This “How To” describes how we build the JS3 (1:3.5) tailplane

Part 2: spars and underside

Once the top side of the tailplane is done (see Part 1), we can start on the spars and the underside.

First we make the following cut outs from the foam core:

• leading edge (3-4mm)(we fill this with epoxy thickened with microballoons to allow us to sand down the leading edge to the shape of the airfoil later)
• location of the hinge (important: remove this bit very carefully, as we will need it later on, we usually make this around 8mm wide)
• spar
• center/position of the screws to fix the tailplane to the tailfin

See the picture below for some of the tools that we use to ensure that the cuts are straight. Applying tape and drawing the cuts on the tape before cutting helps keeping the foam in place. It’s essential that all these cutouts are in the correct position to match the elevator hinge and location of the screws to fix the tailplane to the tailfin.

Once the above sections are cut out it should look like the picture below (picture is from our SB-14, but the JS3 is similar).

Important: sand all the cutouts to make sure that the next layer of epoxy holds!

Now prepare the material to fill up these cutouts as follows:

• leading edge (3-4mm): nothing needed at this stage
• hinge: use a light carbon braided tube to cover the bit you cut out. Getting the foam into the carbon sleeve can be tricky.
• spar: you can use different materials. For the tailplane we usually use two 12K carbon filaments (one on the top and one on the bottom of the tailplane) and a piece of balsa wood in the middle. You can also use a carbon sleeve over the foam cut out, or a carbon rod (filling the remaining space with balsa or epoxy resin thickened with micro balloons).
• center/position of the screws to fix the tailplane to the tailfin: also here you can use different materials, we usually use plywood blocks cut to shape to fit the cutouts.

Prepare the abachi and carbon sheets in the same manner as the top side (see Part 1).

Prepare your vacuum bag and vacuum pump for use.

Now prepare the epoxy resin. You will need:

• Normal epoxy resin, unthickened (we use 120mins hardener)
• Epoxy resin mixed with microballoons (thick enough so that you can still apply it using a syringe)

Using a small paint brush apply a small amount of epoxy to all the cutouts (except the leading edge) and, importantly, also the trailing edge.

Using a paint brush, apply resin to the materials to fill all the cutouts (spar, hinge, screw base) and put these materials into position.

Apply resin to the abachi and carbon sheets (see Part 1).

Using a syringe, fill the cutout for the leading edge with epoxy resin with plenty of microballoons. This allows us to sand the leading edge into shape once the tailfin is finished. Make sure to use enough material amount to avoid pockets of air and have a nice leading edge later on.

Place the abachi with carbon sheets onto the tailfin core. Fix the abachi sheet into position using a few small bits of tape. Then apply the underside negatives, some leftover blocks of foam on both ends of the tailfin (to avoid the vacuum pressing the ends downs), fix with tape and put the whole package including building board into the vacuum bag. Start the pump and wait for the resin to harden out.

Once the resin has hardened out, remove the tailplane from the styro negatives. That’s the “rough” core done!

Sanding

Sand the tailplane into shape. Use 60 or 80 grain to sand down the surface and finer to finish sanding. Be very careful with the trailing edge. This should be razor thin, but only sand top and bottom, not the rear (as you are using the rear to measure the distance to the location of the hinge later!). Also sand the leading edge into shape to match the profile of the tailfin. Then epoxy the balsa/carbon edges to both ends of the tailfin, and sand into shape once the epoxy has hardened out. We also applied a thin piece of plywood to further reinforce the place where it will be attached to the tailfin. The result should look like the picture below. Drill out the holes for the screws to attach the tailplane to the tailfin. Apply epoxy to the holes to ensure that no humidity enters the tailplane and swells the wood.

Covering with glass

If you’re content with a foil covering you can skip the next steps and iron on the foil instead. We always cover ours with glass fiber. Cut out a thin sheet of glass fiber to size and apply thinned down resin with a bit of white colourant using a paint roller to one side. Wait to harden out and do the same with the other side. Once the both sides have hardened out, lightly sand the whole tailplane, ensuring in particular that the leading edge is in the right shape. Apply a filler (using a spatula) to cover up the structure of the glass. Wait for it to harden out and sand the whole tailplane with 300/400 paper (wet). Now your tailplane is ready to be spray painted. After painting we polish it to a shine.

Cutting out the elevator

We make the hinge for the elevator at the upper side of the tailplane. Mark the middle of the cutout of the hinge. Attach a ruler with double sided tape to the tailplane. Cut out the hinge using a dremel (we made a special setup that helps us guide the dremel along the ruler – see the picture below). For the top (hinge) side we use a 1mm cutting bit. For the underside we use a 1.5 or 2mm cutting bit. Make sure that your setup only cuts through the paint/glass/abachi/carbon, and not all the way through to the other side. The pictures below show the JS3 and AvantiHawk tailplanes. Don’t forget to mark what part of your elevator is up and what part is down before you detach it.

Making the hinge

We use silicon to make our hinges. If applied correctly and left long enough to harden out it works very well.

Using tape and a 1mm thick piece of wood, tape the elevator back to the tailplane, in the correct position, leaving a 1mm gap between the elevator and tailplane where we will apply the silicon. Make sure that you can fold the elevator over onto the tailplane so that you can easily apply the silicon.

Prepare a few small pieces of wood with double sided tape on them (to be used to fix the elevator to the tailpane so that the silicon hardens out with the elevator in the right position).

Fold over the elevator and using a syringe fill the 1mm gap with silicon, taking care not to leave any pockets of air. Fold back the elevator a couple of times to evenly distribute the silicon and then fix the elevator in the correct neutral position to the tailplane using the small pieces of wood with double sided tape on them.

Silicon needs enough time to harden out. We usually leave it for a couple of days (even up to a week!) before we very carefully (diagonally) remove the tape. You should now have a fully functioning tailplane and elevator. You may need to sand back a bit the lower part of the elevator to ensure that you have enough space for it to move downwards (the 2mm gap may not be enough). We also usually apply tape to the lower side of the elevator to cover up the gap between the elevator and tailplane. Alternatively, you can make a seal using epoxy with micro balloons.

Done!