500 Trex 500 Rudder Conversion

[Harford]

I have a Trex 500 ESP (Flybarred, GP780). This model comes with a metal rudder control rod and I have wondered if this contributes to the little bit of tail movement this heli has (very minor wagging). I therefore decided to convert from a metal rudder control rod to a carbon fibre rod. I purchased a Trex 500 carbon fibre rudder control rod, made for a later model and adapted it (shortened it) for my heli. Effectively, I still am still using the original metal rudder control rod but it is now runs through a hollow carbon fibre rod with one tail control guide. The end result is a straight control rod which moves smoothly, however it has turned out to be a bad modification. The heli now has poor tail control. I can easily conclude it is the new control rod set up although I don't understand the logic of it. I had at the same time replaced the front tail drive assembly due to a small accident (with the correct parts for this heli), which also involved removing the GP780 to effect the repair.

Do you think this is a control rod issue or have I upset the GP780 somehow ?

I can of course go back to the original rudder control set up.

Many thanks

 

[Geena]

Is the wagging, a fast wag, or a slow "hunting" type wag? Tail wagging can be caused be a couple of things. It can be caused by a stiff moving tail slider, or a poor mechanical setup. Check the tail slider assembly to be sure it`s moving smoothly and freely. Tail wag can also be a gyro gain issue. Which is the reason I asked if it`s a fast or slow wag. If it`s a fast wag, it could be that you have too high of a tail gyro gain. reduce the gain a few points at a time, until it stops wagging. If it`s a slow, hunting wag, it could be that you don`t have enough gain, and you need to increase it until the tail is holding.

 

[Geena]

One more thing. You mentioned you had to remove the gyro to do your repairs. Did you replace the two sided tape, or did you reuse the tape that was on there? If you reused the tape, that could be your issue.

 

[Harford]

It is a slow gentle movement. It isn't an issue in so much as the heli is stable and flies well - it is just that it is not a rock solid tail. I have exactly the same issue with a Trex 450Pro (again flybarred with GP780). I have played with gains, it makes no difference.
The situation has deteriorated sharply since a converted the rudder control. It runs smoothly and freely, additionally I replaced the two sided tape under the GP780. However , clearly I don't have something right so I think I will just go back to the original rudder setup. I am still curious though ...

 

[Smoggie]

Slow wag is often caused by binding or slop in the tail mechanism.. I'd double check that the new rod isn't binding. It can also be due to a tired or slow tail servo. If all that checks out you can try reducing the 'I gain' in the gyro slightly.. Assuming I gain is adjustable independent of master gain (I dont know the GP780)

 

[Harford]

I understand from reading forums that slow wag is often due to mechanical set up issues. I am completely biased in this respect and believe the mechanical set up is not the issue - although my confidence has been undermined following my rudder control modification.

 

[Harford]

Sorry, my reply above was incomplete. The Trex 500 has had very little use (less than 10 flights) so there should be no issue with the tail servo (DS520). I am lost with your reference to master gain. I can change the gain settings in the Tx.

 

[Smoggie]

There are two 'components' to gain in a gyro 'P gain' and 'I gain'. When you adjust gain in the Tx you change both.

The I gain is what gives the gyro 'heading hold'.. If it's too high you get a slow lazy wag of the tail. Usually there is a way that you can adjust the I gain independent of master gain adjustment from the Tx

 

[Harford]

Thank you. I will research the different gain components to which you refer - I was unaware there was more than one component.

 

[Smoggie]

Reading through the manual it doesnt appear that there is the ability to adjust I gain separately. The only option you might try is the small /large heli setting as this may change default gain values.

 

[Tony]

The GP780 is a tail only gyro and there is no way to adjust the P and I gains on this gyro.

What I would do is remove the linkage from teh tail servo and move it by hand. Everything should feel smooth. If it does, great, lets move on.

What are your Tx settings for the tail gain? And what Tx are you using?

 

[Harford]

I am using an original DX7 for this particular Heli. I am not using the Gyro Sense menu to set the gain. I am using the gear channel (5) and the travel adjust menu where I have the gain values set at +40% (HH) and -40% (Rate).

 

[Tony]

And I'm going to assume you have taken the tail rod off and everything was smooth. If that is the case, then start raising the gain by 5%. If you do not see any increase in tail wag speed, then you know it is a tail servo issue and the tail servo is just not fast enough which is unlikely for the DS520 which is what I use on 450 helicopters.

What hole are you in on your tail servo control horn?

 

[Harford]

The rod is operating smoothly. The horn has two holes on each arm. I am using the inner hole (as per manual).
To digress, I am going over the tail setup again and need to centre the rudder electronically ie I have it close mechanically but need to using some sub trim to get it perfectly at 90 degrees. Can you remind me please how to do this using the GP780. Many thanks

 

[Tony]

Set all subtrim to 0 and get the servo arm as close to 90 to the linkage as you can. Once that is done, then use subtrim to completely 90 the arm to the rod. From there you will adjust the linkage rod to center the tail slider on the tail shaft. If all is done correctly, you will have a slight angle on the tail blades where the trailing edge is facing the right when looking at it from the rear. From there, just run through the setup on the gyro and you should be done.

 

[Harford]

Thanks Tony but I am missing a step here (I think). It has been too long since I have done anything with a GP780 - too used to the Microbeast where you have to be in the settings menu when you do these electronic adjustments. When I attempt now to adjust the rudder arm using subtrim, initially the rudder arm does not response when I change the subtrim value, and then suddenly it moves far too far. Do I need to be doing something on the GP780 before I use the subtrim adjustment ?

 

[Tony]

It shouldn't be doing that. This issue sounds like a dirty pot on the 520 which is common. I have two that I have had to take apart and clean, but only do it if you know electronics.

 

[Harford]

Perhaps I am getting closer to why I am having this problem then. Just for the record, I am attempting the subtrim adjustment while the GP780 is in HH mode. I am not qualified to take the 520 apart and clean - unless there is a video somewhere showing the steps. Does this mean a new tail servo and if yes, what should I use please (if not another 520). Many thanks

 

[Tony]

Try this first, set your gyro gain to 0. This will effectively bypass the gyro. Try that and see what it does.

 

[Harford]

I have done this and it all seems to work fine. Mechanically centred as far as possible , applied a little subtrim with no issues. Now at 90. All seems good. I will go through the rest of the tail setup now. Have to say I am a little nervous now that the servo just may be faulty (dirty pots). Many thanks