Category Archives: Building techniques

Diana 4: fitting the canopy

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In-between working on the Elfe S3 I’m also still working to get my Diana 4 ready. It has been sitting in my workshop for a while, waiting for me to finish it.

Fitting the canopy frame is always a bit of a pain and I usually leave it to a day where I feel like having a go at it.With a self-built canopy frame and canopy there are no real shortcuts. It’s a job that requires a lot of time and patience.

Essential is that the canopy frame is sanded back so that the canopy nicely fits onto it and is level (or just a bit inwards) from the fuselage. The worst looking canopies are those that stand outside the fuselage.

Once the frame is sufficiently sanded back, it’s time to cut the canopy to size. I usually do a rough large cut and then try it on, marking it with a marker pen and further cutting it back (I have a nice Tamiya canopy scissors that does a great job). Once you more or less have the right shape, it’s time to make the perfect fit. Before you do so, make sure that you mark the center line front and rear of the canopy so that you always put it in the exact same place. I then use a permagrit sanding block (the really coarse one) to slowly sand back the edges of the canopy. And then just try, sand, try, sand, etc…. It usually takes me at least a few hours to get it right. Once you’re satisfied with the fit I use a fine grit sandpaper to smoothen the edges of the canopy.

Before glueing the canopy to the frame I first make sure that the fuselage is nicely waxed so that the canopy doesn’t glue to the fuselage, but just to the frame (epoxy always has a way of “escaping”).

Not it’s time to glue it on. I use normal epoxy resin, coloured grey (same colour as my canopy frame) and thickened using both micro-balloons and aerosil. Using a pencil I apply it evenly around the frame and place the frame on the fuselage. This is probably the hardest part – too much and you have a mess with epoxy coming out when you place the canopy, too little and you have gaps between the frame and the canopy.

I then carefully position the canopy onto the frame. Make sure you remove any excess epoxy – if there’s some on the edge of the canopy then use some alcohol to make sure that the canopy is clean (saves a lot of work when spraying the canopy edges). If you did your job fitting the canopy really well, it fits perfectly and you can just let it cure. I’ve never succeeded in doing that. So I usually end up positioning the canopy with bits of tape or double-sided tape on ebechi. You can also wait for the epoxy to partially cure and then carefully press the canopy onto the frame in places where the canopy stands out. Then let the epoxy fully cure.

I got lucky and the fit turned out very well. I did have to fill a few gaps between the canopy frame and the canopy with some thickened epoxy, just to make sure that the inside edge is perfect.

Once all this is done I carefully sanded the edges of the canopy so that they are nice and smooth. I also lightly sanded the edge of the canopy to be spray painted. Then I carefully covered the bits that are not to be painted (make sure you use good masking tape, not the 3m DIY stuff!) and first applied a bit of primer, followed by a few coats of white. I’m pretty satisfied with the result.

Elfe S3: making the fuselage mould

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The fuselage mould for the Elfe S3 is done and ready to be spray painted so we can start building the first fuselage.

Here is how we made the fuselage mould:

– Using the right wax in the right places is key. For building the fuselage mould we use: 1) a coat of HP-G1000 priming wax (HP Textiles); followed by 2) three coats of a release agent based on polyvinyl alcohol, dissolved in a mixture of ethanol and water (SCS PVA-folientrennmittel). We’ve used this for building all our recent moulds and never had any issues releasing the plug from the mould..

– Following the fitting of the plug in the parting plane and filling the gaps between the parting plane and plug with blue tac, the separator for the tailfin is mounted, the holders for the centering pins are placed into the parting plane and the edges of the mould are marked by brown packing tape (also marking the part of the parting plane that needs to be waxed). A foam bloc is places in the space for the canopy. Also the various pins for the gaps in the fuselage for attaching the wing and horizontal stabiliser are set and waxed.

