Elevator Trim Servo

Time: 1.5 hrs

Well it’s been over a week since I worked on the Sling. It’s been a combination of being busy with other things and the lack to cash to put towards the plane. Too many things going on financially… wife needed a new car, paid off the Grumman Tiger, estimated taxes :-(, and working being a bit slow. In any case I’ve been trying to get to the low hanging fruit as far as cost is concerned to at least keep working on the build.

The trim servo seemed like a good option. The Sling uses the Ray Allen T2-7A servo and the RC8-7 clevis so I ordered those from Aircraft Spruce. I also purchased a new trim tab hinge since I want to replace the hinge pin with a longer one that I can bend the ends on. I’m not able to drill a hole for a security wire like the other builders have done unless I want to unrivet and install a whole new hinge. As a side note I know that you can just purchase the hinge pin for Aircraft Spruce, but it’s a 6ft piece they’re charging over $25 to ship it (for a $4 part). I found if I buy a 3ft hinge the shipping cost is minimal and the price difference in the part is only $1. So I’ll just remove the pin from the hinge and use that. Thanks to Pascal for the tip on the alternate approach to securing the hinge pin.

Wiring the trim servo was a bit more time consuming then expected. The instructions that shipped with the servo only showed it being installed with the Ray Allen switch and LED position indicator (no of which I’m using) so it didn’t call out what each wire actually did. I referred to the VPX manual which did list out the what the wires were for, but just to double check I referred to the installation of MGL avionics and that didn’t seem to match up colorwise to what the VPX manual showed. The only way to sort it out was to do a test (I posted a short video of that). What it comes down to is that the White/Green is the position and then the White/Orange and White/Blue can technically be swapped around since all that it’s doing is putting power through a potentiometer so ground and supply can be swapped around and it doesn’t matter (unless you use the Ray Allen LED indicator).

Here’s a few photos of the test

At full extension the position resistance is at around 20 ohms.

Fully retracted is around 5k ohms

So centered should be around 2.5k ohms

And the mess of clip leads connecting to the switches. I didn’t happen to have a momentary DPDT switch so I used 2 DPDT latching switches, hence the larger mess of wires. Basically you need to connect one motor wire to 12V power and the other to Ground. You do this with a double pole switch (one side ground, one side power). This gets you to move the servo one way. If you want to go the other way you need another switch that flips the ground and power so the motor runs backwards. If you have a monetary DP switch the it will sit in a no connect state, when you press on the top of the switch then it comments the common (shared) terminals to the position 1 terminals and when you press on the bottom then it connects the common terminals to the position 2 terminus on the switch. I just had to use 2 separate switch to accomplish that.

With the testing and verification of the wires down I wanted to pin it up and install it. The manual says to drill out the mounting hole sin the servo for M3 rivnuts. I’m thinking though the the M3 screws fit nice trough the skin and the mounting holes and then I can just use locking nuts rather then the rivnuts and thread lock. I’m worried that the rivnut is going to crack the plastic mount when I install it and I like using the locking nuts rather then rivnuts anyways. The problem is I only have 2 locking nuts and now M3 washers so I can’t install it today. But with the wiring done and the push rod installed on the servo it will go quick once I get them.

You need to install the pushrod and clevis pin on the servo side before mounting the servo. With the servo set to approximately center I checked that the pushrod is screwed in enough to allow enough adjusting on the trim tab side. With that checked I think it’s safe to thread lock the retaining nut on the clevis (on the servo side).

I put a short piece of wire sleeve and heat shrink on the wires. Also writers are labeled with heat shrink labels and the connector is on on. The last bit is just to wrap the area where the back shell strain relief with some silicon tape and screw on the back shell.