Today I bagged the 2nd upper side of the Diana4 wings, having done the first one earlier this week. Bagging the wing is not all that much work – less than an hour. It’s the preparations that take most time.
After preparing the foam cores (see previous post) comes the incredibly important work of drawing the layout of the wings onto the building board and positioning the foam shells. We build upside down (upper side first). Once all the measurements are done and double-checked I position the foam shells and glue them to the building board using painter’s tape. Then I apply thin brown packaging tape on the edges of the shell on the board as well as to the individual shells of the underside of the wing. It’s important to find a packaging tape that doesn’t stick to epoxy resin – the purpose is to avoid the shells sticking to the wing core or other shells.
Once the board and shells are prepared I prepare the ebechi (abachi) wood. We use 0.6mm ebechi, prepared by a carpenter friend or purchased through RIK Modellbau. This ebechi is cut to size using the plan of the wings – important is to make the ebechi for the upper side approximately 4mm longer (depending on wing profile). I then draw the layout of the carbon inserts onto each of the four ebechi sheets (so I know where to position the carbon when glueing it to the ebechi). We rarely do a full carbon layup – that’s not necessary and would only add weight to the wings. The carbon inserts are just big enough to cover the carbon D-box at the leading edge of the wing, the control surfaces at the back of the wing as well as the positions for the servos and the wing joiner. Once the layout of the carbon inserts is drawn onto all four sheets I carefully sand the inner side (where the epoxy is to be applied) of each sheet and clean off the dust. I then apply a transparent primer (“Hartgrund”), 50% diluted, to avoid the epoxy completely soaking the ebechi. Allow this to dry out and another quick sanding and the ebechi is ready to be used.
Once all that is done it’s time to prepare the resin. I used around 60 grams of resin for the upper side. It is possible to do it with 10-15 grams less for weight watchers. Using a small roller I apply the resin to the parts of the ebechi sheet where the carbon layup is coming. I then put the carbon layup onto the ebechi and push it on using the roller. Then I use the roller to apply most of the resin to the carbon layup. I leave a few grams of resin and add a few drops of foaming agent and let this rest a bit. I then use a hard rubber roller and kitchen paper to roll excess resin off the carbon layup. Then use the soft roller to apply the remaining resin with foaming agent to the parts of ebechi not covered with carbon.
Then put the ebechi with carbon layup onto the shells. I use six or so small bits of double sided tape on the shells to make sure that the ebechi doesn’t shift. Then put the wing cores onto the ebechi, fix the outer ends in the right position with a bit of wax tape and fix the cores together with a bit of the same tape. Then close the upper shells, lightly fixing them into place with painter’s tape. Then bag the entire building board, use a vacuum cleaner to remove most of the air of the vacuum bag and then start up the vacuum pump. Stabilise the vacuum pump at -1.5 bar and let it run for 12 hours. I use a cheap webcam to watch the pressure of the pump so that I can go do something else and occasionally check the webcam on my smartphone. The entire setup runs over a timer that automatically switches off after 12 hours.
When I was cleaning up after bagging the 2nd wing I noticed the appropriate newspaper article used to protect my workbench (see last picture :-)).