Fabricating the Rudder Control Horn

(...or A Tail of Two Horns and a Teeny-Tiny Cockup!)

Dec 2015: I finished assembling the tail fin and rudder some months ago but didn't get around to fabricating the rudder control horns until now as I wasn't confident I could do a neat job. It turns out I needen't have worried, and I'm really happy (and impressed!) with the final results.

The instructions for building the rudder horn in the original Carbon Dragon Builder Manual are somewhat sparce and leave a great deal up to the imagination:

I get the impression that I may not have been the first CD builder to have wished for better instructions, but happily I came across Clint Brooks and Jonathan Pitt's article "Building the Carbon Dragon Rudder Horn", which makes the multi-step process much clearer. I recommend you read their article before going any further.

Clint and Jonathan's article certainly makes the process a lot clearer and easier with the introduction of the tapered balsa wood fillets, but I didn't like the complexity of the jig for supporting the carbon rovings. Shaping those balsa wood fillets to an exact fit looked like too much work and altogether too fiddly to me! My solution was to simplify the jig and replace the balsa wood fillets with expanding foam for an exact fit.

The first thing I did was to make the appropriate cut-outs in the bottom of the rudder pillar and prepare the space to contain the expanding foam in only the places I needed it, using offcuts of foam board and masking tape.

The space shown in the photo above was filled with expanding polyurethane foam. The bottom of the rudder pillar was blanked off with scrap wood to help shape and constrain the expansion of the foam.

After a little cleaning up I had a solid foam plug filling the bottom of the rudder pillar.

A sharp knife is used to cut out the central bulk of the foam plug leaving just two, shaped fillets. My knife wasn't so sharp and I damaged the foam, so I used a styrene based filler (Isopon) to neaten things up. The two plastic spatulas in the photo above are just there to provide nice flat surfaces to mold the filler. They peel off very easily.

Here you can see the tapered shape of the foam/filler fillets.

Final shape of the foam/filler fillets.

Having created the shaped fillets to support the interior carbon rovings of the rudder horn, I now needed to make a male plug to exactly match this space that will later press the carbon rovings into the desired shape. I did this by covering the surfaces brown packing tape, which the next layer of filler will not stick to.

I cut out a number of foam board off-cuts roughly to shape and glued them together into a single block. I then covered this plug in a generous layer of Isopon filler and pressed it into place.

The Isopon filler is allowed to cure.

The next step involves marking and cutting the end of the rudder pillar at a 10° angle. I did this by making up a paper template and then wrapping this around the tube end and marking the angle. Note: there is a 1/4" (6mm) offset between the plane of the rudder spar and the line of the control horn holes (see later photo.)

The cut end is cleaned up with an orbital sander leaving a nice flat surface that allows the foam plug to be glued to the rudder horn jig in the next step.

Here you can see the foam plug (and brown packing tape) has been removed and is ready to be glued to the rudder horn jig.

Here you can see the 1/4" (6mm) offset. See the drawings for full details.

Next, the rudder horn jig is drawn out on a scrap of melamine covered chipboard. The bolt holes are drilled and countersunk to a depth of 6mm. I used a 16mm diameter spade bit to drill the counter sink holes, into which the excess carbon roving is forced, making a very nicely reinforced 'boss'. After drilling the 16mm x 6mm deep counter sink holes, Imixed up some quick-set Araldite epoxy and daubed this into the holes, making sure to generously coat all the edges of the holes. Gravity and natural surface tension results in the epoxy taking on a nice smooth meniscus between the sides and bottom of the holes. You'll see the effect of this in later photos once the jig is removed from the cured carbon rovings.

Four scraps of 3mm thick foam board are glued to the jig and define the thickness of the two rudder horns. Two scraps of 1/4" MDF are cut, drilled and shaped to form the two 'cauls' called for in Clint and Jonathan's article. They get covered in brown packing tape and are used to squash the carbon rovings into shape between the 3mm thick foam board formers.

Here's how the jig looks with the two cauls in place.

Here the jig is fitted to the rudder pillar and carefully aligned while the central plug is glued to the jig base.

I'm afraid I forgot to photograph the process of laying up the interior and exterior carbon rovings (this time around! - see later) but here's what it looks like at the end of the first stage of this wet lay-up. Half the rovings are wound once around each bolt before being spread out over the central 'plug - the ends of one set of rovings overlapping the ends of the other side. You'll see this more clearly later on. The two bolts are given a coating of vasiline to prevent the epoxy from sticking to them. Note that the rovings that have been splayed out across the outer surface of the rudder pillar have been covered with peel ply to aid bonding the second stage lay-up the following day.

Once the epoxy has cured for around 24hrs, the two bolts are easily extracted with a vice-grips and the jig is carefully removed from the rudder. The triangular horns and the rounded 'bosses' have turned out perfectly, with only very little excess carbon getting squeezed out beyond the 3mm foam board molds. This excess 'flash' is quickly trimmed off and the horns cleaned up with a Dremmel tool.