– Next we build up one half of the fuselage mould on the plug and parting plane. The layup of our mould is as follows (note that we only make around 4 fuselages out of this mould, so it doesn’t need to be super-robust):

  • First a coat of special mould resin (we use Formenharz P from Suter/R&G). This needs to cure for a while (1-1.5hrs) before we can start on the next layer;
  • Put a smal strip of 25-30gr glass on the parting plane, tightly around the plug, followed by thickened epoxy resin, so that the edges of the mould are reinforced and minimising the chance of air pockets;
  • Build up epoxy resin thickened with cotton flakes around key areas such as the connection to the wing and around the pins for connecting wing and horizontal stabiliser, but also ridges around the canopy;
  • Apply epoxy resin on the entire plug (we now use this epoxy resin).
  • Apply the glass. Each layer of glass becomes a bit of extra epoxy – enough to fully soak it. The epoxy is evenly distributed using a small roller, which is also used to ensure that no air is trapped underneath the glass. We add the following layers of glass:
    • 2 layers of 165gr glass
    • 1 layer of 280gr glass
    • 1 layer of 280gr glass around the edges of the mould, not on the plug
    • 1 layer of 280gr glass
    • 1 layer of 280gr glass around the edges of the mould, not on the plug
    • 1 layer of 280gr glass
    • 2 layers of 165gr glass
  • Build up epoxy resin thickened with cotton flakes around the pins for connecting wing and horizontal stabiliser as well as around the holders for the centering pins for the mould;
  • Apply tear-off fabric to the places on the fuselage mould where the plywood feet are to be added

We let the first half of the mould rest for about a week. After a few days we added the plywood feet (three).

After a week we very carefully removed the parting plane (leaving the plug in the half-mould!) as well as the tailfin separator. We then added a new tailfin separator as well as the holders for the centering pins and foam for the cutout of the canopy.

The 2nd half of the mould is then built up in exactly the same way as the first half – see above.

We then again let the mould rest for about a week, adding plywood feet after a few days.

After this week the edges of the mould are cut off and rounded, after which the mould is carefully separated and the plug removed.

We then sand out any uneven bits from the mould using 600 or 800 grit sandpaper (wet) and then polish the mould progressively using 1000 and 1500 grit sandpaper (wet) followed by polishing/cutting past. After another cleaning, the mould is now ready to be waxed and then painted for the first fuselage.

Summer repairs: broken DG800 tail boom

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This year’s annual club outing to Hahnenmoos in June was great. We spent many hours flying in excellent weather, especially at my favourite flying spot Luegli. With many flying hours unfortunately also the risks of damage increase – especially landing on alpine slopes. I misjudged the landing of my Chocofly DG800 after a few hours of flying in Luegli and came in too fast on a down-sloping hill. During the landing the plane’s wing caught on a lump of grass, causing the plane to twist in the last few meters of the landing and the tail boom of the DG800 snapped in two. Fortunately it was a clean break and the plane had no significant further damage.

In my collection of cardboard tubes I found one that was approximately the size of the tail boom. I covered this with cling film and then laminated a carbon sleeve around it (covering it with peel ply so that it’s nice and tight and is also easier to laminate into the fuselage). Once the resin was cured I cut the sleeve across so that it perfectly fit the inside of both sides of the tail boom. I then epoxied this sleeve into the plane – taking care that the tail was exactly in the right place. After letting this cure I added a few bits of 50gr glass to close any remaining gaps, together with some resin thickened with micro-balloons and finished it off with a layer of 50gr glass all around the repair. After lots of sanding – primer – sanding I spray painted it with 2 component paint.

The DG 800 flies like a dream again.

Monnett Monerai: polishing

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Although the paintjob on my Monnett Monerai was really good, we like to polish our gliders after they are painted and the paint is fully set (usually takes a few weeks). Polishing the paint gives the glider that bit of extra gloss and smooth surfaces and gets rid of the slight “orange skin” structure in the paint. It’s a lot of work though. I divided it over three days, around 2-3 hours each day.

