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7

 In the above picture, a main spar is being dry fit. The straight edge reference  is the set of blocks held by C-clamps on the left. The blocks on the right are screwed to the worktable and are used to apply clamping pressure. The tapered extension of the web in the foreground will be reinforced and act as one of the supports of the wingtip bow when the wing is assembled.

 

 As can be seen in the  picture, the clamping pressure in the gluing together of the spar is supplied by two tapered blocks which, when shoved against each other, force the capstrip-web-capstrip against the straightedge of the clamped blocks.

The construction of the rear spar is different, as the capstrips are glued to the face of the web, top and bottom. The capstrips must be glued one at a time, with the edges of both capstrip and web being forced against the straightedge while the glue sets up. The picture above shows a spar cap being glued to the web of a rear spar.

Aside from strength, which is supplied by the architecture and material of the spar, the other important attribute of a spar is straightness. This was provided by building the spars against a very carefully aligned straightedge. This consisted of 2”x 4” blocks clamped down along a chalk line and then trued against a 6’ straightedge which was slid back and forth along the clamped blocks. The position of the blocks was slightly adjusted until there was no error. As each set of caps and web was glued together to form a spar, the newly  glued structure was clamped against the straightedge formed by the 2”x 4” blocks to harden.

R-80 Tiger Moth – Wing Spars