An aircraft structural member such as a wing or fuselage is provided with a series of suction ducts formed in transversely disposed structural elements and embodying porous portions through which suction of the boundary layer is effected. The skin consists of outer and inner sheets 2, 13 connected by transverse channel members or, as shown in no. 1, by corrugated sheets 12, the outer sheet 2 being locally thickened at intervals for the reception of strips 1 of sintered bronze, rolled gauze or perforated material. The portion of the sheet 2 below each strip 1, and the walls of the associated duct 5 are perforated, the ducts being placed in communication with one or more suction pumps. In an alternative construction the strips 1 are replaced by sets of spanwise rows of fine, closely spaced perforations in the sheet 2. These rows of perforations may, in a further arrangement, be formed in shallow recessed grooves in the outer surface of the sheet 2, which is covered with a thin continuous sheet of porous material, such as steel or nickel alloy gauze. In another construction, FIG. 2, the outer sheet 2a is formed with ridges 22 defining rectangular or polygonal recesses, a continuous outer layer 2b of porous material being secured by gluing or bonding to the ridges 22. In the case of a wing the members constituting the ducts 5 act as stringers, while in the case of a fuselage they serve as frames. The porosity of the inserts, or the capacity of the pumps may be varied to modify the degree of suction over different portions of the surface.
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