Hello,

I've just had a pretty dramatic event which I'm trying to troubleshoot before flying the heli again.

The heli is a Goblin 700, running a Spirit FBL unit, Jeti radio, Futaba S9255 servos on cyclic, separate 2s lipo with regulator, Kontronik ESC, 10s Lipo. I've had well over 100 flights with no issues at all - everything has been very reliable. No 3D flying, just mild aero's and FFF.

On the last flight, I was about 6 minutes into the flight and travelling in FFF in a mild dive, when the heli pitched nose-up suddenly and dramatically. This was uncommanded, and I was not able to re-level the heli with stick inputs. In my panic, I hit the rescue button and it re-levelled perfectly, back to stable, level flight. As soon as I released the rescue button, it pitched nose-up dramatically again, and would not respond to forward elevator stick, so the rescue button saved the day again. This repeated another 2 or 3 times, by which time I realised the only way it was coming down intact was by staying in rescue mode. So I kept it in rescue mode, and used the motor switch to allow head speed to decay & the heli to descend. Some mild damage to the undercarriage, but otherwise OK.

Once I retrieved the model & got back to the pits, I checked it out fairly closely and could find no mechanical issues, and the swash controls all seemed to be functioning normally. The only issue I could find was that one of the cyclic servos (side servo on 120 degree CCPM) had less torque than the other 2. The S9255 is rated at 9.0kg torque at 4.8v, and my uncalibrated finger would rate this weaker servo at about half the torque of the others, so probably 4 to 5 kg, which should still have been more than adequate. Suffice to say this servo is being replaced. A more detailed analysis back in the workshop has found nothing further. Data log shows vibration generally low with peaks up to 16%, tracking was good, Rx battery 8.1v.

So what happened? My suspicion is that the Spirit may have had a malfunction of some sort. My reasoning is:
- The mechanical control system still appears to be functioning properly. The fact that rescue mode was able to bring the heli back under control also suggests there was no mechanical failure.
- If the weaker servo had insufficient torque to control the swash, the result should have been a combined roll/pitch effect, not just pitch (nose up). Similarly, the fact that rescue mode was able to bring the heli back under control also suggests the servo had sufficient torque.
- The Jeti radio link appears to have remained unbroken, as even when the uncontrolled events were occurring, I was still able to activate rescue mode and switch the motor on & off. Data logging on the Jeti also showed Rx signal strength at 100% for the full duration of the flight.
- In Rescue mode, I have Stick Priority set at 6. So if the Jeti was giving an uncommanded elevator output, there should still have been a nose-up response in rescue mode, which there wasn't.

I've tried downloading the log from the Spirit, which produced the screen message "Log from previous flight is available", however the log file is from earlier today, while trying to connect up to the software (couldn't quite remember the right connection procedure).

I'm very thankful that rescue mode was able to save the heli, but don't want to fly it again without rectifying the underlying issue. Any suggestions appreciated!

Keith

Hello,

from your description I can say with almost 100% certainty the problem was the servo.
Especially when you saw it is weaker, even at the bench it is perfectly clear sign there is something very wrong with it.
When servo become hotter it could randomly loose a torque completely. So there is usually big difference in the air and at the bench.

I can't see any reason why Spirit unit should have a malfunction, especially if Rescue function worked and it was still able to level the model in some way.

Technically FBL unit is the last piece of electronics which could damage "itself". There is nothing (at least in Spirit units) that is wearing off.
Anything that is rotating or moving is wearing with every flight. So probability of malfunction is very high for servos and is increasing exponentially with each flight - depending on used type.

Side note is, if the servo started to behave unexpectedly quite soon (if the servo was brand new) then with very same servo it will happen possibly again sooner or later.
Each servo has limited lifespan, but some can last orders longer.
Elevator servos are the most prone for failure, because torque demands are the highest there. It require roughly 2 times more torque to hold the model than the remaining two.
If it become hot it could loose its strength completely which you can rarely identify at the bench.

