Month in the Patent Office

Abstract

Fig. 2 shows a blade carried by a head of the kind described in Specification. 435,818. The root of the blade comprises a steel tube 32 provided with a fairing 33, Fig. 3, which is a sliding fit over supporting arm 30 and is rotatable to vary the blade pitch. The outer end of tube 32 is secured to blade spar proper 34. The blade is anchored to the hub by a torsionally resilient tie rod 35 screwed at its outer end into spar 34 and secured by a nut and tapered collet device 36. At the inner end rod 35 is secured into arm 30 and secured by a screwed plug and taper pin assembly 37. The blade is of lancet shape and is arranged so that axis B—B of the spar intersects the flapping and drag pivot axes and in the normal mean position of the blade intersects the axis of rotation at the mean centre of oscillation F of the blade pitch control gear. The masses and aerofoil sections of the blade are such that the centres of mass and mean centres of pressure of all the sections lie along axis B—B. The construction of the blade is such that the “ neutral torsional axis,” defined as the locus of points in the chord at which an applied vertical thrust produces equal degrees of flexure of the leading and trailing edges, is at or slightly in front of the axis B—B. In the latter case increase in lift tends to decrease the angle of incidence of the blade as is shown in Fig. 6 wherein C is the centre of pressure, L the lift force, and 0 the neutral torsional axis. In either arrangement aerofoil sections having a stable centre of pressure travel may be employed. In order to bring the neutral axis forward, the nose portion of the blade, in the case of hollow stressed‐skin construction, may be reinforced by additional layers of material or may comprise material having a higher modulus of elasticity than the remainder. In order to compensate the resulting forward movement of the centre of mass, a small amount of non‐structural mass may bo incorporated in the blade. In one form in which the neutral torsional axis is coincident with the B—B axis, the blade comprises a spar and an aerofoil‐shaped fairing of material of the synthetic resin or plastic group of which the modulus of elasticity is so much lower than that of the spar as not to relieve the latter appreciably of its loads. Fig. 7 shows the method of construction of such a blade comprising a steel spar having a moulded fairing. A first mould comprises upper and lower dies 1, 2 and an interposed core 3. Spar 4 is located by pegs 5 and by rows of spaced raised points 6, and is also fluted to key the moulding. Steel wires 8, 9 are strung in the spaces forming the leading and trailing edges. The blade is formed with a solid nose and with internal ribs 10 and webs 11, the latter being produced by slots formed in the upper side of core die 3. After moulding as shown, dies 2 and 3 are removed, pegs 5 cut off, countersunk, and plugged, Fig. 9, and a lower die 13 placed in position and heat applied to unite the lower skin to ribs 10 and to seal the trailing edge. A suitable plastic material is stated to be “ plastic glass.”