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Old 01-12-2020, 08:41 AM   #1 (permalink)
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Default Neutral pulse length and FBL setup

I just read in an OpenTX manual that OpenTX uses a neutral servo pulse length of 1500usec, which is slightly different from the 1520usec Spektrum/JR uses (I believe those numbers are right - correct me if I'm wrong).
Does this mean that I would have to go over the setup procedure of my Microbeasts again after making the jump from Spektrum/JR to FrSky/OpenTX? I guess the Microbeast would interpret that small difference as a bit of trim, which shouldn't be used.
The reason why I'm even asking this is that all but one of my helis are in scale fuselages, and it would be a major pain to have to pull them out just to level the swash. I have replaced receivers in the past without touching the FBL, but that was just a swap within the same brand (Spektrum).

Thanks!
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Old 01-12-2020, 08:51 AM   #2 (permalink)
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The center pulse length is completely adjustable in OpenTx. You can even have different centers for each channel! Look on the Output page and you’ll see the right column to adjust.

But make sure of course to check your FBL’s live view of transmitter inputs. The transmitter channel center pulse length doesn’t necessarily match the servo center pulse. Eg, my tail servo is 760 us but that doesn’t mean I use that value for the pulse length on my rudder channel output.
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Old 01-12-2020, 09:58 AM   #3 (permalink)
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Thank you for the quick response.
The adjustability is good news. Isn't that effectively "trim" then? I'll play around with Companion later to get a better understanding.


You pretty much lost me with your second paragraph, though. I don't see how I could check the input to the FBL unit, unless you are talking about the transmitter outputs to the FBL in the Companion monitor. My FBL units themselves are set up, with the swash level and the correct compensation directions. But if the center pulse length is slightly different, this will probably act like a bit of trim.

Is there any way to make sure that in the end swapping out the receivers is all I need to do (besides setting up the usual stuff in the radio), or do I have to bite the bullet and go through the FBL setup again when it's hooked up to the FrSky receiver?


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Old 01-12-2020, 10:07 AM   #4 (permalink)
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Personally I would bite the bullet and hook up the FBL again. It’s the safest option. Remove blades and all that.

It doesn’t mean you have to redo the entire setup though. You are just verifying and modifying settings as needed. In your FBL setup there should be a way to see what receiver values your FBL is getting. So for example that when your rudder stick is centered, is your FBL seeing 0 rudder value? And if you do rudder stick full right, does your FBL interpret the receiver value correctly, or do you need to adjust reverse and/or endpoint Values in your TX. That’s what I mean by looking at transmitter values live in your FBL.

I suppose you could just look at your physical servos on the heli to see if anything needs reversing in your transmitter... but you wouldn’t necessarily be able to know if there are end point adjustments needed by eye.
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Old 01-12-2020, 12:02 PM   #5 (permalink)
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You are right, it's certainly safer to go over it again. Servo reversal and all that I would check anyway; the main difficulty is the swash leveling/servo centering because I cannot see my swash or all my servos inside the fuselage (I get a front view of it but no side view), and I can't use a swash leveling tool because it wouldn't fit through the opening in the doghouse.
However, I think I have come up with an idea to check the swash leveling/servo centering without removing anything. The key is that mechanically everything stays the same. If I now go into the servo centering menu in the FBL, I just have to make sure that the pitch angle over the nose is exactly the same (ideally zero) as over the tail - if it's not, the swash is tilted to one side. The same applies to blade left vs. right.

I believe that should work.
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