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Nov 12, 2016, 04:39 PM
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Taranis: "SetFail" Flight Mode

When it comes to setting failsafe configurations that determine what our models will do if and when they lose radio contact, we usually just take a wild guess about what position each control surface should take. Then we face the very scary prospect of turning off our radios in flight to test those settings, hoping that we'll be able to regain control in time. I've never had the nerve to try that and I'll bet very few others have either.

Here's my method for setting failsafe:

First of all, on the Taranis MODEL SETUP page for each model, the options for Failsafe mode are Not set, Hold, Custom, No pulses, and Receiver. I had at first set mine to Receiver, which means when you press the F/S button on the receiver it memorizes the current control positions and uses them as its failsafe configuration. I've since switched to Custom and added a note to the end of this post about why that appears to be a better choice than Receiver.

For setting failsafe I used what was my next available flight mode, which was FM8, to which I assigned the name SetFail. I set all four trim settings to Own Trim and set the SetFail mode to be activated by a Logical Switch which I'll call L4 for this example. L4 becomes false and SetFail is deactivated the instant either the aileron or elevator stick position is moved from center. In Mode 2 that means the instant the right stick is moved.

I added these mix lines to determine what the trim switches do while in SetFail mode. I fly gliders, so the control surfaces I need to set are aileron, elevator, rudder, and flaps.

The aileron trim doesn't need any special mix but I needed more elevator trim range, so I added this mix to the elevator channel:
CH05 (Elev)
 := TrmE Weight(-50%) Flight mode(SetFail) NoTrim [Setfs]
This allows me to set enough down elevator to compensate for the effect of flaps.

I also added a similar mix to the rudder channel to trim it so that the glider yaws in a circle without rolling:

CH06 (Rudd)
 := TrmR Weight(+50%) Flight mode(SetFail) NoTrim [Setfs]
My throttle trim is re-purposed for other functions in normal flight, but I use it to set my flaps while in SetFail mode:

CH11 (FlapCm)
 := TrmT Weight(+100%) Flight mode(SetFail) Offset(-100%) [Setfs]
I find it easier to set the failsafe position of my flaps before I launch. I choose a position where the flaps are down far enough to bring the model down but not so far that a flap servo might break on landing. I also preset the elevator to a down position that looks about right for the starting flap position, but keep in mind that these are only first approximations to be adjusted further while in the air.

For my models with motors nothing special needs to be done because I'll have the motor off when I actually set my failsafe. The ESC channel's failsafe will be set to -100%.

Here are the Logical Switch settings that activate and deactivate SetFail:

I use an Edge function to activate SetFail when I hold SH↓ for one second.

L1  Edge  SH↓  1.0 (instant)  FM0  0.0  0.0

("AND FM0" prevents the sticky from being set while not in flight mode 0.)
For deactivating SetFail I detect right stick movement with two Logical Switches.

L2  |a|<x    Ail  10  ----  0.0  0.0
L3  |a|<x    Ele  10   L2   0.0  0.0
If the right stick is not centered then L3 is FALSE (i.e. !L3 is TRUE.)

L1 sets Sticky L4 and !L3 releases it.

L4  Sticky  L1 !L3 ----  0.0  0.0
And on the Flight Modes page, FM8 (SetFail) is set to be activated by switch L4.

Here's an equivalent TTL logic schematic diagram:

To set my trims for failsafe I begin by flying my model to a safe, high altitude and I make sure it's trimmed for level hands-off flight. Then, without touching the sticks, I hold SH↓ until the radio switches to SetFail mode, and then I adjust the trims. If the model gets out of control I grab the right stick and the radio immediately returns to normal flight control.

Once I've set my failsafe positions to the best hands-off flight my model can manage, I land, go back into SetFail mode, and use those position settings as my failsafe for that model.

To make 100% sure that failsafe works I can turn off my transmitter, but now I can do it while the aircraft is safely on the ground.

Addendum: On 12/26/2017 Kilrah posted a couple of things I hadn't thought of concerning failsafe:

First, there are advantages to using the Custom failsafe mode instead of the Receiver mode:
Originally Posted by Kilrah
You can set Custom the exact same way, except this way you know it's stored on the radio and set everytime, and you don't risk anything should you rebind, press the receiver button by mistake, or change receiver without remembering to check.
Second, there's an easy way to transfer the current control positions into the radio's Custom failsafe memory:
Originally Posted by Kilrah
The button navigation in there is a bit complex due to the many choices...

Long enter to go between the different types, but once you're on the custom value mode it's short enter to get to editing it, and then it's long enter to store stick position (short enter exits with the set value without using stick position, which you'd use if using the +/- keys to adjust).
So the procedure I settled on is to use SetFail to get my control positions right, then land, go back into SetFail mode, change Failsafe mode to Custom, switch to the Set screen, and individually set each channel that's in use with the procedure that Kilrah described.

Last edited by Miami Mike; Sep 15, 2019 at 10:00 AM.
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Dec 13, 2016, 12:19 PM
Ok that's high enough
FabFlight's Avatar
Very clever. Will adopt.
Dec 13, 2016, 05:27 PM
e^(iπ) + 1 = 0
Miami Mike's Avatar
Thread OP
Here's an extra note about failsafe.
Last edited by Miami Mike; Sep 06, 2018 at 10:53 PM.
Sep 15, 2019, 11:41 AM
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jims123's Avatar
Amazing! soon as I get a chance Iím implementing this Thanks ,

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