...resulting in a pretty decent looking first stage lay-up! The eagle-eyed amongst you will have spotted that I forgot to start the lay-up with a layer of peel ply, so this surface needs to receive a little sanding to roughen it up before the second stage lay-up. This is no big deal, especially if the second stage lay-up is done withing 24hrs to give the two layers of epoxy to cross-link.

The second stage lay-up involves re-using the jig to apply two layers of carbon cloth to both the bottom and outsides of the control horns so that all the first stage carbon rovings are fully covered.

I covered the outer layer of carbon with plastic and smoothed it down carefully with a spatula. When peeled off, the result is a nice smooth, shiny surface.

Another few minutes with the Dremmel tool and I have a pretty good looking pair of rudder horns!

Ta daaaa! The finished rudder horn, with the rudder mounted on the tail fin. I was *completely* happy with the result and not a little stoked that it turned out so well, having taken just two evenings to fabricate!

It is at this point that reality made an inconvenient and most unwelcome visit into my world of smug self-satisfaction!

Mounting the tail fin and rudder on the tailboom of the glider I discovered, to my horror, that the bottom edges of my beautiful control horns were hitting the tailboom before the rudder had reached full deflection! The bottom section of the rudder pillar was exactly 5mm too low. But why??? Colourful vituperations filled the air around me like New Year's fireworks while I tried to figure out what I had done wrong. By comparing the original drawings and my actual rudder pillar I discovered that I had made one critical measurment from the wrong side of the bottom 5mm thick rudder pillar bulkhead.

Oh drat and poo, I thought as I reached for my hacksaw. I'd have to start the whole job again.

Happily I'd learned a trick or two the first time round, so the second attempt went much quicker and, if I'm honest, turned out even better than the first!

The bottom of the rudder pillar, after trimming off the horns and the excess 5mm. Note I have maintained the 10° angle.

After removing the first set of rudder horns and trimming the end of the tube, I took the opportunity to re-create the two shaped fillets that support the carbon rovings on either side of the inner surface of the pillar. I had run out of expanding foam so I used solid Isopon filler instead. This actually worked better than the foam!

Much neater and altogether much more better than the first attempt!

Again, the area is built up with 5mm foam board to help shape the central plug I'll make next, and the whole area covered in brown plastic packing tape. This central void is now over filled with Isopon (or any stiff filler) so that the excess will squish out at the edges and ensure a perfect fill.

A piece of scrap plywood, covered in brown plastic packing tape is pressed over the end of the rudder tube so that the excess filler squidges out at the edges and ensure a perfect fill.

Once set, the temporary end plate and central plug can be removed and tidied up, and the packing tape peeled off.

I made up a fresh jig base. Note the 1/4" (6mm) offset between the spar line and the line of the rudder horn. Again I used quick-set Araldite epoxy to coat the 16mm diameter countersunk bolt holes and to create a gentle meniscus curve all around the bottom of each hole. This worked really well.

The central plug is placed back into the base of the rudder pillar and glued to the jig base plate in situ to ensure perfect alignment.

The rudder horn jig base with central plug glued in place.

3mm thick foam board off-cuts are glued in place to shape the wet carbon rovings.

All parts of the jig that will come into contact with wet epoxy are covered in brown plastic packing tape to prevent it sticking. The area marked XXXX is a reminder to me that the wet carbon rovings must all be spread out below this area (towards the rear half of the rudder tube.)

The two 'cauls' are put in place and the whole assembly is offered up to the rudder tube to make sure everything fits perfectly.

The original plans call for four bundles of 6  x 12k carbon tows to make up the two rudder horns. I felt this was not enough and (as I had it handy) I used four bundles of 6 x 24k carbon tows, effectively doubling the amount of carbon in the horns. There was certainly no shortage of carbon in my horns!

On each side, one bundle of 6 x 24k carbon tows is wound once completely around a bolt and then draped and spread out over the central plug, making sure that it stays below the area marked XXXX. The ends of the tows from one side overlaps the ends of the tows on the other side. It is important to spread them out as flat as possible. These are the 'inner' tows.

The 'outer' tows (which will end up spread out over the outer surface of the rudder pillar) are similarly wrapped once around a bolt and then draped carefully to one side.

The cauls are put in place and tightened up as tight as possible with a spanner. This forces the carbon rovings into the space between the 3mm foam formers. The next step is to install this assembly onto the end of the rudder pillar and spread the outer rovings evenly over the outer surface of the pillar.

With the outer rovings spread out over the outside of the tube, they are covered in peel ply and held in place with masking tape. Vertical pressure (to flatten and tighten the inner rovings) is provided by the cloth strap.

24hrs later, the cauls, bolts and jig are removed. This lay-up is much neater than my first attempt!

Here you can see the peel ply finish which leaves the outer surface ready for the second stage lay-up.

Two layers of carbon cloth are placed over the end of the rudder rube and horns, and the jig is re-introduced. Two layers of carbon cloth are also applied to the outside, over the bolts, to cover and protect the outer rovings.

A layer of plastic is placed over the outer carbon cloth and smoothed into place with a flat spatula to ensure the smoothest possible finish.

This time, when I mounted the tail fin and rudder on the tail boom there was no interference. The rudder can move to full deflection left and right!

Proof positive: practice makes perfect! :-)