Installing wing servos and IDS

In all my recent builds I’ve used Chocomotion servosĀ  (supplied by in combination with the IDS system supplied by 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.

Elevator and Cockpit

I’m not very good in making nice cockpits. Although I admire colleagues that do perfect scale cockpits, I just can’t be bothered to take the time to do it myself and usually do a very rudimentary job. I’m too keen to get onto the next project or go flying :-). The Urupema cockpit was a bit of an issue, since we had no pictures whatsoever of what the original looks like. While Richie made a all of us very nice seat pans, we re-used the instrument panel from our Diana2, which looks to fit quite nicely. I also lack the skills to make a nice seat, so I usually just cut a piece of felt to size.

As our Urupema is built for higher speeds than usual, having a robust elevator is important. Usually we attach our elevators with plastic screws (allowing them to break with rough landings on the slope), but this time we attach it with a metal screw, but the front of the elevator is attached with a carbon rod (allowing it to break there if the elevator gets caught in a rough landing). The control horn is glass fiber. As the elevator hinge is on the topside of the elevator, the lower side doesn’t need an epoxy seal, I just use tape.

Setting the seals for the wing control surfaces

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.

Maiden flight of the Chocofly Diana2 – Glider version

When there is a stable high pressure area north of us, over Germany or eastern Europe, we usually have a stiff northern breeze, called “Bise”. Most of my favourite slope soaring spots are on north-facing slopes, i.e. time to head out. Unfortunately, being a northern wind, it’s also a freezing cold wind, which means that this early in the year you can add a significant wind-chill factor to temperatures already barely above zero degrees Celsius. In cycling we always say that there is no such thing as bad weather, just bad gear. Following that wisdom I went out on Tuesday to my favourite slope, with chilly but perfect wind conditions. The slope is perfect Diana territory – I usually fly one of my Dianas (Diana 1 or 2) there. With the whole Covid-19 lock-down I drove up alone and had to hand launch the plane myself. This is one of the reasons that last year I ordered an extra fuselage for my Chocofly Diana2 (1;3.5), and built it as a light glider without the usual FES (5.5 kilos). Normally I maiden my planes on a nearby airfield, using a towplane. But extraordinary circumstances require extraordinary actions. And as I know the Diana2 pretty well by now, having clocked many hours on the various version I have, I hand-launched it myself on the slope for the maiden flight. It flew off without any bother and I spent a wonderful 75 minutes in the freezing cold. The nice thing about the Diana2 is that it’s such an amazingly versatile plane. It thermals amazingly well and loves low as well as high speeds. At 5.5 kilos it’s not as fast my heaviest version, which weighs almost a kilo more, but still very fast and a lot of fun to fly. I’ll be taking this one to the slope more regularly.

On the slope I met up with my mate Richie, who was also flying his Diana2 (the Chocofly Diana2 is the commercial version of the design from Richie and Georg). Hence the photo below of the two sisters together. His Diana2 is even lighter, well below 5 kg, and on the slower side, but goes up like a little balloon in the least bit of upwind.

Urupema: cutting the wing control surfaces and setting the hinges

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.

Urupema back from the paintshop and fully polished

Earlier this week I also got the Urupema fuselage back from the paint shop. I spent quite a few hours sanding and polishing it to a shine. I first used 600 grain sanding paper, then 800, then 1500 and then a 3000 diamond polishing pad. After that I treated it with machine polish and shiner, using the polishing machine.

The paint shop and especially the polishing afterwards is always very unforgiving when it comes to small mistakes in the building. There are the inevitable spots where it’s not quite perfect, it’s obviously made by hand, but I’m satisfied. I’m particularly happy with the spray filler that I’ve used for the first time on the seam on the top of the fuselage and part of the wings. I was a bit nervous trying this out, hoping that it wouldn’t react with the final coat of paint. It came out well though. I couldn’t resist sticking a few of the decals on and putting the plane together for a photo session.

Now the hard work on finishing the wings starts. Next step is cutting out the control surfaces. No mistakes allowed there…


Watching Paint dry

Watching paint dry is not really my thing. Andi managed to secure a slot in the paintshop on Saturday and spray painted the wings and all small parts. The fuselage should be done in the next few days. I’m now waiting for the paint to harden out, after which the hard work of polishing starts.

I’ve been using the time to build a small glider, based on the mini-Uhu, but remote-controlled. Originally this was for our upcoming indoor glider-tow competition, which has now been cancelled due to the Corona crisis. I’ve also built a new Clik21 indoor plane, which had been lying around in my workshop for a while. I love that small indoor plane. I’ve built (and sold) multiple versions of it already, always trying to make it lighter improve it further. My current one weighs 93gr (no battery), but with a pull-pull rudder and elevator it wasn’t very precise to fly. Hence my wish to build another one. The new version looks again much improved compared to the previous one, with more stable ailerons and rudders, and a total weight of 95.6 grammes (without battery, i.e. flying weight 110gr).

