RC Plane Dual Rates and Expo: Where to Set Them on Your Radio

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Most new airplanes do not need more stick time to feel right. They need five minutes in the transmitter menu. Dual rates and expo are the two settings pilots adjust most after a new radio system is bound, and they do different jobs. Mix them up and a perfectly good plane ends up either twitchy or mushy on its first flight.

This is a researched guide built from the radio makers' manuals and from the setup pages of E-flite airplane manuals, not from our own flying. Where a number appears, it comes from one of those documents, and we say which kind. Percentages on a screen mean little until you measure what the control surface actually does, which is a point the manuals themselves keep making.

What each setting does

Dual rates change how far a control surface moves for a given stick movement. Per EdgeTX's manual, the weight on an input line is the "percentage value of the stick travel to use (often referred to as 'rates')." A 70% low rate gives you 70% of the travel you set up with 100%.

Expo (exponential) changes the shape of the response, not the end points. Per the Spektrum NX8+ manual, "Exponential affects control response sensitivity around center but has no affect on overall travel." It softens or sharpens the feel near neutral while full stick still gives full travel.

Two things sit underneath both:

• End points and mechanical set-up come first. Jeti's guide says to start by setting the basic deflection mechanically through the servo arm and control horn ratio, so full servo travel is used. Futaba's manual describes the AFR (D/R) function as "normally used after End point has defined the maximum throw directions." If a plane is simply moving too far, fix the linkage, then adjust rates. Our servo guide covers arm and horn geometry.
• Heavy expo shifts the work to the rest of the stick. If the middle is softened a lot, the remaining travel has to deliver the response, so the outer part of the stick can feel abrupt. Test both the center and the edges.

Where each radio keeps them

The sign of expo is the trap. Several makers state it differently, and one (Futaba) does not state it in the manual text this guide reviewed.

The practical rule: after you change the sign, look at the curve on your radio screen, or move the stick on the bench and watch the surface. A minute of checking is cheaper than a nose-down first flight.

Spektrum AirWare

On the NX8+, Rates and Expo sit in the Function List and apply to the aileron, elevator and rudder channels. You pick the channel, assign a switch, then change the dual rate and expo values. The manual's definitions are quoted above. E-flite's own airplane manuals walk NX owners through "D/R (Dual Rate) and Expo" in the Function List and iX owners through "Model Adjust, then Dual Rates and Expo."

The NX8+ specifications list 20 channels, DSM2/DSMX, integrated telemetry and a 3.7V 2000 mAh Li-ion battery. Availability at retailers moves around, so check before you plan around it. Search for the Spektrum NX8+ on Amazon

A rule worth underlining for Spektrum AS3X and AS3X+ aircraft: several E-flite manuals say that to keep the stabilization working properly you should not lower rate values below 50%, and that smaller throws should be made by moving the pushrod on the servo arm instead.

EdgeTX and RadioMaster

EdgeTX has no screen called "Dual Rate," although the manual notes that the Inputs section "is also commonly referred to as 'Dual Rates'" because it was called that in earlier OpenTX versions. Set the input's weight for the rate, set its curve type to Expo, and add extra input lines with different switch positions for low and high rates. The manual describes an input line as having a source, weight, offset, switch, curve, side and trim.

The RadioMaster TX16S Mark II is a common example of an EdgeTX radio, listed as a 16-channel, Mode 2 transmitter with a 4-in-1 module and Hall gimbals. Retail stock varies. Search for the RadioMaster TX16S Mark II on Amazon

FrSky ETHOS

ETHOS does rates inside the mix itself. Per FrSky's v1.6.3 manual, a mix can have weights (rates) and curves, conditional on a switch, logical switch, trim position or flight mode. The default weight line applies when no other line is active, and each extra line adds another rate.

The manual's own example shows three rates on one three-position switch: the default line at 100% weight with 50% expo, the middle position at 70% with 40%, and the down position at 50% with 30%. That is a software illustration, not an airplane recommendation. The manual also says the recommended order of actions is Slow, Curve, Weight, Differential, Offset, then Trim, and that the curve is applied before the weight.

