Horizontal/Vert fin fairing

One of the last "big" jobs I need to finish is the fairing between the Vert fin and the horizontal. There is a natural gap between these two parts and passing through this space is the elevator. A slot will be cut in the fairing to allow for full up travel of the elevator. Because I cannot make a lightweight "insitu" fairing out of foam, I decided to make a full mold. The fairing will be laid up in this tool with one 3.2 oz fiberglass surface ply and one layer of 8 oz graphite.

Tape being applied to exposed plywood

 The project started by covering all of the exposed plywood with aluminum duct tape. We used to call this 500 mph tape at Cessna, we used it to attach external instrumentation wires on the Citation 3. The white strips on the middle of the Vert fin leading edge are adhesive backed 3" wide 60 grit sand paper. I will uses these to contour the floral foam blocks that are attached later. I have used this technique to make contoured sanding blocks for automotive body repair. If the car I am working on was too severly damaged in this area, I would find an umdamaged car and make contoured sanding blocks out of styrofoam.

1 lb/c ft Floral foam blocks

The area where the fairing is to be built is stuffed with floral foam blocks. Notice how the sand paper has some foam dust on it. The blocks are rough cut using a wood saw and a hacksaw blade. The first contour cut will be the side profile of the fairing. Before starting the foam blocking, I used a piece of welding rod and taped it to the horizontal and the leading edge. I used two pieces of tape on each end so the wire would have an established tangency. The fairing needs to smoothly blend between these surfaces.I made a tracing on a piece of poster board and I will use this as a guide when I cut the side profile.

Foam after contouring

After the side profile was shaped, the foam was sanded to a pleasing shape using another block of foam. Once I was satisfied with the shape, I laid up 2 light weight plys of fiberglass and let everything cure overnight. The next day, I started coating the glass with Bondo. I have gotten pretty good working with Bondo. I attribute this to having grown up in the Detroit area. As a 16 year old driving around, every spring was spent repairing rust from all the salt on the road.
The technique for using Bondo is to lay on multiple thin layers. Sanding in between each coat.

multiple layers of Bondo

36 grit paper on an air powered file board is used to rough contour the bondo. As the shape gets closer, finer and finer sandpaper is used. The last grit will be 180. There is no need at this stage to use any finer. A word about materials here is in order, do not rely on the chain stores (Auto Zone...) for your materials. Find a body shop supply business. They will have the professional grade body fillers and sand paper. The price is always less than the inferior commercial products.

Final Bondo Coat

After you are satisfied with surface and it is wave and pit free, block sand the surface with 220. The next step is to coat the surface with professional grade Spot putty. Spot putty is a lacquer based product that dries by evaporation. The thicker the coat, the longer it takes to dry. Spot putty is not used to contour the surface, its used to fill the sanding marks an very slight depressions.

Spot putty applied

I usually let this dry for 24 hours .

Final sanding after application of Spot Putty

This will give you an idea about how much of the spot putty is left after sanding. The spot putty is first sanded with 220 grit, and finish sanded with 320. at this point I taped up and papered the rest of the plane and applied 5 coats of carnuba mold release paste wax.

PVA Poly vinyl acetate

On top of the polished wax, 6 coats of PVA were sprayed on. The coats were allowed to thoroughly dry between applications. Once the surface has been coated, the PVA forms a film that will act as the release surface

Texas AutoClave

Between coats, I rolled the fuselage out into the "Texas Autoclave".  85 degrees F here today, in the middle of October. Texas is a wonderful place to live 3/4 of the year. True Texans however are solar powered and don't care a whit about the heat of summer, in fact they say its a natural Yankee barrier!! They curse the day Airconditining was invented!!!

Gel Coat being applied

Using my Gel coat gun, I sprayed 1/2 a pint of Gel coat. the gel coat is allowed to cure until tacky and then raw resin is applied over the gel coat and pieces of non directional mat is applied. It took about 2 hours to cure enough so I could pull the mold off.

Fairing Mold

Tomorrow I will lay up the fairing.














Fairing in position






 Finished fairing!!! 1.0 ounces.

The fairing will be bonded to the horizontal