes can quickly recognize the symptoms of out of trim flying -- the plane constantly veering off course in the same fashion (left, right, up, or down) then being abruptly corrected, then veering off again -- the cycle being constantly repeated. In such a situation, the pilot is more fighting the plane than just flying it! And flying an out of trim plane is not an enjoyable experience!

If you relate to all this, seek out a more experienced flyer to help you get the beast in trim, and to show you ways to check it for yourself. You might be amazed at how much easier flying your plane can be! ... Clay