Taranis: Adding V-Tail Differential - RC Groups
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Jan 15, 2017, 12:11 PM
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Taranis: Adding V-Tail Differential


Adding V-Tail Differential to Mike Shellim's F3J/TD Setup, Version 2 or 3

Last updated on January 16, 2017.

If your V-tail glider changes pitch when you apply left or right rudder, the cure is to add V-tail differential. Here's a mod for Mike Shellim's F3J/TD Setup for the Taranis, Version 2 or 3, that not only adds that capability but also provides convenient in-flight adjustment!

There are other radio brands that have V-tail differential capability but I'm not aware of any that provide a way to adjust it in flight. The usual method is to try a setting, land, tweak the setting, launch, and try again. This modification features a way to avoid having to make several test flights and landings. Instead, you'll be able to adjust differential in flight with the throttle trim switch (TrmT).

To add this feature you'll need to free up a global variable (specifically GV6) that's currently used to set reverse differential, which is a setting that applies when both ailerons are raised during Landing mode (known as "crow"). After you've found the best V-tail differential setting you'll have a choice of several options, including restoring GV6 to its original use.
  • Free global variable GV6
    The first step is to free up GV6. Currently it's read by the two aileron channels, CH01 and CH02, and can have any value from 0% to 100%. Personally, a reverse differential setting of 100% seems fine to me, and the setting won't even matter unless you have crow enabled in Landing mode. I'll use 100% in this example.
    1. Edit the line in CH01 that starts with "CH19" by changing "GV6" to +100% or the value you prefer: <- Correction: Just set it to +100% as shown.
      Code:
      CH01 (LtAil)
          [I3]Ail Weight(+100%) NoTrim Diff(GV9) [Aileron]
       += TrmA Weight(+100%) Curve(6) [AilTrim]
       += CH10 Weight(+100%) [AilCm]
       += CH19 Weight(+100%) Flight mode(Landing) Function(x<0) [RevDiff]
       := Ail Weight(+100%) Flight mode(CAL) NoTrim [Cal]
       := Ail Weight(+50%) Flight mode(CAL) Switch(L7) NoTrim [Cal50]
    2. Edit the line in CH02 that starts with "CH19" by changing "-GV6" to -100% or the negative or the value you prefer: <- Correction: Just set it to -100% as shown.
      Code:
      CH02 (LtAil) 
          [I3]Ail Weight(-100%) NoTrim Diff(GV9) [Aileron]
       += TrmA Weight(-100%) Curve(6) [AilTrim]
       += CH10 Weight(+100%) [AilCm]
       += CH19 Weight(-100%) Flight mode(Landing) Function(x>0) [RevDiff]
       := Ail Weight(+100%) Flight mode(CAL) NoTrim [Cal]
       := Ail Weight(+50%) Flight mode(CAL) Switch(L7) NoTrim [Cal50]
      Update: As Mike Shellim points out in his post #5 below, if your optimum reverse differential setting is other than 100% then there's a way to enter that number in place of GV6 without having to enter it in two places. Mike says, "The easiest way to adjust RevDiff is to leave the CH01 and CH02 modifications in place and instead change the weight value of the first line in CH19."
      Code:
      CH19 (RevDiff)
          [I3]Ail Weight(nnn%) NoTrim [Ail]
       *= CH16 Weight(-50%) Offset(50%) [Spoilr]
      Sounds good to me! Be sure to read the rest of his post, which explains how reverse differential should be set.
      Continuing...
    3. Switch to the Flight Modes page and change the name of GVAR6 from "RevDif" to "V-Diff."
    4. Go through Flight Modes tabs 1 - 8, and on each tab, change the "Own value" setting for GVAR6 to "Flight mode 0 value."
  • Add differential function to the ruddervator channels
    On the Mixes page, edit the first lines of CH05 (RtVee) and CH06 (LtVee) to add differential:
    Code:
    CH05 (RtVee) 
     CH12 Weight(-100%) Diff(GV6) [VeeAlt]
     += CH13 Weight(+100%) [VeeCm]
     := Ele Weight(+100%) Flight mode(CAL) NoTrim [Cal]
    CH06 (LtVee) 
     CH12 Weight(+100%) NoTrim Diff(GV6) [VeeAlt]
     += CH13 Weight(+100%) NoTrim [VeeCm]
     := Ele Weight(+100%) Flight mode(CAL) NoTrim [Cal]
  • Add in-flight adjustment of GV6.
    At this point, V-tail differential could be activated by directly entering a value for GV6 on the Global Variables page. A value of zero disables V-tail differential, negative values produce negative differential (ruddervators go up less than down, needed if the glider pitches up when rudder is applied), and positive values produce positive differential (ruddervators go down less than up, needed if the glider pitches down when rudder is applied).

