What is the currently recommended auto-takeoff method for a flying wing?

Hi, I’m about to maiden a pusher-type flying wing and would like to ask about the recommended takeoff method?

TL:DR

Frisbee-like wing toss seems too risky and throwing it traditionally is out of question because there is nothing to grab onto.

I’ve seen a couple of videos, mostly with INAV, where shake-to-wake together with an overhead throw is recommended.

That approach seems feasible but with Arduplane I noticed some caveats.

  1. Shake-to-wake seems to be incompatible with crash detection by design.
  1. Throwing it overhead and hoping it starts the motor also seems to be a recipe for crash unless safety checks are disabled.

OK,
lessons learned after a beyond repair maiden:

  • no reflex for an over the head throw
  • less throttle
  • no tarmac within 50m

I use shake-to-wake on all my hand launch planes. On my AR-900 I would use the Frisbee style throw because I didn’t like the over the head launch. I know lots of people do it, it just doesn’t work for me. And the EBee style forward throw puts a spinning propeller way to close to critical parts of my body.

What worked for me

  • Set the shake to wake so the motor started slowly before I threw it. This way I knew it was ready to go.
  • Set the slew rate so I could hear the motor speeding up. I would time my throw so the motor was ~3/4 of max when I threw it. My AR900 was only on 3S so there wasn’t much power to spare. YMMV.
  • The throw needs to be much flatter than you think. Not too hard with the frisbee method. But really critical for a good launch.
  • The throw needs to be as straight as possible. This I found to be the hardest part of the frisbee method. Holding the plane on the leading edge it was really hard to not give it some back-spin.
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After some crash analysis, it seems I experienced the famous stall and torque roll on the AR Pro.
Over-the-head throw with takeoff mission item, no conditions set, motor running at 85%.
Let go too early, pitch 60deg, no impulse visible on yaw and roll from the throw, managed 25-30m/s² acceleration.
Instead of nose-down due to the CG, it nose-up again by motor thrust after the throw, then stalled, torque rolled to the left, changed direction completely and turned for the tarmac.

QED: too much pitch, too much throttle, too much tarmac

I was able to salvage and move almost all of the electronics to another KIT.
Just had to order a new analog camera because that was hit directly before it split the main fuselage diagonally. Surprisingly not even a scratch on motor and prop. Wings are also ok except that the winglets came off where factory glued them.

I’m preparing to maiden the next iteration of that plane and I believe I found out what effectively crashed the first one. Insufficient throws for elevator down.
With the default FF and P for pitch, the elevons barely move in elevator down direction when I pitch it up in FBWA. I can move it part of the remaining deflection with the stick.

I had to double the values to make the plane move the elevons in full deflection without me touching the stick and I assume if I would have had the radio in my hands at that moment, I would have been able to save the plane by pushing elevator down.

This must be addressed in the wiki IMO, otherwise it’s a recipe for disaster.

Did you turn up the MIXING_GAIN at all? I seem to remember the default is low for some planes.

Nope, I left it at 0.5 and was going to adjust it later if necessary. Have to be once in the air, trimmed and at least half-way tuned for that decision to make.

The thing is that having 1-2mm reflex up takes those 1-2mm from elevator down and with default FF and P, it moved maybe 3mm of the 12mm+reflex it should have in that direction.
I practiced throwing over-the-head in FBWA and I can see that the elevons follow and deflect enough to pitch down, even if I throw it at 60deg pitch again.

I also set CG to balance at the front end of the CG marking with 120g (50g more) of lead in the nose as a last line of defense against this sort of stall.