Production of AIREX® panels

We need several ribs to support the center section. These have to be installed before the upper and lower halves are joined. The main ribs conduct the wing’s shear to the spar bridges (shear load of about 5000 N  or 1100 lbf). Other ribs are needed to support the shape of the center section, e.g. to be able to walk on top to enter the plane. The ribs are subjected mostly to shear, which means that the fibers should be oriented ±45° in respect to the chord line. Thin plates tend to buckle under shear. Hence, we decided to use a 6 mm sandwich with biaxial non-crimp carbon fabrics face sheets. To prevent splintering in case of an accident, the carbon is covered by a layer of flax fabrics.

Though a honeycomb core is well suited to create ribs, the much cheaper and easier to process AIREX® PVC-foam cores are a good alternative. Their weight is somewhat higher, but considering the small area of the ribs, their contribution to the final weight is small.


AIREX® provides a good and long processing guide (see for example copy at R&G). Essentially these foam cores are suited for almost any available processing method. This does not mean that the different methods do not result in different qualities. We compared two processing methods: vacuum resin infusion and usual vacuum bagging.

Vacuum resin infusion

We created small resin channels with a screwdriver. This way we hoped to prevent having to use a flow media.


Trimming the face sheet material (100 g/m² biaxial non-crimp carbon fabric and 100 g/m² flx fabric):



Vacuum resin infusion with HP-E300GL epoxy resin and MTI® hose:




The face sheets look perfect. The surface was covered with peel ply to obtain a rough surface for further processing. The flax fabrics are almost not recognizable besides at the edges. It would be perfect if there wasn’t a small „but“: low fiber volume ratio (fvr). The final panel weights 1550 g (55.6 oz) for 0.66 m² (7.1 ft²), which means that it weights 2.3 kg/m² (7.8 oz/ft²). Too much! I expected roughly half that weight, which means that the fvr is catastrophic. But wait, vacuum resin infusion is known to produce high volume ratios! So how is that the result is overwetted with resin? Where’s the resin gone? It was sucked by the porous surface of the foam core! This is the crux when vacuum infusing foam cores. Some people fill the surface of the foam before infusing, but filler weights also and makes processing more elaborate.

In total: it worked alright without flow media, but keep an eye on temperature (not too cool). It is a good alternative for applications were weight is less critical, but it is certainly not suitable for airplanes.


Vacuum bagging

The face sheet material was separately wetted with HP-E200GL epoxy resin and vacuum pressed to the core. Processing is more elaborate than in infusion, but this approach prevents the porous surface from filling with resin.


Peel-ply, perforated release film and absorber material were used on top of the face sheets. The panel was then pressed between two standard shelf boards. A strong vacuum with a pressure lower than 20 mbar (0.3 psi) was applied, which is rather unusual in vacuum bagging.


The face sheets were almost as nice as in vacuum infusion. More important: much less resin consumption and a higher fvr. A fvr of over 55% was achieved with 200 g/m² carbon face sheets. This is an excellent value! For 100 g/m² carbon we achieved over 45%, which is still a very good value for vacuum bagging. A panel with 200 g/m² carbon and 100 g/m² flax weights about 1.3 kg/m² (4.3 oz/ft²), which is not that bad even compared to a honeycomb core.


Use vacuum bagging with very low pressures if weight is critial. It is more elaborate than vacuum resin infusion, but much higher volume fraction ratios can be achieved.


I uploaded a video of the vacuum infusion. Enjoy!

Almost there

The top side of the center section is ready! It was a long and difficult path to get there…

We tried to prevent some problems we had last time with the lower side. Most prominent is that we produced a left and right side for the inner face sheet. Last time we had problems with wrinkles while draping the geled sheet. Using two halves reduces the risk substantially and is much easier in handling.

Production of the inner face sheet started with a dry layup on top of the pattern. Last time we used a drawed tamplate while laminating the sheet, which was difficult to set correctly up. It is much easier to make a dry layup on the pattern.

Flax fabrics:




Originally we wanted to vacuum infuse the face sheet and join it with the honeycomb core when geled.  However, we had massive problems to get a sealed bag and the infusion of one half failed. I think the following picture shows my mood that night:


Already here it showed that having two halves is better than one, as we lost half of the sheet instead of all. The problem was that the bag was made of the wrong material: Never use a single layered PA bag for infusion, use a three layered PE/PA/PE bag. No matter if the PA bag is slightly cheaper. Usually a small leak is not a problem, as long as the part produced is not something like a visible carbon surface. But, this time the leak was directly at the suction line, so that the resin was not sucked properly and the laminate was not wetted. Anyway, we made the dry layup of one side again, we wetted the old way with a roller and vacuum pressed them for about twice the resin’s pot time.

After having glued the geled face sheet with 3M 9323 epoxy-resin on the honeycomb:


The whole thing was cured in moderate vacuum for 24 hours and then heat treated for 15 hours at 60 °C (140 °F):



We are proud of the result: very light and extremely nice



We have to cut and level the joining edges of both the upper and lower sides. Also some ribs are needed to support the shape, in particular where the pilot will walk on top to enter the plane. Having done this, we’ll be finally able to join these into one piece. I guess the picure above gives anyway a good impression of how it will look like.

Merry Christmas


Glass bubbles

Not many recipies are found when it comes to filling honeycomb cores with glass bubbles thickened resin. R&G provides here a list of reciepies for usual additives. They advice to use 30 g of 3M 0.2 g/cm³ glass bubbles for 100 g of resin. We will denominate this as 30% fraction from here on, though officially the mass fraction is only 23%.

The recipie of R&G is too thick to fill the small channels of a 3.2 mm honeycomb core. As a result, air is trapped in the core weakening it. Take, for example, this carbon sandwich:


Using less bubbles provides a better mixture, which fills easily the channels. However, less glass bubbles means also more weight. So what is the best mixture?

To answer this question, we made some systematic tests with a low-viscosity resin from HP-Textiles designed for vacuum resin infusion (HP-E300RI). Four different fractions were tested:


Cross sections show a separation of the mixture into phases of pure resin and glass bubbles:


This means that the mixture was oversaturated. Pure resin does not only add unnecessary extra weight, but also tends to have additional shrinkage leading to marks on the finished surface.

An estimation of the volume fractions and extrapolation delivers the optimal amount of glass bubbles (about 18.5%):


Having a figure for the optimal recipie, we made a further test in which the effect of adding a thixotropic agent (1.5% fraction) was investigated:


It looks that we found the best recipie!

With a good feeling, we filled the core of Schneewittchen: