Having completed the basic build of the wings I first spent a morning sanding down the ebechi and getting the leading edge into the right shape. A lot of very dusty work, best done outside on a sunny day. After that it was time to fit them to the fuselage. Lots of measuring and trying, before glueing the 6mm aluminium pins into place. To ensure a perfect fit with the fuselage I then first closed the end of the wings with a bit of carbon and with a bit of epoxy with lots of microballoons filled any remaining gaps between the wings and the fuselage. Following that I completed the wingtips, in two steps, using small bits of balsa wood, glued on using thickened epoxy resin. Following all that was another coat of transparent primer to ensure that the ebechi doesn’t absorb too much epoxy.

Then it was time to cover the wings in glass. We use 49gr glass, applied at a 45 degree angle to improve the torsional stiffness of the wings. We apply it using a paint roller and an old anti-stick frying pan. The epoxy is mixed with 30% methanol, so it gets really watery. First the underside, then let the epoxy cure (with the wings hanging leading edge up, to avoid deformation). Then the upper side, again letting the epoxy cure with the leading edge up.

We build our wings “upside down”, i.e. the top side first. That’s the easy bit. The underside is much more work. First comes all the measuring and drawing out the position of the main spar, wiring channel and the spars at both sides of the hinge of the control surfaces. I mark the right position of each on packing tape, doing both wings at the same time and regularly cross-checking to make sure that all is in the correct place.

Then comes cutting out the foam for the three spars. Foam is a killer for blades. As we re-use the foam cut out of the two rear spars (carefully “dug out” using a sharpened screwdriver), it’s important that these cuts are clean. The same goes for the cutout of the main spar – too big or too messy and the calculations for the amount of carbon rovings no longer work, or the main spar may end up positioned slightly skewed. To make sure all is cut straight we use a thick board that is placed horizontally using a small inclinometer and cut along the edges of the thick board. I also cut out a bit of foam at the leading edge, so that I can fill it with micro-balloons in resin (easier to sand the leading edge into shape). Make sure that all surfaces that were glued before are carefully sanded and cleaned of dust so that the next layer of epoxy resin sticks. Using thick plywood I also prepared to bits that will be glued into the root of the wing and into which the two 6mm aluminium pins will be glued to attach the wing to the fuselage.

I glued together the foam cutouts of the two rear spars using 5 minute epoxy and pulled over a carbon sleeve. The core of the main spar is made out of Rohacell – a dreadfully expensive material, but very easy to sand into the right size. The size for the Rohacell core of the main spar can easily be calculated using the really cool excel sheet by Christian Baron (link to the 2013 version, example filled out for an ASK18). In this case I used the layout that was used for the Ventus 2c built by Georg over 20 years ago – with a few minor modifications. The spar will be much more robust than needed, but since I plan to mostly use this plane on the slope, the extra stiffness is welcome. I’ll have 2×20 1600k carbon rovings at the root, reducing by one roving every 10cm, finishing with 2×2 rovings at the start of the penultimate wingsegment. The bits of the main spar are also glued together using 5 minute epoxy and then covered with a carbon sleeve.

The channel for the wiring was cut out using a small soldering iron with an attachment made by our building team mate Richi.

Once all the bits for closing the underside were prepared it was time to prepare the workshop, including setting up the tool for adding resin to the carbon rovings. All is now set, I hope to close the first underside in the next few days.

The upper sides of both Ventus wings are now done. This is the “easy” bit of the wing building. Before starting with the epoxy I first prepared the foam shells and cores. Using a brush on the vacuum cleaner I cleared the “angel hair” remains of cutting the foam. The parts of the upper shell are taped to the building board (which has carefully measured markings for the right position of all parts) and glued together with UHU POR. The leading and rear edge of the shells, as well as the area underneath the main spar, are covered with packing tape (make sure you find a version that doesn’t stick to epoxy). I also applied a few bits of double-sided tape to ensure that the ebechi stays into place.

The ebechi is painted with a primer to avoid it from absorbing too much epoxy. After the primer has cured, the side where the carbon and foam are applied to is sanded and cleaned.

