Step 4.5, Trim tab

Well, no more putting it off.  The time has finally arrived.  Time to build the dreaded trim tab.  Quite a lot has been written (on the internet) about the difficulties of constructing this diminutive, yet all important component.  First of all, the build manual instructs one to bend the skin to the final angle.  Then just bend the tabs over.  After all of the drama bending the elevator tabs, I was seriously considering cutting these off and building replacement ribs from the start.  But, I thought I would give the Van's method one more try and so I did.

I had the same difficulties with the bending block moving in, but the results were somewhat less catastrophic this time so I'm going to just live with it.  Since practice makes perfect, I figure that in ten or twenty more planes I could make the Van's tabs look pretty good.

The next step is to attach the tab actuator flange to the bottom of the trim tab skin.

Once that is done then the tab hinge and the tab spar are match drilled and then riveted.

The completed trim tab is then ready for installation on the left elevator.

Here is a view after riveting the tab hinge to the elevator.

Next, the trim tab servo is fitted.  It took a considerable effort to get the servo to fit through the access door and to prevent interference between the elevator actuator rod and elevator skin.  The build manual just barely mentions that it may be necessary to trim the actuator rod egress area.  Out comes the cut-off wheel...

Trim...

Trim...

Trim...

Trim...

And Trim!

What is really annoying is all of the test fitting between each cut-off wheel attack.  It was most of a night's work on this tiny weeny little part.  Finally, the result:

  

Since the build manual fails to give any guidance whatsoever on the proper way to adjust the trim tab servo linkage, I came up with this method:

1.  Using a 12 volt power supply, run the servo out to its furthest extent and measure the arm length.

2.  Run the arm back in to its minimum length.  Take half the difference and then add it to the minimum length to find the exact midpoint of the servo's extension.

3.  Run the arm out to the midpoint.

4.  Cut off the actuator's threaded rod at a length that allows it to be half way in the clevis fork with the trim tab in trail with the elevator.

This method insures the maximum travel possible with the supplied servo.  There is quite a lot of deflection available.

For me, the trim tab was not nearly as horrific at the elevator tabs.  Your mileage may vary, of course.  I did have a minor mishap while fitting the servo access plate though.  One of the #6-32 screws broke off in a nut plate.  Oh, $%@#!

I replaced the nut plate rather than risk damaging the backing plate.  In all, it was a ten minute detour on my way to finishing this airplane.  Not too bad considering I probably still have most of 2000 hours to go.