700 My New T-Rex 700X Build

[PLANETGETLOW]

Both helis look great. I was at a fun fly yesterday and MOST people are flying 700's. Quite a few X's, L's, and E's. Great performing helis. I watched a guy put a 700X in inverted about 5' off of the deck into dirt and the only damage that it suffered was the tips off of the tail blades were broke.

 

[Smoggie]

So I managed to maiden the 700X on Sunday.. A mixed report, first the good news:

Maiden went perfectly. I started with a governed throttle % at 70, 75 and 80. For sports and mild 3D 80% seemed about right (I'm guessing 2000RPM). She flew real nice, felt quite light in the air. For the second flight I bumped up the tail gain (I'd started with it deliberately low) and re set my throttle % to 75, 80 and 85. I also turned down startup power in the ESC to '2' (default is 3) to address a slight kick on spoolup. The changes worked well and she was flying perfectly.

Now the not so good On spool-up for the third flight (after changing nothing at all) she became unstable, wobbling on all axis. I managed to catch it and powered down. I power cycled the heli and everything seemed to be working fine so i put it down to maybe the FBL not initiating properly. She spooled normally and was flying perfectly until about 30 seconds into the flight doing a slowish straight and level pass right in front of me control was suddenly lost and she started pirouetting wildly. I hit throttle hold but couldn't do anything to prevent a crash. Fortunately it wasn't going too fast and I hit TH pretty quick but 700 size helis don't like being thrown into the ground tail first! The damage runs to: tail boom, torque tube, tail blades, tail shaft plus bearings, tail skid, front tail drive gear, front landing gear snapped, bent landing gear skid pipe, very slight chipped tooth on auto gear, cracked canopy, slight chips on one main blade.

The most frustrating thing is that I cant figure out the cause of the crash. It certainly wasn't 'dumb thumbs' as she was just flying straight and level when all hell broke loose. After recovering the heli the servos were not responding normally to tx input, they seemed to be going in random directions as if the FBL wasn't set up. When I stripped it I didn't find any mechanical problems, no loose screws or detached links or cables and all servos appear fine. The FBL data log was unfortunately inconclusive because I spent so much time testing the heli afterward that I must of overwritten whatever the data relating to the crash. There were multiple missed signals in the log but that may have related to after the crash.

My best guess is that it's a FBL fault. The Vbar that went in it was bought second hand and this is the first time I've used it, it came from what should have been a reliable source but you never can be 100% sure. Strangely though it seems to be working fine again now. Fortunately I have another spare FBL and the sat receivers I ordered for the heli will be here soon (the ones installed were borrowed).. so I can change out both FBL and receivers. Getting the spares to make the repairs is not so easy because the dealers here don't have much in stock for the 700X so it will likely be grounded for a while
Some photos:




I have videos of the first two flights and of the problematic spoolup but frustratingly the camera battery crapped out seconds before the crash so I only have the take off on the last flight. i'll post the vids one I have chance to upload them.

 

[D.O.G.]

That sucks big time Smoggie. Especially if you don't no the cause I hope you can find out what happen and get that bird back in the air so you can enjoy it.

 

[Heliman450]

Hi Steve, oh dear! Oh dear! (or some alternative, unprintable version). I've got the parts but I'm afraid you can't have them. What a disaster for you, I hope you're managing through it ok. Maybe I should order up a spare 700X just in case I need something........... there's a thought.

In my short career in the hobby, nothing's more frustrating than not knowing why. Early on every crash was like that but at least I've become aware enough to know what went wrong when it involves pilot error. I'm assuming that you're extremely experienced with the Vbar set up that you didn't miss something along the way. The closest I ever got to something similar was when I tried to set up the Spirit Pro governor option and doing it wiped the fail safe settings and I didn't realise that it might happen and nothing is mentioned in any of the instructions as warning other than the overall statement to repeat set-up in a general sense.

At least you're looking towards a re-run with a different gyro. Is it also a Vbar?

Good luck and keep us updated on it.

 

[PLANETGETLOW]

Damn that sucks. So many variables but the FBL is essentially the Logic of the heli. Could be the FBL but could also be bad sat, or low voltage, etc

Hang in there dude

 

[trainrider06]

Dang Steve, hate to hear this! Bad enough crashing one but a brand new one? And it sounds like the Vbar was the problem! I would have smashed it with a hammer for doing that to my bird! It had to be that, especially since it did its weird deal on the 3rd spool up! That might be a lesson for us all, have a strange glitch like that don't fly it!
It had to be the unit, with it acting freaky after the crash even! Really what else could it be? Sure one could suspect something in the tail failing with how it acted when it went down, but how the FBL unit acted pretty much over rules that I'd think.
Sorry man, but you'll have her back good as new and flying right in no time!
!

 

[Smoggie]

Thanks for the words of support guys.
Truth is I've been lucky for the last few years not to have a crash with my big helis so I'm taking this one on the chin, it was sure to happen sooner or later. The fact that I can't get the bits I need and not knowing the exact cause are the most frustrating parts. I'm pretty sure that it wasn't a setup problem because the heli had two perfect flights under it's belt and I wasn't doing anything out of the ordinary when it went 'pear shaped'.

Rodney, you are right in hindsight I should have called it a day when it did the wobble on spoolup but even with hindsight I'm not really sure what I could have done other than checking the heli over and testing it to make sure it was working ok, which I did. I kind of convinced myself that the wobble was probably just the wind (it was pretty windy).

