Setting Up Control Throws: A Guide to Dual Rates and Expo on Your RC Plane

If your remote control airplane exhibits jittery and extreme sensitivity to your control, then the trouble certainly isn't your skill as a pilot but your configuration. Control throw, dual rates, and expo are the key elements that, if mastered, can easily translate an unrefined flight into an elegant one. This tutorial will guide you step-by-step through the necessary configurations to help you tailor your airplane’s control to your reflexes. After the tutorial, you can be sure of how your transmitter can be harnessed for your benefit, making your flight safe and fun.

What's the Basics of Control Throws?

These "Control throws" are the maximum distances that the control surface—the aileron, elevator, and rudder—travel as you push the stick on your transmitter to the fullest.

The Impact of Throw Size

When the control surface has a long throw, it is more forcefully caught by the air. As a result, the plane starts rolling, pitching, and yawing very quickly. This gives it tremendous agility, which is awesome during 3D aerobatics. What is more, the throw is short, making the aircraft more docile and easier to handle.

Mechanical vs. Digital

In an ideal setup, it is best to establish the basic control throw mechanically first. This is achieved by adjusting the pushrods on the servo arms and control horns. When the basic mechanical throw is established, it is necessary to adjust it further with software on the transmitter. Although it is possible to improve it further on the transmitter, doing so may decrease the resolution of the servos.

Unlock Flight Modes with Dual Rates

Dual rates are arguably the most useful feature on modern computerized transmitters. They allow you to define two (or sometimes three) different limits for your control throws on a single physical switch. Think of it as having a "mild" mode and a "wild" mode available at your fingertips.

When you flip your switch to "Low Rate," the computer limits the servo output. For example, even if you move the stick 100% of the way, the servo might only move 70% of its total travel. This is perfect for takeoff, landing, and smooth cruising where you want stability and predictability. It prevents you from accidentally over-rolling the plane when you are low and slow.

On "High Rate," the servo uses 100% (or more) of its travel. This unleashes the full aerobatic potential of the remote control airplane. By learning how to set dual rates and expo on a transmitter, you effectively give yourself a safety net that you can turn off when you want to perform loops, rolls, or spins.

Tame a Sensitive RC Plane with Expo

Although dual rate restricts the maximum extent of the control surface, "Exponential" (or Expo) varies the rate of control. Without the expo, the motion of the servo is linear. So, if you move the control stick 10%, the servo will also move 10%. Nevertheless, the average human thumb is not rock-steady, particularly when nervous.

Softening the Center

Expo smooths out the control response near the middle of the stick. With the expo on, it may take very little movement of the stick to result in only a very small movement of the control surface. Expo eliminates the "twitch" associated with unsteady thumbs.

Retaining Full Control

Most importantly, expo does not affect the total throw. When you move the stick to the very end, the curvature ramps up, and yet you will have maximum power and full deflection. It is akin to a progressive steering setup, which is soft in the middle when on the highways and sharp on the corners. This is the best tool when it comes to precision flying.

Get Your Transmitter Ready for Programming

If your rc plane is electric, remove the propeller before powering it up on the bench. This ensures that if you accidentally bump the throttle channel or reverse a setting, the motor won't spin up and cause injury.

Power on your rc plane controller first, followed by the aircraft's receiver. Ensure the plane is bound and all surfaces are moving in the correct direction. Navigate to the "D/R and Expo" menu (often found under the Function List or Model Setup menu). Move the sticks to confirm you are adjusting the correct channels for rc airplane control (Aileron, Elevator, Rudder). You want to see the surfaces moving in real-time as you adjust the values to visually understand the changes you are making.

How to Set Up Dual Rates on Your Transmitter

This procedure is slightly different between brands, but the principle is the same. Here is a generic guide on how to program an RC transmitter to set up your rates.

Assigning the Switch

First, identify the option to assign the switch to your D/R setup. Most pilots like to assign the positions of all rates (aileron, elevator, and rudder) on a single switch. This is often marked "Combi" or designated on a dedicated toggle such as Switch F and/or Switch G. More advanced pilots assign each rate separately, but it is easier to keep them together to lower the workload.

Defining the Percentages

With the switch set to the "High Rate" mode (typically up or backward), set the percentages to 100%. You can now set the maximum mechanical throw. Turn the switch to "Low Rate" mode. Turn the percentage increment downward. Starting with 70%, which is a common rate to start at when flying a trainer or sport plane, indicates that when the switch is engaged, full stick motion will only actuate the control 70% of the maximum. Repeat this process for the aileron, elevator, and rudder channels individually.

A Practical Guide to Programming Expo

Since you now know what your limits are regarding traveling, it is time to work on the middle. You can locate the Expo function on the same menu option where you changed the dual rate.

Understanding Positive and Negative

Be careful here: different manufacturers use different signs. For Spektrum and JR, positive (+) expo softens the center. For Futaba and Hitec, negative (-) expo softens the center. Look at the graph on your screen. You want the line to be flat in the middle and steep at the ends. If the line spikes in the middle, you are making it more sensitive, which is dangerous.

Setting the Values

A good starting value for most pilots is between 20% and 30%. This flattens the curve enough to hide minor mistakes without making the controls feel sluggish or disconnected. You should apply expo to both your High and Low rate settings. Some pilots prefer a higher expo on High Rates (e.g., 40%) because the control surfaces move so far that the center sensitivity is even more pronounced.

Find the Perfect Settings for Your Aircraft

There is no "correct" percentage to which all remote control airplanes should respond. What works best is greatly dependent on the plane and on individual flying styles. A 3D aerobatic plane with very large control surfaces could require 60% expo to hold a steady course, while a sailplane may require 10% expo, if any.

Of great importance is also your individual preference. There are pilots who like the strong, dynamic, and responsive control, and others who want to experience the soft, mushy feeling. You should consider the provided values (70% dual rates and 30% expo) to be the best starting point.

Test Fly and Fine-Tune Your Controls

The real test is when you're up in the air. Take off with the "Low Rate" mode, because it is the most stable mode. When you are at a safe altitude, which is "three mistakes high," turn the switch to "High Rate." Notice the difference between the speeds when it rolls and when it pitches. Does the plane respond too slowly when it is at low speeds? If so, land the plane and raise the low-speed percentage to 75% and 80%. Does it respond too erratically when you are trying to keep it on course and it is near the middle of the turn? Raise your Expo by 5%. Land the plane and alternate between these small changes.

FAQs

Do I need to set dual rates for the throttle channel?

No. Dual rates and expo are usually only utilized for the primary control surfaces, and that would be the aileron, elevator, and rudder. Your throttle needs to be linear (0-100%) at all times, as it would provide reliable acceleration, except for advanced pilots who use something called 'throttle curve,' which would be a different function.

Can I use too much expo?

No. If the expo value is set too high (above 60% for normal flight operations), the stick will be "dead" in the middle. As soon as you extend the stick, the control response becomes aggressive. This makes the aircraft difficult to control, especially during landing.

What if my transmitter doesn't have a screen?

If you are using a basic/non-computerized RC plane transmitter (commonly shipped with Ready-to-Fly radios), you may not be able to control the percentages specifically. But most basic remotes come equipped with the "High/Low" RATE Switch. Simply refer to your user guide for the values that the maker preset for the RATE Switch.

Should I use dual rates for my maiden flight?

Absolutely. It is an extremely good idea to land and take off on Low Rates for the initial flight. You are not sure how sensitive the aircraft will be. You are much safer starting out with less throw. Once you are comfortable, start experimenting with the high rates.