I’ve had requests on tools and materials we use to build our wings, as well as a more detailed description on how we build them. I’ll be doing a detailed description on how to build the wings for my “how to” section on this website in the next few months. Here’s already the overview of tools and materials.
Foam cores: we’re using foam boards from Schurg (https://schurg.de/), approximate size: 100x50x6cm
UHU Por to glue together the foam shells (upper side)
5 minute epoxy (I like UHU Schnellfest) to glue together bits of the wing spars
A good thin brown packing tape to apply to the edges of the foam moulds to prevent the ebechi being glued to these by excess epoxy resin. Note that not all brown packing tapes are equally good. We use the cheap one from our local “Landi”
Vacuum Pump. I use two different pumps, a small and a bigger one. The smaller pump is very good, but when you build the wings the extra power of the larger version gives you a lot of extra security in case of minor leaks.
Vacuum bag, tube, connector and seal: all these bits can be found through R&G. I close the vacuum bag using a U-profile with a fitting tube. Press the bag into the u-profile and then press the tube into the U-profile to close it.
Today I bagged the 2nd upper side of the Diana4 wings, having done the first one earlier this week. Bagging the wing is not all that much work – less than an hour. It’s the preparations that take most time.
After preparing the foam cores (see previous post) comes the incredibly important work of drawing the layout of the wings onto the building board and positioning the foam shells. We build upside down (upper side first). Once all the measurements are done and double-checked I position the foam shells and glue them to the building board using painter’s tape. Then I apply thin brown packaging tape on the edges of the shell on the board as well as to the individual shells of the underside of the wing. It’s important to find a packaging tape that doesn’t stick to epoxy resin – the purpose is to avoid the shells sticking to the wing core or other shells.
Once the board and shells are prepared I prepare the ebechi (abachi) wood. We use 0.6mm ebechi, prepared by a carpenter friend or purchased through RIK Modellbau. This ebechi is cut to size using the plan of the wings – important is to make the ebechi for the upper side approximately 4mm longer (depending on wing profile). I then draw the layout of the carbon inserts onto each of the four ebechi sheets (so I know where to position the carbon when glueing it to the ebechi). We rarely do a full carbon layup – that’s not necessary and would only add weight to the wings. The carbon inserts are just big enough to cover the carbon D-box at the leading edge of the wing, the control surfaces at the back of the wing as well as the positions for the servos and the wing joiner. Once the layout of the carbon inserts is drawn onto all four sheets I carefully sand the inner side (where the epoxy is to be applied) of each sheet and clean off the dust. I then apply a transparent primer (“Hartgrund”), 50% diluted, to avoid the epoxy completely soaking the ebechi. Allow this to dry out and another quick sanding and the ebechi is ready to be used.
Once all that is done it’s time to prepare the resin. I used around 60 grams of resin for the upper side. It is possible to do it with 10-15 grams less for weight watchers. Using a small roller I apply the resin to the parts of the ebechi sheet where the carbon layup is coming. I then put the carbon layup onto the ebechi and push it on using the roller. Then I use the roller to apply most of the resin to the carbon layup. I leave a few grams of resin and add a few drops of foaming agent and let this rest a bit. I then use a hard rubber roller and kitchen paper to roll excess resin off the carbon layup. Then use the soft roller to apply the remaining resin with foaming agent to the parts of ebechi not covered with carbon.
Then put the ebechi with carbon layup onto the shells. I use six or so small bits of double sided tape on the shells to make sure that the ebechi doesn’t shift. Then put the wing cores onto the ebechi, fix the outer ends in the right position with a bit of wax tape and fix the cores together with a bit of the same tape. Then close the upper shells, lightly fixing them into place with painter’s tape. Then bag the entire building board, use a vacuum cleaner to remove most of the air of the vacuum bag and then start up the vacuum pump. Stabilise the vacuum pump at -1.5 bar and let it run for 12 hours. I use a cheap webcam to watch the pressure of the pump so that I can go do something else and occasionally check the webcam on my smartphone. The entire setup runs over a timer that automatically switches off after 12 hours.
When I was cleaning up after bagging the 2nd wing I noticed the appropriate newspaper article used to protect my workbench (see last picture :-)).
The summer weather is still not showing signs of coming to an end and I’m trying to squeeze out as many hours of flying as possible. Shorter daylight hours however provides an opportunity to use mornings and evenings to start preparing for the coming winter’s projects. l’m expecting a nice Chocofly surprise in the next few weeks/months that will have priority, but shouldn’t be too much work. The main project for coming winter will however be the Diana4, which I’m hoping to maiden in Spring 2024. Work on the Diana4 already started last year. The fuselage is as good as done (just need to fit the canopy): I also finished the elevator and rudder, built the wing joiner and prepared the sleeves.
The next step is building the wings. We cut the styrofoam cores for the wings at the end of last year. The original Diana4 will have a wingspan of 18m, that means that our scale 1:3.5 will have a wingspan of 5.14m. As my car is just large enough to accomodate 2.5m wings, I will build them in one piece.That makes the wings easier to build and, importantly, saves weight.
Today I started preparing the building board. I’m normally using an 18mm MDF board of 260x40cm to build the wings. For the Diana4 I have extended the board with another 30cm. I’ll also need to extend my working table to accomodate the longer building board (I’m using IKEA tabletops, which are light, incredibly stable and perfectly straigth). After preparing the board and table I started preparing the foam cores. Using a vacuumcleaner with a brush I removed the “angle hair” on the cores, resulting from the hot wire cutting. I also removed the last 4-5mm of the trailing edge of the wing cores to ensure that the trailing edge of the wings is nicely sticking together and can be sanded really thin.