I'll post the videos soon as possible, it's a pity that I didn't catch the crash on video as that might have helped with diagnosing the cause, just another example of sod's law at work!

 

[trainrider06]

Well it's one thing if you dumb thumbed it into the ground, but this obviously sounds like an electronic issue, which like you said can be a worrisome thing not knowing at least the exact cause. I am wondering if somehow when you started adjusting the gains if somehow that triggered some sort of glitch within the electronics of the unit? I guess too we gotta realize there is probably a few units out there with issues in em no matter what brand they are.

 

[PLANETGETLOW]

I'm paranoid of a mechanical failure. I still remember my first crash with my shuttle z, ground wire came lose and it went down. Even though I was progressing it was a solid confidence killer. Since then I do a pre and post complete check of all bolts, screws, linkages, and connections but if something is gonna fail, it's beyond our grasp.

 

[Smoggie]

Rodney,
Between flight #2 and the fatal third flight I didn't adjust anything at all. All I did was give it a good look over for any loose screws etc. (there were none).

 

[trainrider06]

Ah ok, just thinking if so maybe some trigger within the unit took place. I guess another possibility could be the EsC? Servo high draining possibly? A very odd and scary thing with unknown cause.

 

[D.O.G.]

Hey Smoggie, I had a pirouetting wildly go wrong with my bird once. My ball link that connects the tail rotor control arm to the tail rod, for no apparent reason, broke during flight. Just like you and probably everyone else, I've checks everything after a couple of flights for any loose parts. Everything was fine and then the next thing you know BANG the bird is down. Sometimes parts just had enough abuse that it has no choice to break. Especially me with some of my crashes Lol. Good luck on the process of elimination on determining the cause of the crash. I feel for you man.

 

[Smoggie]

First think I thought of was the tail pitch rod coming disconnected but I'd checked it before the flight and all was secure. Ball links were all connected after the crash except for the main rotor pitch links which popped off as result of the crash. The link on the tail control arm was still attached to the arm but had broken off from the pushrod, that's not surprising as the boom was bent at 90 degrees!.. It has clearly broke off in the crash because the threaded rod sticking out of the end of the pushrod was also bent.

Rodney, overloading the BEC is always a possibility but the BEC in the Hobbywing is rated at 25A so would be very hard to overload. Add to that I just flying straight and level when it failed so the servos were virtually static. I'm pretty much sure this was an electronics fault of either the Rx or FBL... So I'll change both items, I'll also add a capacitor bank as additional brownout/back EMF protection on the basis that it cant do any harm and I already have some suitable capacitors.

Sorry, still no videos because the internet is down at home so I cant do any uploads (I'm really not having any luck with electronic stuff lately!).

 

[trainrider06]

Capacitor bank? Is that something similar to this?

 

[Heliman450]

Wow Rodney that's a big photo!

Maybe we should ask Tony to edit the title on this thread to 'RC-Help.com T-Rex 700X Team Build' rather than 'My New T-Rex 700X Build'

 

[Smoggie]

'capacitor bank' was perhaps too grand a title. The reality it is just two 4700uf 10v electrolytic capacitors wired up to a short servo extension cable which connects to a spare port on the FBL. This helps dampen out momentary dips or surges in voltage. Mikado did lots of testing which showed them to be highly effective, for example: http://www.vstabi.info/en/node/1330

 

[Smoggie]

Maiden flight:
[video=youtube_share;lFZ6RdlsUIc]https://youtu.be/lFZ6RdlsUIc[/video]

Second flight to follow

 

[Heliman450]

Hi Steve, just tried to watch your video but it's asking me to sign in??

- - - Updated - - -

A couple of photos to update you on my progress...................


Before Supra X After Supra X


Just the ESC to do now and of course, the second pair of batteries and ESC for the 550. Two pairs of charging leads also ready for use

No real problems other than my lack of soldering practice, particularly with electronics stuff and quite big cables. After a few expletives, it became much easier. Anyone need something soldering?

 

[Smoggie]

Phil, some things that may save you some time that i learned from my two flights :

• If you wire up the motor with matching colours on ESC and motor it should spin in the correct direction (or at least mine did).
• ESC timing was left default (15 degrees), this worked fine
• I set the Governor to Gov-Store
• I set startup power to '2' down from default value of '3' (avoids startup kick but you need the latest firmware to see this setting)
• ESC throttle curves: 75%, 80% and 85% (all flat line 'idle up')
• Tail gain set to 65% in Tx flew ok(this will vary according to Tx and FBL)

 

[Heliman450]

Looks great Steve! Well done. I must have been trying to view as you were loading it. Unfortunately, now that we know the next chapter in the story it's a bit sad to watch it

- - - Updated - - -

Phil, some things that may save you some time that i learned from my two flights :

• If you wire up the motor with matching colours on ESC and motor it should spin in the correct direction (or at least mine did).
• ESC timing was left default (15 degrees), this worked fine
• I set the Governor to Gov-Store
• I set startup power to '2' down from default value of '3' (avoids startup kick but you need the latest firmware to see this setting)
• ESC throttle curves: 75%, 80% and 85% (all flat line 'idle up')
• Tail gain set to 65% in Tx flew ok(this will vary according to Tx and FBL)

Cheers Steve, I'll keep the E-Mail to save the settings.