Wing Bonding Process
At today’s composite wings, flexing forces are mostly absorbed by the main spar, in particular by the upper and lower spar cap. Assuming an upward flex, the upper spar cap is compressed, while the lower cap is stretched. To compensate, both caps try to move towards the centerline of the spar. This is inhibited by the spar’s shear web, resulting in a primarily compressing force within the shear web. In addition the spar caps try to bend sideways, attempting a screw-like distortion of the spar. This is inhibited by the torsion stiffness of the wing shell. Also the shear web is contributing, which causes diagonal forces in the shear web. The load on the wing in free flight is fairly complex; however, the upward flex during a fast recovery maneuver represents one of the highest load cases.
This short look at wing loads explains why the bonding of the main spar during final wing assembly has this extraordinary importance; complicated by the need for a blind bond on a high-load component. The designer has the opportunity to mitigate the problems by a wise placement of this blind bond. Based on the usual design of the main spar with upper and lower spar cap 
connected by the shear web, four potential placements of the blind bond are possible (see figure):
1. Between upper spar cap and the upper wing shell.
2. Between the lower spar cap and the lower wing shell.
3. Between the upper spar cap and the shear web.
4. Between the lower spar cap and the shear web.
Variant 1 and 2 allow a separate production of the main spar outside of the wing molds. The disadvantage is the need for a blind bond with one of the wing shells. Under normal operations, variant 1 is more critical. Under strong upward flexing (recovery maneuver),
the upper shell attempts to delaminate from the spar cap in form of compression-induced wrinkles. This causes areas of tension forces in the blind bond, the most unfavorable load case for a glue bond, mandating a top quality bonding process.
At variant 3 and 4, the final assembly of the main spar occurs together with the final wing assembly. The important advantage of this method is opportunity to create a very intensive bond between the spar caps and wing shells under completely controlled conditions. The difficulty of these variants is the precise manufacturing of the shear web. But since the bond between spar cap and shear web is only experiencing compression and shear loads, it is more forgiving to manufacturing tolerances. This is in particular true for variant 4, which represents the most favorable placement of the blind bond regarding normal operations of the aircraft.
Wilhelm Dirks has always realized variant 4 in all his designs.
Manfred Koethe