I also finalised the Urupema decals and spent much time making the rudder control horns. Georg helpfully documented how he made his control horns, which I’ve copied. I’ll post a picture of the painted Urupema as soon as the fuselage comes back from the paintshop.


Urupema: ready for the paint shop

The filler/primer on the wings was sanded down again, to get a really smooth surface. Last week we also got the canopies, which we had produced by a friend, using our mould. I spent quite a bit of time fitting it to the canopy frame, before fixing it with epoxy resin. It looks quite good. I also fitted the towhook and bungee hook, as well as an extra bulkhead to hold the seat inlay. Today I prepared all the parts for spray painting (canopy is always a lot of work to prepare), as well as the tools to hold the parts during spraying and letting the paint harden out. Tomorrow I’ll bring all of it to Andi, the master painter in our building team. I hope he can get a slot in the paintshop over the weekend. I am very curious to see how it will come out…spray painting tends to be quite merciless in showing where you made mistakes……


Urupema wings and elevator: sanding the glass and applying filler/primer

The 49gr glass covering wings and elevator hardened out nicely, and I couldn’t help putting the plane together to see how it looks, now also with the landing gear :-).

Today I sanded the wings and elevator and cleaned them. After that I applied the primer/filler. First with a roller and then pulling it deep into the (few) pinholes using a spatula. I hate this part, as the primer stinks like hell. While it needs around 20deg C to harden out I cannot apply it outside, so I spent the morning in the workshop using a face mask. Now the primer can harden out over the weekend. On Monday it’s sanding time again….

Urupema landing gear and wheel cover

Fitting the landing gear required fiddling a bit with the correct position of the two spants to ensure that the center of the wheel is 2cm behind leading edge of the wing. Then cutting the hole at the bottom of the fuselage to the right size so that the wheel fits through. The suspension is a couple of rings of inner bike tube, which we also use for our retract able gears. I always first provisionally fix the spants with a few dots of 5minute epoxy, and once everything is in the right place I fix it using 12hr epoxy.

Fitting the wheel casing is done with two 4mm screws. I first cut some plywood to size, fixed it in the right position on the fuselage with a few dots of 5min epoxy resin, then drilled the holes, removed the plywood from the fuselage and fixed it to the wheel casing. Using a syringe and some coloured 12hr epoxy resin I filled the small gap between the wheel casing and the fuselage to ensure a perfect fit.

Urupema: covering wings with glass and building canopy frame

After giving the wings a final fit, filling some minor uneven surfaces on the lower side of the wing with polyester putty, the next step is covering the wings and elevator with 49gr glass fiber (lower side of the wings first). Before doing so we apply a watery primer (to avoid too much resin being absorbed by the abachi wood), which is slightly sanded and then the wing is wiped clean of all dust. The 49gr glass is applied with a roller and resin that has white colorant added as well as around 15% methanol (50gr resin, then add hardener etc. per wing). The methanol makes the resin very watery and easy to apply using the roller (and an old teflon frying pan).

I also finished the canopy frame and started builing the “lock”. The canopy frame looks quite good already, but will need a layer of filler before I can spray paint it. It was built by applying tape around the canopy base and two coats of liquid wax on theĀ  areas where the canopy frame is lying on the fuselage. The frame is then built using first a bit of thickened epoxy (with micro-balloons and a bit of aerosil), then adding a bunch of carbon fibers. It is then further “filled out” with thickened epoxy. After giving it a rest of a few hours, allowing it to harden out a bit, I first rolled the upper side of the frame (after first applying cling film), to even out the top surface. I then filled any small uneven bits with thickened epoxy again and applied two layers of glass (280gr and then 160gr), and allowed it to harden out like that.

Urupema landing gear assembly

We wanted our Urupema to have a landing gear with suspension (as we often fly on pretty rough alpine slopes). Georg designed it, and Richie built the mould for the cover. We spent a lot of time sourcing the right size of the various bits and pieces. The sides are made of 5mm glass fiber. A friend was able to mill the glass fiber sides on the basis of our drawing, and another friend helped us thread and cut aluminium parts to size. Today I assembled my landing gear. Fits perfectly.

Urupema: motor bulkhead done and wings, rudder and elevator first fit

Last week we built my second wing, it came out of the bag very nicely. Georg also built the rudder of my Urupema (in the mould). I spent a few hours sanding the abachi wood of the elevator and wings, and had a first fit. I also cut off the nose and installed the bulkhead for the motor, which also worked well.

