431 hours - Measuring and drilling holes for trim servo mounting brackets

I ended up posting the last photo in my previous post out to VAF to solicit some responses about the offset lignment of the threaded rod as it runs from the attach point of the trim tab control horn down into the tapered opening in the elevator skin. See it here. It simply is about 1/8 of an inch too far over to one side. From VAF I received 3 options:
- Leave the rod straight and trim away material from the elevator skin, support bracket, and cover plate to make room for the output shaft of the trim servo and the rod
- Center the position of the trim servo and then joggle or bend the rod over as far as necessary to align it with the control horn
- Center the trim servo, leave the rod straight, and bend the control horn to align everything.

I decided that option 3 is just not an option. Bending the control horn presents all sorts of possible fatigue and failure scenarios as far as I am concerned that I just won't do it. This comes mainly from a lot of RC airplane building experience where bending a control horn to a point where it is out of alignment with the hinge line of the control surface causes fatigue to the horn, the clevis pin, and the clevis, and may also put strain on the servo or on the hinge of the trim tab.

Option 2 is what many people have done, but others warn that bending the threaded rod causes stress points in the rod where the threads are forced closer together, and this may lead to failure of the rod - also not a good outcome.

So I decided to explore option one a bit further. One comment I received from another builder hinted that you might not know just how centered or uncentered everything is without putting it all together, mounting it in the elevator, and checking it then. This was sound advice adn is the main reason I decided to proceed, even though I knew that te rod was off center from where I expected it to be.

The following pics describe a very handy process that I learned from Steve Riffe. It basically involves trsansfering the position of the rod as it aligns with the control horn onto the trim servo cover plate, and then making a template on a piece of cardboard or similar material that contains the outline of the cover plate, and an exended line for the pushrod location, using the line marked on the cover plate as a reference.

It starts by screwing the cover plate into position, attaching the rod and one clevis to the trom tab control horn, laying the rod over the access plate, adn drawing a line a cross the top. This is the reference line that shows the position of of the rod as it enters the area where the trim servo will be located. Use a stright edge to transfer the line from the rod to the access plate:

Next carry the marks for the end of the reference line on the top of of the cover plate down the edges and then draw the same line across the bottom of the cover plate.

Turn the cover plate over so that the bottom faces up. Take a piece of carboard or similar that is large enough to transfer the line for the rod, position the bottom of the cover plate about half way off of the edge the cardboard, and trace the outline of the plate into the cardboard. The reason for tracing only half of it and not the whole thing will become apparent a bit later. IN this next pic, I show the cardboard traces of the cover plate, one in its entirety and the other half way down. Also note the line that I have extended from the cover plate onto the cardboard for the rod location.

Clamp the cover plate over the tracing, again - bottom side up.

Next, cleco the trim servo to the left and right mounting brackets, measure the offset distance (3/8 of an inch from the rear of the plate, and draw that line on the plate. This is where the back edge of the mounting brackets should sit on the cover plate. Take the trim servo assembly and place it on the cover plate. Align the servo output shaft and jack screw with the line drawn on the plate for the rod, then clamp the brackets in place on the cover plate as shown in the next photo:

This next one shows just how far off center the rod and the output arm of the servo is when compared to the rough opening in the cover plate. Not quite centered in the cutout hole:

Next I removed the trim servo from the brackets and drilled the rivet holes to #40. I used a sharpee to draw the outline of each flange of the mounting brackets so I could monitor their position while drilling.  Sorry for the blurry pic, but you get the idea...

Remove the card board and reclamp the assembly to the drill board and drill the holes. Put clecoes right into the wood to hold the brackets in position while the remaining holes are drilled.

This next pic shows  closeup of an additional short line that I drew. This is the reference line that Van's tells you to draw to align the trim servo. Basically, if you use this line, you will end up about 1/8 inch off in the other direction (on the other side of the rod exit hole. It is best to follow this exercise to find out exactly where the servo mounting brackets need to be positioned.

This next sequence of pics shows you what I ended up with after I remounted the trim servo to the brackets and installed the unit in the elevator. You guessed it - still off center. At least now I can see how far of center, and if I might still be able to trim some material away to clear it. The concern is maintaining correct edge distance of the rivets that are set along the same side. It turns out to be very close.

You can see from the sharpee marks that I made how much material I would need to remove to clear the output shaft at its most extended range of travel. Guess I forgot to mention earlier that you want to fully extend the shaft of the trim servo using a 9 volt battery so that you can see the impact of the fully extended arm.

Now I get to decide - - trim the matieral and leave everything mounted as is, or reposition. I have already ordered a new cover plate from Van's, so I will contemplate what I want to do while I wait for the part to arrive.