I then prepared 45gr of epoxy resin (make sure you measure and note this down for the other wing shells – 35-40gr would have been ok as well). Using a small soft roller I applied the epoxy to the parts of the ebechi to be covered with carbon. Then I put the carbon in place and again applied epoxy to the carbon using the roller. After letting it rest for a bit I then used kitchen paper and a hard roller to remove excess epoxy resin from the carbon. I then added a bit of foaming agent to the remaining epoxy and applied that epoxy to the remaining areas of the ebechi wood (and also parts of the carbon).

I then put the ebechi with carbon into the foam shell (sticking it to the shell using the bits of double sided tape), after which I added the wing cores (make sure you position them carefully) and the top of the shell. I added some foam bits to the corners of the wing (to avoid them being pressed down too hard in the vacuum) and insert the whole board with wing into the vacuum bag. Using the vacuum cleaner I created a vacuum, and then attached my new vacuum pump. I stayed with the pump for a while to make sure it stabilises the pressure at -0.15bar and then I left it to run for around 12hrs (making sure that the room temperature is around 21 degrees). I use a timer to turn off the pump after 12 hours (usually late in the evening), leaving the wing in the bag until the next day.

After removing the board and wing from the vacuum bag I used a sanding block to remove any bits of ebechi or carbon sticking out (the carbon pins can be really nasty).

Building the canopy frame is always a bit of a pain. Here’s the method we use. First applying loads of tape to protect the fuselage in the parts around the frame. Then apply three layers of wax (leaving each layer to dry and polishing it carefully with some soft cloth). Then apply some thickened and coloured epoxy resin (with a “fast” hardener) and wait for the epoxy to cure a bit. Then I applied a layer of 100gr glass fiber, followed by four carbon rovings (entire length). I again waited for the epoxy to cure a bit. And then the “construction work” begins. Using coloured epoxy, thickened with lots of microballoons (so it stays in shape when applied) I built up the canopy frame on top of the carbon rovings/glass base. The result usually looks quite messy, but that doesn’t matter. Just make sure you apply enough material, so that you can sand away enough to get the frame into shape. I usually wait a few days for the epoxy to fully cure before sanding the frame into shape. Before sanding, make sure you drill the holes for the front pin and the rear lock. I always sand the inner part of the frame before releasing it from the fuselage. I use 2mm steel wire for both the front pin and the lock.

With the fuselages and tail sections of both the Diana 4 and Ventus 2c well advanced, it’s time to start on the wings. I will do the wings of the Ventus 2c first. All previous wings I built with my mate Andi. These ones I will be doing by myself.

The first step is to mount the cut styrofoam parts to the building board (which takes a lot of measuring), clean them and glue together the upper part, prepare the obechi wood, draw out the layout of the carbon on the obechi (which takes even more measuring) and cut the carbon for all four surfaces.

We always build our wings “upside down”. I will first be doing the two upper parts, and then I will add the spars, in the same step as closing the lower part of the wing. All in all four “vacuum sessions”.

Each wing will be built in one piece. Once the wing is sanded and glassed, I will cut off the two outer segments and glue them back at the right angle.

A few details on how we build our wings:

• We build the “traditional” way, around a foam core, cut with a hot wire. It allows for a light wing, perfect for the type of flying we do. The foam we use is from Schurg Modellbau in Germany. Our club purchased a truckload of the stuff decades ago, but it’s now running out, so we’ll be looking for a fresh supply – ideas for suppliers in Switzerland are more than welcome!
• The foam cores are glued onto 0.6mm obechi (abachi) wood. A good supplier in Switzerland for this is RIK Modellbau in Mosnang.
• We use carbon cloth to reinforce the wings at the front (carbon D-box) and rear (control surfaces), as well as around the wing joiner and underneath the servos. We’ve experimented with different types of carbon cloth over the years. Our favourite is 100gr Carbon Biax supplied by swiss-composite.ch. It’s good value, fibres are at a 45 degree angle, and it’s easy to cut and use.