The FrSky Tandem X20S is an ETHOS radio with a dual-band 900 MHz and 2.4 GHz internal module and an 800 by 480 colour touch screen. Amazon showed it as unavailable when this guide was researched. Search for the FrSky Tandem X20S on Amazon

Futaba 16IZ and 16IZS

Futaba splits the job in two. In the Model menu, the Dual rate entry chooses the function and the switch. The rate and EXP values themselves are set on the AFR screen while the D/R switch is on. Per Futaba, up to six rates can be added for each flight condition, D/R is set per condition and is not reflected in others, and the D/R at the top of the list has priority.

The manual gives the adjustment range as minus 200 to plus 200% for rate and minus 100 to plus 100% for the EXP rate, with initial values of plus 100% and 0%. It calls the EXP1 curve effective for smoothing the start of aileron, elevator and rudder movement. Because the sign is not spelled out in the pages reviewed, move the slider and confirm which way the curve bends.

Futaba lists the 16IZS as an 18-channel (16 linear plus 2 switch) FASSTest system with a 4.3 inch transflective colour touch screen, S.BUS2, and a USB-C port that can act as a Windows joystick for simulators. Search for the Futaba 16IZS on Amazon

Jeti Duplex

Jeti keeps dual rate and expo under Fine Tuning. Each function supports up to three deflection levels in a 0 to 125% range, where 100% equals the deflection set in Model/Servo Setup. Expo runs from minus 100 to plus 100%, with positive values reducing sensitivity around center. You can switch with a two- or three-position switch, or with any proportional control if you enable the Prop. option.

Jeti also says not to use the normal Dual Rate function with receivers that have integrated stabilization, such as the ASSIST series. The DS-12 has a 3.5 inch colour display and 8 GB of SD storage per Jeti's catalogue page. Search for the Jeti DS-12 on Amazon

Starting points from airframe manuals

Rates and expo are not universal presets. The most reliable starting point is the manual for your own airplane, which gives rate and expo together with measured throws. These examples from E-flite manuals show how much the numbers vary.

Three things stand out:

• The same percentages mean different movement on different planes. A 100% rate on the Conscendo is 26 mm of aileron, while on the Eratix flat foamy it is 88 mm. Match the measured throw in the manual, not the number on the screen.
• More expo goes with more throw. The aerobatic flat foamy and the bush plane use more expo than the trainer, glider and warbird in these manuals.
• Low rate is where the first flights happen. The Apprentice STS manual says to make the initial flights in low rate and use high rate elevator for landing. Several other E-flite manuals say the same.

For a pilot stepping up from a trainer, a three-position low, medium and high arrangement is a gentler progression than a single big jump. It also pairs well with the aerobatics routine once the basics feel solid.

A bench set-up sequence

1. Set the mechanics first. Center the servos, square the arms and horns, confirm direction, and bring the surfaces to the throws in the manual.
2. Set end points before rates. Rates scale the pilot command within the range the end points allow.
3. Program a safe low rate and a high rate. Add a middle rate if you have a three-position switch.
4. Add modest expo using your radio's sign convention. Check the on-screen curve.
5. Watch each surface while you flip the rate switch. Confirm that high rate really moves more than low rate.
6. Measure with a ruler or throw gauge wherever the manual gives millimeters or degrees.
7. Check stabilization rules. On AS3X and AS3X+ models, keep rates at or above 50% per the manuals, and on Jeti ASSIST receivers do not use the normal Dual Rate function.
8. Fly the first flight on the maker's conservative setting, then change one variable at a time.
9. Use rate for authority and expo for feel. If you need more authority at the edge of the stick, fix the rate or the linkage rather than piling on expo.
10. Practice before you fly. A few sessions on a flight simulator let you try different rate and expo values without risking a wing, and the beginner flying guide explains what the controls should feel like.

Common mistakes

• Assuming one expo sign fits every radio. The table above shows how the wording differs. Check the curve.
• Using expo to hide a mechanical problem. Misplaced horn holes, slop in a linkage or the wrong servo arm should be fixed physically.
• Assuming more expo is always safer. A soft center can make a recovery input feel sharp at the edge.
• Copying numbers from another model. The Conscendo and the Eratix use the same kind of switch but very different throws.
• Ignoring stabilization instructions. AS3X, AS3X+ and ASSIST systems have their own rules about lowering rates.
• Sticking to two positions out of habit. If your radio has a three-position switch, use it.

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