Throttle punch caused backward pitch & crash?

[cjohndavies]

I finally finished building my v4 tricopter (we had some uncharacteristically good summer weather here in Scotland so I spent a lot of weekends recently flying quads rather than building my tri!) & took it out for its maiden flight today.

I flew 3 sets of 3S packs (I’m using smaller packs in pairs in parallel) & everything went remarkably well – much appreciation to everybody who has worked on the PIDs, (tail) tuning, etc.!

Then I tried flying 4S & had a small incident. I took off, hovered for a few seconds, then punched the throttle. The tri immediately pitched backward where it stayed until it hit the ground tail first. I had a GoPro on the tri, so there is some footage to analyse, but alas I didn’t have the GoPro on my head like I did for the previous flight – that would’ve been a lot more diagnostic I think! You’ll see/hear that I nudged the throttle again after it had pitched back in an attempt to get some altitude, but it was too late.

https://www.youtube.com/watch?v=sFCv7Lrt_40

When I retrieved the tri from the grass, the tail prop was broken & had obviously hit the VTX (which incidentally wasn’t wired up/in use) which had slid back on the tail arm. However I think that was a result of the crash, not the cause. The fact that the tri pitched back but stopped before going all the way over & still seemed to be stable in its pitched back orientation until it lost enough altitude to hit the ground indicates to me that the tail prop was still intact – otherwise it would’ve completely flipped, crashed immediately, etc.? The prop was intact on one side of the hub but broken down to about 1″ on the other side & I really don’t see how it would’ve kept spinning & kept the tri in a stable backward pitch if that had happened before it hit the ground. I was flying in angle mode, so to me it looked like a control/software/IMU bug that caused the tri to pitch back to the maximum angle imposed by angle mode, rather than a physical failure which would’ve caused a much less controlled crash?

I put a new prop on & gave it another go & everything seemed fine, as you’ll see from the second part of the video. The only oddity is that when I connected power to the tri, after fitting the new prop, it didn’t bind to my Taranis. I switched from my 4S model on the Taranis to my 3S model (I have two identical models that differ only in the voltage alarm) & it bound straight away. Then I switched back to the 4S model & it too bound straight away. I did another short flight & repeated the same little throttle punches & everything seemed fine.

I’m using the standard electronics kit – BE2217 1300kv, 30A LittleBee flashed about 2-3 days ago with the latest BLHeli & I have the BMS-210DMH servo. My F3FC board is flashed with the latest Triflight from the github page (triflight_0_5_F3FC.hex) & I copy/pasted the CLI PIDs from the tricopter product page. I’m using a Taranis & a brand new X4R-SB set up on UART1 with SmartPort telemetry on UART2. The batteries were a pair of HobbyRC (Tattu factory) 1400mAh 4S 45-90C in parallel, both had the same voltages after the flight.

I don’t like not knowing the cause for a crash, because it means I can’t address it to prevent it from happening again in the future. So does anybody have any idea what could have happened?

PS – if you’re wondering why my XT60 is on top of the tail arm, it’s because David’s build video shows him soldering it like that & I didn’t realise the mistake until it was too late for me to bother correcting it!

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[RCExplorer – David]

To me it sounds like the back ESC is loosing sync for some reason.
Not sure why it would do that as it’s normally fine running that setup on 4S.

Could you check the soldering on the back ESC?

If you know how to use BLHeli suite you can always try to lower the timing a tad on all ESC’s. Might fix the problem.

Does the back motor spin freely? No resistance?

/David

[cjohndavies]

Thanks for the response David. My solder joints look fine, not my best work because there isn’t much space to work with on the F3FC, but all shiny & with good continuity according to the multimeter.

If it is an ESC sync issue, should I be able to recreate the issue on the workbench if I take the props off & repeatedly punch the throttle? Or might it only happen if there is actually resistance on the motor from a prop? I’ve tried this & couldn’t get it to stutter at all.

No resistance on turning the back motor, even after it’s little crash!

[RCExplorer – David]

The soldering between the ESC and motor is what could cause this (if it’s solder related)
If you have it open it wouldn’t hurt to reheat them to make sure they aren’t cold jointed.

You will need the prop on the motor to recreate it. But yes, repeated quick throttle changes will trigger it, especially going from low to high quickly.

[JRach]

I had the same problem happen to me. It turned out to be my ESC. I went through 2 sets of ESC’s before this stopped happening. (I am running a mini tri with 1900kv motors so if my ESC choice seems odd it’s not.) My kiss 18a’s were unreliable and made the left arm drop on 4s. My rotor geeks rg20a’s were awesome for like a month until it made the back motor drop on 3s(causing many crashes) someone on these forums told me it was a bad FET. I am now using little bee 20 amp ESC’s and they work great. ALSO, IT could be your motor. To check spin one of the motors on the front with your hand. Make sure the copter is off and has sat for 5 min without power. If the motor spins freely without too much resistance after your hand is not in contact with the motor, it passes. But If you spin your back motor and it stops spinning as soon as your fingers come off the shaft then it is most likely that you have a short in your motor. I had to replace a motor because of this. Sometimes the motor would “stall” as I increased the throttle or did a backflip. Causing it to scare me and do a double flip. I also experienced this as I went inverted and then when I came to flip out the motor would stall and the copter would fall for a few seconds. Hope this helps! Happy flying!
– Jacob Rachwal

[cjohndavies]

Thanks for all the input, finally had a chance to do some more investigating.

I clamped the ‘copter down to the bench so that I could test the tail motor with the propeller attached. It behaved fine as I punched the throttle up & down a lot more aggressively than I ever would when actually flying, so I think I’ll try flying it on 4S again next time I go out.

What I did notice though, is that all 3 of the motors emit a high pitched squeal when they reach about 1800 throttle input on 4S (calibrated 1000 to 2000). I’ve read that this can be due to ESC timing, so I tried all 5 options in BLHeliSuite under ‘Motor Timing’ but they all still made the squeal. Is this just normal for these motors? Should I try to oil the bearings?

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