700 Trex 700E DFC with 3GX - Backwards drift

[Erfan]

Hi guys!

I have a issue with my Trex 700E DFC (3GX v5). When hovering it starts to drift backwards. I can really see the whole hell starting tilting back and it drifts backwards...
I have checked the swash with a swashlevel tool, the servos are at 90 degres (as good as I could see) and I have made the entire 3GX setup. I have checked the servos for binding and there are non.

This only happens when the hell is up in the air, I have spooled it up on the ground at almost mid stick, just before it lifts and looked at the swash to see if there are any vibration issues or anything that would suggest why the swash tilts back but the swash is completely level on the ground (checked this for 1 minute, no movements on the swash), this happens only in the air...

I have also checked CG with canopy and battery on and it looks good, no tilting forward or backward.

Some forums say play with the trims but I'm almost 100% sure that this is not a good option with 3GX, others say that I should extend the elevator link a little bit to compensate for this but that also sounds like a bad suggestion, no?


Any thoughts or ideas?

 

[Tony]

What I would do is use sub trim to adjust a little forward elevator into the gyro. You can use sub trim on the 3GX, but do not use trim. And, make sure you reinitialize the gyro after every change so it knows where it's centered. My guess is it's just slightly tail heavy. Even with a perfectly level swash, a little tail heavy and it will drift to the rear.

 

[Erfan]

Hi Tony

With "reinitialize the gyro" you are talking about doing the DIR settings again?

 

[Tony]

No, just unplug the battery, then plug it back in and wait for the swash to jump up and down 3 times. No need to redo any of the settings in the gyro.

 

[Erfan]

Cool, I'll try that later today and let u know what happens!

 

[Lee]

How are you checking the CG?
Have a look at my video in RC TIPS & TRICKS at the bottom of my post.
This is the most reliable way to check CG.

 

[Graham Lawrie]

Sounds like vibration in flight getting to the 3GX

 

[Erfan]

Hi Lee, I will check your video, I just do a "regular" CG check, holding the main rotor head (blade holders) and check if the heli tilts in any direction, I do this with everything on (Canopy, battery etc...).

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I tried to do your CG check but there is no way I can do it with a 700 size heli, I just don't have the strength to hold it up like you do in the video
Have u tried that with a 700e? Mine ways 5.2kg (11.45 lbs) with everything on

I did it with my 450 and that's ok, it does not way so much...

But doing it regular it looks kind of good, actually it has little bit tilt forwards so if anything it should have drifted forward and not backwards

I will try to fly today and see if changing the elevator sub-trims a little will help as Tony suggested... Next thing I guess is to look for vibration issues for the 3gx but I have tried to see if there are any by spooling up on the ground almost to the point it lifts and let it run for 1 min and looked closely at the swash, no movements there so I don't think it's vibration issues, but then who knows...

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I was thinking, could it be interference between the servo wires and esc/motor wires? They are "touching" each other and the elevator wire has a minor servo extension wire attached to it.

I have attached a pic on the wiring.

 

[stokke]

This might be a pretty bold claim, but if your mechanics are set up right, your radio and dir setup is correct - the flybarless unit should compensate for any uneven cog.

Please review my guide for setting up a FBL head with the 3GX - see if you come over anything you might have overlooked. (link to guide in my signature)

- - - Updated - - -

If the guide does not unveil any error in your setup:

Check that no trim-lever is enabled on the Tx
Do a stick calibration on the Tx

 

[murankar]

One thing to think about, is if the heli has a natural tilt. When sitting on the bench does the heli rock when you push or lift the tail?

When you leveled your swash did you use a swash level tool or something like a soko heli tool?

Why do i ask? When using zip ties, level tools that slide on the main shaft will 90 the swash to the main shaft. What this could mean is that the swash is not level to the flying surface, which in turn could be causing your backward drift.

With something like a soko heli tool you swash will be level to the flying surface if done correctly.

This is just me thinking out loud if you will, i am tired and want to help.

Sent from my LG-E980 using Forum Runner

 

[Dirk]

I don't think the wires running across your your servo wires are the problem. Your routing is not ideal and I would re-route the wires. On my 700 my elevator servo wires lie next to my motor wires and it is perfect.

