View Full Version : Slow Gyro Drift in HH Mode --- Mystery

01-16-2008, 06:12 AM
Howdy All ---- well here is another challenge. I now have my Trex450SEV2 together in a preliminary lash up and hovering. Flight characteristics are great and the set up went per text book instructions.

There has been a strange mystery though. I first put the heli together using a 401 gyro and had significant heading drift in flight, in spite of adequate gain in the gyro loop. After much frustration trying to figure out why the heading would not hold steady(and the gyro is indeed in HH mode), I switched the gyro to a JR G500T and the heading drift slowed to a very small amount, yet it is still there. I am using a Futaba S9650 digi-servo for the tail rotor.

The mystery is that I cannot find the source of the heading drift. The gyros (both) have no heat source near them. Both had way beyond enough gain to wiggle the tail in flight --- so I can only assume that possibly I have a voltage problem from the ESC causing the drift (?). Everything else is functioning very well and is set up correctly ---

Has anyone had this similar problem??? ????This is a hair puller. I am just short of calling Align and Futaba and discussing it with them, but wanted to try here first.

What say out there pilots???? :blah:

Thanks --- Bob

01-16-2008, 08:10 PM
I'm not very knowledgeable about the two gyros, but it sounds like if the heli is still doing it after a servo change that logical deduction would point to your Tx set-up?

In AVCS mode all trims & subtrims MUST be nulled and the servo centered, etc, or it will reset the rudder neutural position and thus give you a drift (I would imagine).

Post your fix if & when you find one!

01-17-2008, 04:17 AM
Howdy Curtis and all --- well I think I found the problem. I switched out the receiver as an experiment. While the system was functional (plus drift) with the first receiver (Spektrum 6100), it became properly functional with the second one. I spoke with Spektrum about it, and they said it could have been a "fluke" receiver that was not providing sufficient drive output on the rudder port to properly supply the gyro and servo. Unfortunately there is no way for me to verify that exactly --- guess I should just enjoy the receiver as it is and go for it :red: --- I intend to do some experiments later on with the other receiver and a smaller servo just to see if that helps or verifies anything.

The good news is, my Trex450 is now behaving as a HH mode bird should.

Thanks / Cheers --- Bob :ws:

01-17-2008, 04:48 AM
I'm just curious, is the rudder servo digital?

I knew someone that put a 6100 in a small sport scale A/P and he had glitches, etc, that eventually crashed it. Seems the 6100 was too small for the demands placed on it - power-wise.

From what I have heard and read, you have to be careful with the BEC's used with digital servos when used with gyros. I'm still studying up on all that stuff!

Glad all is well, Bob!

01-17-2008, 12:48 PM
Hi Curtis -- yes, the present servo is a digi. I found out from a group of heli flyers at my LHS that this particular one is being used extensively as a tail servo (Futaba S9650). I plan to run some experiments using an analog servo to measure different behaviors as well. I have learned too that the qualification of many different arrangements is done empirically --- you never know what might interact positively or negatively with so many different components available.

In talking with the good folks at Align, they did not feel there was any issue with their ESC (its BEC) -- but at least I am going in the right direction ;)

Thanks for the interest --- Bob

01-17-2008, 06:18 PM
Hey Bob,

If you mount an analog servo be sure to NOT run the gyro in digital mode - there are warnings all over the manual that it will destroy an analog servo.

I can't imagine that an analog will function as well as the digital, the speeds are not even close. What might be more revealing is another digi servo? I see the 9254 is popular, too.

I guess another main concern is the operating voltage available. The digi servos run at 4.8 volts, and what I have read indicates that they need a very controlled power supply and that a lot of people use a voltage regulator to ensure proper operation.

Just some thoughts.


01-20-2008, 07:52 PM
Seems as if you let the 401 warm up for a few minutes (powered up but no throttle) and then unplug and reboot the sytem (401/9650) You get a lot less drift. I have tried multiple ar6000 Rx's and still get a tiny bit of CC drift. Warming and rebooting helps to minimize this. Anyone in the warmer climates experience this?

01-20-2008, 08:28 PM
have you gone through Finless' set-up videos for the 401?

01-20-2008, 08:40 PM
I had the same problem for months,
I had 4 veteran heli pilots troubleshoot the
system and later found that the
"pot" on the rudder stick on the transmitter
was bad, sending an changing value and an
erroneous center,
it was probably worn or dirty.
The substituion of another TX with
a good functional pot on the rudder stick
permanently fixed this pesky glitch.

01-20-2008, 08:48 PM
have you gone through Finless' set-up videos for the 401?
Oh yeah, To the "T". I have a mechanical center within 3 points Tx trim. Maybe try another "slide" One turn on the link gives me about 12 points Tx trim.

I will check the rudder pot. Pretty new dx6 but I will give it a wiggle and maybe a blast of contact cleaner. Thanks

01-21-2008, 01:12 AM
I just realised that I am running 5v BEC on my align 35G ESC. Futaba GY401 states to use 4-6v and the s9650 states optimum 6v.
I have another heli coming soon and will have the 35X ESC with 6v BEC. Maybe this will cure the drift. I will post back.

01-26-2008, 06:53 PM
Yep, same gyro and servo and no drift on the 6v bec. slight drift on 5v