I have started stripping and repairing the damaged wings. These wings are made with normal expanded poly-styrene foam cores skinned with an obechi wood sheeting and bonded on with a layer of glass and epoxy resin.
On removing the covering it is clear as to why this wing failed . Under load the wing flexes and in the case of a loop the bottom skin is under tension and the top skin compression. The greatest load in normally found just after the joiner tube and this drops off exponentially with very little load at the tip. Generally when laying up carbon tows as spar caps we try match the loads and would typically lay more tows in the first quarter then in steps less and less tows until the tip. A big change in structure/material strength is normally the place where things break.
This wing failed under stress because the only thing holding the top skin apart/together is polystyrene. I will route out a groove and install a lite ply shear web and a new set of carbon spar caps. A new aluminium joiner tube will be installed, easy as this wing is flat. Also note the wiring was installed in a tube made with drinking straws and this will need to be replaced. I will install a piece of fishing line through this tube as a pull through.
The damaged area is cut out and a new piece of foam glued in with 5 minute epoxy.
A rib template is tacked on and using a hot wire bow the wing core is cut to the original shape. A little sanding and a new piece of balsa skin is epoxied back on and clamped until set.
Both sides and some new leading edge needs to be done, next I will tackle the smaller wrinkles at the tips, sand everything down and cut through the wing for the shear web which will go out about 1 mtr with the joiner tube.
In the meantime the canopy form has has been layed up with 3 layers of 163gr glass and epoxy. As usual the foam was covered with packaging tape to aid easy release.
Thank you. Any questions please feel free to ask.