RV-12 Trim Speed Adjustment

Has anyone had trouble adjusting the trim speed on their RV-12? It was very touchy when I adjusted the pot, but seemed to work fine for about 20 hours. Then the trim tab stopped working completely. The motor works when connected to a 9V transistor radio battery (ULs fly like that with these servos if they don't have an electrical system) and pulls about 200 Milliamps, but when I hit the trim button to move the tab I only get 0.2 Volts and no movement. With the trim at full travel (no current flow) I get about 9.6 Volts. I find it hard to believe the control circuit just died after only 20 hours, so I'd like to know if anyone else encountered this. Next step is to pull the glareshield and start poking around the trim adjustment. In a worst case scenario I'll bypass the speed control circuit.

I would suspect a loose pin wiring connection on the D-Sub, pins 30? 31? Where are you measuring the voltage?
Doug Dahl

It was a big problem for early builders - -

They changed the control board which gave more power to control the trim motor. The motor is not totally smooth and the gears bind a little. With 12 volts it zips along, but with limited power it can stop. You have an early control panel ?

John Bender

JBPILOT said:

They changed the control board which gave more power to control the trim motor. The motor is not totally smooth and the gears bind a little. With 12 volts it zips along, but with limited power it can stop. You have an early control panel ?

John Bender

Click to expand...

Actually John, the speed control circuit is pretty much the same for the D-180 and Skyview equipped RV-12's.
The only real change for for Skyview was to use a 25 turn potentiometer so that the the adjustment wasn't so sensitive.
The actual out put of the circuit is no different.

The reason the voltage reads low when checking the output with the speed adjusted to a slow value is because you are measuring a pulsed DC voltage. When measured with a DC volt meter you are looking at the RMS value of the pulsed voltage, which will read much lower than the peak to peak value.

BTW, the lack of smoothness in the motor when moving slowly is related to the (variable) internal friction of the trim motor, not the output of the control circuit.

Here we go again - but - -

I could not get mine to control properly originally. Van's sent me an exchange board, and it did work. I was told the very early ones had an issue in some cases. The facts anyway.

John Bender

JBPILOT said:

They changed the control board which gave more power to control the trim motor. The motor is not totally smooth and the gears bind a little. With 12 volts it zips along, but with limited power it can stop. You have an early control panel ?

John Bender

Click to expand...

JBPILOT said:

I could not get mine to control properly originally. Van's sent me an exchange board, and it did work. I was told the very early ones had an issue in some cases. The facts anyway.

John Bender

Click to expand...

Ok, I will fill in the fact gaps.

A small # of PCB's in the initial production batch got built with an incorrect component that caused poor performance. Only a few builders received one.

No change was made other than to install the correct component that was originally intended. No changes were made to provide for more power (it was that statement that caused me to reply John)

If the O.P. received one of the very first avionics kits shipped, it is possible he has one, but since it worked fine for 20 hrs, I think that is unlikely.

OK - -

That is a good explanation. I had asked if he had an early panel. I did not find out what the problem was, and originally there was a hesitancy to replace mine, but it finally happened.

Satisfied.

John Bender

thanks

I won't get a chance to check the trim board until I get back from OSH, but you guys have given me some ideas. My board was received in early 2010, so it's probably not an old one. I measured the voltage at the servo connectors in parallel with the servo while I toggled the trim switch. The pulse DC circuit description makes sense. I thought it might just be a voltage regulator, but pulsing makes sense. Maybe I'll drag out the o-scope that has been gathering dust in my hangar and see what it's up to. Anybody see any problems with bypassing the circuit if necessary - other than faster trim response?

Thanks for the idea on the pins. One was not seated properly. BTW the guy who put the potentiometer under the D180 should be sent back to Design 101 to learn about maintenance.

RFSchaller said:

...... BTW the guy who put the potentiometer under the D180 should be sent back to Design 101 to learn about maintenance.

Click to expand...

AMEN to that!

Yeah, and

I think we'd all like to have a few words with him. Cost me a couple of fuses in the process of getting it set right. Somewhere in the archives thate's al long discussion about setting the pitch trim.

Wayne 120241/143WM

Is the potentiometer in a better position in the Skyview aircraft?

John

Look at this post, the pics in the middle. http://www.vansairforce.com/community/showthread.php?t=82782&page=17. See the box that has all the cables plugging into it vertically? It has a label reading Vans Aircraft. Coming out the far side of it facing towards the centerline of the a/c is a row of pots. So you can adjust them one of two ways - one is to remove the top cover as in this picture and reach them from above, the other is pulling out the Skyview screen from the front and reaching in. There is slack in the cable to the Skyview so this is not a big deal. It is tough to see them and know that you have aligned the little plastic screwdriver with the slot in the pot.