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Cabin’s mold is ready

Many hours to produce the plug and to make the mold were needed to get to this point. The three piece mold is finally ready to be used! We had several small problems during mold making, so that it was unclear until the end if the result would be acceptable. The pictures speak for themselves:

In that photos the border is still untrimmed and the molds still have a coat of PVA release agent. That is why they look somewhat frosted. The border is now trimmed and the molds washed and they are perfect! The plug survived without taking any harm, which cannot be taken as granted.

We made again a tutorial video:

It’s the fourth and last part of a short series that documents the whole process, i.e. from drawings to mold. The process is long, elaborate and full of possible drawbacks. So if you plan to make a larger mold, I really recommend to watch the full video series. Take your time to understand the process and adapt it to your special situation. We are happy to answer questions. The whole thing is time consuming and you really don’t want to take too high risks and invest weeks of work for nothing. To give you an idea, that mold is more than 4 m² (4.8 yd²) and we needed more than half a year to make it…

Though the materials used and methods applied are state of the art, it is nowadays not difficult to get access to them—at least when living in the European Union. We had several suppliers, which I can fully recommend (in alphabetical order): Bacuplast Faserverbundtechnik GmbH (release agent), GRM Systems s.r.o (carbon and glas fabrics, resin, Sorotec®, …), HP-Textiles GmbH (resin, vacuum pump, MTI® and other consumables), Mühlmeier GmbH & Co. KG (glas and carbon fabrics, Compoflex®, …), R&G Faserverbundwerkstoffe GmbH (tools and consumables).

The other three parts of the video series are here (also on YouTube):

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Wrapping a special kind of Easter egg

Last week we were on vacation and had plenty of time to work on the cabin’s mold. After a couple of 10 hours shifts, the mold is almost ready. Only the vacuum infusion of the right aft part is left to do:

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We started by making the parting surface which separes the aft section. This surface splits the mold in two symmetrical pieces and has consequently a very long and curved edge. It was somewhat tricky to get it right, in particular splitting the thin trailing edge was delicate. Like last time, the gap was filled with putty and sanded flat resulting in a very good edge.

Having the parting surface waxed and fixed on the plug, we started with preparation of the materials needed for the first shot. These are mainly carbon rovings to strengthen and round the corners, and a couple of layers of glass and carbon fabrics to make an airtight tub. We prepared again patterns of Compoflex®, which were used for both mold sides. This way we were able to easily cut the materials beforehand:

There was no turning back after everything needed was ready. The surface was coated with a thin layer of tooling gelcoat (Formenharz P + EPH 161, R&G Faserverbundwerkstoffe GmbH) after a layer PVA release agent was applied. Every time we have done this, it has been a strange feeling to spread that black nasty looking gelcoat on the nicely shining surface of the plug. What must be, must be. The whole process took about 10 hours including two hours of gelling time. That brush shows pretty well how we felt afterwards. Nevertheless, the effort was worth it:

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The tub for vacuum bagging was ready, so we made the dry layup, which is again a couple of carbon an glass layers and a 4 mm Soric® XF core:

Everything was covered with Compoflex® SB 150. Last time we had problems to get a good flow rate, so we decided to use an additional flow media besides Soric® XF. Infusion worked pretty well this time. Several batches of resin (HP-E120RI, HP-Textiles GmbH) were mixed and degassed during infusion to prevent overheating.

Demounting of the parting surface is always a moment of tension. The result looks very good and convinces:

 

Though we really took care to get an airtight border, we had some problems with vacuum consitency. It’s probably contributable to the coarse carbon fabric used (388 g/m² or 11.5 oz/yd²). We made the next and last tub slightly different:

  • thickened resin was spread along the border between carbon and glass to fill eventual gaps
  • the carbon was trimmed shorted than before, so that it doesn’t reach out to the border
  • a surplus layer of glass was laminated on the carbon along to embed it along the border in tight glass fabrics

These measures should end up in a much airtighter border. We’ll see it in the next infusion…

Happy Easter!

