Evolution

The proof of concept of the seat’s spar worked pretty well allowing to start with the design and production of the final part. The central idea is to make a carbon composite with birch plywood, AIREX® and aluminum core. The aluminum parts are fittings to connect the seat’s spar to the two wing’s spar joiners (steel tubing framework). These critical pieces transfer the load produced by the pilot’s and structure’s weight to the wings (roughly 8 kN or 1800 lb). Machining them was not easy, as a couple of different steps were needed—including reclamping.

A couple of takes were necessary until we had the quality we wanted. Any negligence during programming or machining ends up in a worthless piece. Here’s how the first piece evolved from left to right:

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The alloy is AW 7075, which is very strong and easy to machine (6 mm solid carbide tools at 24’000 rpm, 1’000 mm/min, 1 mm cutting depth and 25% overlapping). We started with a 10 mm plate by leveling an making the pockets on one side. Two holes were made for repositioning after turning. To get a high accuracy, the holes are finished with a reamer and a dowel pin is used to reposition the piece. Having turned and reposition the plate, the other side is equally machined and the final contour is milled. The spar joiners and aluminum fittings will be joined by a fitting screw (M5 Ø6 mm).  Hence, the hole will be finished later with a reamer during assembly.

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So much on the fittings. As mentioned above, the spar will be a carbon composite with a mixed core. The spar caps will be made of 8×0.8 mm pultruded unidirectional carbon profiles (DPP®, R&G Faserverbundwerkstoffe GmbH), which are laminated to sufficient thickness. The core is mainly made of 8 mm birch plywood of aircraft quality. To reduce weight, holes were milled and filled with machined AIREX® discs. The whole package will be wrapped with ±45° carbon fabrics. We’ll use LG 735 Aero epoxy resin from GRM Systems s.r.o.. A little bit of Elan-tech® AS90/AW93 will be used to join the spar caps to the aluminum fittings. To prevent contact corrosion, a thin layer of glass will be wrapped inbetween carbon and aluminium.

Lamination of the spar caps comes next…

By the way, we got the steel tubing for the spar joiners. 14 meters of very strong 1.7734.5 / 15CDV6 tubes:

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The material 1.7734.5 is substantially stronger than the usual 1.7214 / SAE4130. It has a yield strength of 790 MPa against 480-520 MPa of SAE4130.

Slowly coming up together

Since the canopy’s die is ready, we had „plenty“ of time to work on the details of the cabin. To the most prominent details count probably the spar joiners and seat. Somehow the pilot’s weight has to be lead to the spar joiners and finally to the spars. The idea is to make a spar on which the seat is installed and the weight of the pilot rests. Before we start to work on the shell of the center section, it’s better to make a proof of concept. We have the mock-up spar joiners made by Siggi, and we made a proof of concept with cheap popplar plywood:

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We recycled actually the seat of the wooden mock-up:

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For the first time we could seat comfortably in the center section:

 

The cabin offers plenty of space and a very good sight:

The view angle corresponds roughly to the angle of attack at cruise speed. On the ground the angle will be somewhat higher (roughly 8° against 2°).

You probably haven been asking how hard it is boarding the wing. Well, boarding is easy, but getting out is harder. At the moment it’s even harder than necessary, as the shell does not offer the stability and stiffness which it will have later. So, we have to take really care when we entering and exiting the cabin at this stage:

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Not that easy but doable. Later, when the shell is completed, we’ll be able to walk on it and the border of the cabin will offer a natural handle to pull up yourself.

The true material for the seat spar is ordered: 8 mm birch plywood of 12 layers (Thoma Balsa GmbH)  and plenty of pultruded carbon profiles (R&G Faserverbundwerkstoffe GmbH). This plywood is in combination with PVC structural foam (GRM Systems s.r.o) the core material of the spar. Bending stresses will be taken up by carbon spar caps and everything will be wrapped in ±45° carbon fabrics. The stresses in the caps are quite high and will reach up to 400 MPa (58’000 psi). The spar will be joined to the steel spar joiners with an integral piece of CNC machined aluminum (EN AW 7075), and it will be cemented to the shell of the center section.

Having found a solution to the seat spar, most open questions regarding the spar joiners are answered. Hence, the design and drawings can be slowly finalized:

Hauptholm

We are about to order the material (1.7734.5 steel tubing). If everything goes as expected, the joiners will be ready this winter.