A short “how to”: First we get rid of the orange skin structure by wet sanding all painted surfaces with 800 grit sanding pads (Georg even starts with 600 grit sandpaper), followed by 1500 grit and then a 3000 grit sanding pad (all by hand). After that we machine polish all surfaces using a fast cut compound, followed by a machine polish compound. See the last picture below for the materials used. In Switzerland we purchase all materials through sury.ch – unfortunately only in larger quantities, but then they last a lifetime of building gliders.

 

 

How to: make a carbon wing joiner

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We use a standard carbon wing joiner for most of our projects. It’s slightly conical and has a 6 degree V-shape/Dihedral. See the picture below for the sizes of the joiner resulting from the mould. The mould for the wing joiner is close to 20 years old. It’s a pretty simple setup, made of wood and aluminium. It has been used countless times and is still going strong. For the Elfe S3 we reduced the dihedral of the mould to 1 degree (the mould allows for this).

Here’s a description of how we build wing joiners out of this mould. All references to materials are to the R&G Webshop.

Setup:

The setup consists of 1) a roll of CF-Roving Tenax-E HTS40 F13 24K 1600tex (Art. 205.0024), mounted on an easy-rolling dispenser; 2) a 70ml syringe with a short piece of tube (I use motorcycle fuel lines) mounted in a self-made holder (to drench the roving with resin)(the syringe is held by a standard broom holder); 3) a small turntable to help cut the carbon roving to the right size; and 4) the mould. See the pictures below for more detail. The syringe and turntable are screwed to my workbench to make sure they stay in place.

Waxing the mould:

We apply three coats of liquid wax. Allow each coat to dry and then lightly polish with a piece of soft cloth. Apply some bicycle grease to the screws to close the mould (to avoid them getting stuck due to a bit of wayward resin).

Prepare the core of the joiner:

Previously we used a rohacell core for our wing joiners. For more recent builds we’ve found that it’s much easier (and cheaper) to use a balsa wood core. We use either 4mm or 6mm balsa. This is put into a 3K carbon sleeve (35mmØ 3K, Art. 200.4008).

Epoxy Resin:

I use two small joghurt cups to prepare the resin. Amounts needed:

  • 4mm Balsa Core: Approx 120gr Epoxy (90gr Resin + 31.5gr Harder)
  • 6mm Balsa Core: Approx 110gr Epoxy (80gr Resin + 28gr Harder)

I use black colourant to get nice and black wing joiners.

Set aside and thicken some of the resin in the 2nd cup (Aerosil) to apply to the mould and cover of the mould.

Building the joiner:

Apply epoxy resin to the balsa core (in the sleeve) using a small brush.

Apply thickened epoxy resin to the mould (this is to ensure that the surface of the wing joiner is nice and even, without air bubbles)

Then roll carbon rovings onto the turntable. I roll them in sets of 8. Use scissors to cut them into separate rovings at the right length.

Evenly put the rovings into the mould, starting at the edges.

Regularly spread out the rovings in the mould using an old credit card or a piece of wood .

The amount of rovings to use is as follows:

Wing Joiner with 4mm Balsa Core:

50 Rovings above and + 50 rovings below the core. In addition, add on each side of the core 4x13mm und 3x 17mm rovings in the center

Wing Joiner with 6mm Balsa Core:

40 Rovings above and + 40 rovings below the core. In addition, add on each side of the core 4x13mm und 3x 17mm rovings in the center

Note that you may need up to three rovings more on each side, depending on how much resin the rovings absorb (this can be adjusted by squeezing the tube coming out of the syringe – see picture)

Once all rovings and the core are in the mould, close the mould and let the resin cure for a few days.

 

Monnett Monerai: sanding and fitting the wings

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The main advantage of using ebechi to cover the wing cores is that it makes it easier to get rid of any uneven parts. These usually occur on the underside of the wing where the main spar or the two spars on either side of the hinges of the control surfaces are epoxied into the foam. The 0.6mm thick abachi usually gives enough “margin” to sand the uneven bits out.

