Dual Rates and Exponential (and other things)

by Richard Lindberg

If you have a computer radio, then you may be familiar with the terms ‘dual rate’ and ‘exponential’—then again, you may not! After all, these functions were only present on the high-end analog radios of a few years ago, and certainly not in the form found on your new computer radio. In addition, I’ve found that these functions aren’t used as much as they should be—even though they’re two of the most desirable functions to be found on your radio! Let’s find out about them, discover how they work, and why you should be using them.

Dual Rates. When you set up your control surface throws on your (new or otherwise) airplane, you used the Travel Adjust/Adjustable Travel Volume/End Point Adjust function on your radio. Doing this actually set up one rate, or control authority, for each control surface. A given stick movement thus resulted in a certain control surface movement, from minimum to maximum. Dual (or even triple!) rates provide you with a means to instantly change that control authority by flipping a switch*.

The question arises: why would you want to change rates? Most models respond more quickly to a given control input when they are flown at high speeds. The same stick movement at high speed as given at a lower speed might result in over-controlling your model—a prime cause of ‘unscheduled landings’—or perhaps a case of PIO: Pilot Induced Oscillations (also leading to undesirable results). Having the ability to REDUCE control authority would reduce your model’s sensitivity to stick movements, resulting in a smoother, more controlled flight.

The following graph depicts how changing rates affects a servo’s response. If, say, a 50% rate is programmed, the servo will only move 50%, or half, or the distance it would otherwise move if the rate were not activated. Most radios will let you set a different rate anywhere from 0% to 100/125/140/150%, depending on the manufacturer.

Changing rates is usually done by a switch, as mentioned earlier. Some radios have dedicated switches for each primary control surface, e.g., elevator, aileron and (perhaps) rudder. The (usually lower) rate you select depends on how your airplane responds to controls at high and low speeds. Some airplane model manufacturers are beginning to include recommendations for lower rates; check your instructions. Barring this, I usually recommend 70-75% for elevator and ailerons; about 50% for rudder.

One problem with dual (or triple) rates is that, once that rate is chosen it’s in effect until the switch is used to select another rate. Furthermore, if that chosen rate is a lower rate, you can see by the preceding graph that you no longer have full throw capability! While this might be fine for high-speed flight, you might need more control surface movement for takeoffs and landings. Of course, all you would have to do is remember to flip those switches after takeoff and before landing. Is it possible to have the benefit of reduced sensitivity AND full throws? Yes, it’s called…

Exponential. This function is more clearly shown in the following graph (…a picture is worth a thousand words…).

This is the ‘usual’ exponential that is described in your manual—as the graph text says, "…increasing exponential values gives a smaller response for the same stick movement around neutral." For JR, this exponential is called Positive, and you should enter positive values to achieve it. For Futaba and HiTEC, this exponential is called Negative, and you should enter negative values to achieve it. For other manufacturers, please check your manual for the correct inputs.

Note the curve(s)—indeed there is a smaller response around neutral, just like dual rates, but unlike dual rates, full deflection is available near extreme stick positions. Kinda like having your cake and eating it, too…this is why I think exponential is perhaps the most useful of the functions in your computer radio!

Your manual will probably state that values of exponential can range from –100% to +100%. Ignoring, for the moment, which brand of radio we’re talking about, what does the ‘other’ exponential curve look like, and why should you use it (or NOT!)? Check out the following graph. Note the servo response around neutral—for a VERY SMALL stick movement, the servo response is HUGE! This is why you should be very careful to input the correct values (either positive or negative, depending on your radio and the softness/sensitivity desired)!

When would you choose to use this kind of exponential? As the graph text states, perhaps for airplanes that exhibit sluggish response, or for 3-D models, or (so I’m told) for some helicopter tail rotor applications.

Exponential values are usually assigned as part and parcel of assigning different (dual) rates, and therefore are controlled by the same switches. You can, for instance, set exponential values for one rate (such as the high rate) but not for the other, then compare the response when you fly. (Remember, however, that lower rates don’t give you full control authority…!)

There are variations of these functions. One of these is called:

Variable Trace Ratio (VTR). This is a ‘poor man’s exponential, and is shown in the following graph:

The ‘curve’ is really two straight lines; the ‘break point’ is programmable, as is the slope of the first segment (i.e., the percentage reduction, just as you would set a dual rate). Your radio may not have this function—it was an early ‘feature’ in some computer radios.

There are other variations, such as "Expo-Linear" and "Linear-Expo". Their graphs are shown here for completeness. Again, your radio may not have these functions.

As I’ve stated previously, I think the Exponential Function is perhaps the most important function in your computer radio. I use it for all of my airplanes, along with dual rates. I think you should, too!

† Models that are ‘sluggish’ in their responses are sometimes found to have large gaps between control surfaces and their corresponding fixed counterparts (e.g., ailerons and their wings). Seal those gaps and you may find that you have a different airplane on your hands!

* Most computer radios provide dual rates for all three primary control surfaces (aileron, elevator, and rudder); hence, you’ll have three switches with which to contend. This usually isn’t a problem.