How to Set RC Airplane Control Throws, Dual Rates, and Expo

Control throws determine how far the ailerons, elevator, and rudder move. Dual rates let you switch between different amounts of that movement, while expo changes how quickly the controls respond near the center of the sticks. They work together, but they are not the same adjustment.

The safest place to start is the airplane manufacturer’s recommended setup. Once the airplane is mechanically correct and you have flown it, you can fine-tune the feel a little at a time. Starting with somebody else’s favorite numbers—or simply turning everything up—can make a perfectly good airplane difficult to fly.

The short version

1. Use the airplane manual’s recommended throw measurements.
2. Center the servos and make the surfaces mechanically neutral.
3. Confirm every control moves in the correct direction.
4. Adjust linkage geometry before relying on extreme transmitter endpoints.
5. Measure high and low rates at the control surface.
6. Add only the recommended amount of expo, with the correct sign for your radio brand.
7. Check every switch position, mix, and flight mode for binding.

What control throw actually means

Control throw is the distance or angle a surface moves from neutral. A manual may specify it in inches, millimeters, degrees, or a combination of those. Measure at the exact location shown in the manual—normally near the widest part of the surface—because the number changes depending on where you place the ruler.

Do not assume the two directions are equal. Hinges, linkage geometry, differential, and servo-arm position can produce different measurements even when the transmitter shows the same travel value.

Start with the mechanical setup

Set trim and subtrim near zero, power the receiver, and allow the servos to center. Install the servo arms as close to 90 degrees to the pushrods as practical. Then adjust clevises or pushrod length until the surfaces are neutral.

This preserves electronic adjustment range and gives the linkage better geometry. Large amounts of subtrim may be convenient, but they can create unequal travel and reduce the available movement on one side.

If you need a refresher on servo arms, linkages, torque, and installation, use the RC airplane servo guide.

Change throw with linkage geometry

Try to get reasonably close mechanically, then use transmitter travel or endpoint adjustments to finish the measurement. Avoid geometry that forces a servo to work hard, makes the linkage sloppy, or puts the pushrod at an extreme angle.

Set the recommended high and low rates

Dual rates are simply two—or sometimes more—available amounts of control travel. A low-rate position makes the airplane less responsive because the surface moves less. A high-rate position provides more authority.

If the manual provides both settings, measure and program both. Label or assign the switches consistently across your airplanes when practical. Consistent switch habits reduce surprises during takeoff or a maiden flight.

Do not use a generic percentage as a substitute for an actual surface measurement. Seventy percent travel on one transmitter installation may not produce anything close to seventy percent of another airplane’s throw.

Add expo after the throws are correct

Expo changes the relationship between stick movement and servo movement. With the usual “softening” setup, the surface moves less for the first part of stick travel but still reaches the programmed endpoint at full stick. It does not replace dual rates and it does not reduce maximum throw by itself.

The sign used for expo varies by radio brand. A positive value softens center response on some radios; other systems use a negative value. Check the transmitter’s servo monitor and move the stick slowly. The surface should respond gently near center and progressively faster toward the end.

Start with the airplane manual’s recommendation. If none is provided, use only a modest amount and make changes after flying. Too much expo can make the airplane feel disconnected near center and then suddenly responsive farther out.

Check direction before worrying about feel

Stand behind the airplane and move one control at a time:

• Right aileron: the right aileron rises and the left aileron lowers.
• Up elevator: the elevator’s trailing edge rises.
• Right rudder: the rudder’s trailing edge moves right.
• Flaps: both move in the intended direction and to the expected positions.

Reverse the channel in the transmitter when the primary control direction is wrong. Do not correct a reversed primary control by crossing linkages or adding a mix. If a gyro or stabilized receiver is installed, the stabilization response must also be checked separately; correct stick direction does not prove correct stabilization direction.

Check for binding in every configuration

Move each stick through its full range slowly. Repeat the check on every rate, flight mode, flap position, and mix. Listen for buzzing and watch for a pushrod bending, a clevis touching the horn, a surface hitting the airframe, or a servo continuing to push after the surface stops.

Binding wastes receiver-battery current and can damage a servo or linkage. Reduce travel or correct the mechanical interference rather than accepting a constantly loaded servo.

Common setup mistakes

Before the first flight

Finish the rest of the airplane with the RC Airplane Setup & Preflight Center. If the airplane later needs a large amount of trim or feels unusually sensitive or sluggish, use the trimming and control setup guide and the nose-heavy versus tail-heavy guide to diagnose the cause instead of hiding it with transmitter adjustments.