Category Archives: Building techniques

Ventus 2c: preparing to “close” the wings

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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.

Ventus 2c: Building the Wings (upper side)

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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).

Ventus 2C: Canopy Frame

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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.

Ventus 2c: preparations for building the wings

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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.

New Vacuum Pump

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With my previous projects I built the wings and stabilisers together with Andi, using his workshop and pump, or borrowed a vacuum pump. As I hope to build quite a few more planes in the future and wanted to have a go at building a glider all by myself, I though it was time to purchase my own vacuum pump. For building the horizontal stabilisers I used a small KNF vacuum pump that I purchased second hand a while ago. It worked well with the smaller vacuum bags, but it will be at it’s limit with the larger bags that building the wings of the Ventus and the Diana 4 will require. Last month I thus ordered a larger pump through Lindinger – it’s also a KNF pump, made in Germany, and distributed through R&G. It took almost a month to be delivered, and seeing the production date on the pump it looks like it was made to order. Am looking forward to trying it out on the wings of the Ventus.

Diana 4 and Ventus 2c: tail sections

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Earlier this week I picked up the rudder for the Diana 4. Fellow builder Georg built three of these for us, using the mould he made for the JS3 rudder. As usual it’s very nicely done and extremely light.

The tail sections of both the Diana 4 and the Ventus 2c are now as good as ready (of course the horizontal stabilisers and the Ventus rudder still need covering and painting):

  • 45 degree angle reinforcement to avoid torsion in the vertical stabiliser (3mm balsa, covered with 49gr glass on both sides);
  • vertical reinforcement to close the vertical stabiliser before the rudder
  • rudders fitted and brass tubes (pull-pull system with kevlar wire) fitted
  • elevator servo installed, including wiring

The pictures of the (transparent) tail section of the Ventus show best what’s where. We always prepare a sheet of 3mm balsa wood with 49gr glass on one side (glass applied in a 45 degree angle). The balsa wood reinforcements are then glued in with thickened epoxy resin and 49gr glass (also in a 45 degree angle). With the Ventus I also applied two thin carbon rovings – that’s because I felt like it and had it lying around, but it isn’t really needed ;-). Together, the two reinforcements give enough stability/torsional stiffness to the vertical stabiliser.

Ventus 2c and Diana 4 – fuselage servos

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It’s often the small things that take up a lot of time. I spent quite a few hours building the frames/setup for the servos in the fuselages of the Ventus 2c and the Diana 4, as well as the bulkheads for the landing gear and the towhook of the Diana 4.

The picture below shows:

  • The two units for the rudders. I will use the usual kevlar wire pull-pull system, using a small pulley that was designed and built within our club ages ago.
  • The servo frames for the elevator servos. These are built into the vertical stabiliser and then connected to elevator using a carbon rod with 2.5mm (Ventus) or 3mm (Diana 4) threaded metal rods glued into the ends.
  • The bulkhead for the towhook servo and the towhook entrance.
  • The landing gear with bulkhead and servo setup.

Diana 4: installing the landing gear

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The Diana 4 will get a retractable landing gear. We’re running low on the stock of landing gear made by our club ages ago and only have a smaller version left. It requires an 89mm wheel rather than the 103mm wheel we used for the JS3 (which uses the same fuselage as the Diana 4). It still looks nice enough though, and the difference will be hardly noticeable. To install the gear I cut out the doors from the fuselage using my dremel with a 0.8mm milling bit. To give the doors a bit of extra stiffness I covered them with a layer of carbon fibre (before cutting them out of the fuselage). The doors are then attached to the hinges, made of steel wires through a brass tube. The landing gear is attached to 4m plywood.

Diana 4: 2nd fuselage

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Two days ago we built the 2nd Diana4 fuselage (mine). Today we released it from the form. It’s the best one we built so far. When we built the first one a few weeks ago we used a new resin for the first time(Hexion EP-Harz L285 LF and Hexion-Härter LH 285 (LF1), both from Suter Kunststoffe AG), one that has a much shorter processing time (50 minutes). The advantage is that the when layering the glass, the first layers are more stable and less likely to shift as you’re building up the layers. The disadvantage is that you really need to mix only small quantities of epoxy (we mix 100+40gr) and must time the mixing right. We quickly learned this when building the first fuselage. For the second fuselage we got it right from the start. The result is a fuselage that has much fewer airpockets (haven’t found any so far) and one that’s also quite a bit lighter (1288 instead of 1428gr).

