TruTrak Autotrim option

[Tankerpilot75]

I have the opportunity to purchase a TruTrak automatic pitch trim module at a very reasonable price (lunch) for my Vizion 385 from a friend who chose not to make it part of his autopilot installation (he has manual trim).

Two years ago I converted my RV7A from manual trim to electric and installed Pat Hatch?s Ultimate Trim Module board under the copilot?s seat (I love this trim hub board and Pat stepped through the installation with great patience). At times (when climbing/descending) I would appreciate automatic pitch trim capabilities for airspeed changes.

However, I?m honestly more than a little concerned about the wiring changes required and the placement of the module. If I?ve read through the installation instructions correctly, the module must be installed flat. I have literally very little space available behind my panel, a little under the pilot seats, and if I install the module behind my cargo panel near the elevator servo then a very long set of wires is required to connect the module to the autopilot control head and would require me to drill a new hole through my wing spar to run these wires to my autopilot control head.

I?m not even sure how this module works. Normally I would expect it to send commands to the elevator electric trim (which I installed on the self below the vertical stabilizer and use a push/pull cable to transmit trim changes to the elevator trim tab). But looking at the installation instructions I almost think it actually moves the autopilot elevator servo which I think would aggravate an out of trim condition.

Can someone explain to me how this auto trim module works and does anyone make a conversion cable for this module installation? I?m really reluctant to mess with my electric trim system because it was such a challenge for me to install it and get all the other stick buttons to work correctly. I may be trying to make my little RV more complex then it really needs to be. After all, it?s not a jet.

Advice sought!

[flyinga]

" I may be trying to make my little RV more complex then it really needs to be. After all, it?s not a jet."

The RV disease.

[BobTurner]

I have the Trio Pro with auto-trim, so yours may be different.
The Trio trim function connects to the trim tab motor, not the elevator servo.
The Trio has a pulse-width modulated speed control (programable) so the trim motor runs slowly at high airspeed, full speed at low airspeeds.
The Trio trim can be used when the servos are not engaged (not sure if TT does that) to trim manually.
Assuming you also have stick or panel switches for trim, you need to install some sort of relay logic circuit to prevent accidental shorts (if the autopilot is calling for nose up while you push a switch for nose down, for example) unless the TT box has inputs for these switches.
I love the auto-trim. I can start a coupled ILS at 140 kias, later ease off on the power, until down to 75 kias. Trio tracks the ILS perfectly, and, when I disconnect it for landing, the plane is in trim.

[Tankerpilot75]

The ?accidental shorts? you referred to are supposed to be eliminated by the ?Ultimate Trim Control Boad? I referenced earlier.

If Jesse St. James or Walt Arron is reading this I would appreciate hearing from you on this topic.

[Walt]

You have to have a trim sensing servo for starters, the basic operation is the module senses servo torque and then adjusts the trim tab to relieve the force. Sounds like you a non standard trim setup with a drive cable, but that really shouldn't change anything. I've only installed one TT module years ago but don't recall it being to bad. You do have to run some wires up front and others to the servo. The unit usually mounts next to the elevator servo.

[Jesse]

Is there a Jesse St. James, or are you referring to me?

The auto trim module for the TruTrak can be mounted in any orientation, and can go right back by the pitch servo. It doesn?t need to run to the control head. This is assuming you have a Vizion 385 with trim sensing in the pitch servo.

The installation manual shows a fairly easy to make harness. I could make an interconnect harness that would be plug-and-play except tying to the trim wires. If you are using the Ray Allen 5-conductor Cable, you need to strip the insulation for about 4-6 inches and isolate the 2 white wires, which will be cut and run to 4 pins in the auto trim module, 2 Ins and 2 Outs.

[Jesse]

From the TruTrak Auto Pitch Trim Manual:

Autotrim Installation
-The autotrim module can be mounted in any location, in any orientation. Generally it is installed near the pitch servo to minimize the wire length required for installation.
-The LED status lamp can also be installed in any location desired. However, it must be visible for the setup procedure. The lamp will also indicate a failure so having it visible even during normal operation is beneficial.

[Tankerpilot75]

Thank you both Jesse and Walt. Sorry for the name screw up. I obviously need to do more studying.

Walt, I do have a non-standard trim setup with drive cable. I chose to install it this way to avoid having to both rebalance my elevator and also avoid modifying the already painted horizontal stabilizer. This non-standard setup works quite well.

Jesse, my autopilot is the Vizion 385. I will call you soon and discuss the custom interconnect harness.

[penguin]

Do you need autotrim?
I would suggest it will be almost redundant in practice as the trim requirements are low.
More weight and something else to break/need maintenance.

[Tankerpilot75]

Originally Posted by penguin

Do you need autotrim?
I would suggest it will be almost redundant in practice as the trim requirements are low.
More weight and something else to break/need maintenance.

I?m not sure yet. That?s why I?m thinking about it and asking these questions.

One thing I do recognize is that keeping the aircraft in trim during descent and on approach when coupled to the autopilot is something I also have to bring in my cross check otherwise I can experience unexpected autopilot pitch jerks.