El Gato Dos

On Saturday I got the mounting for the trim servo done and was ready to install it into the left elevator. However one last thing remained to do, add the wiring to connect it up through the horizontal stabilizer and so it can be wired up to the control stick eventually, but that’s after the fuselage gets built.

Here is the servo on it’s mounting plate, you’ll notice that the leads are a little short to reach the cockpit at this point.

The first part is to extend the leads on the servo so that they have enough length to reach through the HS and make it to a connector I will mount in the tail of the fuselage.

For people not familiar with wiring, here’s a brief tutorial… first I figure out how long the wiring need to be to reach from the servo’s mounting position into the fuselage through the HS, I leave it a little longer than I estimate, just in case. Then I cut some of the appropriate size wire for the amount of current that will be on the wire and for the length of the total cable run. For the trim servo it’s not much current on two leads that operate the motor and the other three leads are for position signalling and have even less current so the guage [wire is sized in ‘guage’ or AWG] of the wire doesn’t have to be much. I’ve selected 22 AWG for this application.

Also note that the wire is white… that means that I have to put tags on each lead so I can tell them apart later in the wiring harness. Not much problem now but later on I’ll have hundreds of wires and tracing them out if they were unlabelled would be a major pain. The wire is white because it has insulation sheathing made of Teflon. Ordinary plastic wire isn’t allowed in aircraft since the plastic burns and produces toxic fumes in a fire or a short. Don’t wan’t to add that to any potential in flight emergency.

Next all the wires are stripped to expose the conductor, the and each extension are twisted together neatly. Solder is applied to make a good solid mechanical and electrical connection. After each wire is soldered heat shrink tubing is slipped over and shrunk with the heat gun to insulate the connection. Finally all of the wires are fed into a larger piece of heat shrink tubing to gather them into a neat bundle and they are fed through the grommets in the elevator.

Here is the servo installed in the bottom of the left elevator. One remaining task is to fit a small fairing over the pushrod. Every little bit of drag reduction and streamlining makes the plane go faster and saves fuel!

On the left elevator of the RV7 tail there is a small trim tab. The tab requires a that the builder fold the ends to complete it. The best way to accomplish this is with a sheet metal brake. That’s another one of those nice to have tools that I wish I had in my shop! But the builders manual from Van’s tell you how to do it the cheap hombuilders way.

First thing you make a fixture out of wood, a simple V notch to pin the end of the trim tab in place with the portions to be folded protruding.

Next with the ends to be folded securely pinned in place in the jig the protruding ‘ears’ are bent. They can be hammered, but that makes for a kind of ‘beaten’ over crummy look. I prefer to use a heavy weight to gently pressure them into shape. I have a 4″ diamater by 1 foot long piece of cold rolled steel rod that makes a great, though very heavy folding tool.

Here’s the end result, a neat smooth fold.

And finally I can match drill, and fit the trim tab in place.

I’m almost finished with assemby of the left elevator. It’s a bit more complicated than the right one since the left one has a trim tab and houses the trim servo. The skin is riveted to the skeleton and the final step is to close the leading edge of the elevator. To do that I need to roll the skins so they meet and can be riveted together. Rolling is done by attaching the skin along it’s edge to a dowel (a broom handle works well). You use duct tape along the edge of the skin to attach it to the broom handle, then you use the broom to create a nice rolled bend to the skin.

You can see the rolled edge ready to river together in the above picture. In the next photo you can see the opening in the botom of the elevator where the trim servo will mount and the notch in the trailing edge is where the trim tab will fit. More on this when I start bending the trim tab and fitting the servo.

Now that the empennage is built, I have to add the tips to the rudder, stabilizers and elevators. They are supplied in the kit as rough fiberglass parts that have to be fitted, cut to size and attached.

Below is a picture of the tip drilled into place on the horizontal stabilizer. At the moment it’s simply drilled and held with 3/32 clecos and is open on the back side.

