625,778. Rotary wing blades. CIERVA AUTOGIRO CO., Ltd., PULLIN, C. G., and SHAPIRO, J. S. March 31, 1947, Nos. 8723 and 31926. [Class 4] A rotor blade for a rotary-wing aircraft is made up of two principal components, a tubular metal main spar 11, Fig. 1, the section of which conforms to the leading portion of the aerofoil section, and a light metal skin element 12 that defines the remainder of the aerofoil section, the two components being bonded together over a large contact area by means of a metal-to-metal adhesive 15 requiring no pressure for bonding and completely filling the space between the surfaces. The main spar component 11 may be of carbon steel, alloy steel or high-strength non-ferrous alloy. It may be made from strip metal shaped in dies and closed by means of a seam preferably located on the internal web portion 13, formed from seamless tube and tapered over a mandrel, or built up from two or more tubes fitted one within the other. The skin component 15 of thin light-alloy sheet, which may be of composite type, having a layer of light low-strength material such as balsa wood, or calcium alignate sandwiched between thin metal sheets which may be in one or two parts with seams at the trailing and/or leading edges. The skin component may be in two parts 12 and 17, Fig. 3, joined at the webs 18 and 19 by the same bonding agent 15 as for the main spar or may be wrapped around the front of the main spar component, as in Fig. 6, with channel-section auxiliary spars 25a and 25b, and a light alloy extrusion 23 secured to the trailing edges. The bonding of the two components at 15 must be by an adhesive with " gap-filling " properties to ensure no small voids are left in the bond. A junction of this type may be obtained by forming a plastic layer on the metal surfaces with phenol-formaldehyde, or the material known by the Registered Trade Mark " Formvar," and disposing between them a layer of phenol-formaldehyde, urea-formaldehyde, or a resorcinol formaldehyde derivative. Alternatively, the "hot" rubber process or the adhesive known by the Registered Trade Mark " Araldite " may be used. All the bonding materials must be electrolytically inert. The root fitting of the rotor blade is in the form of a tapered cranked sleeve of high-strength material 27, Fig. 8, made in two halves and clamped to the root part of the main spar 11a by bolts on the edge flange. The sleeve 27 has at its inner end a circular collar 29 with an internal shoulder 30 for connection to the blade-mounting stub. The rear skin component of the rotor blade is tapered along its length, Fig. 10, to a greater degree than the main spar.