CASE STUDY Aircraft wing sine wave spar

Proposed designs for composite wings often consist of a number of spars with laminated skins. A multispar design means that load on the individual spars is low and buckling tends to be the dominant design criterion. A structure of sine wave configuration is ideal for this application. The design of the sine wave configuration itself is influenced by a number of factors:

• The size and spacing of fasteners through the flanges.
• The width of the web.
• The critical buckling load.
• The ease of processing.

Different geometries are possible but it is found that a wave configuration based on arcs which are not tangential, but separated by a small flat region is the optimum of the alternatives. Of the other options tangential arcs pose tooling difficulties and a true sinewave has insufficient buckling stability. A typical spar cross-section is shown below.

Typical wing spar cross section

The webs of the spars have three layers; two CFRP cloth plies with fibres orientated at [±45°] to transmit shear loads, and one unidirectional CFRP ply in the centre to provide vertical stiffness. The [±45°] layers are folded over to form the flange. Additional reinforcements are applied each side of the web, orientated at 90°, to transmit the load from the fasteners into the web. The structure is completed by capping plies on the tops of each flange. The stability of the web as a function of the number of laminate layers is shown below.

Stability of web designs for aircraft wing sine-wave spar

Matched metal tooling can be developed for the sine wave spars, as, in detail, the spars are not symmetrical about a centre-line and the tools must therefore be capable of splitting into several parts.

Tooling for aircraft wing sine-wave spar