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2

First mold of cabin

Last weekend, we vacuum resin infused the first mold of the cabin. It’s the first and most crucial one of three pieces, because it will be used to make the die for the clear canopy. We had to take two shots, as the first vacuum infusion failed. More on the reasons below.

Everything began with the dry lay-up of the sandwich. We made some simple tempates using Compoflex® SB 150. This peel ply is ideal for making templates because its drapable, but it does not deform as easily as woven fabrics, and it’s a consumable.

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After having made the sandwich’s dry lay-up, we used the Compoflex® templates to make the peel ply layer. There is a Compoflex® variant specifically desgined for vacuum resin infusion (RF 150), but we have only the one for vacuum bagging (without flow mesh).

We used this type of Compoflex® as a substitute for usual peel ply, as the forces needed to peel it are much lower (prevents premature sepration of the mold). Though it has no flow mesh, we didn’t expect to get into trouble, beacuse the Soric® XF core should act as a flow medium. We were proved to be wrong, as we painfully recognized later…

As always, we used the MTI® vacuum line by DD|Compound. It makes a resin trap unnecessary, and we’ve used it in all our infusions.  Highly recommendable.

Having vacuum bagged everything, we started to mix the resin and degas it. We used HP-E120RI from HP-Textiles. It’s an epoxy resin with 200 minutes pot life. About a month ago we made an infusion test together with the Soric® XF core. It worked pretty well, but we saw that it takes quite some time to degas. Degassing of 1.7 kg (3.7 lb) of HP-E120RI took us about 30 to 35 minutes, and we needed roughly 6.8 kg (15 lb) for the mold! To make the story short, it took ages to get the resin ready:

After opening the feed line, it became clear that we’ll get into real trouble. The flow speed was incredibly slow. If everything works well, it’s possible to infuse larger amounts of resin before it starts to heat up. We’ve done this, for example, in the mold of the center section.

Epoxy resin reacts with the hardener and creates heat. It’s an exothermic reaction. The larger the amount of resin is, the more it heats up because less heat is dissipated to the environment. Heat speeds up the reaction ending up in more heat being produced (pot life is halfed every 10 °C or 18 °F more). If you have bad luck, it can heat up so fast, that it hardens within minutes, although it takes usually one day to room temperature. This is exactly what happend in our first infusion: We needed very long to degas the resin, infusion speed was too low,  the resin started to heat up quickly, and it „boiled“ at some point. Lucky us that it was cold and snowy outside and the resin did not catch fire.

We had to stop the infusion, although only 1/3 of the mold was ready. Usually this is a killer for a laminate, because infusion is usually used to create nice looking laminates. Here, we are only interested in properly wetting the sandwich, and we do not care about the looks. This is why we decided to let the aborted infusion cure and to infuse on top of it in a second shot.

Before we made a new infusion we had to understand why the first one failed. The main problem was probably the low permeability of the sandwich. If the feed line has direct contact to the Soric® core, low permeability shouldn’t be a problem. Here we had no direct contact to the core: The resin had to flow through a layer of Compoflex® SB 150, two layers of 105 g/m² glass and two layers 380 g/m² carbon. This was proably too much friction for the somewhat viscous HP-E120RI. The solution was, thus, to use a flow mesh the next time. We also decided to take the slightly less reactive and less viscous HP-E300RI with 300 minutes pot life:

The second photo shows clearly how much the infusion fronts outside and inside the core differ. We’ll probably not rely solely on Soric® in our next infusions and will use always a flow meash. We documented both vacuum infusions in a short video:

Here are some pictures of the mold after the laminate cured and the peel ply was removed:

We are making now the next parting surface. This surface will separate the aft part into a left and right mold.