VT-16 Orlik: installing wing servos

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The resin on the seals of the wing control surfaces of the Orlik finally hardened out enough to sand the seals into shape (it takes 3-4 days to fully cure when you use white colourant and micro-balloons and your workshop is not that warm). Finally I had some time off and a few rainy days:  time to install the wing servos.

We’re using the usual setup for our Scale 1:3.5 gliders: Six control surfaces (3 on each wing), connected with an Integrated Drive System (IDS). We never use airbrakes on modern wing profiles – butterfly is better for landing on the slope and with modern profiles the ability to camber the full wing makes a much more performant glider.

As servos we use the Chocomotion FOX 10/10 and 8/6. We’ve used these servos on all our builds for the past few years and with many flying hours never had one fail on us. New for the Orlik are the IDS aluminium servo arms and the new glass/wood servo frames with ball-bearings kindly provided by Chocofly. The new frames are easier to install than the plastic ones we used earlier, and the aluminium servo arms are a perfect fit with the Chocomotion servos (unlike the plastic ones we used earlier) and also very robust. For the rest I used IDS pieces I still had left over from earlier builds. Rather than building the connectors on the control surfaces within the wing, I’ve placed them externally. The reason for this is that the control surfaces are quite large and I’d like to somewhat reduce the power required by the servos to move them.

Fitting all is a lot of work and careful filing all the openings. It almost took me two full days. After getting all openings and pieces to fit, I first fix all the bits with 5 minute fast-curing epoxy. At the end of the day I add slow-curing epoxy resin thickened with aerosil, to make sure that it all holds. The epoxy will cure overnight.

Next step is finishing the wiring in the wing and programming the plane….

VT-16 Orlik: cutting the wing control surfaces

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There is still a lot of work that needs to be done once a plane comes back from the paintshop. Most frustrating is that after the big “wow” of putting it together, the next steps are barely visible and yet there’s a risk of really messing things up. Probably the scariest thing is cutting out the control surfaces on the wings. If that goes wrong (not a straight line, wrong place), you at least have a very visible mistake and even risk ruining all the work and having to start again.

A crucial step for cutting the control surfaceds is always made during the building of the wings, where we drill small holes in the abachi between the two spars that mark the division between the control surface and the wing. These holes need to be kept open during all subsequent steps, so that you can find them back! With these holes you know that you’ll be cutting in the right place.

For the actual cutting we use two different methods. The first is an adapted Dremel with a 0.8mm mill bit, that’s pulled along an aluminium ruler which is in turn stuck to the wings with bits of double sided tape. I used this method for the Urupema wings. The second is a nifty little gadget with a small motor running on a 2S LiPO battery, running on an aluminium “track” (see picture below), which Andi and I used for the Orlik wings. Both methods require a steady hand and double-checking before you cut. Fortunately Andi is an expert in this and perfectly cut our control surfaces. Using wooden templates we also cut out the openings for the servos.

Following this there’s a lot of work cleaning out the foam. Then we sand back the upper part of the wing so that the gap between control surface and wing is around 2 to 3mm wide. Then we sand the part where the control surface goes under the main wing back so that you have a sharp edge. Finally, we apply 49gr glass inside the main wing so that it’s protected against humidity and slightly reinforced.

VT-16 Orlik: fitting the canopy

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The general view among builders I know is that fitting the canopy is their least favourite (most hated?) task in building a model aircraft. The canopy frame is not too hard to build (see earlier post), but cutting and sanding the canopy to a perfect fit, and then glueing it on is a real pain. It takes a lot of time and patience and very few things are so visible and ruin a build like a badly fitted canopy.

I use a pair of Tamiya curved lexan cutters and a Permagrit sanding block to cut and sand the canopy to size. Important is to mark the center both at the front and rear of the canopy as well as the fuselage to make sure that you always place the canopy in the same position. I do a first a “rough” cut using the Tamiya cutters and then using the Permagrit to sand it into perfect shape (being VERY careful that you don’t slip and scratch the canopy!!). Then it’s try, sand, try, sand…..repeat. It took me a bit over two hours on the VT-16 Orlik to get to an acceptable cut.

I then wax the seat of the canopy frame in the fuselage (three times, using liquid wax, slightly rubbing it with some soft cloth in-between coats). I then prepared epoxy mixed with micro-balloons and aerosil, fairly thick, but thin enough so that I could apply it using a syringe. Using the syringe I applied the epoxy evenly to the canopy frame and gently placed the canopy onto the frame. Using pieces of waste wood with double-sided tape as well as lots of wax tape I made sure that the canopy is in the right position and nice and even with the edge of the fuselage. Now it’s time to let the epoxy cure. Fingers crossed that it will come out well tomorrow….