Next step is to draw out the wing layout on the table and prepare the foam shells.
For the Ventus2c I will for the first time be using the new Chocofly LDS PRO system. The wood/carbon servo frames, as well as the perfectly fitting aluminium servo horns I already used for the Orlik. The carbon control horns and the aluminium/steel/brass connectors are new. Although heavier than the system I previously used, it’s also much more robust. Most importantly, it’s easier to install as you can shorten the 3mm threaded rod to the right length and slightly adjust it to the perfect centering position once the servos are installed.
Another important change compared to previous builds is that I prepared all the openings in the wing and wing control surfaces before setting the hinges and seals. In the past I’ve ended up damaging the seals or even the wings when making those openings afterwards. Doing it before makes the process much less risky and results in a cleaner build.
I’m finally starting to get the hang of doing the seals for the wing control surfaces. In the past I tended to make the epoxy resin mix too thick, which resulted in uneven seals. The trick is to leave the epoxy resin mix relatively liquid – it should be thin enough to spread out evenly, but no so thin that it flows over the tape. Here’s how I do it:
make sure you clean the gap between the wing control surfaces and the wing – any dirt, especially carbon dust, risks leaving black dots in your seals;
apply a thick PET Tape (see for instance here – with thanks to Martin E.) to the control surface, leaving just enough space so that when the control surface is at around 20-30 degrees down deflection the seal is nicely underneath the overlap on the side of the wing;
position the wing in a position that you can apply the epoxy resin to the tape so that it flows a bit more towards the side of the control surface;
prepare epoxy resin, thickened with a bit of aerosil and lots of micro-balloons as well as a bit of colourant (I use white) – the resin should be thin enough to spread out evenly, but no so thin that it will run off the tape;
apply the resin to the tape using a syringe – I also use a thin pin or metal stick to make sure it spreads out evenly;
wait until the resin is cured enough so that it no longer runs off, but still soft enough so that you can mould it;
move each control surface upwards and carefully slide the PVC tape under the overlap on the side of the wing – I use a long ruler for this. The control surfaces should be in a down deflection of around 20-30 degrees, the tape will round itself and ensure a nice and even round seal;
allow the resin to fully cure;
once the resin is fully cured, sand it back to the correct depth so that you have the deflection that you require on each rudder, leaving enough so that there is no gap between the seal and the wing for normal downward deflection (of course this doesn’t apply to the downward deflection of the brake flaps in butterfly mode)
Below are some pictures that hopefully clarify the above.
My mate Andi did a great job spray painting the Ventus, with an excellent finish. I could have left it at that (as we did with the Orlik), but a polish makes it just that bit nicer. Polishing the paintwork is a lot of work. It basically took me almost two half-days and a day of sore muscles in-between. But the result is pretty neat – a wonderful glossy finish.
To polish the plane we first sand it (wet, by hand) with either 800 grit paper or pads (some of us first use 600 grit) to remove the “orange skin” surface and make the paint perfectly even. Then another go at it using a 1500 grit pad and then a 300 grit diamond finish pad (also wet, by hand). Once that’s done the shine is already pretty nice. To finish it we use a machine to apply fast-cut compound and finish off with a machine polish. The products we use are shown on the pictures below (all 3M).
While waiting for the Ventus 2c to come back from the paint shop I’ve been working on the Diana 4 and the Monerai. For the Diana 4 I started work on the canopy frame. First I applied good quality painters tape to protect the fuselage from resin spillage. Then I applied two coats of liquid wax on the area where the canopy frame is built. Then came a coat of thickened epoxy with grey colourant. Allow some time for this to cure until it’s less liquid but still sticky. Then came a layer of 49gr glass, followed by a five carbon rovings going around the entire canopy (staggered so that they end/start in different spots). Then I built up the rest of the frame using epoxy with grey colourant and very much thickened using micro-balloons. It’s not sitting in the workshop to fully cure (needs a few days) before I can sand it into shape.
While waiting for the canopy frame to fully cure I also built the seat insert for the Monerai, using an old mould for the SB-14. The procedure here is very similar to making the canopy frame. The mould is waxed. Then apply slighly thickened epoxy with grey colourant. Allow to cure a while, but it still must be sticky. Then apply two layers of 160gr glass, also with epoxy with grey colourant.
As you can see in the pictures below I didn’t wait long enough to apply the glass and had a few nasty air pockets in the folds of the seat pan. These can be easily corrected with surplus epoxy thickened with micro-balloons. Allow it to cure, sand to shape and it’s good to go.
I’m only planning to finish the Monerai next winter and hope to maiden it in Spring 2024. Richi, our shaper, however already finished the mould for the Monerai’s fuselage pod and built the first pod. He also kindly waxed and spray painted the mould for me, and pre-cut the glass, so that I could easily build the fuselage pod in the mould myself.
I’ve built fourteen fuselages together with our “master builder” Georg, but never did one all by myself. A small fuselage pod for the Monerai was thus a great opportunity to try it out. Even with such a small fuselage it took me over three hours (even with three instead of the usual four layers of glass), not including preparations and cleaning up. I also made a few mistakes along the way, but fortunately realised and corrected them on time.
After leaving the pod to cure for three days I released it from the form. The result was much better than expected – just a small airpocket in the nose (where FES spinner will be anyway), the rest looks very good. I’m pretty pleased.
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.
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.
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.
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.
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 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.
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).
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….
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.