How fast is fast (controllability)?

coyoteAnd more importantly, how fast should it be?

Note: This article is what the site is, “RC-Thoughts”, they are my thoughts about an issue that is widely talked about, “speed of servo’s” or “speed of radio” and so on. This article is not about the absolute truth, it just gives one point o view.

Wait, what?

Whats RC-flying (and any RC-hobby actually) about? I think it’s quite much about user experience, usability of the controlled model regardless of what it is, a plane, heli, drone, car, truck, tractor or anything RC-controlled object. When we are talking about usability and the actual user-experience we are sooner or later coming in to the term “speed of control”. What we are talking about is the time from the moment user gives the command to move and the user experiences the model to respond.

The “chain of command”

Let’s think of the chain of events leading to  a hypothetical situation where a RC-controlled car is powered and stands still. We want it to go forward, let’s think what are the events leading to the actual forward-movement:

  1. The idea of “let’s go forward” in user’s head
  2. Brain commands finger to do the movement
  3. Finger does the movement
  4. Transmitter reads input value of a control-device (in this case throttle lever)
  5. Transmitter processes data
  6. Transmitter sends data to receiver
  7. Receiver get’s the data
  8. Receiver processes the data
  9. Receiver commands speed-controller
  10. Speed-controller processes data
  11. Speed-controller commands motor
  12. Car moves forward

The point is, all of these twelve points take some time. Some of them more than others but either way, every step takes time. Let’s say we have a flying thing we need to control then the thing is even more complicated. This is due the fact that response-time of the flying thing is slower since air where the model is moving in is not solid. There is always some time between the (for example) movement of aileron to the moment where the ailerons changed angle creates force and moves the wing.


What we are talking about are really small steps in time, some of the steps are fractions of milliseconds but they still are there. On top of that they are not always the same. Things affecting the speed of all these steps are almost endless, for example:

  • Users overall health
  • Users reaction speed (affected for example age, medical conditions)
  • Users speed according external  influence (weather, any disturbing things like bugs, people, excitement due  performing in a show etc)
  • Used radio-equipment
  • Used settings in radio equipment
  • Used servos or other control devices (speed or force of the servo)
  • Reaction speed of the controllable device (engine, turbine, speed-controller etc)
  • Used geometry in physical connection to controllable part of device
  • In some cases size or shape of controllable part of device

This really is getting a true jungle due so many variables.

User experience

It really comes down to user experience. One common topic to talk about regarding user experience is the question of “when can user notice a difference in speed of <insert something here>?” This is mainly a two-part issue:

  • Total reaction speed (measurable)
  • The “feel” of continuous control-experience

We need to separate these two. Let’s think of driving a car or flying an airplane and nothing surprising happens. The movement of the model is based on control-inputs by the user, model responds to those inputs and moves accordingly. If there is too much time between input and actual movement the user experiences by watching the model it “feels sluggish” or “there’s too much lag”.

If something surprising happens then it’s more about pure reaction speed combined (in many cases) to perception of the orientation of controlled model (a plane is coming towards you, left is right, right is left). All of that is once again very personal due the different variables regarding the person itself. One cool thing you can do is to go to different reaction-speed test-pages that are on the world wide wait. Do not expect any 10 ms results…

roadrunner Now we’re getting somewhere, finally.

If the time between control input and the user-experienced movement is short the model is experienced as “fast” and “responsive”. Of course if the case is opposite the model is “pure crap” or “it’s just slow” and the overall user-experience is bad. Can you feel the difference in the same controlled model after changing from 0.13ms servo’s to 0.04ms servos? Some say they do but I’m personally leaning towards the thing that they are not feeling the difference per se but are having an overall better user experience due faster overall controllability.

What is “fast” and “responsive” in measurable terms?

That is the million dollar question. When we humans do different things we “demand” different time-frames the desired event should occur in. Boil an egg, we know and accept it takes minutes. Use a GUI on a device, it is known by several researches the response-time of (for example) an application needs to be 0.1 seconds (100 milliseconds) or less so the user experiences the usage as “instant”. Wait for an webpage to load, this should be done within a few seconds. Flying an RC-heli? I’d like it to be “instant” but it’s not, the time is “something”. But it still feels “instant”, sometimes. For example let’s say we are hovering a heli with big rpm. We know that when I move the pitch up the heli moves up. If I move the stick fast the heli goes up fast but it does not start on that specific moment when I move the stick, it is VERY close to that but it’s not the SAME point in time.

Some ideas of what the total time is about

Let’s face it, servos for example are fast. Or putting it more precise they’re FAAAAAST. It’s not uncommon for a normal-sized servo to be able to move the 60 degrees range in 0.04 seconds or even less. That is 40 milliseconds. Since we rarely need to move from -100 to +100 the actual time in normal situations is even shorter.

Radio? Well here is once again a true jungle. It is very hard to get any comparable results what comes to total time between control input and actual command sent out of receiver purely due the fact that it is rare for someone to have access to several brands / models and to measure them on the same measurement equipment. One of the exceptions is a thread on Runryder.

The value we should concentrate (in my opinion) is the average time. What we can see is that they’re quite a lot a part, fastest are around 8 milliseconds and slowest are over 72 milliseconds. One important thing those measurements also demonstrates is that settings do make a difference, look at Futaba’s values with settings.

Can you make out the difference between say, 10ms radio and 70ms radio? I actually think you can. If we consider the time between control input and the experienced movement for example of a steering input of a rc-car the time-difference of 60ms is pretty big when looking at the total long-term user-experience. Just moving the steering-wheel once right-left most likely does not give any thoughts to one direction or the other but using the car and driving around most likely will give you the earlier said feeling of “fast and responsive” or “sluggish”.


Let’s face it. Most of us have not given this that much thought. Many of us buy “the best available” and some get “just something that works”. What we also need to understand is the fact that we do look for different things in this hobby of ours.

One example. A competition heli-flyer once while asked told what servo’s he uses. Nothing fancy, basic servos with mediocre performance to say the least. And that dude flies smack like no-one else.  It’s more of “knowing your model” and “feeling your model” than anything else. You use your equipment long enough, you practice long enough and the result is that you know exactly how the model behaves in different situations and can use it in the best possible ways. Most often with some absolutely great results.

Market-oriented performance?

“Fastest <insert product here> available!” or “True speed <insert product here>!” or any other commercial you’ve read recently. Sound familiar? Yes. Two things sell good in male-dominated hobby, it’s sex and speed. Strangely there’s no rock’n’roll :)

Do you NEED the fastest available? Some do, most don’t. It’s most often more “I want” than “I really need”.

Now guys. (Any girls out there?) Grab a cup of coffee, charge your batteries while the model’s batteries are in the charger. Get on the field and put the models to work!

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