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Weak sick week

Not much to report on mold-making today. Since we started with the mold last weekend not much has happend. It has been a weak week, because we have a heavy cold. Last saturday we coated the plug with tooling gelcoat:

It’s strange to put put this intimidating tar-alike stuff on the high gloss plug. The gelcoat works perfectly (Formenharz P from R&G GmbH), but I forgot how nasty it is. Use always a good mask when working with it!

After two hours, the gelcoat geled and we laminated two layers of 105 g/m² glass and one layer of 388 g/m² carbon on top:

The outcome looks quite promising:

This lay-up builds a sort of air tight tub on which the rest will be vacuum infused: Soric® XF 4 mm, two layers of 388 g/m² carbon and two layers of 105 g/m² glass.

Though I’ve been sick this week, I finally found some muse to modernize the vacuum regulator of my small pump. You’ve probably already seen my old regulator:

I know it looks unprofessional, but the features of the regulator were ideal: hysteresis regulator, set-point setup over a potentiometer, display of current pressure and pressure history. To make it short: Great firmware running on an unprofessionally mounted hardware. A comparable commercial product costs around 500 EUR, which is ten times more compared to my custom solution.

Some weeks ago I stepped on the regulator and destroyed the display. Shame on me. This is why it was time to build something new. The pump still looks quite strange, but the regulator got a nice case and new components. I lost the code of the firmware and have to start again from scratch, which is demoralizing. Anyway, it is not always bad to start again, and I use anyway a different microcontroller now (SAMD21 Cortex M0 against two ATmega328P before). The pressure history is still missing in the firmware, but at least the regulator works as it should:

2

Material finally arrived

Ready and set to make the cabin’s mold! Tuesday was a great day, though the delivery was posponed and split into three trips because the load was too large and contained dangerous goods. More about the dangers later.

We were expecting something big, but this exceeded our expectations:

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Inside the smaller and lighter box, some Soric® XF 4 mm core and 105 g/m² glass fabrics were delivered.  It also included some goodies, in particular a very nice spread carbon fabrics with IMS 65 fibers:

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It’s very dense and very light (80 g/m² or 2.4 oz/ya²) but still very tough. We bought a couple of meters and a sheet of Rohacell® IG-F 31 to make some tests for the wing shell. Further details on the test will be provided later in another posting.

Back to the large and very heavy box. It contained a large roll of thick carbon fabrics (388 g/m² or 11.4 oz/ya²):

It’s so much, that I had problems to lift it into the house (over 130 m² or 155 ya²!). I leave the maths to you, if you want to know how heavy the roll is. This material is obviously too heavy and thick for the airplane, but perfect for making though and stiff molds. Though it looks coarse, it is a special kind of twill weave and very flat. We got a very good price for it and we had to have it. We’ll probably make all subsequent molds with it and there’s enough for a couple of friends too.

Now to the dangerous goods. I love it when companies are creative while packaging a delivery. I can literally see the employee looking for empty boxes at the grocery store :

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I leave it up to you to decide which good on the pallet is most dangerous…

Anyway, we bought some of a very special epoxy-resin system: LG 735 AERO (GRM Systems). This resin is a non-toxic—probably more precise to say less toxic—substitute of the well known Aradite® LY 5052 from Huntsman. It makes extremely tough carbon laminates and the producer states that it’s even tougher than the LY 5052. We’ll test it together with the spread fabric and the Rohacell® core. Let’s see how it performs…

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Ready for mold-making

The plug of the cabin is ready for making the mold. After the surface was shot several times with filler and sanded, a layer of clear coat was shot on top to get a hard and robust surface. This layer was sanded with 1500 grit and waxed with a Carnauba wax several times: A thin layer of wax is applied with a piece of cotton and after 10 minutes it is polished with fresh cotton. A couple of hours should be inbetween applications to let the wax harden. One should keep on waxing and polishing until the plug looks glossy:

We made again a video on the whole process:

 

Last weekend we made the first parting surface, which separates the front from the aft. Another surface, which separates the aft part chordwise into two pieces, will be made after the first mold is finished. The edge of the parting surface can be created by protecting the plug’s surface with tape and using putty to fill the gap. The edge is sanded flush after the putty hardened. Minor gaps can be filled later with some wax dough.