Pretty excited, this is going to be a beauty. A lot of work to be done still though, experience says that once the wings are fit it’s about a third of the work done. But I hope to have my Urupema airborne by early June.

How and why do we do it?

We’re regularly getting questions not just on how but also why we build our own gliders, and if it’s really worth the hundreds of hours of work that go into them. On 14 December 2019 we were invited at the 35th international aeromodelling symposium in Winterthur (CH) to give a presentation on our work. Unfortunately I was unable to attend, but the other three members of our building team, Georg, Andi and Richie, explained how and especially also why we do this. I put together the slides for the presentation, which are now uploaded here. It’s mostly pictures, with some text in German. The two videos were included at the end of the slide-show are available on my Vimeo channel:

JS3 slope start

JS3 on our airfield


Georg’s Urupema

While I’m still working on the wings of my Urupema, and Andi is working on the fuselage of his, our “Master Builder” Georg has been making excellent progress on his. He’s finished the rudder (with Rotational Drive System – RDS) and elevator installation and his plane is just about ready to be spray painted. It looks awesome.

We also got most of the parts of the landing gear together, including some bits that were cnc milled by a colleague. I’ll need to do some work on the missing bits today.

Georg also checked the weight of his Urupema. He’s expecting a take-odd weight of around 6300 gr., which means 63 gr/dm2. With the ballast in the chambers it will be around 78 gr. /dm2. Another plane that will be great both for thermals and heavier slope work.


Urupema decals

In the meanwhile I’ve also been searching the web for Urupema pictures to help decide what we want our Urupemas to look like. As the plane was built in so many version, with so many different decals we’ve decided that we have a bit of liberty with this, but will allow ourselves to be inspired by the version that’s most photographed on the internet (see for instance here). We will go for a Brazilian design. I’ve identified the fonts of both the logo under the cockpit (Microgramma bold-extend) and the registration number (Bahnschrift Semibold Condensed) and done a first design of what our decals can look like. We now need to decide on the registration numbers we want to use. I’ve also ordered a couple of Brazilian flag stickers, which we may stick to the tail fin if they look nice enough. Note that the below file links through to the PDF version of the graphics, which are free for anybody to use.


Urupema build progress – wings

It’s been a while since the last update on our Urupema build, and a lot has happened. We finished building the four fuselages (Georg and Andi pushed out the last three while I was on vacation). Two sets of wings are done. Andi and I have been working on the third set of wings (mine) over the past week. Yesterday we bagged the first, which came out very well. We hope to bag the second wing of my Urupema sometime next week. We’ve been using 160gr carbon biax cloth for the full surface of the wing (top and bottom), in addition to progressive carbon rovings (37 at the top and bottom for the first 10cm, reduced to one at full length of the wing), and of course the usual carbon sock around the wing spar and control surface hinges. We’re also building ballast chambers on both sides of the wing joiner. The result is an incredibly stiff wing, with a nice flexibility at the tips, exactly how we wanted it.

In-between: another Diana2….

With the JS3 maiden flight still to go, my favourite plane by far remains the Diana2. So far I had three versions: my 1:3.5, 4.28m and 5.65kg scratch-built (my favourite!), the Chocofly derivative of our scratch-built (identical size and profile), with 6.3kg (Alpine version) as well as the Baudis/Chocofly 1:3, 5m version (8.2kgs). The great thing about the Diana2 is that it’s such a versatile plane and can be flow in weak thermals and in extreme slope conditions. It performs very well at low weight (a real “balloon”) but also carries ballast and higher weight extremely well, when it becomes a real racer. I’ve been wanting to built a “light” version for a while. Chocofly was able to supply me with an extra fuselage, which I’ve now built as a light glider. It’s ready to maiden, and I hope to do a first aerotow soon, and use it on the slope afterwards.

Swift S1 Maiden flight (OO-YES)

And finally the winter fog cleared! Yesterday afternoon I took the Fridayfly Swift S.1 (2.8m, 4.6kg) to our airfield for its maiden flight. Start was using our club’s catapult. The leomotion motor (L4031-2550, with 6.7:1 gearbox and RFM 16×13 Prop, running on 6S) has plenty power. It needed a bit of up elevator, and the throws are quite large. Unfortunately the conditions were far from perfect for flying, incredibly bad visibility and a low sun meant being very careful. And this time of year you can just about forget about any thermals, especially with a heavy plane like this of course (it’s meant for the slope, not our airfield). It flies incredibly well though, and I’m looking forward to taking this to one of our local slopes soon, and Hahnenmoos in June….