Month in the Patent Office

In order to avoid the formation of ‘knuckle’ lines at the edges of stiffeners bonded to a panel when the skin is bent to shape, the flanges of the stiffeners are pre‐curved in the sense in which they will be curved in the final assembly and then bonded to the skin while in a flattened condition. In the construction shown in FIG. 1a a panel 20 is provided with longitudinal stiffeners 21 of rounded ‘top‐hat’ section, which are passed between rollers to bend the flanges 22 in the required direction (FIGS. 1b and 1c) to an extent slightly greater than that required in the finished assembly. They are then bonded to the skin 20 by a thermo‐setting resin, the bonding being carried out in a press on a flat platen under a pressure of about 100 lb./sq. in. applied to the upper surfaces of the flanges 22 to flatten them against the skin 20, which latter may also have been pre‐curved in the desired direction. The panel is then bent to its final shape, as shown for example in FIG. 1d, in which stiffeners of the form shown in FIG. 1b are arranged on the inner surface of the sheet 20. As shown in FIG. 1e the method may also be used when the stiffeners run in the direction of the panel curvature, the stiffeners 61 then being pre‐curved longitudinally before being bonded to the skin 60.