 

The material for the mold should come next week. The layup will be glass, carbon, Soric, carbon and glass. Part of the mold will be vacuum resin infused. As before, we’ll document the process and provide here information.

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Skin white as snow

„Skin white as snow,
lips red as blood,
and hair black as ebony.“
, Jacob Grimm

To be honest, today only the skin of the cabin’s plug became white. The lips and hair of Schneewittchen will follow some day. Promised.

The day started with moving the plug into the other room, where the center section was waiting patiently for the cabin. We had to tidy both the room und the garage up. Having the room empty we had to clean it throughly, so that we don’t get dust on the plug while shooting.

The plug was then ready to get the first layers of filler shooted. Tomorrow, after the filler hardens, we’ll sand the plug and shoot a couple of layers more.

 

 

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It seems to go on forever

„It seems to go on forever,
but it’s not done
until it’s all one color
and it’s all smooth.“, Mike Arnold

Very true words, those of Mike Arnold, the designer of the AR-5 and 6. Some postings ago I mentioned and included his video on plug making. The Arnold Company has some other very interesing videos online and it’s worth to take a look at their channel.

Finishing the plug truly seems to go on forever: We’ve consumed until now more than 4 kg (9 lb) of putty and the plug is still not „all smooth“. We started by sealing the wooden surface with Clou® Schnellschleifgrund, which is a nitrocellulose based pore filler. After that, we applied two thick layers of putty and sanded everything flush with 80 and 120 grit sandpaper between the coatings:

As soon as wood is seen, sanding should be stopped. This is where abachi venner is better compared to glass fabrics: It’s not too tragic, if some of the venner is sanded.

Having two layers of putty on the plug, the long lasting detail work starts: A dry guide coat from Mirka hepls to make the low spots visible. The nice thing about dry guide coats is that they are ready to use right after application. Guide sprays need to dry before sanding. Stroking with a pencil on the surface is also a good technique to identify large low spots, but you’ll probably miss the small ones:

Merry Christmas!

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Hard work and patience pay off

The cabin’s plug is ready for finishing! It still has to be filled with putty and painted, but we have about 80% of the job done. Last time we reported how we started to prefabricate an abachi veneer layer for the plug. Now, the abachi veneer is on the plug and sanded. It was worth the effort:

Though the veneer layer needs time to get ready for cementation, the result after grinding is great. Not much putty will be needed to get a perfect surface:

We made also a video tutorial which provides many interesting and helpfull hints on how to make such a complex plug:

 

 

2

Patience …

… is a gift and usually pays off.

The rough XPS core of the cabin is ready to cover. I hoped to be able to use few pieces of abachi veneer to cover it, but it turned out to be impossible. Thus, I remembered a technique I’ve used several times in rc-models. The basic idea is to use small strips of wood to cover the spherical surface. Take for example the prototypes I made for the S12 (1:6), the Beluga (1:4), and Schneewittchen (1:4):

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The principle is always the same: Use thin strips to cover the surface.  If you try to cover a spherical section with a large sheet of abachi veneer, balsa or plywood, you produce huge shear loads in the sheet and most likely you’ll fail to get a good surface. The reason is that spherical surfaces have gaussian curvature. It’s very easy to cover a cylinder or a cone, because these lack gaussian curvature: Cones and cylinders are curved only in one direction, whilst the other is straight. This is not the case for the cabin of Schneewittchen, which has for sure gaussian curvature.

I used the same technique. The difference is that I have to prefabricate the curved venner sheet before it is cemented under pressure to the XPS core. This is how it looks at the moment:

There are still a couple of few square meters to cover. Though it will take a couple of days to finish, it is a nice relaxing job which is best done with some music, and of course with plenty of patience.