Click image to enlarge.

In this shot the elevator "regular" trim lever has been set for full down elevator trim. You might use this setting if your model tends to climb while in level flight. The key point in this photo is that the position of that elevator trim levers gives you a rough idea of how much trim you are using on the aircraft.

With digital trims, there is no lever to show the position of the control surface, only a push-button switch as shown on this Hitec Eclipse RC transmitter.

If you wanted to know how much elevator (CH-2) trim you were using on your Hitec Eclipse transmitter, you would have to look at the LCD screen. In this case "TRIM — 50%" is the same as full down-trim. If it were full up-elevator trim it would read "TRIM + 50%."

Q-20: I hear RC modelers refer to 'regular trim' and 'digital trim.' Since I’m new to the hobby can you explain the difference to me?

A-20: Regular trim has been used for many years. A small lever located on the outside of the RC transmitter is connected to a potentiometer control on the inside of the transmitter. It provides for a vernier adjustment of the particular channel functions, such as the aileron, elevator, rudder, and throttle controls.

Let's use the elevator as an example. The servo may move 45° on either side of the neutral position. The trim lever can relocate the neutral position of that same servo by, say, plus or minus 15°. If your airplane climbs slightly by itself, adding a small amount of down-elevator trim will make it fly level. The time-honored “regular trim” has always been operated by a small lever; its position makes it obvious how much trim travel has been set in.

Modern RC transmitters have been going to “digital trim.” With this concept, the trim lever is replaced by a push-button switch. You keep pushing, or pulsing, the trim button to achieve new trim positions. But since it is a button, you really don’t have any reference to the actual trim position on the model. Some transmitters have bar graphs that appear on the LCD screen showing servo position. Some others show the trim as a percentage of travel.

Digital trim has upset many old-time RC fliers. It took me a while, but I now have to admit I like digital trim. It also makes for much easier storage of trim positions in the transmitter’s computer memory circuit.

—Bob Aberle