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

Part 1: design, foam core and upper side

Tailplane Airfoil and Shape

Most of our gliders use the HN477S airfoil for the tailplane. It’s an airfoil that has proven to be a good fit on our JS1 and JS3 and that we’re also using for our new Embraer EMB400 Urupema.

The shape of the JS3 Tailplane has four sections on each side. The three inner sections are based on a foam core, the outermost section is two layers of balsa wood with a sheet of carbon epoxied in between. Although the airfoil is symmetric, the tailplane isn’t. As we make the hinge of the elevator at the top side, this side needs to be straight. The hinge would not work if the top side of the tailplane would be curved. See the picture below (front/rear view of the aircraft with the tailplane on top).

For a 1:3.5 scale glider you will need to enlarge the size of the tailplane compared to the original glider. We usually enlarge it by around 10-15%. The picture below contains the layout and sizes of the tailplane we built for our JS3. The inner section is 149mm, the second section is 109mm, the third section is 69mm, and the final balsa edge is 22mm.

Profile and Cutting Templates

For building the JS3 fuselage plug, you will need a template of the HN477S airfoil of the JS3 tailplane. You can download the PDF of this airfoil here. If you print it out in full scale it can be used to produce the necessary template(s) to build the fuselage plug. For building the plug it’s important to note that we usually set angle of incidence between the tailplane and the wing at 0.8 to 1 degrees. In combination with a center of gravity set as far back as possible this makes for a fast and agile plane with excellent flight characteristics, including in thermals.

For cutting the styrofoam cores, you will need the cutting templates. You can download the PDF of our cutting templates here. If you print them out in full scale then you can use them to produce the cutting templates for the three sections of the elevator. We cut these templates by hand out of plywood. Note that these cutting templates are based on using 0.6mm Abachi, which is then sanded down to ensure that the elevator is nice and even, after which we cover the elevator in glass, filler and then spray paint it (see below).

Materials

A list of the main materials we use for tailplane:

– Carbon fiber cloth, as light as possible, 60-90 gr./m2, laid out in +/-45° angle (note: we usually only use this for the part of the elevator);

– Carbon roving for the wingspar, we only use one on top and one on the bottom, with balsa wood in between, 24K roving, wingspar is around 1/3 of the width of the tailplane;

– Glass or Aramid cloth, around 100gr/m2 for the area where the tailplane joins the tailfin;

– a very light braided carbon tube for the area where the rudder joins the tailplane.

Cutting the foam cores

Using the templates, we cut the foam cores. Note that we always build our tailplanes and wings “upside down”, i.e. with the top side below, and starting the building with the top side. Also note that we always use the rear edge of the tailplane/wing as the reference for all our measurements (i.e. avoid any excessive sanding during the build!). This is important for how we cut the foam cores.

The foam cores should of course be thick enough to allow us to cut all six (2×3) sections. The size of the foam cores is determined by the lenght of the section (See above) and the depth of the airfoil of that section. Note that we use on both sides of the airfoil a 1cm surplus, to facilitate the entry and exit of the hot wire (so the size of the styro block is the space between the two red stripes in the picture below).

We cut the top side of the airfoil first, and then the underside of the airfoil. It’s helpful to fix the template to the foam core with two bits of tape. Important is to put a mark on the entire foam block so that you can ensure that the core remains centered as you cut both the top and bottom of the airfoil. Also important is to mark the rear of the airfoil/profile (green stripe in the picture below). And don’t forget to mark which side of the foam block is the top and which is the bottom (as the cut is not symmetrical). See the pictures below.

 

After we cut both the top and bottom of the airfoil into the foam, we cut off the last bit of the foam block at the end of the airfoil to ensure that the rear edge is precise enough to allow us to do all our measurements from the back of the foam block. And of course be careful to ensure that you mirror the two sides of the elevator. We usually cut the foam with two persons and count out the marks on the rib to ensure that we cut in parallel and have minimal distortions of our profile.

Building the top side of the tailplane

The next step is to remove the foam cores from the foam blocks. Clean them of any “debris” from the cutting. Then glue the six “top section” negatives together with any glue that doesn’t melt your foam and fix the six sections with tape in the correct position onto a building board. Apply brown packaging tape to the front, rear and ends, as well as over the seams of the foam negatives. See the picture below (picture is from building the wings, but it’s the same method). Also apply brown packaging tape to the edges of the negative of the upper part of the tailplane. The purpose of this packaging tape is to stop the resin from gluing your wing cores to the foam negatives. Note that not all brown packaging tape is good for this – test which one the resin doesn’t stick to. Important also is to apply a few small bits of double sided tape to  the negative (on the brown tape at the end of the airfoil). We use this double sided tape to ensure that the abachi stays in exactly the correct position on the negative as we put the entire building board into the vacuum bag later on.

Once the foam is prepared, you need to cut the abachi wood to the correct size. As mentioned above, we use 0.6mm thick abachi wood, which we have one of our local carpenters prepare for us. While the front (leading edge) doesn’t need to be that precise, it’s important that the rear (trailing edge) fits the negative exactly. You can also impregnate the abachi with a thin transparent base coat to avoid it absorbing too much epoxy during the build.

In addition to preparing the abachi, you also need to prepare the carbon sheets. What and how much carbon you use depends on what’s available, and how heavy (or light) you want to make your elevator. Remember that every gram extra weight at the rear needs to be compensated factor 3-5 at the front of the aircraft. We usually only apply carbon to the part of the elevator. You can also apply it for the area between the front of the airfoil to the main spar if you want a carbon D-box for your elevator. For the center part (where the tailplane is connected to the tailfin) we use 100gr/m2 glass or aramid cloth. See the picture below (picture is from building the wings, with carbon at the locations in the center where we put the servos, but it’s the same method).

Next step is to prepare the vacuum bag and a vacuum pump.

After that prepare the epoxy resin (we use a 120min hardener) and apply sparingly (using a spatula) to the abachi, place the carbon sheets and again apply epoxy resin sparingly (using a spatula). Once the sheets are impregnated with resin you can put a few layers of absorbing kitchen paper on top and using a roller have it absorb any excess epoxy.

Place the abachi with carbon into the negative (make sure that it sticks to the bits of double sided tape so that it stays in the correct position). Then put the tailplane cores onto the abachi and finally cover it with the negatives (from the lower side of the tailplane). Use tape to keep the whole thing in place. Add a few waste foam blocks at the two ends of the tailplane to avoid the ends being pushed down by the vacuum and also fix them with tape.

Then place the entire board into the vacuum bag, start the pump and make sure that the vacuum remains stable at maximum -0.15 bar. Leave it under vacuum to harden out. That’s the first part done!