Elevator Trim Tab

Today was a rainy day in Los Angeles; perfect day to be building. I knew it was going to rain, because the original internet (the sky) sent two bits of data yesterday from Santa Barbara saying it would be so. Here is a picture of the data; or maybe this was the dashed line you see on the weather maps.

So the elevator trim tab went well but I did not follow the directions verbatim. They say this is the part that everyone has trouble with, and if I had followed the directions exactly as written, I would have had trouble too. But planning this out yesterday, while the internet data was being received, I decide that there was a better way.

I followed the plans as written up until the trim tab horn is dimpled. I chose to dimple and rivet the trim tab horn after the ends were bent over. But let me walk through the order I did things.

I matched drilled the holes and deburred per the plans, but did not dimple any holes yet. I bent the trailing edge to the correct angle as per the plans using a 4 foot section of  2x 4 and some el cheapo hinges from the aviation aisle of Lowe's, as per the plans.

Then I built the bending blocks for the ends as per the plans.I practiced on some scrap aluminum until I felt comfortable working on the expensive stuff. I left the blue plastic on and used double sided tape to keep the blocks from moving as I dressed the ends.  This didn't work completely as planned, as the trailing edge in the forming blocks still moved a little as I bent over the ends; but still came out acceptable. I used a piece of 1" x 2" wood to start the bend. Then I used the flush rivet set in the rivet gun and really gently smashed over the edge till 90 degrees. I had previously filed in a 1/16" radius in the bending block to form a nice radius in the finished bend. I worked the gun back and forth until the bend radius was even without smashing it completely.

The skin area for the trim tab horn was not dimpled or riveted earlier (as the plans would have had you do) because then the trim tab wouldn't sit nicely in the forming blocks. So now I went back and dimpled the trim tab holes.It is interesting to note the #40 holes in the spar and the skin seemed to already be punched to final size, but I went back and reamed them anyway to clean up the holes.

Then I riveted the trim tab horn to the skin, except for the trim tab to spar holes, per plans. There is a spare to skin hole outside of the trim tab that I didn't dimple before the trim tab horn was riveted on. So I had to dimple this after the trim tab horn was on. This presented a problem in that there was no enough room next to the horn for the dimple die to sit down properly. I ended up drilling a hole in some scrap aluminum the same thickness as the trim tam horn and dimpled these two sheets at the same time In the picture below you can see the hole and the scrap sheet. Next time, I will dimple this hole before the horn is riveted on. I also bonded the two halves of the trim tab together, using JB Weld, to keep the moisture from wicking in between and possible causing corrosion.

Next I deburred and dimpled the #30 holes in the ends. These get pop rivets later.

When riveting the spar to the lower trim tab skin, the tungsten bucking bar is essential. I don't feel this can be done without a tungsten bar because the normal bars are just too big.

Everything else that followed, was done per the plans. nothing special.

I should be seeing the replacement parts for the elevator tip rib in a few days, so I will be finishing that up.

Elevator, Left Side

The left side elevator started out much smoother than the right. I was able to get the stiffeners back riveted no sweet by using the same method as the right elevator. Here is a picture of how I did it. The plans says to use rivet tape and tape all the rivets in place. I don't like that. My general rule is to have the pieces tight together before riveting. So, I used a completely different method since I know more than the guys at the factory who have built 20 more years of airplanes than me.:-)

So my method uses a cleco in the hole adjacent to the one I am riveting. The rivet plate is taped to the bench and cardboard is used to hold the assembly up off the table. The cardboard has a cut out for the clecos that hold the stiffener to the skin. I start at the trailing edge and insert a rivet one at a time, back rivet and then advance to the next hole. Since I am not using tape against the rivet, the rivet gets marred by the vibration of the rivet gun; but I am painting, so I don't care. The clecos keeps the skin straight as I flex and contort it to get the gun on the rivets.

The spar went well but I learned from the right side and made all the bottom rivets the pull type (with the larger dash 4 hole size) I guess if I really wanted to, I could  squeeze dash 4 solid rivets in the holes I could get too, but that seems like over kill.

But again, riveting  on the tip gave me trouble. More so than the previous elevator tip. The new parts will be here this week. We don't need to discuss this further, but lets just say, the new tools (1/8" double offset rivet set) will also be here this week. And another lessons learned, make sure the bucking bar is on the rivet and not sitting on a cleco holding the skins together. Last lessons learned, take pictures of riveting setups. Part of this trouble came about because I forgot the exact setup I had used successfully on the right side for the same rivets. Moving on...

I have started to work the elevator trim tab. It is not as bad as others make it out to be... yet. But it seems many before me have had trouble bending the skin tabs down. Knowing this, I will probably make up so samples and perfect my technique before doing the flight parts. Now I am off to cut the grass, paint the house, loose weight, before I start building again.... OK maybe just cut the grass... Naw, lets get building!

Elevator, Right Side

The right elevator is coming along but I had trouble getting to the two rivets shown in the picture below that are back in the gap at the top left of the blue-covered skin. Turns out I was assembling the tip rib incorrectly; the tip skin goes underneath the main skin, I had the main skin underneath the tip skin. By doing it the right way, the main skin can be opened up, providing good access to these rivets. When I followed the directions, it was easy.

On the bottom skin, when attaching it to the spar, the instructions said I could use either all pop rivets or a mix of pop rivets and the much stronger squeezed rivets. I chose to use a mix of pop and squeezed rivets. This made it much harder. I think if I were to build another elevator, I would just use all pulled rivets for the bottom skin to spar; much easier and less grief than having a combo of the two.

For bonding the trailing edge, I used the T88 epoxy instead of the tank sealant. The T88 is mentioned on the plans as an alternate material. I hoped it will provide less pillowing and better bond on the trailing edge than what I had on the rudder. But I have a feeling the pillowing and bonding is due to the metal being riveted so close to the edge, not the bond material. OK I give, the main reason I used the epoxy is because it is a 1 to 1 mix, whereas the tank sealant is a 4.5 to 1 mix. I didn't have a good enough scale to weigh out the sealant and I didn't want to wait three days for Amazon to bring me one. Previously on the rudder, when I mixed the tank sealant, I discovered my Target scale did not read fractions of a gram, so I guessed the mix ratios and luckily got it right.

In summary the right elevator took quite a bit longer than the rudder; mostly because there are many more rivets and the lessons learned on the rudder provided a bit of education and familiarity.

Once the epoxy dried, I riveted the trailing edge per the instructions. This went much better than the rudder, but still had the same seperation of the skin very slightly from the AEX tapered strip. I eliminated puckering between the rivets by gently squeezing out the epoxy with a seaming pliers. I think the only way to keep the skin tight tot the trailing edge is to use some structural adhesive, like JB weld, or to edge break the trailing edge before dimpling. I will try the edge break method on the left training edge.Edge breaking puts a slight crease in the skin edge so it sits tight to the AEX tapered piece. This is described in the plans.

Overall I am happy with the right elevator. I think I could do a perfect job on the left now with this experience. (Right !!! wishful thinking see next post.)