Click image to enlarge.

A close up of the elevator dual rate (on/off) switch on a Hitec Eclipse 7 RC transmitter. Bob points to the elevator switch, which has a short handle. To the right with a longer handle is the rudder dual rate (on/off) switch. When one of these switches is turned on, the amount of control throw can be reduced according to your preference.

This is what the dual rate LCD screen looks like on the Hitec Eclipse 7 transmitter. In this sequence the CH 1 or rudder has been selected and the control surface is still set at it’s normal or 100% position.

Here CH 2 or elevator dual rate has been selected and the elevator control-surface throw has been reduced to 80% of normal so the elevator throw will be cut back by 20% with the elevator dual rate switch in the on position.

This shows the expo rate function on the Hitec Eclipse transmitter LCD screen. The CH 2 or elevator function and 30% nonlinearity has been selected. This is moderate expo rate control.

This chart, copied from the Hitec Eclipse instruction manual, explains the difference between linear- and nonlinear-type control. The 0% is linear, 25% is moderate expo (or nonlinearity), 50% is substantially more, and 75% is a large amount of nonlinearity. You would use that much expo on a fast-moving, highly maneuverable model such as an E-3D Electric.

Q-27: I hear the expressions "dual rate control" and "expo rate control." Are they the same thing? Is one form of control better than another?

A-27: This is an excellent, often-asked question. Dual rate control and expo rate control work differently, yet they do tend to achieve close to the same results. With dual rate control, you have a switch for the function; one switch is usually for rudder or aileron and the other switch is for elevator.

With the switch in the off position, you will obtain full control throw on that particular channel. When you turn the dual rate switch to the on position, you can reduce the amount of servo control throw or travel to at least 50% of the full control.

Let's say you have a fast, heavily loaded airplane. When it flies at slower speeds for takeoffs and landings, it requires a great deal of control travel. But when it gets up to a fast flying speed, it needs far less control throw. At the faster speeds you want less control to maintain a smooth flight. This is where the dual rate functions come into play.

You make your takeoffs and landings with the dual rate turned off. Then during high-speed flying, you turn on the dual rate switch, which reduces the maximum amount of servo travel (control). The key to this is that you must physically throw that switch while in flight to obtain the reduced control. Keep in mind that while that dual rate switch is on, you can not obtain full control throw. So if you needed more control in a hurry, you must first turn off the dual rate switch.

Exponential rate control (which is referred to as "expo") does not have to be operated by a switch. (It can be, but doesn’t have to be!) Nor does it cut back or reduce your maximum servo control throw. Expo rate basically provides for a nonlinear-type control around the neutral transmitter control-stick position. In normal linear control, if you move the transmitter control stick 50% of its normal travel, the servo that it operates moves 50% of its travel. In other words the control response is perfectly linear.

With expo rate, as you begin to move the transmitter control stick, the servo will begin to respond very slowly. By the time you move the control stick 25%, the servo it operates may have only moved 10%. The control response is nonlinear. As you get up to 50%, stick movement, the servo may be up to 40%. When you get to full control-stick movement, you get full servo control.

The degree of nonlinearity can be adjusted. You can set the expo rate for very little nonlinearity, which may be hardly noticeable. Or you can set the expo rate for maximum nonlinearity, which means that when initially moving the control stick, the responding control surface hardly moves at all.

Expo rate control works best on high-speed, highly maneuverable model aircraft. An airplane that appears “jumpy” in high-speed flight will become quite smooth when a good amount of expo rate control is added.

The two best features of expo rate compared to dual rate control are that you get full control throw when you need it and you don’t have to remember to keep throwing a switch in flight.

The relatively inexpensive computer transmitters I wrote about in my "From the Ground Up" series (February 2004 Model Aviation) describe these functions in detail. Even beginners or relatively new RC pilots can benefit from both dual and expo rate control. However, keep in mind you must pay slightly more for an RC system that provides such features.

—Bob Aberle