Sanding the ebechi is a messy job. I usually do this outside and was lucky that today was relatively warm and windy. To sand the ebechi I use my favourite Permagrit (the really rough side), my trusted old bosch sanding machine (80 grit sandpaper) as well as a block with 100 grit sandpaper. Most important is to sand the leading edge of the wings into shape. During the building I put a few mm of resin thickened with microballoons at the leading edge, which makes it much easier to sand. I also use a small template to check the profile of the leading edge at different places of the wing. The template I used for the Monerai is the same as the one I will use for the Diana4 – the wing profiles are not identical, but close enough. A good trick to make sure that you sand the wings evenly is to mark the entire wing with a pencil. Once the pencil stripes are sanded away you have sanded enough and any uneven parts are most likely to have been sanded out.

Sanding reduced the weight of each wing by almost 40gr. Both of them now weigh exactly 482gr each (I weighed the wings during sanding to make sure that they weigh the same).

After sanding the wings I fitted them to the fuselage. The wings are attached to the fuselage using 6mm aluminium pins that are glued into small bits of plywood that I inserted into the base of each wing during the build. On the side of the fuselage they fit into short aluminium tubes that I glued into the fuselage earlier. To ensure that these pins are in the right place I first make the two holes in the fuselage for the wing joiner and make sure that the openings in the fuselage are just big enough to keep the wings in the right place. I check the position of the wings using my laser, as well as by measuring the distance to the table and to the nose of the fuselage. I then fix the first wing using tape, remove the other side, and use a long 5.5mm drill (thanks Aliexpress) to mark where the hole for the aluminium pin needs to come. I do the same for the other wing. I then drill open the holes for the four pins, making sure that there is enough play to enable the pins to set themselves in the exact right position. After waxing the fuselage around the area where the pins go in I then glue the pins into the wings using thickened epoxy resin (with cotton flakes), put both wings onto the fuselage and wait for the epoxy to cure.

Monnett Monerai: Main Spar Buildup

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Today I spent a few hours preparing the 2nd wing of the Monerai for closing. Measuring, cutting out the foam and especially preparing the Rohacell core of the main wing spar always takes quite a bit of time. All is now set for closing the 2nd wing, which we will do tomorrow.

I got a question via the comments form on the buildup of the main spar, especiall how I connect the sleeve of the wing joiner with the rohacell core of the main spar. I always glue the different core bits of the main spar together using 5 minute epoxy, fixed with a small piece of painters tape (picture below). This is really only to make sure that the core of the main spar stays together while handling. It doesn’t add to the stiffness of the wing. The stiffness of the wing is provided by the carbon sleeve around the entire core (from wing root to wing tip), by the carbon rovings below and on top of the main spar (for the Monerai 2×19 at the root and 2×2 at the tip) and of course the carbon layup underneath the ebechi (note that the carbon layup continues under the main spar for the entire length of the wing, in the first 75% of the wing covering the entire width of the wing, in the last 25% only the carbon D-Box as well as the area of the control surfaces).

So to be clear, the buildup from inside to outside is as follows:

– in the area of the wing joiner: carbon wing joiner (stays in the joiner sleeve during the building of the wing); kevlar sleeve wing joiner; 3k carbon sleeve; 19 carbon rovings; carbon layup (100gr biaxial); ebechi.

– in the area after the wing joiner (from inside to outside): rohacell core; 3k carbon sleeve (reducing to 1k towards the tip); 19 carbon rovings (reducing to 2 at the wing tip); carbon layup (100gr biaxial); ebechi.

– Important: once the Ebechi is sanded into shape I’ll also add a layer of 50gr glass (diagonally), before applying primer and then paint.

Also see my earlier post on building the wing of the Diana4 with a list of materials used.

I hope that the drawing below provides more clarity.

