Since it's been 5 years, I've revised the SetFail flight mode for 2021. Please see OpenTX SetFail Flight Mode (revised) instead.

The old version:

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 OpenTX 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.

Very clever. Will adopt.

Here's an extra note about failsafe.

Amazing! soon as I get a chance I’m implementing this Thanks ,

Mike.
Thanks for the description.
However, I am not sure what "Set Fail Mode" is.
Is it something in OpenTx? Or is it something that you create by naming a flight mode accordingly and selecting it as required?

I did do a search on Google and in OpenTx, but could not find anything.
Sorry if this question is revealing my utter ignorance (which apparently has no limits!) and is wasting your time.

Jim.

Yes, it's a flight mode. OpenTX supports up to nine flight modes numbered FM0 to FM8 and my glider setups use them all. They are:

• FM0, "Cruise" - the mode with the lowest priority, so it's automatically selected when no other flight mode overrides it. Most of the time I'm flying in this mode.
  
  
• FM1, "Calibrate" - for making adjustments on the Outputs page, basically the same as in Mike Shellim's templates. It has the highest priority but is entered with a special combination of a switch position and stick movement that's unlikely to happen by accident.
  
  
• FM2, "Flap" - activated when the flap stick is pulled down with SH↑. FM2 and FM3 have priority over FM4 - FM8 to ensure that when the flap stick is pulled down the flaps will go down.
  
  
• FM3, "Crow" - activated when the flap stick is pulled down with SH↓. Flaps go down, ailerons go up.
  
  
• FM4, "Launch" - activated with SE↓ only on my winch-launched gliders. SE controls throttle on my electric-powered gliders.
  
  
• FM5, "Zoom" - activated with SE↑ only on my winch-launched gliders.
  
  
• FM6, "Thermal" - wing is cambered for flying slowly and circling in lift.
  
  
• FM7, "Speed" - wing is reflexed for speeding through sink.
  
  
• FM8, "FailSafe" - described above. Because it has the lowest priority, it can only be activated while in FM0, "Cruise" mode.

I know you've seen this before. It shows how the priority of flight modes can be used to make sure you're always in the mode you want:

Thanks Mike. I will study it.

Mike,
Thank you for putting this together. I have been wondering how to check my failsafe settings. Sure enough I just took a guess and set my control surfaces where I thought they'd work. Who knows if they'll bring the airframe down easy? I look forward to giving this a go.

So, on this same subject, I happened to find 'SetFailsafe' under Special Functions under Model Setup in the QX7 Aloft manual for OTX 2.2.
It states that failsafe can be set with a switch while flying or on the ground. It can be enabled or disabled with a checkbox on its own line in Special functions. Are you familiar with this option?

I need to absorb all this good information. Thankyou. Good code from Mike and Bill's function description.

Have I missed the point that for "fail safe to work" the system must have integrity?

What actually triggers Fail safe? My understanding is that like computer communication the RF is just the transport carrying the ONEs and ZEROs.

So what actually happens when a single packet has corruption. Can it not be discarded? As I understand a 9milli second glitch.

The old FM system where the Carrier was spiked with the servo pulse train a single glitch just caused a very short small jerk in the model flight line.

One computer I serviced the single bit error correction electronics was more unreliable than the memory card it was correcting. (8 bit data 1bit parity and 3 bit for correction.)

Is there an answer to this conundrum?

Take care. jq