Main Rotor Hub and Blade Assembly
The main rotor hub and blade assembly is a pre-coned, underslung, semi rigid rotor system composed of two metal blades that are rigidly mounted to the main rotor hub assembly.
Rotor hub
The rotor hub provides attachments for the main rotor blades, a means to change the pitch of each blade continuously and independently and a flapping axis mechanism for the main rotor. It is driven by the output shaft of the main transmission which gives the main rotor blades the necessary rotational power to provide lift for the helicopter.
The rotor hub has the following components:
Main rotor hub assembly
The main rotor hub assembly is mounted at the top of the mast of the transmission assembly and attaches the main rotor blades to the main rotor shaft of the helicopter. It provides a power path from the main rotor shaft to the blades. The parts of this assembly are made of aluminum alloy and steel. Moving parts are lubricated with grease. It consists primarily of the parts that follow:
Yoke assembly
The yoke assembly forms the main structural member of the main rotor assembly. It has two hollow spindles that form the surface on which the blade grip assemblies are mounted, and the two pillow blocks that, with the trunnion, form the flapping axis mechanism for the rotor head. The hollow spindles and pillow blocks are oriented at 90 degrees to each other. Fastened to the underside of the yoke are two blade stops that contact the main rotor shaft if excessive flapping occurs. The parts of this assembly are made of aluminum alloy and steel. There is one yoke assembly per main rotor assembly.
Blade grip assemblies
The blade grips assemblies attach the main rotor blades and provide a mechanism to rotate the blade grips over the spindle of the yoke. They are cylindrical in shape and rotate about the hollow spindle of the yoke by means of two bearings. They are held into the main rotor assembly by the tension-torsion straps. Attached to each blade grip assembly is a pitch horn which provides the means to attach the control rods that change the blade pitch. For proper lubrication, packing and seals keep the lubricating grease contained inside the blade grip assemblies. The parts of this assembly are made of aluminum alloy and steel. There are two blade grip assemblies per main rotor assembly.
Blade bolts
The blade bolts attach the rotor blades to the blade grip assemblies. This is the single point of attachment for the rotor blades. The parts of this assembly are made of steel. There are two blade bolts per main rotor assembly.
Tension-torsion straps
The tension-torsion straps are flexible structural tension assemblies which attach to the blade grip assemblies using a steel bolt and to the yoke assemblies using a steel pin. They hold the blade grip assemblies into the main rotor against the forces that pull them away from the center of rotation while allowing the blade grip assemblies to rotate about the axis of the spindle. They are mounted inside the hollow spindles of the yoke. The parts of this assembly are made of flexible steel cable and a composite binding material. There are two tension-torsion straps per main rotor assembly.
Pitch horn assemblies
The pitch horn assemblies attach to the blade grip assemblies and provide an attachment point for control rods from the swash plate which move vertically to change the pitch of the blades. The parts of this assembly are made of aluminum alloy and steel. There are two pitch horn assemblies per main rotor assembly.
Pillow blocks
The pillow blocks and the trunnion provide the flapping hinge mechanism for the main rotor assembly and are located at the top of the main rotor shaft. The pillow blocks attach the outer race of the bearing (that is between the pillow blocks and the trunnion) to the yoke. To maintain proper lubrication, packing and seals keep the lubricating grease contained inside the pillow blocks. The parts of this assembly are made of aluminum alloy and steel. There are two pillow blocks per main rotor assembly.
Trunnion
The trunnion serves as the attachment point of the main rotor to the main rotor shaft. It is attached to the yoke assembly by two pillow blocks and mates with the splined main rotor shaft. The trunnion is held in place on the rotor shaft by the mast lock nut and cone set. The trunnion and the pillow blocks provide the flapping hinge mechanism for the main rotor assembly and are located at the top of the main rotor shaft. Each spindle of the trunnion mates with the inner race of the bearing that is between the trunnion and the pillow blocks. The mast nut and cone set hold the trunnion in place on the splined portion at the top of the main rotor shaft. The parts of this assembly are made of aluminum alloy and steel. There is one trunnion per main rotor assembly.
Mast nut
The mast nut is a threaded cylindrical fastener. At the top is an eyelet and at the bottom is a disk with external splines. It secures the trunnion and the flap restraint assembly to the top of the main rotor shaft. Due to the high torque required, this part requires application with a hydraulic torque wrench. This part is made of stainless steel. There is one mast nut per main rotor assembly.
