Part VI: Trimming your RC model
You can learn a lot about trimming by experimenting with inexpensive toy gliders.
Trimming” means adjusting your model so that it flies correctly (as opposed to trimming by choosing a paint scheme). A model that is badly trimmed might fly terribly or even be uncontrollable. Yet that very same model, properly trimmed, might fly delightfully. If you have an instructor, he or she will help you trim your model. If you are on your own, these hints should help.
One of the best ways for learning about trimming model planes (or any planes, for that matter) is to use inexpensive "toy" balsa or foam gliders. The kind where you can move the wing forward and backward is best.
You can make effective rudders and elevators by taping aluminum foil tabs on the models. (This is a lot easier than trying to bend the balsa or foam plastic.) You can add non-hardening modeling clay (plasticine) to the nose or tail (or even to a wing tip) to see what happens when a model is in or out of balance. First, get the model to have a nice, smooth glide by leaving all the tabs straight. Next, by adjusting the aluminum control surfaces, gliding (not throwing) the model, and watching what it does, you can learn a little about what to expect when you operate the controls on your RC model. It is also interesting to watch what happens when you make the plane balance too far forward or backward.
If you are just getting started in aero modeling, I really hope you will experiment with a toy glider for a while. Once you get your RC model off the ground, you will have to trim it in the air, using the trim controls (often called "the trims") on the transmitter. It helps to be able to recognize how an out-of-trim plane flies and to have some idea of what to do to fix it.
Basic Transmitter Configurations
Sideways movements of the stick roll the model left or right. Fore and aft movements “push” the nose down or “pull” it up.
This two stick transmitter shows the basic 4-channel configuration most popular in the US.
Three-channel single stick transmitters are very popular for simple models such as park flyers and basic trainers.
The Trim Levers
When your hand is not on the stick, a model plane that was flying straight
and level should continue to do so for a while. A good trainer will fly "hands-off" indefinitely, while more advanced models are less stable and will stay straight and level only briefly. This doesn’t bother advanced fliers, as it allows the planes to be more aerobatic.
Usually, when you fly a plane for the first time, or after there's been some change to it, you find that you have to hold the stick (the right stick on most two-stick transmitters) a little forward or back or to one side of center to make the plane fly correctly. This is difficult to do. If you find that you have to hold the stick to the right, move the roll trim lever a little to the right. You adjust the roll trim so that you can leave the stick in the center when the plane is flying with its wings level.
To adjust trim, you move the trim control in the same direction you moved the stick to correct the out-of-trim problem. On some transmitters, you have to click the trim lever repeatedly to one side or the other to adjust trim. The principles are the same.
I speak in terms of "roll" because that is what sideways motion on the stick does. When I say stick, I mean the right joystick on two-stick radios, and the only joystick on one-stick radios. Sideways motions of this stick are used to control roll whether the servo it operates is connected to the ailerons in an aileron-equipped model or if it is connected to the rudder in a rudder-elevator controlled model. In either case, you use sideways motions to cause the plane to bank, and then you pull on the stick to get around the turn. This is why I don't like to call its sideways motion the "rudder" stick or the "aileron" stick because it's neither: It controls *roll*.
The same procedure is used for trimming in pitch. You move the trim lever forward or backward until you no longer have to hold the stick. There is a tendency among newcomers to "lean" back on the stick. A good instructor will catch it, but you can do it yourself if you use the trim levers to make the plane fly hands-off.
A very few transmitters have auto-trim buttons: Check their manuals to see how this feature works. When available, it can make trimming a model much easier and safer.
WHAT IF THERE IS NOT
ENOUGH TRIM CONTROL
If you move a trim lever all the way to one side and the plane needs still more adjustment, it is time to land the plane (an expert pilot is very helpful in this circumstance) and make adjustments on the ground. This usually means making the control linkage longer or shorter.
Let us say that a rudder-elevator controlled plane was turning right and you tried to correct this by moving the trim lever all the way to the left, but the plane continued to turn right. In that case, you'd adjust the linkage to the rudder by making it shorter or longer (depending on which side the rudder horn was located) to move the rudder to the left. Then you'd put the trim lever back in the center on the transmitter and try flying the plane again.
It is a good idea, if this happens, to check to make sure that some other problem isn't causing the turn, such as the wing not being centered, the fin not having been put on straight, or the wing being warped.
TRIM AND THROTTLE
Adjusting Linkages
METHOD 1. Screwing or unscrewing a
clevis at the control horn.
METHOD 2. Loosening a pushrod connector and moving the control rod back or forth.
METHOD 3. Adjusting a bend in a wire.
Trim on the throttle is used differently than the other trims. On fuel-powered planes, we usually use the trim lever as a "kill" switch. We set the motor so that its idle occurs with the throttle trim lever forward or nearly so. Moving the throttle will then allow you to fly with the engine at any speed from idle to full power. However, if you want to stop the engine, you pull the throttle stick to idle and then pull the trim lever back, which completely closes the throttle and the engine stops.
On electrics, we sometimes ignore the throttle trim lever because many ESCs automatically set themselves to your transmitter's throttle range. In some cases, you will have to pull the throttle trim lever to a low position before the ESC will allow the motor to work. This is because a very fine safety feature won't allow the motor to run if the airplane’s electronics are turned on while the transmitter throttle is in an advanced position. Before this feature was available, electric planes sometimes started their motors when the power was turned on, and when the throttle stick had been left forward or accidentally bumped into that position. Especially on the more powerful models, this could be very dangerous. So, if your electric motor won’t start, yet everything else is working and hooked up properly, try putting the throttle trim lever all the way down before you turn on the aircraft's electronics.
WHY LEARNING TO FLY RC
IS NOT EASY
I think that it's worthwhile to step back from the nuts-and-bolts of learning to fly to explain why nearly everybody finds learning to fly RC a tricky business.
The first reason has to do with what made it hard to develop the technology originally. The Wright brothers succeeded in learning to fly because they were the first to understand how a bird or plane turns. The idea of banking the wings and using their lift to pull you around in a circle was far from obvious and is very different than the way that cars and boats turn.
Some recent trainers have tried to make planes turn by using two motors that can be run at different speeds to pull one side of the airplane or the other forward. While this does make turning easier, other aspects of flying become more difficult. For example, you do not have an elevator control to slow the plane for a landing. Whether it is better to start with a conventional model or with a differential thrust model is not clear. There may not be a single answer, as different people learn in different ways. Nonetheless, I suspect that a conventional model with stability augmentation might be the best way, and such models will be discussed in a future column.
The second reason why flying RC is tricky has to do with an optical illusion: You cannot see surface markings on a plane at some distance. It becomes a silhouette. The image of a plane, in silhouette and at certain angles, is ambiguous: It could be flying either toward or away from you. You (and anyone else) may not be able to tell which way it is going by looking at it. Also, when a plane is turning rapidly, it is hard to judge which way it is going.
At these times, expert pilots are relying not on their sense of sight, but on their memory of what direction the plane was flying! It takes time to learn to trust your brain instead of your eyes, and a lot of experience. Many beginner crashes, and even some expert crashes, are caused by this seldom-recognized phenomenon. When a beginner whose plane I have not been watching asks for help, I sometimes have to try a turn or two to figure out which way the plane is flying.
This illusion means that it will take time and practice to become a confident pilot of RC aircraft. Practicing on a
simulator is nearly as good as flying a real model to learn this particular skill, so long as you stick to the ground-
based view of the model when using
the simulator. But stick time in the air is
also necessary.
