This Blog will attempt to chronicle the repair of Malcolm’s big Impala wing after it got badly damaged during a bailout after launch at Chapmans Peak. A Port Jackson tree did a good job of dissecting one of the wing panels.
The basic wing construction is of a low density polystyrene foam core with what looks like 100g/m2 glass fibre cloth at 45° bias with a layer of 0.6mm Obeche wood veneer as a surface skin. There is a Meranti leading edge but no wing spars.
The repair process will be as follows:
Assess the damage
Cut out most of the damaged material
Insert lite ply braces to breach the damaged area
Fill the gaps with new blocks of polystyrene foam
Sand the polystyrene to the wing contour
Insert a Meranti leading edge
Epoxy a 200g/m2 carbon fibre cloth with a 49g/m2 glass fibre veil cloth over the repair area
Sand the repair area smooth
Use filler to fill any low spots
It is quite a nasty break as you can see in the photos.
Basically, this is how it looked when it arrived.
After removing the fibre tape, the wing came apart.
I used a razor saw to make a clean cut towards the flap split line. The bottom looks quite bad, with a large delamination.
A closer look at the problem area. If I cut this whole piece out, the flap will have to come off.
I made another cut with the razor saw, removing most of the “mush”. The bottom shows a missing piece by the flap.
Here is the missing piece. The servo tray is visible. All I can do is to glue this piece back.
The piece is epoxied and taped into position. Sand bags and weights are used to apply even pressure on the glued piece.
After the epoxy cured overnight, the two wing pieces were lined up by clamping an aluminium straight edge to the trailing edge of the wing.
The position for the 3mm lite ply spars were marked with a long ruler. The 3mm slots were cut very carefully and precisely to accommodate three ply spars that will breach the gap between the two wing pieces.
Here you can see how a scrap piece of 3mm ply fits in one of the slots.
Bit hard to see what is going on here, but I basically used a bunch of straight edges to clamp the wing pieces together so that they are perfectly aligned for gluing the spars in.
The spars are dry fitted, ready for gluing tonight.
This is what it looked like after the spars were glued in place. I used Balcotan polyurethane glue. It has a slight foaming action that fills any gaps between the bonding surfaces.
I used a razor saw and a sanding block to shape pieces of low density packaging polystyrene I had lying around into puzzle pieces that fit snugly in-between the spars.
I drilled a hole in one of the blocks to allow the servo cables to pass through.
Here all the pieces are dry fitted.
The foam blocks are glued in place with polyurethane glue.
It took about 1 hour of sanding with a long sanding block to get the foam to the wing contour.
On to the Meranti leading edge. It was cut slightly longer than the break area to help distribute the stress area. A cutout was made to accommodate the piece.
The piece was glued and clamped with packing tape.
Next time the leading edge will be sanded to shape and some of the veneer will need to be stripped back to allow me to sand the foam to the correct level. I suspect that it is going to be tricky to get right, so let’s hope it works out…
I used a small palm plane to shape the leading edge.
I used a marker to indicate where the veneer must be stripped back.
The idea was to strip back the veneer to expose the glass layer underneath around the damaged area so that the carbon/glass skin has a good surface to bond to and to ensure that the final top surface of the carbon/glass is on the right level (not above the veneer level).
I tried to peel back a small piece of the veneer but it is bonded very well to the glass underneath and de laminated the glass layer from the foam. I already anticipated this kind of difficulty, so I switched to plan B. If the veneer is well bonded to the glass to the foam, then it should be no problem to make the carbon/glass patch bond to the veneer. I just need to sand all the paint off the veneer in the area of overlap to ensure a good bond and to make it easier to feather the scarf joint between the carbon/glass layer and the original top surface. I am not too worried that the patch will leave a ridge since the wing surface is not very smooth to start with and there is a large enough area to feather the join area.
After some serious heavy sanding to get rid of most of the paint around the patch area, we are left with a good surface for the skin to bond to. The bottom of the wing is very undulated with lots of high and low spots. The low spots will need to be filled in.