Do you have some slack in the servo wires where the go into the 3GX? If too tight, that will also introduce some unwanted vibes int the 3GX.

 

[Erfan]

Hi Stokke

I will have a look and see if you're doing something I have missed, thx!

This might be a pretty bold claim, but if your mechanics are set up right, your radio and dir setup is correct - the flybarless unit should compensate for any uneven cog.

Please review my guide for setting up a FBL head with the 3GX - see if you come over anything you might have overlooked. (link to guide in my signature)

- - - Updated - - -

If the guide does not unveil any error in your setup:

Check that no trim-lever is enabled on the Tx
Do a stick calibration on the Tx

- - - Updated - - -

Hi Muranker

I always use a "real" swash level tool (the Align swash leveling tool), no zip tie method. And its completely level at mid stick (50% pitch). Even the blades are almost at 0* at mid stick, the mechanical seems perfect if u ask me
But obviously something is less perfect

One thing to think about, is if the heli has a natural tilt. When sitting on the bench does the heli rock when you push or lift the tail?

When you leveled your swash did you use a swash level tool or something like a soko heli tool?

Why do i ask? When using zip ties, level tools that slide on the main shaft will 90 the swash to the main shaft. What this could mean is that the swash is not level to the flying surface, which in turn could be causing your backward drift.

With something like a soko heli tool you swash will be level to the flying surface if done correctly.

This is just me thinking out loud if you will, i am tired and want to help.

Sent from my LG-E980 using Forum Runner

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Hi Dirk

Yeah, the wiring is not the most beautiful I've done
The idea was the make it safe enough for getting the bird in the air and optimize all the 3gx settings before I come up with a better wiring, for now its just easiest wiring if I need to move stuff around. I will try later to do the wiring from inside the frame...
IF you have a pic on how u have done it I would love to take a look!

All wires have good bit of slack because I have heard they need that to prevent vibrations on the 3GX.

I don't think the wires running across your your servo wires are the problem. Your routing is not ideal and I would re-route the wires. On my 700 my elevator servo wires lie next to my motor wires and it is perfect.

Do you have some slack in the servo wires where the go into the 3GX? If too tight, that will also introduce some unwanted vibes int the 3GX.

 

[Dirk]

Good one on the slack on the wires. I tried taking some pictures of the wiring on my 700 but to no avail. They are all routed inside the frame. Can't see much.

 

[Erfan]

Hi Stokke

I just started doing everything from scratch again when it comes to the servo setup. What ever I do I cannot go below the "50 steps" of subtrim in order to get the servo arms to 90*.
Specially the pitch servo.

I have tried to swap arms but still not even close to "below 50 steps".
I got the following:
AIL: -62
ELE: -2
PIT: -129

This is done in a Futaba T14SG transmitter, not sure if these steps are different from Speaktrum which you had in your howto.
Do you suggest getting new arms or is it possible to fly with these settings? There are no binding issues or anything like that...

This might be a pretty bold claim, but if your mechanics are set up right, your radio and dir setup is correct - the flybarless unit should compensate for any uneven cog.

Please review my guide for setting up a FBL head with the 3GX - see if you come over anything you might have overlooked. (link to guide in my signature)

- - - Updated - - -

If the guide does not unveil any error in your setup:

Check that no trim-lever is enabled on the Tx
Do a stick calibration on the Tx

 

[stokke]

Hi Stokke

I just started doing everything from scratch again when it comes to the servo setup. What ever I do I cannot go below the "50 steps" of subtrim in order to get the servo arms to 90*.
Specially the pitch servo.

I have tried to swap arms but still not even close to "below 50 steps".
I got the following:
AIL: -62
ELE: -2
PIT: -129

This is done in a Futaba T14SG transmitter, not sure if these steps are different from Speaktrum which you had in your howto.
Do you suggest getting new arms or is it possible to fly with these settings? There are no binding issues or anything like that...

Well since you have a different Radio, sub trim steps MIGHT not be equal to the values I refer to. It all depends on how far the servo arm travels - e.g if 129 steps of sub trim is equal to say 30° rotation on the arm - that is a lot! But if it is equal to maybe 3-5° rotation, that shouldn't be a problem.

Excessive sub trim WILL mess with the FBL-gyro.

 

[Erfan]

My transmitter goes up to 240 steps... I will try and see how it feels... a