VT-16 Orlik: Covering Elevator and Rudder

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I’ve used many different methods for covering aircraft in the past. Among my favourites are Oralight and Oratex. Both are relatively easy to apply, with Oratex being very robust and oralight very light. Most recently I used Oralight for my Inside F5J and Oratex for my EcoBoomster Towplane. Both however have as a disadvantage that the surface tension is good but not brilliant, and you regularly need to do another pass with the hot air gun or iron to iron out new wrinkles – especially when you store your planes in an unheated room like I do.

When building my Moswey III, I was shown another method by Georg, the designer in our building team, and an experienced builder. This method not only gives great surface tension (love to drum my fingers on it :-), it’s also very light and, importantly, doesn’t require you to regularly straigthen out wrinkles. For the VT-16 Orlik we decided to use the same method again.

I’m sure that there are many different ways to cover a glider. This is the one we use for the Orlik. Materials to use: SIG KOVERALL covering fabric; Dope (we use Fuller-MZ), UHU Hart glue, nitro thinner and acetone. Method: 1) make a mix of 50% (by weight) UHU Hart and Dope and use a brush to apply one or two coats to all areas that the Koverall tissue need to stick to, allow this to dry; 2) cut Koverall into rough shape; 3) put the Koverall on the area you want to cover and fix the edges (not inner parts!) of the cloth to the wood using a small cloth with acetone (the UHU Hart/Dope mix will dissolve and the Koverall will stick to the wood); 4) use a hot air gun to carefully tighten the Koverall and remove any wrinkles; 5) using the cloth with acetone attach the rest of the Koverall to the inner ribs/surfaces; 6) apply two or three coats of dope, letting it dry out in-between and making sure that the covered surfaces stay straight.

While the Moswey III was hand-painted (to get that Oldie look), the intention is to spray-paint the rudder and elevator of the Orlik. More on that later.

VT-16 Orlik: Canopy frame

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One of the things I like least is building the canopy frame and fitting the canopy. It’s a lot of work and so much can go wrong. The canopy frame is now done and ready to fit the canopy. The frame was built on the fuselage, as follows: 1) prepare the front pin, so that it can be epoxied straight into the frame (3mm steel, bent at the end); 2) carefully wax the fuselage on the area where the frame is built up – I use three coats of wax, slightly polished in-between; 3) apply thickened epoxy resin on the area where the frame is built up, let this cure a few hours; 4) prepare 10 carbon rovings (24K, 1600tex) for the full length of the canopy frame (around the entire frame) and soak with epox; 5) build the frame with the carbon rovings on the thickened epoxy (I apply two rovings at the same time) and let the epoxy cure again for a few hours (so that the rovings don’t “swim away” during the next step); 6) build up the rest of the frame using epoxy thickened with lots of micro balloons (and also a bit of colorant if you wish); 7) let the whole thing cure for 12 hours at least.

Important: after curing, but before releasing the frame from the fuselage, I drill the hole for the canopy lock. The canopy lock I made with a 1.5mm pin, soldered with a brass tube to a 1.5mm steel wire.

After this, it’s lots of sanding the frame to shape, then painting it, before glueing on the canopy. More on that later.

VT-16 Orlik: Fuselage Glass and Carbon Layup

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In my previous post I mentioned that we’re using a new glass and carbon layup for the fuselage. The picture below explains how we built it (in German). Previously we only used glass, but we decided to use carbon on this fuselage as it’s very long and thin, with a short nose, and we want to keep the weight of the tail as low as possible. The new layup is heavily inspired by one of Chocofly’s main builders and designers, who gave some very useful tips on how he does the fuselages.

As the first fuselage came out at 1.4kg, with the center of gravity at the trailing edge of the wing, it looks like we got it right. We need to check if we’re happy with the robustness of the fuselage and might make some small changes for the next one we built, but so far it looks ok.

VT-16 Orlik: first fuselage!

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On Thursday we built the first fuselage for the VT-16 Orlik. The first fuselage out of a new mould is always a bit of an adventure, as you need to get to know the mould and the “difficult spots”. Also, we were experimenting with a new glass and carbon layup (see next post), including using for the first time carbon in the fuselage. Today Georg cracked open the mould and shared pictures of the result. It came out very well! Weight of the empty fuselage is 1.4kg, and the center of balance is just at the trailing edge (which means we got the weight of the tail right). Two more fuselages to build over the next few weeks.

