I tell you what, I spent hours on discovering how it works. This is very tricky and I don't fully understand why. Here are my results.
I tested all of this on Ethos. Probably it is same for OpenTX but I'm not 100% sure.
Also, this is with Spirit GT and Archer RS, and FrSky x20s radio. I have to retest if it will be same for other models I have. But so far my experience is that other FBL units and other receivers don't make a difference.
Also, I really hope that "Diagnostics" page in Spirit shows correct results, because I fully relied on this. It would be great to see numbers with fractions there (like 100.0%), this would help.
AIL, ELE, RUD
These channels center themselves when powering on Spirit, and this is already reflected on the "Diagnostics" page. What this means, is: if you change centering (and you see non-zero values on "Diagnostics" page), after you restart Spirit, you will see zeros again.
So PWM / center / subtrim doesn't matter for them. But I think they should all be set to same PWM center (for pitch - see later).
My setup that works:
Min: -82.9 Max: +82.9 Center/Subtrim: 0 PWM Center: 1500
Pitch
It seems that "Pitch" also centers itself at the startup of the radio, but not from its own channel (#6) which is of course not possible. It rather somehow uses combination of RUD, AIL and ELE to center itself. If you would start FBL with RUD or AIL or ELE non zero, pitch centering will be affected.
But what is weird, it offsets this by some value. So to get "0" pitch in Diagnostics, when radio sends "0", I have to offset PWM center for Pitch (comparing to RUD, ELE, AIL).
This works for me:
Min: -82.9 Max: +82.9 Center/Subtrim: 0 PWM Center: 1507
Why? I don't know, but it works.
Gyro Gain (#5)
Here I can't get perfect setup, where each % value sent by radio would match same value on "Diagnostics" page. But this is not very needed for this channel. This works well:
Min: -95.2 Max: +95.2 Center/Subtrim: 0 PWM Center: 1518
Why this channel needs such big range comparing to other - no idea.
Channels 8-16 for custom functions
If you use custom functions, then this is what I found workin:
Min: -82.5 Max: +82.5 Center/Subtrim: 0 PWM Center: 1518
Best test is using "Pirouette Consistency", because with range 50-250 it spans over 201 values, just like radio with -100..+100 %. With this setup I get exact match, so -100% = PC 50, 0% = PC 150, +100% = PC 250 and changing value in radio by 1% changes pirouette consistency by 1 across the whole range.
This part is important for me, because I control some values with channels, like "Cyclic rotation rate" and I really want to be sure that if I set my models same, to have - for example - cyclic rotation of 13, I get same thing in every model. So I'm very happy I found some consistency here and I hope it works for all models. Changing PWM center was key here.
Throttle
Here I don't know. I didn't want to mess with this one, to not recalibrate ESCs. But maybe I check one day, as it would be good to have a proven set of numbers, which can be copied anytime you setup a heli.
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