In the following steps I’ll be setting it up to be attached with #6 stainless screws and closing up the open back side for drag reduction and to keep anyting from nesting in there when the plane is hangered. Since the open end doesn’t hold the tip to the shape of the opening we’ll have to create a support to allow it to keep it’s shape when removed. Below you can see a small former that I’ve made with fiberglass and place in the open end to cure after the tip has been cut to fit.

After this cures I’ll remove the tip and add fiberglass to cover the entire opening then sand the tip to have a nice streamlined shape. Below is the tip removed and I’m laying fiberglass cloth with resin into the open end to close it.

The resin I use is a West epoxy resin, it is available with mixing pumps that meter the resin and catalyst in proper ratio for mixing.

After the fiberglass sets it requires a lot of filing and sanding to create a nice smooth finish. Below is one tip in progress after one application of filler and a bit of sanding. Lots of fill and sand operations to go yet until this is finished.

The elevator skeleton is a little different from the skeleton of the horizontal stabilizer (HS) that they attach to. The HS skeleton has ribs that form the edges and an internal structure to support the skin. The skin of the HS is thicker also, so when it is riveted to the skeleton it forms a strong rigid structure. The curvature of the HS to create the airfoil also contributes to it’s strength.
No so the elevator which has a much thinner skin, a skeleton that only forms the perimiter and has a flat surface profile. The elevator’s surface is given rigidity by means of small stiffeners made of angle aluminum that are riveted to the skin.

Here’s a photo of the right elevator skin with the angles cut to the correct shapes and match drilled to the skin.

After cutting to shape and drilling to match the skin the ribs need to be sanded to remove sharp edges and the holes need to be deburred. Cracks start from the little nicks and cuts left by the forming operations. So the next step is to use an abrasive wheel and a deburring tool; seen at the right.

In the last step the ribs are attached to the skin. In the photo the skin has been deburred, dimpled and primed and is ready for attachment of the ribs. The ribs are attached by a method known as backriveting. The rivets are inserted into the skin and taped in place with ordinary celo tape. The skin with the rivets is places upside down onto a smooth metal plate (a backriveting plate) and the rib placed over the rivets. The last step is to use the rivet gun to peen the heads of the exposed rivets.

The trailing edge of the rudder is a somewhat complicated step. The right and left side skins of the rudder are joined using a method called ‘double flush riveting’. This involves pounding a rivet against a back plate to form the head and tail of the rivet flush with the skins on both sides. The big deal here is that the edge of the rudder remains straight within 0.1 inch - anything more and you invite handling issues in flight.

So the edge is glued with a goop called ‘Proseal’ that is also used to seal the fuel tanks against leaks. It is a sticky gooey two part mixture that all builders hate since it sticks to everything and is an unholy mess to clean up. Frequent rubber glove changes minimize the amount you end up spreading around.

Mixing proseal

Here’s a shot of the Proseal mixed and ready to be applied to the trailing edge of the rudder.

Proseal applied and the rudder clecoed to a piece of angle aluminum to keep it straight whilst the Proseal cures.

I spent a lot of time in the garage recently and the two major components of the tail are finished! Having built a previous airplane has really helped to streamline and be much more efficient at building, machining and reading blueprints. On the first plane it took over a month to get these parts finished.

Today I’m skinning the rudder skeleton that means in 2 short weeks of work I am almost finished with the tail kit. Something that took me 4 months on my first plane! I must have learned a few things since then.

The rudder skeleton, the internal structure of the rudder.

The right skin being riveted on.

My faithful co-builder George.

Here’s the tail kit…it’s small enough to be delivered by FedEx, the only kit section that will be. The rest arrive in huge crates at the local truck terminal.

The tail kit is the ‘weed out’ part of the kit, if the builder can learn enough and perservere to create the tail most likely he/she has the skills and determination to complete the plane. It is in small sampling of all the skills required, riveting, filing, cutting, shaping, bending and forming aluminum.
First however is the inventory of all the bits to see that everything is here.