Diana4: closing the first wing

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Today I closed the first wing, with the kind help of my mate EP. While EP inserted the carbon rovings into the opening for the main spar (using the tool with the syringe on the picture to apply the correct amount of epoxy resin to the rovings) I applied epoxy to all the spars and in all the cutouts of the wing. Once he finished inserting the first set of 26 rovings we inserted all the spars as well as the ballast tube and plywood. While he then inserted the remaining 26 rovings on top of the main spar I prepared the ebechi with biaxial carbon. We used three batches of 80 grammes of epoxy in total, around 40 grammes of which was thickened with micro-balloons to fill out the leading edge, cutouts between the control surfaces and levelling out the space between the main spar and the top of the wing (where needed). We then closed the wing and inserted into the vacuum bag. Here it will remain for the next 12 hours, under close watch of my webcam. Tomorrow I can unbag the first wing and then start preparing the 2nd wing for closing – hopefully sometime next week.

Diana4: preparing to close the first wing

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The preparations for closing a wing require quite a bit of very precise work. First I roughly mark the areas where spars, ballast tube, wiring and other bits need to come. Then I cover these areas with brown packing tape. Using waterproof pen I mark out the areas where I need to cut out foam. Cutting out the foam is done with a very sharp carpet knife (using multiple blades – they go fast). Cutting out the main spar is done along a level wooden board to make sure that the cut is perfectly straight and level.

Once the styrofoam is cut I carefully remove the bits that are no longer needed using a sharpened screwdriver. The foam of the two small spars on either side of the hinge of the control surfaces is used again to fill the carbon carbon sleeve and goes back into the wing. Also make sure you cut out a bit of the foam where the cuts between the three control surfaces are coming (so the edges of these control surfaces are epoxy with micro-balloons rather than foam). I then sand and clean out the areas where the foam is removed.

Next is the preparation of all the inserts. The two spars nearest the trailing edge of the wings consist of the R&G 200.5211 carbon sleeve, filled with foam that was cut out there. The main spar consists of three different carbon sleeves (R&G 200.4008 for the inner third of the main spar, EMC VEGA SC1062 for the middle third and R&G 200.5211 for the outer third). At the inner side of the wing the carbon sleeve starts around the wing joiner and its kevlar sleeve. From the end of the wing joiner onwards I prepare a core consisting of Rohacell. The right thickness of this core can be calculated based on the number of carbon rovings and the thickness of the sleeve. Our wing designer Georg usually prepares balsa inserts of the right thickness that you can put into the cutout of the main spar and simply sand the rohacell level with the wing foam core. At the inside of the wing I also insert two bits of plywood where the two aluminium plugs that will connect the wing with the fuselage will come. For the ballast tube I prepared two bits of balsa, one on top and one below, to make sure that it’s snugly in the middle of the wing next to the main spar. I also cut out the wiring channel using a small soldering rod and the two silver bits that Richi, another building team member, prepared. To ensure that I find the right place to cut out the control surfaces once the wings have been painted I drill four small holes 2mm from the trailing edge between the two rear wing spars at either edge of the control surfaces. Finally, I take out 2mm of foam at the leading edge of the wing, which is later filled with epoxy resin with micro-balloons to allow the leading edge to be sanded into perfect shape.

Once all bits are prepared I apply a bit of masking tape next to the main spar and mark out the number of carbon rovings that are to be pulled into the wing before closing it (the Diana 4 has 26 rovings at the first 4cm, decreasing to 1 roving for the last 10cm or so, on both sides of the main spar). Then I prepare all the tools for closing the wing.

Wing building technique: materials and tools

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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.

Materials

Tools

Diana 4: upper sides of the wings

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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 :-)).


Start building season Winter 23-24: Wings of the Diana4

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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.

 

Ventus 2c: installing the wing servos (1)

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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.



Ventus 2c: seals for the wing control surfaces

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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.




Ventus 2c: polishing

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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 3000 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).

Diana 4 and Monerai: canopy frame and seat insert

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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.

Monnett Monerai: building the fuselage pod

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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.

 

Ventus 2c: fitting the wings and covering them with glass

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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.

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).