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Weird(?) gyro issue

Old 04-11-2011, 02:00 PM
  #1  
ululi1970
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Default Weird(?) gyro issue

HI,
I have just put together a t500 clone and I am having a weird issue with the gyro (JR 7000T + Hitec HS-6965HB)
For the first minute of flight I get strong tail jerks (20 degrees or so) every few seconds, both in rate as in HH.
I moved the gain to low too.
I get the same result even if I take the main blades off with the heli clamped to my turntable (which lets the heli rotate freely).
So I have to hover for ~ 1 minute fighting it like a bronco, before I start flying it. After that,
the heli flies beautifully. I have tried an old CSM 200 and it works fine, so I think that would rule out
mechanical problems (the blades are balanced and the tracking is fine). Plus, usually mechanical problems
do not disappear with time. The belt is properly tensioned and lubed. The problem with static electricity
should get worse with time...


The ESC outputs 5.52 V, and the JR gyro says 4.8 - 5.1 V. Maybe those 0.3 V make the difference?
Also, could it be that it does not like the Hitec servo? But then why the problem goes away after a minute?

At the end of my wits....

Thanks
Alberto
Old 04-11-2011, 02:07 PM
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BarracudaHockey
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Default RE: Weird(?) gyro issue

My guess, and this is only a guess based on your post, is that the servo is getting too much voltage till it burns off a little.

The only real way to tell is to put an inline volt meter between the servo and the gyro. The gyro can take the voltage, its the servo that will act up.
Old 04-11-2011, 04:30 PM
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dooleyje
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Default RE: Weird(?) gyro issue

The 6965 is rated at 6V and should yeild a speed of .10 and at 4.8 is will give you a speed of .13. This is rather slow by todays standards. It is workable as long as you use 6V. The problem I suspect is your servo arm is too long. For a 500 size heli I would use about 10-11mm at the most. Now if you are using a DX8 your rate would be about 27% and not above 50% for HH. Your clue was that it wags in rate mod. Also make sure your gyro know you have a digital servo.

As far as set up, you should have made sure your servo arm is 90% to the tail boom and the slider centered. The difference in the two gyros id due to the fact that not all makes give you the same throws and servo control and response. Read up on the T7000 to see if you have it programed for your equipment.

Jim
Old 04-11-2011, 05:45 PM
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ululi1970
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Default RE: Weird(?) gyro issue

Thanks all for the comments.
I would not describe it as a wag in rate mode. It is really a sudden jerk but maybe this is what Jim means.
As for the setup,
the arm is set up properly (90 degrees), in fact I set it up mechanically at zero gain to balance the torque in hover (or very nearly at least). I measured the distance of the servo arm which is ~13 mm, in fact I had to reduce the throw quite a bit using the pots. So that is too long? I will move it in starting from the innermost at ~7mm. Specwise, the 6965 is on par if not better than
the JR 8700G, which is the servo recommended with this Gyro.
I think the radio is set up correctly. Once the "bronco" phase is over, I do not need much gain (in Rate or HH mode).
But what I do not understand is why the problem disappears after some time in flight. Does the JR Gyro go through a learning process? There is no mention in the manual of anything of that sort, even though it would make a good bragging point...

Alberto

Old 04-12-2011, 04:37 AM
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BarracudaHockey
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Default RE: Weird(?) gyro issue

No the JR gyros assume a center pulse which is why they drift on the bench if you dont clean it up with some sub-trim
Old 04-12-2011, 01:42 PM
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ululi1970
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Default RE: Weird(?) gyro issue

Well,
I tried. I shortened the servo arm to 7 mm but no luck. It still violently jerks to the right. At this point, I am inclined to think the problem is with the Gyro itself...

Alberto
Old 04-13-2011, 06:10 AM
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ululi1970
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Default RE: Weird(?) gyro issue

Good news... . Just as I was about to write the Gyro off, I rechecked my notes and realized that the problem started after doing some changes to the throttle
curve in normal mode. Without realizing, I had set things up so that the heli would hover just before the curve went flat. Apparently, the governor mode of the ESC (a
Turnigy 85A ESC/SBEC, but this is a budget machine) did not handle that too well, likely causing power to fluctuate rapidly, sort of like a nitro engine misfiring. I think the CSM gyro did not react fast enough to make
that noticeable, but the JR did. As for why it seemed to disappear with time, I am inclined to think that as the ESC/motor warmed up, I had to apply slightly move throttle/pitch to hover, and so I moved into the flat section of the curve.

So, once I changed the curve to be flat at take off and hover, the jerking disappeared as well.
The silver lining is that I learned about the effect of control arm length on the gain.

Thanks again for the input. Jim, If I'm ever near your neck of the woods I'll make sure to pay a visit to your store.

Alberto
Old 04-13-2011, 07:21 PM
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Default RE: Weird(?) gyro issue

Come on over and fly with me.

Jim

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