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.
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!