When big is not big enough

No matter how large your heating oven is, at some point, you’ll make a piece that won’t fit in:

 

The canopy’s die is almost ready. It just needs to be heat treated and trimmed. The resin used (RIM 935) is brittle when cured at room temperature and has to be heat treated at 50 °C (120 °F). Later, before the die is used, it needs a further treatment at 140 °C (280 °F). This is done by the company that makes the canopy (Plexiweiss GmbH).

Last posting we reported how the die was vacuum infused. We’ve been looking forward to see how well the surface became. However, before the die can be demolded, an inner supporting structure has to be made. This took us a couple of days and sessions to finish, so we had to wait somewhat longer to see the result…

We started by thread milling plywood and adapting the shape of the pieces as good as possible to the inner surface. Having fixed the front and aft bulkheads with thickened resin (RIM935 + cotton flakes + thixotropic agent), the rest of the structure was much easier to adapt and resinate. The surface of the wood has to be sealed with resin to prevent FOD during canopy production.

 

Today was finally the day to demold! All the work and waiting was rewarded by a perfect result:

 

The laminate is transparent and those black markings are on the inside! Only the rough surface of the inner side blurs somewhat the appearance. Peel ply had to be used inside to get good adhesion of the wooden structure. Else the die would be transparent as thick glass. Heat treatment will have finished by tomorrow and the die will be ready for getting its border trimmed. Soon we’ll contact Plexiweiss GmbH again and commission the canopy.

No matter how complex a problem is, there is always a pragmatic solution to it:

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Infusion of the canopy’s die

About three quarters of a year transcurred since we started to make the mold of the cabin. Since it’s ready, we’ve been eagerly waiting to make the canopy’s die. Some open questions regarding release agents and spray adhesives delayed the production. Yesterday and today have been the days: dry layup and vacuum resin infusion of the laminate.

Making a die is a huge enterprise, as material costs and effort are considerable. The die has to have a flawless surface, it should be a full laminate (no sandwiches allowed), have a thickness of more than 6 mm (1/4 inch), should be produced in one shot, and should have no air bubbles. Looks like vacuum resin infusion is the way to go to fulfill these requierements.

The resin for the die has to have a high temperature resistance (glass transition temperature over 140 °C or 280 °F). We decided to use RIM 935 from HEXION Inc., an epoxy resin designed for infusion and high temperatures. We were lucky that a local company (Lange+Ritter GmbH) distributes it. I took my bike and got back with roughly 7 kg (14 lb) of resin and a couple hundred € less in my pocket. Temperature resistent resin is not cheap!

 

Don’t let you fool by the nice blue color. That stuff is nasty and should not be handled without precautions!

The layout of the laminate is pure glass fabrics, starting with some lightweight stuff and going over to heavy fabrics to build thickness up. Originally, we thought of using a couple of layers of 105 g/m² Interglas 91111 and then 600 g/m² Interglas 04367 for the thickness (both from GRM Systems). However, MÜHLMEIER Composites GmbH was so kind to provide a thick 600 g/m² glass frabric for free. So we made a nice mixture. More on that below.

Before we took a shot on the die, we made a small test with a cheap infusion resin (HP-E120RI from HP-Textiles):

 

Besides a small leak, the test was very successfull. Usually the dies are hand laminates of milky/frosty appearance. This one looks almost transparent, though the laminate is about 7 mm thick. The test helped to know that infusion works good for such thick laminates and provided an estimate on the needed amount of resin (about 7 kg per m²). The surface was perfect, and we were ready to go for the big one. That’s what we thought—at least. In contrast to the small test, the fabrics need to be fixed for the full die…

Normally, the layup for resin infusion is fixed using spray adhesive. The adhesive should dissolve in the resin without effecting the quality. That may be right when you have the right adhesive and the surface does not have to be perfectly shiny. Before we spend hundres of Euros in material and the adhesive turns out to affect the surface’s quality, we made a small test with carbon frabrics. No comment on the outcome:

 

The test was actually part of another one we made to decide how to make the shell of the wings. More on that on a later blog entry. Anyway, the used adhesive was INFUTAC from Diatex. It’s very strong, but it affected the surface and should not be used on the surface. The problem is that the spray’s aerosol is too gross. Lucky that we tested it before using that expensive RIM 935 resin. We bought another adhesive form HP-Textiles, with a much finer spray. We also used the adhesive only at the edge of the die, so that it should not affect the surface at all.