Futaba S9255 is quite inadequate servo unfortunately. It is very slow for FBL usage so also performance was likely unoptimal. If you can replace them sometime in the future there are servos with 3x better specs for similar cost. Flight performance can be also significantly better together with increased safety, because almost each todays servo is designed for FBL helicopters.

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Spirit System developer

Thanks your your prompt response!

I hope you're right about the weak servo being the problem. And yes, the 9255's are slow, and are now going to be replaced with something faster.

When I initially found one servo to be weak, I assumed it was the problem, but then some nagging thoughts made me question that:
1) If the servo was so weakened that the heli became uncontrollable, how could Rescue Mode bring it back under full control?
2) The weak servo was one of the side servos (120 degree swash), so it's effect would be mainly in the roll axis. The uncontrolled deviation I observed appeared to be in the pitch axis (nose-up). There may have been a roll component as well, but it didn't register in my brain.

Those two points made me suspect the servo may not have caused the problem. Hence my wondering if the deviation may have been commanded by the FBL unit, due to processor error / gremlin / who-knows-what.

Keith

1) If there is some problem with servo, two remaining servos will still push the model in more or less correct direction, only collective pitch might not change correctly.
2) Because counter action of remaining servos it can do an unexpected movement, because all 3 servos are required for steering the model. When it is not moving as desired FBL unit is trying to compensate it with remaining two. Where model will tilt depends completely on what forces are the biggest. Then it will push model in some direction which require the lowest force.
There are various failures of servos. Some could cause complete loss. If for example there is problem with potentiometer inside the servo it could regain full power in extents which were not used that much. Usually middle area of potentiometer has the biggest wear. So when Rescue is activated, the servo could move in correct direction.
Since I have no idea what is wrong with the servo anything is possible. In any case any deviation from normal servo behavior when compared with other is very suspicious.

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Spirit System developer

Come on, let's not joke.
on a 700 you can't put a servant with those kg of torque.
you think about a 600 usually 20kg is used (more or less), on my goblin raw 580 I have servants 8.4v 22kg 7.4v 19kg 6v15kg .
your servant It could go well if your model It has a flybar but the flybarless models they need of a few kg of torque higher on a 700 I recommend 20kg or more but the minimum torque to fly 15kg. (only if you fly normally no 3d flight).

@franz88: even 9kg is not enough for sure on a 700 (no doubt about that)... but we have been flying hard 3D with servos ~12-15kg for years (only some 5-7+ yrs ago this was still standard, like top-class Futaba: BLS273SV). so "minimum torque to fly 15kg. (only if you fly normally no 3d flight)." is simply not true. i am flying my Diabolos (all 700s) with manufacturer-recommended servos (= "Futaba BLS BLS471, BLS272, BLS273", all 12-15kg class) since years and of course 3D. and max IBec of 10-12A even in fast TicTocs shows me that they are far from their limit...

btw there is more about a servo than just no-load-torque (look at load/torque/speed graphs )... and that since some yrs servos have (luckily) gone much more powerful, does not mean that you cannot fly 3D without 20-25+kg torque... just look at all top pilots i.e. 7-10 yrs ago. they have not won 3D masters or IRCHA with just flying around normally our 3D-machines have not gone useless from one day to the other, just because stronger servos have shown on the market. me too, i will be using them for future build most probably too, but not switch a 5-700EUR servo set for stronger ones...

anyhow clear: 9kg is really too weak for a 700. and Sorry for a bit off-topic...

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cheers
Michael

Well just to follow up on this thread, I replaced all 3 cyclic servos this week with some 0.085s / 15kg Savox brushless servos. Lots of bench testing and ground run up without blades - no odd behavior noted. Two test flights yesterday, and all went well. So hopefully that's the end of the issue.

Thanks again for the assistance.

great you got it sorted out!

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cheers
Michael

Please make sure that Cyclic Ring range is high enough too.
This uncommanded nose-up problem can happen basically with any helicopter in a fast forward flight when collective pitch is too high, but cyclic pitch range is not high enough.

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Spirit System developer