Installing wing servos and IDS

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In all my recent builds I’ve used Chocomotion servos  (supplied by www.leomotion.com) in combination with the IDS system supplied by Servorahmen.de. When deciding to install the IDS system, we’ve found that 1) you need good and strong servos; and 2) at ridiculously high speeds even the strongest servo may give up as the lever from the IDS system is simply too short. In our earlier builds we used Futaba 3174 and KST x08 (aileron) wing servos. Unfortunately I’ve had quite a few serious burn-outs and control surface flutter at speeds over 240kmh on my scratch-built Diana2, even resulting in one Futaba servo catching fire (fortunately the plane is still alive!).

The Chocomotion servos are much stronger than the Futabas (no breakage so far, and the newer versions now also have little or no play). For the Urupema’s flaps and flaperons we use the Chocomotion 10/10, as with our JS3. For the ailerons we’re for the first time using the Chocomotion 8/6 instead of the KST X08. The “normal” X08 is simply not strong enough for the ailerons using IDS in larger planes when you want to fly it fast (I’ve killed way too many of those…an they’re not cheap). The X08plus is much better (no breakage so far), but in view of our good experience with the Chocomotion brand, we decided to try out the Chocomotion 8.0/6.0 instead, as it should be a bit more powerful than the X08plus.

As we have quite large control surfaces on the Urupema, we also decided to replace the standard control horns on the control surfaces with our own, using a bigger lever to reduce stress on the servos. It was quite a bit of work to cut them out of glass board (by hand), but I hope that it will allow the servos to hold at higher speeds.

Setting the seals for the wing control surfaces

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Learning how to make the seals for the wing control surfaces has been a bit of a process for me. We always use a silicon hinge (see two posts earlier), usually at the underside of the wing. This means that a nicely set seal really helps the plane look good, but of course also helps flight characteristics. In the past I’ve had to redo the seals quite a few times when they didn’t come out well, or I had to spend a lot of time sanding them. The Urupema, my third fully scratch-built plane, has the nicest seals I’ve made so far, and I’ve learned again for next time.

What I’ve learned so far: of course the tape is really important. We use a relatively hard PVC tape that is easily removed once the epoxy resin has cured. Also important is cleaning, especially when you have carbon in the wings. It just takes a bit of carbon dust to give you ugly black spots in the nice white seal. The first few seals I set I made the expoxy too thick, which meant that it didn’t distribute nicely the seals looked ugly. I still thicken the epoxy with micro-balloons and a bit of aerosil (and of course apply colorant), but it can stay quite liquid. Use a good syringe to apply it on the tape. Wait for it to cure to a point where it’s still elastic enough to bend, but no longer liquid. Then insert the tape under the wing (I use a ruler) putting the control surface at around a 20-30 degree downward angle. Fix into position and let the epoxy cure. If you apply colorant, you may want to give it some extra time to really harden out (depending on the temperature in your workshop). I usually leave it for two days, then remove the tape and sand the seals down to size (allowing enough upward movement).

For the Urupema I had a lot of work shortening the seals to get enough upward movement in the control surfaces, something to take into account for next time. They’re also quite thick, but then weight isn’t as much of an issue with this plane. I’m pleased with the result.

Urupema: cutting the wing control surfaces and setting the hinges

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For the past few projects I always had one of my friends help with cutting out the control surfaces in the wings and elevator. It’s something that easily goes wrong and then ruins lots of hours of work on the wings. With the Corona lock-down rules, having seen it done a few times and actually having all the required tools in my own workshop I though it was time to do it myself. And it went surprisingly well, without any major hiccups, and was easier than I feared – although it does need a steady hand! I used an ancient Dremel with a 0.8mm bit for the underside of the wings (hinge) and a 1.5mm bit for the upper side of the wings. One of my colleagues in our club made a very clever add-on for the Dremel that allows you to guide it along an aluminium ruler. The rules is positioned and glued to the wing with bits of double sided tape. My mate Georg also very helpfully provided a template for cutting out the servo openings, which made it easy to do those.

After cleaning out the foam, I sanded down the upper leading edge of the wing, so it’s razor sharp, and applied a strip of 60gr glass to protect the exposed abachi wood and give the edge a bit more rigidity. Once that was hardened out I set the hinges using silicon glue. This will now need to rest for approximately 5 days, after which I can start doing the seal on the upper side of the wings.