500 Trex 500 Rudder Servo

[Harford]

Re Trex 500 Flybarred with GP780

Following from my earlier thread regarding tail issues with this heli. I still have issues. Spooling up the heli, the tail is constantly moving from side to side - not substantially, but it is not solid and not flyable. It is doing this because the rudder servo arm is constantly moving back and forward slightly. I have installed a new rudder servo but this makes no difference.
Just to recap I have recently changed the front torque tube drive gears due to a small accident which also necessitated the removal and reinstallation of the GP780. I used new Align foam pad on the gyro.

I am not sure what to look at next and would appreciate some help.

Many thanks

 

[Tony]

Sounds like you need to adjust your tail gyro gain. If it is a fast wag, you need to lower it, and if it is a slow wag, you need to raise it.

 

[Smoggie]

A video is always of great help in diagnosing problems, if you can get one taken?

 

[Jimbo]

Smoggie has a eagle eye for picking things up from a picture or video. Go grab a friend and shoot a video.

Jim

 

[Harford]

Thanks for your replies. I have lowered the gain, it made a marginal difference but did not eliminate the movement. I have taken a video which shows what I am talking about - the website says it is too large to upload. I will work it out and get it posted.

 

[Tony]

You will need to first upload it to youtube and then post the link here. Our server doesn't have enough space to allow for video uploads.

 

[Harford]

Ok - I will work on doing that. Thanks

 

[Harford]

Here is the link Very short I know, I can put up a longer one if needed

 

[Tony]

When you stopped the rotors, it stopped vibrating. I think you have a pretty bad vibration on that helicopter causing this. That or your gyro is not happy where it is.

What is your gyro gain in the Tx?

 

[Geena]

Tony is right. There must be a bad vibration that the gyro is unhappy with. Check the spindle shaft in the head, to be sure that it is not bent. Easy way to do that is remove the blades, and turn one of the bolts that hold the blade grips in place. If the spindle shaft is bend, you you see the blade grips wobble as you spin the spindle shaft. If the shafts are all good, I would look at the Align foam tape. BeastX used foam tape in the beginning too, and it cause all kinds of gyro issues. 3M tape was the cure, so that`s something to consider too.

 

[Harford]

Many thanks for this. Gyro gain is at 35% in the videos. I previously had it at 40%. I think the problem must be related to the accident (tail strike) where I broke two teeth off the front torque tube drive gear (not the mushroom gears but the gear which is driven by the autorotation tail drive gear). I replaced these 3 gears but not the autorotation gear as it does not appear damaged. I did not replace any bearings. The 3 new gears ran very smoothly when installed in the tail boom mount but I do notice a certain coarseness once they are installed in the heli and driven by the autorotation gear. Perhaps I should now replace the autorotation gear. I assume spindle shaft is the same as feathering shaft. I will check this but the heli has not had anything happen with the head. I am also suspicious of the foam gyro mount - I replaced it with the Align foam part (the pack came with 4 foam pieces and a metal piece the same size as the foam pieces). I used one foam piece only. I sure I will have some BeastX foam I can use.

 

[Tony]

The metal piece may be wise to use. It is there so you put down a foam pad, then the metal plate, then another foam pad and then the gyro on top. This is done for high vibration conditions.

Check and see where your bearing is in your tail boom. It should be just slightly off of center in there. And it needs to be glued to the torque tube shaft so it will not move.

Yes, spindle and feathering shaft are the same. Check that and also the main shaft. this is a pretty severe vibration, something somewhere has to be bent.

 

[Harford]

Thanks Tony. The torque tube bearing was glued in place when I built the heli several years ago. However, I will now dismantle the heli and check everything. Again thanks to you all for all the advice. Will report back.

 

[Tony]

You could be having to go through all that I'm having to do on this new helicopter lol. Talk about work...

 

[Harford]

Serves me right for a careless landing maneuver .. I mentioned there is a coarseness when rotating the main shaft - main gear not moving but autorotation gear and tail assembly is. I have traced this to the two mushroom front torque tube drive gears. Both are new. I should have picked this up when I replaced this front torque tube drive gear set.

 

[Harford]

Referring to the problem above with the jittering rudder servo. I have been through the tail to ensure everything is running smoothly. This included new bearings for the front umbrella gear in the torque tube folder, replacement of the complete rear tail torque tube unit, dismantling tail blade holders and control mechanism to make sure all straight , moving freely and smoothly, removing torque tube and checking bearing, removal main shaft to check it was straight (it was), attached gyro with one piece 3M double sided tape and then with 2 pieces of the tape with the metal piece in between. I still have exactly the same problem.

Another thing which happened when powering on to spool up, with the gyro in HH mode, the rudder servo moved from 90 degrees to 45 degrees. This is when the heli was absolutely still and no inputs from me. This happened several times. This must be significant and it cannot be the servo as it is brand new (and the same thing happened when the original rudder servo was on the heli).

I am guessing I have no option but to try it with a different gyro. Do you agree, and if yes, can you suggest a suitable replacement for the GP780 please.

Many thanks

 

[Jimbo]

Do you have the correct pulse/Hz set for that servo in the fbl? I entered the wrong pulse rate on mine, my mistake and it jumped to 45 like yours did.

Jim

 

[Harford]

Hi Jim
I have the gyro set as follows :
Band setting - green light 1520
Servo type - green light digital
Rotation - this is correct (servo moving in correct direction as tail moves0
Limit - these are set so no binding
Delay - Green light for 500 size
Many thanks

 

[Admiral]

I had a problem a while back after changing the Torque tube drive gear, it turned out to be a faulty drive gear, it was genuine Align and new but it had a tight spot in it which caused a serious vibration on spool up, too severe to fly.

I could not get another gear as quickly as I wanted so I removed the assembled housing, placed a Philips screw driver shaft into an electric drill and into the torque spline and ran flat out for about 10 minutes, result was a smooth running gear and no vibration. Worked so well I'm thinking of doing it whenever I need to change a torque gear just in case, kind of like bench running a motor before installation.

 

[Jimbo]

Admiral: That was clever and sounds like a great idea.

Harford: Does the servo require 1520/333 or 1520/200?

Jim