(a) Field of the Invention
This invention relates to bearingless rotor hubs to which the rotor blades of a helicopter are attached and more particularly to rotor hubs providing an element having bending and torsional flexibility about certain axes for connecting the blades to the hub.
(b) Description of the Prior Art
A helicopter rotor blade is formed such that its cross-section defines an outer contour that is airfoil-shaped. Due to the rotation of the blade, the forward flight of the helicopter and the angle of attack existing between the ambient airstream and the mean chord of the airfoil, an aerodynamic lift force is developed along the blade length. This lift varies along the length as a higher order function of the velocity of the airstream relative to the blade, and likewise varies around the rotor azimuth in response to relative velocity variations. In addition to local relative velocity variations, other parameters affecting lift are also changing. For example, a rotor blade is generally formed so that its airfoil thickness contour varies uniformly along its length; the lift is proportional to airfoil thickness provided other parameters are held constant. The control system alters the local angle of attack around the rotor azimuth in a manner that produces the minimum angle at the rotor position where local airstream velocity is maximum and conversely a maximum angle of attack where airstream velocity is minimum.
These and other design parameters are adopted in recognition that lift on the rotor has an inherently changing value both along the blade length and around the rotor. The intent is to produce a more nearly uniform lift on the rotor blade regardless of its position or the speed of the aircraft. This desired result has, of course, not been attained due to the unpredictable variations in the conditions under which the aircraft must operate. Furthermore, since drag on an airfoil is proportional to lift and directed perpendicular to the lift vector, in-plane load variations similar to the normal-plane effects of lift forces are produced as well.
Since the normal and in-plane forces are distributed along the length of the blade, bending moments, in addition to the various blade, planar and normal-plane forces, one of which is lift, are produced at a given rotor blade radius station due to the forces outboard of the particular radius station. These bending moments, which increase in magnitude at the inboard regions of the blade radius, as do the planar and normal plane forces, would be transmitted to the aircraft by way of the attachment of the rotor blade to the rotor hub and thence to the vertical shaft, transmission control system and eventually to the aircraft cabin structure and fuselage. It has long been recognized that this is an undesirable result since it introduces excessive vibration to the aircraft and, because of the great magnitude of the bending moments so produced, tends to control the attitude of the aircraft by external flight conditions rather than by the operations of the pilot on the control system.
Therefore, in order to reduce the magnitude of the moments actually transmitted to the aircraft by these bending moments, a rotor was developed that incorporated at least two hinges at the inboard region of the rotor blade, the axis of one hinge being generally normal to the rotor plane and the other generally in the rotor plane. The effect of this development was to reduce the bending moments to zero at the hinge since a hinge can transmit no moment about its axis, but the direct in-plane and normal-plane forces are capable of being transmitted past the hinge joint. In this way, the moments actually transmitted to the aircraft are, in magnitude, the product of the direct forces at each hinge and the radial offset of the respective hinges from the vertical shaft-rotor hub attachment joint. This lesser moment is known as the control moment and is instrumental in maintaining attitudinal control and disposition of the aircraft. The radial position of the hinges can be chosen by design to produce the optimal amount of control moment for the requirements of the aircraft. Typically, the inner most hinge is located between two and four percent of the rotor radius for tandem helicopters and between five and eight percent of the rotor radius for single rotor helicopters.
More recent developments in rotor hub and blade design have allowed the elimination of the rotor hinges and the production of so-called bearingless rotors. This advance has been largely made possible by the introduction of the fiber-reinforced resin-matrix materials, which allow the stiffness properties of the rotor blade root adjacent its attachment to the rotor hub to be adjusted to suit certain optimal and desired values. Rotors having these properties are characterized by being relatively flexible in the in-plane and normal-plane bending sense and relatively stiff about the other axes and senses of structural displacement. However, design advancement, even with this newer concept, has been curtailed by a tendancy of bearingless rotors to exhibit a effective or virtual hinge radial offset in excess of ten percent of the rotor radius. The hinge offset is virtual, only, because, since no actual hinge or bearing is employed, the flexible bending nature of the blade root produces a sharp gradient of bending rotation in the vicinity of the blade root. This gradient is so pronounced as to give the appearance and the operational effect of the conventional normal-plane and in-plane hinge bearings. The resulting ten percent hinge offset is realizable, furthermore, only by employing complicated shear bearings and other devices that tend to reduce the magnitude of the offset conventional rotor design has heretofore demonstrated to be optimal.
A second device, known as a "fixed shoe" provides a preferred inner contour to which the deflected rotor blade must conform, which contour produces the desired radial offset. The fixed shoe as well as shear bearings result in the addition of considerable complexity and high wear between the chafing surfaces of the bearing and between the outer surface of the blade root and the shoe contour. It was the original intent of eliminating hinge bearings to eliminate their associated design complexity and the concomitant fatigue difficulties that rubbing, fretting and chafing surfaces are well known to cause.