Tru Trak pitch servo

[Rallylancer122]

I have an early trutrak install in my RV8 and I'm wondering about the pitch servo. The motor is loose on the gearbox, meaning that I can rock it back and forth about an 1/8". I seem to recall asking about this before and was told it needed to be this way to sense load for trim indication or something like that. Still doesn't seem right. Can someone confirm or deny this?

I took a video but am too technologically challenged to figure out how to post it.

 

[moespeeds]

I've got one sitting on my desk that was acting up. I can rock the motor in it's mount relative to the gear box, but only about 1/32", or the thickness of my AAA card as shown.

This servo was working, but unpredictably. It was on my roll axis, and turbulence or wind gusts would sometimes cause the AP to go off on its own, rolling the airplane over. A generous member here sent me a loaner servo which mostly solved the problem.

 

[Stevea]

I've got one sitting on my desk that was acting up. I can rock the motor in it's mount relative to the gear box, but only about 1/32", or the thickness of my AAA card as shown.

This servo was working, but unpredictably. It was on my roll axis, and turbulence or wind gusts would sometimes cause the AP to go off on its own, rolling the airplane over. A generous member here sent me a loaner servo which mostly solved the problem.

Just have to ask….is the motor rocking on the gearbox because there are two socket head cap screws missing at the corners, or is something else going on? There should be screws in all four corners of the motor.

 

[moespeeds]

All screws are in place

 

[Stevea]

All screws are in place

OK. Different screws than what are in mine. TruTrak must have made some design changes sometime after mine were built.

Assuming all 4 motor screws are tight, something is clearly not right with the assembly. Try opening it up by removing the 4 motor screws and the 4 gearbox cover plate screws. There are not many parts inside and nothing is breakable, or spring loaded (to jump out and fly across the hangar....never to be found again).

Here are some photos of what mine looked like:



Note: this is one of the original "DSB" TruTrak servos. It is not a "DSP" version used to sense pitch trim conditions.

 

[blaplante]

The pitch servos are designed to rock, but not easily. If you open up the gearbox you'll see the motor is mounted on a web, that allows flex. I've seen two where the web has broken one or two legs. Then they wobble quite a bit!

Take a look. If you have a cracked web, contact me via PM.

 

[blaplante]

My comment applies to the "DSP" servos. If you have an Alt hold only (Altrak), you'll have a DSB servo in the pitch position. DSB don't have out of trim sensing, and hence the motor is hard bolted to the case.

 

[vinzer]

Most likely this is a design of your servo rather than an issue. Motor should be able to move a bit exactly as blaplante said.
This is made to protect the motor and internal components from damaging.
I have similar pitch servo in my airplane and motor moves a little as well.

 

[Rallylancer122]

Removed the servo and took it apart. It is broken exactly as blaplante described. Explains why I was getting constant trim requests from the autopilot. This was not one of TruTraks better ideas.

Not sure how I'm going to fix it yet. I suspect any used replacement servo I buy will have the same problem and I'm not sure new servos will be compatible with my old system. May just have to repair this one.

 

[blaplante]

I believe the new BK xCruze 100 servos are compatible.
TruTrakAndrew (on this forum) would know.

 

[Rallylancer122]

Ok, I got another servo from a friend and robbed the web out of it. Put it back together and installed. However now I have a new problem. On the ground it's calling for down trim. It takes a fairly healthy pull on the stick to null out the indication. In flight same thing. I can trim the stick to a neutral force but as soon as I engage the AP it calls for down trim. If I trim it down to the point the indication goes away it's right on the cusp kicking out from an overload, and even a small gust or pitch upset will cause the AP to kick off. The result is a rapid pitch down as the aircraft is trimmed heavily nose down. Not fun!

Short of trial and error, which is going to be a tedious process of opening the tail, removing the servo, taking it apart, adjusting the little tang on the trim sensor, then reverse..... and doing that a bunch of times, is there a spec or way to set that trim sensor?

 

[Capt]

I don’t have the trim function on mine, I like mechanical trim, Armstrong design, less to go wrong and always works