I was asked about the difference between RC rate and Pitch/Roll/Yaw rates, so here is my understainding 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
What is Pitch/Roll/Yaw 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” – 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 were closely linked to the 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 effect 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 – S.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 S. Rate. I expect more naming changes might happen until things are future standardized.
How to set RC Rate and Roll/Pitch/Yaw Rate?
I mostly fly FPV in acro mode with Baseflight/Cleanflight. I always prefer to tune RC rate and RC expo first, for normal flying and levelling (no flips/rolls). Then I crank up Roll, Pitch and Yaw rates for flip, rolls and acrobatics, until I am happy with the speed of rotation. But of course you need to adjust Pitch/Roll/Yaw expo as well at the same time, make sure it’s not too twitchy around centre stick.
As Pitch/Roll/Yaw rate will boost stick sensitivity around stick centre, I might want to go back and adjust RC rate and expo to get a good balance.
One thing I notice from all the different flight controller software is, they all have different ranges of PID and rates numbers. So I never bother trying to relate those values from one to the other. Just whether the value should be smaller or bigger.
Also, avoid setting expo on the radio (TX), because it reduces your stick value resolution. ONLY set Expo in the flight controller
For LOS quads (although I don’t fly LOS much), I was recommended to use higher Roll/Pitch rate and lower RC rate as it makes the movements looks crisper and more precise. (but it would look a bit jerk-ish and robotic in FPV)
Update 14/09/2015 – Danny Cruz from Multicopter International Group mentioned a discussion between him and Boris B. Boris pointed out that in PID C1 and C2, you can forget about RC Rate. Just set it to 1 for example, then adjust roll / pitch rate separately (reason mentioned above post already). I thought I wanted to mention this as it will make a lot of people’s lives easier.
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.