RC Rates & Exponential: In a Prop-crack

Radio Control Expo Rates

This article will help you understand about Dual Rates and how they can help you fine tune the response of your drone, aircraft, helicopter or robots. These settings can be done on your transmitter. If you are getting into flying drones, it is important to invest in a good transmitter. In this article, as usual we will break down the points you need to know so that you don’t have to read several paragraphs to understand what you need to do.

How do I know if I have a good transmitter?

A good transmitter would require you to have

  1. Good Range
  2. Multiple channels
  3. Adequate switches and knobs
  4. Light Weight
  5. Inbuilt Screen with backlight
  6. Good Program and programming options.
  7. Low cost.

What Are Dual Rates?

  • Controls total deflection of the control surface.
  • Allows you to change how responsive the place is to your joystick movements.
  • You can change the responses during midflight by programming it to a switch.
  • This is useful when you want to convert flights from normal to sport or aerobatic mode.
  • The response from the joystick is tagged to your control surface.
  • At low speeds you require MORE control surface movements
  • At high speeds you require much less control surface movements.

Dual Rates Practical Scenario – Rudder

  • 100% Dual Rate : You can set 10mm movement on your joystick, moves 10mm of the rudder.
  • You can now change the setting.
  • 50% Dual Rate : When you move 10mm on your joystick, and it moves 5mm on the rudder.
  • Stick movement and surface movement remains proportional.
  • Control and Response are proportional and linear.
  • Therefore, if you move your stick 20%, you will get 20% of rudder movement.
  • Linear Graph (Straight line at 45 degrees)


What is Expo (Exponential Rates)?

  • Finer control.
  • Exponential changes the relationship between stick movement and surface movement.
  • When using expo, the stick movement and surface movement will no longer be linear.
  • Expo allows you to shift the control surface response so you get a different amount in the early part of stick movement as compared to later part.
  • Alters default linear curve to one that is less sensitive or more sensitive around the middle point.
  • Gives you varying levels of control surface reaction without needing to flip switches.
  • Compensate non-linear control surface response.
  • Smooth change in rate.
  • We get a curved exponential line graph.
  • The further we move the stick, the less linear the relationship between stick and surface.

Exponential Practical Senario – Rudder

  • At 100% stick movement, you get 100% rudder movement.
  • Shifting joystick to 50%, Only will get 25% rudder movement.
  • This depicts low rates on the first half of stick travel and high rates on the second half of stick travel.
  • Softer response on the stick center Area and faster response towards the end.
  • Also used for helicopter flying, where you want it to slowly take off at 25% throttle but go for more power when it is in the air.

Practical Senario 2

  • Standard Setup expo is 0% , this is the normal linear response we know.
  • Now, -50% expo set
  • For up to 50% movement, I want to get 25% surface travel max.
  • When 100% movement, I want 100% surface movement.
  • If +50% expo set, for the first half travel of joystick it would be very responsive.
  • This could be 75% travel on the control surface.

Expo Recommended Settings

  • Less Sensitive Reaction on plane movement –Positive Expo
  • More Sensitive Reaction on plane movement- Negative Expo
  • Typically 25-40% for beginners
  • However when you experience gusts of wind or unbalanced flying, you may cause issues for yourself. So be very aware of your stick settings.
  • Typically set at 35% expo.

You must remember that all these don’t matter if you have not setup your surface controls well. Proper rigging of servo motors, control horns and control surface movement will allow you to accurately control Dual rates and expo. Many times, incorrect rigging leads to crashes. Always check that your servo creates equal travel between the servo midpoint positions. This technically means, up elevator and down elevator must have equal travel.

Leave your comments below.

Here’s another video to help those still flying in space…

Author: adlabs2

Arrowdynamic Laboratories Pte. Ltd. (ADLABS) was founded in 2008 anticipating imminent technological pervasion. ADLABS operates with two wings, Education and Engineering. Education involves the development of skills training programs and STEM-based enrichment programs for everyone. The engineering wing conducts research and development activities on new prototypes with potential to form into a new branch startup. Our engineering team also offers specialised services for startups, companies and institutions.