    The next step is to make this value adjustable in flight.

    In the original setup, TrmT (the throttle trim) is used in Flight mode 5 (Landing) to adjust elevator compensation and in Flight mode 7 (T3-Speed) to adjust Snap Flaps. This will add a third function to TrmT that adjusts V-tail differential while in Flight mode 0 (T2-cruise).
    1. On the Outputs page, name CH21 "SetVDf."
    2. On the Mixes page, enter this mix for CH21:
      Code:
      CH21 (SetVDf)
       TrmT Weight(+50%) Flight mode(T2-cruise) [SetVDiff]
      The value of 50% can be changed if desired, a larger value for more range or a smaller value for finer adjustment.
    3. On the Special Functions page, add this special function:
      Code:
      SF14   ON - Adjust GV6(CH21)
  • Fly the glider and adjust V-tail differential
    TrmT will only adjust V-tail differential while in Flight mode 0 (T2-cruise) but unlike the other two applications of TrmT, the adjustment will apply across all flight modes. Once the best setting is determined, land the glider and check the value of GV6 on the Global Variables page for reference.
  • Choose what to do now that you've determined the setting you want.
    1. You can leave things as they are, but this might result in the differential setting getting accidentally changed.
    2. You can disable the TrmT function. A simple way to do that is to go to the Special Functions page and remove the ON check mark for SF14. This will still allow directly editing GV6 from the Global Variables page.
    3. As long as you know the value you want for V-tail differential, you can restore GV6 to its original reverse differential function or use it for another purpose. To do that, enter your numerical value directly into the CH05 and CH06 mixes in place of "GV6."

Comments are welcome here and in all of my blog posts. Please let me know if you have any questions or suggestions, and whether this was helpful.

Last edited by Miami Mike; Jan 16, 2017 at 10:12 PM.
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Jan 15, 2017, 06:49 PM
Pompano Hill Flyers
mfwilliams's Avatar
Thank you Mike!
Mike W
Jan 15, 2017, 07:09 PM
Registered Taranis User
Miami Mike's Avatar
You're welcome!

Your Gillette is a V-tail, isn't it? Is it the one at the back of this photo from your blog?



If you're flying it with your Taranis then you should be able to use this setup if you have pitch changes and need it.
Latest blog entry: Taranis: LiPo Fuel Gauge!
Jan 15, 2017, 09:35 PM
Pompano Hill Flyers
mfwilliams's Avatar
I can use it on the Gillette, Falco and my Destiny. It will be very useful.

Mike
Jan 16, 2017, 10:06 AM
Registered User
mikeshellim's Avatar
Mike, regarding RevDiff: the correct value depends on how crow is configured, ideally it should be adjusted per model. Using your modified code, and in the absence of GV6, the easiest way to adjust RevDiff is via weight in CH19:

Code:
CH19 (RevDiff)
    [I3]Ail Weight(nnn%) NoTrim [Ail]
 *= CH16 Weight(-50%) Offset(50%) [Spoilr]
Adjustment method:
  • Apply full crow and full aileron
  • Adjust weight so that the downgoing aileron dips a little below the centre
Jan 16, 2017, 11:20 AM
Registered Taranis User
Miami Mike's Avatar
Got it. Thanks!
Latest blog entry: Taranis: LiPo Fuel Gauge!