Yesterday we started to make the dry layup and today we were ready to infuse the laminate. We used two layers of 105 g/m² 91111, four layers of 600 g/m² 04367, four layers of MÜHLMEIER’s 600 g/m² donation, and two final layers of 04367. This should end up in a thickness of 6 to 6.5 mm:

 

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The layup is quasi-isotropic, which means that always two layers of orientation 0/90° and ±45° are used. A quasi-isotropic laminate tends to deform less when it’s heated up. That’s what we need for our die.

Having a solid vacuum buildup, we were ready to make the infusion. Again, that RIM 935 looks really nice, but take care when handlig it! We made three batches of about 2 kg (9 lb), which were degassed as usual. Degassing of RIM 935 worked pretty well. It needed slightly less time than, for example, the HP-E120RI. No matter how much resin is degassed, count that it will take about 30 min to be ready. That’s why I would not use a resin with a pot life of less than an hour.

Infusion worked as expected:

 

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From mixing the first batch till the end, the whole process took about four hours. We’ll see how the die turned out 🙂

 

Testing structural foam sandwiches

The shell of the center section is made of a honey comb sandwich. Very stiff and still light, however, very work intensive: The core has to be cemented to the face sheets and the inner face sheet has to be installed in a geled state. This is not a problem, when the shell is only a couple of square centimeters large, but gets really involved when it’s several square meters. That’s why we want to make the next shells with structural foam cores. At least one step less is needed to produce a good sandwich.

Large unsupported shells tend to buckle when a certain critical shear stress is exceeded. Buckling does not mean automatically failure, but can be avoided by properly choosing the flexural stiffness of the shell. Usually the stiffness is estimated from old measurements conducted by Idaflieg in the 1980’s. Those values are fine, but they were not measured in sandwiches and they certainly do not consider how well or bad we realize the sandwich. This is why I prefer making own test specimens and measuring the mechanical properties myself.

The flexural stiffness D depends strongly on thickness of the shell (to the power of three). The cores differ slightly in thickness, so that comparing directly D is not meaningfull. This is why it is better to consider the effective elastic modulus of the face sheets. Having this modulus, the thickness can be chosen so that the necessary flexural stiffness is achieved.

The diversity of materials is high: PVC, PMI, carbon, glass, aramid, etc. There are also many epoxy resins. We decided, thus, to test several mixtures. Once we started, we took it very seriously and have tested four different core materials, over five different face sheets and three core shapes:

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The measurement setup follows Herbert Funke’s approach: The shell is used to seal a vacuum chamber and the pressure difference creates a uniform load. The maximum displacement allows to estimate the flexural stiffness of the plate and the shear modulus of the core. Weight and fiber volume fractions are determined by weighting the raw materials.

In theory the displacement should depend linearly on the pressure difference. Our measurments show that up to a neglible non-linear deviation this is true:

Stiffness is mostly determined by the slopes of the above curves. The flatter the better. The test specimens differ in weight and in achieved effective modulus. We are still evaluating different core materials. So far, we’ve made test specimens for the following foams:

  • AIREX® C70.55
  • ROHACELL® 51 HERO
  • ROHACELL® 31 IG-F
  • ROHACELL® 51 RIMA

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PVC foams (AIREX®) can uptake quite some energy and have consequently a high strength, while PMI foams (ROHACELL®) tend to be stiffer but somewhat more brittle. This is in particular true for the 31 IG-F. Evonik has invested research into creating a strong PMI foam, which is equally good as aramid honey comb. HERO is the result of this research and it aims mainly to fullfil the requirements of aerospace industry. RIMA is a small cell sized foam with very low resin uptake designed specifically for vacuum resin infusion. The specs show a very reasonable strength and being an industrial non-approved material, the price should be much lower than for HERO. Thanks to Evonik, we got some samples. They are ready to be measured.

AIREX® is compared to ROHACELL® cheaper.  So why not taking AIREX? ROHACELL is lighter with equal mechanical properties and the quality of the foaming is much better. All sheets of C70.55 we’ve purchased have imperfections, which can be of substantial size:

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We have not fnished our evaluation and have not decided which structural foam to use. Both AIREX and ROHACELL are still in…

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):

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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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.

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:

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…