I was asked about the difference between RC rate and Pitch/Roll/Yaw rates, so here is my understanding of what they are, how they affect your quadcopter based on Betaflight/Cleanflight firmware, and how to set them up.
You might be also interested in PID tuning.
What is RC Rate and Expo?
In a nutshell, RC rate set the speed of rotation for stick travel. Expo adjusts the centre-stick softness against full-stick speed.
RC Rate is the multiplier on your Radio Transmitter inputs for all 3-axis: pitch, roll and yaw (one value for all 3 axis). If you increase this your quad will respond more aggressively to your stick inputs. This is basically the stick sensitivity setting.
Generally, Expo is a value between 0% and 100%. By increasing it, it reduces the sensitivity of your stick input around the centre stick (imagine a U curve, where it’s flatter in the middle). 0% = no Expo, it means no sensitivity reduction, and you get a V curve.
Expo is often used with high rates, so the pilot can have precise control near centre sticks, but yet can do more aggressive aerobatics with stick end points.
There is no right or wrong expo value, as long as it suits you. I have shaky fingers, so my expo is a bit higher than other people’s. :p
History of Pitch/Roll/Yaw Rate, and Super Rate
Pitch, Roll and Yaw rates change the speed at which the quad moves in a given angle or rotation. For example, if you increase ROLL RATE, the quad will spin around roll axis faster and more than at a lower rate.
You might wonder, we already have RC rate, why do we have another rate for each axis? Long story :)
When P/R Rate Was Introduced, Thanks to “Super Expo”
As we know the origin of Baseflight, Cleanflight and Betaflight were Multiwii, and back then it only had RC rate and Expo, that was it.
Later on, more and more crazy pilots started flying those little beasts called “Mini Quad”. They found Multiwii didn’t spin fast enough when doing flips and rolls, so additional rates were added in MW2.3 PID controller, to relaxed the PIDs as stick deflection increases, and increase the maximum rotation rate at extreme stick deflection.
This effect is known as “Super Expo” at the time – basically it means you can have relatively moderate stick sensitivity around mid sticks for “normal flying maneuvers”, and yet super fast roll/flip at stick end points.
With MW2.3 PID controller, Pitch/Roll/Yaw Rates have some influence on PID Controller. For example by increasing pitch/roll rate, you get faster spins, but your multicopter might feel less in control at stick end points. Also, it affects all-round stick range and not just the end points!
No more Super Expo in Cleanflight – P/R/Y Rates are now just Multipliers
Some time later, Cleanflight introduced more PID controllers, such as Re-Write and Luxfloat. In these PID controllers, P/R Rates no longer have an effect on PID values (however it still does have effects for controller 0, 4, 5 and 6).
P/R rate setting was still in the software for Re-Write and Luxfloat, although they won’t give you the “super expo” effect like they were designed to do. They simply linearly increase the overall rate on top of existing RC rate. Same with R/P Expo.
As for Yaw Rate, it’s explained in Cleanflight documentation that, “in PID Controllers 0 and 5, it acts as a PID reduction as explained above. In PID Controllers 1-4, it acts as a stick sensitivity multiplier, as explained above.”
Update 07 Oct 2016 – “Super-Expo” is Back in Betaflight – Super Rate to Replace P/R Rate
Flyduino’s KISS FC is another branch of Multiwii. They inherited the “Super-Expo” feature in their PID Controller, and it was particularly popular within their users.
At the time, Boris (Betaflight developer) was focus on Re-Write and Luxfloat PID controller (by removing all other PID C’s). As we mentioned earlier, these PID Controllers don’t have “Super Expo” effect. But due to how cool it is, he decided to implement it for those PID controllers in Betaflight.
And he later changed the naming of P/R rate to Super Rate.
What is Super Rate
Super Rate modulates the end points of the stick curves. When adjusting Super Rate, only the end points of the curve changes, the center remains the same.
It basically makes the end portion of your sticks more or less sensitive, so you can control the speed of rotation only at the end of the stick.
How to set RC Rate/Expo and Super Rate?
I mostly fly FPV in acro mode and I always tune RC rate first by cruising around (no flips/rolls).
Then I crank up Super Rate for flip, rolls and acrobatics, until I am happy with the speed of rotation.
Finally I will add a little bit of RC Expo to have a bit more fine control in the mid stick.
You might want to further fine tune RC Rate/Expo and Super rate to make sure the quad isn’t too twitchy around the centre stick.
One thing I notice from all the different flight controller software is, they all have different ranges of PID, rates and expo number scaling. So I never bother trying to relate those values from one to the other, just whether the value should be lower or higher.
Also, avoid setting expo on the radio (TX), because it reduces your stick value resolution. ONLY set Expo in the flight controller
Throttle Mid and Expo
Lastly, Next to RC rate and expo, you also have Throttle Mid and Throttle Expo.
THROTTLE MID is default at mid throttle, but in my opinion this should be set to a value where your quad hovers. If you have to raise your throttle stick past the centre to hover, increase it. If your quad has higher power-to-weight ratio than 2, (hover at less than 50% throttle), you might want to drop it, so it’s easier to control your altitude. You need to use this with some throttle expo, otherwise it won’t do anything. If you set Expo = 0, you can ignore throttle mid.
One funny situation I see all the time is, some people fly a over-powered mini quad, and their throttle mid is way too high than its hover point, and they cannot land it without crash into the ground. :D
THROTTLE EXPO is how flat you want your throttle mid to be on a curve, so the flatter curve, the softer response it will give you. I personally use no expo or just a little expo (<10), you get better resolution with throttle control. Especially useful when flying proximity and racing.