Need advice on tail wag for trex450

[sireric]

Hi.

I don't remember if it did this when I started flying it this weekend (I don't think so), but it's certainly doing it now. I get tail "wag". I've got the AVCS gain set to 70% and it oscillates back and forth about 1~2 inches during flight. Does it around once per second.

If I drop the gain, the movement becomes larger and slower, but doesn't go away. Changing delay doesn't seem to make any difference.

This is with a GY401 and a Futaba S3101 servo -- I was seeing it my HS-50 servo before (I had to switch it because it was making odd noises at one end of the movement).

Things I've tried:
1) Re doing tail assembly and making sure that movement is smooth.
2) Pulled out tail from body about 2 mm, to really tighten up the belt
3) Play with distance of rod to center of servo (tried 5 mm, now set a 7.5 mm)

None of the above 3 things made a significant difference. I haven't done any calibration of the GY401.

I'm considering getting a s9560 digital servo to try. Any advice?

Thanks!

[sireric]

More things tried:

1) Loosed up belt -- No real change.
2) Tried gain from +10% to +95% -- Very fast oscillating wag at high end, large slow wag at low end (also harder to control)
3) Increased throttle curve to match Align trex instructions (from linear to 0/40/65/85/100) -- Perhaps a little better; I can tell that headspeed is much faster.
4) Checked main blade tracking it's very good (on top of of each other)
5) Delay back to 0 -- No difference.

Also, this only happens once in the air, n hover. On the ground or very close (skittish area), it's stable.

I'll check headspeed tomorrow, but s9650 is starting to sound like my next try.

[Danyboy]

Hello sireric!

Just a "quickshot from the hip"...

When you increased the distance of rod to center of servo, did it get worse?
Did you try to decrease that distance, once?

What you describe, sounds like either an overreacting control or a massive play.

If you can - I do not know the 401 up-close and personal yet -, try to decrease the amount of correction it gives.
Check the whole control-train from servo up to the tail-prich-rodends for play.

I also expiriened once, that the tail-ptich-slider evolved radial play on the axis, which resulted in a play of about 5° pitch on the tailrotor... I imagine, this would also induce tailwagging on a quick-acting HH-Servo...

Good luck!

Cheers, Daniel

[sireric]

Thanks for the advice.

I think I found the problem - a 30 second flight showed no issues.

I was reading a lot on this, and found multiple people talking about a smooth motion being required on the tail pitch movements. I found that both arms that hold the links were tight and not allowing the ball links to move freely. Some sanding fixed that. Then I found one of the ball/links was much tighter than the other. Cleaned up the link (had some plastic in it) and gave the ball-link assembly a squeeze with pliers -- More balanced now. Looks like the whole thing got much smoother when I manually moved the control rod. Put it back together and it flew fine.

I might still drop by grandRC tomorrow and check out rear servos.

Thanks for all the advice -- Thanks Bayou!

[BuckyBlade]

Hello sireric!

Turns out I had the very same problem you described on my 450XL. I have an SE that's comp quality, and decided to buy a less expensive XL for my "experimental" flight sessions. For the XL I went as cheap as you can go for all the parts, execpt I had a spare GY401 for it, so I didn't need to buy a gyro.

I had constant tail wag, exactly as you described. Could not figure it out until I spoke to the good folks at HeliHobby.

I replaced my inexpensive tail-servo, with the Futaba 9560 and my problem was solved. It seemed my "bargain" servo had neither the torque nor the speed to correct my issue.

After quite a lot of tuning, simply making this change corrected my problem, and my XL is now as solid and stable as a rock in the tail department.

Hopefully this helps!
Cheers,

Bucky