Torque Reacting Means for the Hubs of Cross Beam Rotors

1. Field of the Invention

This invention relates to cross beam rotors for helicopters which utilize composite rotor spars.

2. Description of the Prior Art

Heretofore helicopters having cross beam spars of composite material were attached to the rotor hub by a large number of bolts because these bolts were required to transmit torque between the rotor drive shaft and the blades and hence were subjected to heavy shear loads. Also the lug areas where the bolts passed through the material of the composite spars were required to be reinforced to transmit this hub to spar torque. Due to the large number of bolts required too many critical longitudinal fibers of the blade spars were interrupted by the bolt holes, requiring many cross patterned layers of composite build-up to strengthen these areas as a shear lug definition and for bearing of the bolts. This is an expensive time-consuming, weight-penalty operation.

It is an object of this invention to provide an improved construction for reacting the torque between the rotor hub and the blade spars of a cross beam rotor in which composite spars are used.

It is a further object of this invention to provide a cross beam rotor construction in which the tension bolts which pass through the blade spars for securing them to the rotor hub are not required to react the rotor torque and consequently can be fewer in number and smaller in size for a four-bladed cross beam rotor.

It is another object of this invention to greatly simplify the fabrication of the composite spars in the hub area while at the same time providing superior torque transmission between the rotor hub and the blade spars.

To this end the confronting surfaces of the blade hub structure are provided with intermeshing ribs and grooves which greatly increase the shear area in the composite material of the blade spars.

FIG. 1 is a plan view of a cross beam rotor, the outboard portions of the blades being broken away to facilitate illustration;

FIG. 2 is a sectional elevation of the rotor shown in FIG. 1;

FIG. 3 is an exploded view of the hub portion of the rotor shown in FIGS. 1 and 2; and

FIG. 4 is a partial section of a blade spar showing the raised rib composite construction.

Referring first to FIGS. 1 and 2, 10 is the rotor hub lower element which is made of metal, such as aluminum alloy, and is fixed to the upper end of drive shaft 12 by a plurality of bolts 13, two of which show in FIG. 2. Upper and lower cross beam rotor blade spars 14 and 16, a spar-to-spar filler piece 18 and a top bearing cap 19 complete the rotor hub elements. Tension bolts 20 pass through hub elements, 19, 14, 18 and 16 in that order and are bolted with threaded nuts below a flange 21 on lower hub element 10 as shown in FIG. 3.

Blade spars 14 and 16 each are of composite material construction with a thickened and widened hub portion. The flat upper surface 22 of rotor hub lower element 10 has two semi-circular grooves 24 machined therein which mate with rubs 26 laminated as part of the under side of the hub portion of lower composite blade spar 16. These ribs extend longitudinally of the blade spar 16. The upper surface of blade spar 16 has two semi-circular ribs 30 laminated as part of the upper side of its hub portion which are parallel with ribs 26. Ribs 30 mate with grooves 32 machined in the under surface of filler piece 18. The upper surface of filler piece 18 has two grooves 34 machined therein which are at 90.degree. to grooves 32. Grooves 34 mate with ribs 36 (not visible in FIG. 3 but shown in FIG. 2) laminated as part of the lower surface of the hub portion of upper composite blade spar 14. The upper surface of the blade spar 14 has semi-circular ribs 38 laminated as part of its hub portion which mate with grooves 40 machined in the lower surface of top bearing cap 19. It will be noted that ribs 26 and 30 on composite blade spar 16 and ribs 36 and 38 on composite upper blade spar 14 are directed longitudinally of the blades and since they are laminated as part of the composite material of the spars no fibers are cut by these ribs.

Holes 42 for the four bolts 20 are provided at the four corners of the rotor hub elements. These holes in the composite material of both blade spars and the blade-to-blade filler piece are oversize so that the bolts 20 cannot transmit any torque loads directly to the blade spars 14 and 16. It will be noted that the oversize bolt holes pass through upper and lower composite blade spars 14 and 16 outside the critical longitudinal fibers.

While in this preferred embodiment two mating ribs and grooves have been shown between the confronting surfaces of the rotor hub elements, it will be understood that a larger number or a single mating rib and groove could be used; also instead of mating ribs and grooves of the semi-circular cross section a different cross sectional shape could be used. Formed stainless steel chafing strips may be bonded to all mating rib and groove surfaces in the composite material if desired. In the rotor illustrated these chafing strips are not needed and have not been shown.

In the operation of our improved rotor, all torque reactions between the blades and the rotor hub are taken by the ribbed and grooved surfaces of the various hub elements. The oversize holes in the composite material for tension bolts 20 prevent these bolts from transmitting torque loads into the composite material of the blade spars. Since these bolts are essentially tension members, only four bolts are needed and the four bolts shown are located in the corners of the widened and thickened hub portions where their holes do not sever critical longitudinal fibers of the blade spars 14 and 16. The lug areas surrounding the bolt holes 42 in the composite material do not have to be reinforced in making the blades, since they have no part in transmitting torque loads.

While we have shown and described in considerable detail one embodiment of our invention, we do not want to be limited to the details elaborated herein as many variations in construction will occur to those skilled in this art which fall within the scope of the appended claims.