19,550. Zens, P. Aug. 20. Screw propellers.-The blades of screw propellers for aerial machines are constructed of the desired curvature, or are rendered capable of being adjusted as to curvature or pitch, by composing them of ribs made up of upper and lower laths, which are connected together so as to be adjustable longitudinally, but are relatively fixed at their extremities. In cases where the surfaces are made of material such as veneer or aluminium, the upper and lower faces are made to play the part of the laths of the ribs, which may be considerably reduced in thickness, or dispensed with. The tendency of the surfaces to become altered in curvature under the action of the wind may be controlled by elastic devices connected to the ribs. In the form shown in Fig. 1, the construction is such that the distance between the upper and lower laths remains practically constant during variations of curvature, and comprises an upper lath a, secured to beams b, b<1>, and a lower lath divided into two parts c, c' which overlap. The part c<1> is also connected to the beams b, b<1>, while the part c is connected to the lath a directly at the outer end, and at its inner end by one or more brackets g<1>, which are secured to the lath c and slide on the lath a. The laths a, c are further connected together by straps g and distance-pieces h, which are fixed to the lath a and slide on the lath c. Variation of curvature is effected by means of a cam i which is rotated by a spindle j for adjusting the parts c, c<1> longitudinally. In modifications, the upper lath, instead of the lower lath, or both laths, may be divided into overlapping parts Moreover, the front part of the surface may be constructed in a similar manner to the rear part, so that the alteration of curvature takes place along the whole rib. In the form shown in Fig. 6, the parts a, c<1> of the divided laths are fixed to one or both beams b, b<1>, and the other parts a', c are connected by a bracket g, which is acted upon by a cam or crank i, which bears against the bracket g as shown in Fig. 1, or is connected thereto by a link i<2>. In cases where the variation in curvature required to be produced in the surface is uniform along the whole surface considered in the direction of the beams b, each of the ribs of which the surface is composed is given the same amount of warping by arranging the cams i in the same phase along the operating-spindle j. Where, however, the variation in curvature is to be of a helical nature, the various cams i are arranged slightly in advance of each other. In a modification, the warping may be effected by securing the material which forms the surface at one end to one set of overlapping parts of the laths and then passing it around so as to cover the whole of the ribs and connecting it to a roller arranged adjacent to the fixed end. In a further modification, the upper and lower laths of the ribs may be kept from moving apart during alterations of curvature by means of bands or sheaths of material which are wrapped around the laths in a direction transversely of the ribs. In the form shown in Fig. 9, which shows the application of the invention to a balancing-surface for aeroplanes, the cam i is fixed in the position shown, so that the curvature is altered as indicated bv dotted lines, when the surface is adjusted about its pivot. Springs i' ensure the return of the surface to its normal position. The cam i may be replaced by a cam-groove, or by a pair of links i<2>, Fig. 10, which are connected to the pivot or to a rod i<3> passing therethrough. In order to avoid creasing of the material forming the surface as the curvature is varied, such material may be made wholly or partly elastic, or the adjustable element of each lath may be extended so that it overlaps the stationary element to the end of the rib, and when the ribs are warped, the upper and lower surfaces of the material overlap. Fig. 13 shows one form of elastic device for controlling the alteration in curvature of the surface under the action of the wind, a piece of elastic l being arranged between the adjustable bracket g and a fixed part b<0>. In modifications of this elastic control, the resilient material may be arranged between the beam b and the rear ends of the ribs. In building up propeller blades, a number of ribs are arranged at suitable distances along a beam, and are then given the desired curvature and covered with material. The pitch of the blades may be adjusted by the means described above for varyirg the curvature of the ribs.