Mast nut lock
The mast nut lock is a flat U-shaped plate with two teeth. It is secured to the support of the flap restraint assembly with steel fasteners. Its teeth engage the splines of the mast nut to prevent the mast nut from loosening. This part is made of steel. There is one mast lock nut per main rotor assembly.
Cone set
The cone set is a split disk shaped part with splines on its inside surface and a tapered surface on its outside surface. Its tapered surface accurately and securely locates the trunnion in place on the top splined portion of the main rotor shaft. This part is made of steel. There is one cone set per main rotor assembly.
Blade latches
The blade latches are adjustable mechanical stops located at the ends of the bolts that hold the tension-torsion straps near the blade root. They also position and hold the blades so as to allow the precise adjustment of blade lag. There are four blade latches per main rotor assembly.
Flap restraint assembly
The flap restraint assembly is fastened to the top of the trunnion by two steel fasteners. The main rotor shaft passes through its center. The flap restraint assembly and trunnion are both held in place and attached to the rotor shaft by the mast nut and cone set. The parts of this assembly are made of aluminum alloy and steel. There is one flap restraint assembly per main rotor assembly. It consists of the parts that follow:
Support
The support is the main structural member of the flap restraint assembly. It is a rectangular flat machined plate on which is mounted two arm assemblies and which attach to one end of both the springs. The parts of this assembly are made of aluminum alloy and steel. There is one support per flap restraint assembly.
Arm assembly
The arm assemblies are hinged levers. They are pivoted bearings near their bottom and attach to the counterweights at the top and provide a cam surface at their bottom. Near the top of the lever, they attach to the springs, which pull to keep the arm assembly positioned against the stops. The parts of this assembly are made of aluminum alloy and steel. There are two arm assemblies per flap restraint assembly.
Counterweights
The counterweights are physical masses attached to the top of the arm assemblies. They are made of flat steel washers and retention hardware. There are two counterweights per main rotor assembly.
Springs
The springs attach near the top of the arm assemblies and at the support. They exert a force to keep the arm assembly positioned against the stops. These parts are made of spring steel. There are two springs per flap restraint assembly.
Rotor blades
The rotor blades are lifting surfaces and structure attached to the main rotor hub assembly in such a way that they turn with the main rotor hub. When the main rotor hub turns, the interaction of the rotor blades with the air causes a lifting force which is transferred from the rotor blades to the main rotor hub. Each of the two rotor blades are underslung with respect to the rotor hub and are mounted on opposite sides of the rotor hub.
Two main rotor blades are used on the helicopter and both are individually interchangeable. The blades are mechanical assemblies composed of aluminium alloy, steel, composite parts, and a honeycomb core. To provide lift and optimize the stability and performance of the helicopter, the rotor blades have a purposeful and precisely engineered airfoil cross section. Each blade is attached securely to the main rotor hub. The leading edges have a strip of corrosion resistant steel which protects it from damage caused by sand, water, and other abrasive particles. Inboard and outboard trim tabs are provided for tracking adjustments. The blades have weight lugs on either side of station 28 and tip weight supports that are set with required weights from the factory and are not adjustable. The weights for balancing are housed in the blade bolt. The blades are flexible so as to withstand the cyclical forces acting on them.
Operation
Power:
Power is transferred from the main rotor shaft to the trunnion. The trunnion then transfers the power to the pillow blocks that are firmly attached to the yoke. The yoke then transfers the power to the blade grips which transfers it to the blades.
As the main rotor blades rotate, they react with the air which results in a lifting force generally upwards.
Rotor control:
The angle of attack of the blades are changed by the up or down movement of the pitch links which rotate the blade grip assemblies about the spindles of the yoke. Since increasing the pitch of the blades increases the lift and vice versa, varying the pitch of the blades cyclically allows directional control of the helicopter.
The mechanical structure in the blades transfer the lifting force to the main rotor hub.
Flapping:
The main rotor is underslung from the flapping hinge mechanism which provides the means for the main rotor to rock in a see saw fashion about the main rotor shaft. The flapping hinge mechanism is composed of the trunnion and the pillow blocks and a shared bearing. Since the trunnion is firmly attached to the main rotor shaft and the pillow blocks are firmly attached to the yoke, the flapping hinge also serves as the mechanical connection of the main rotor to the main rotor shaft. Being underslung, the movement of the center of mass of the main rotor is minimized during flapping which reduces rotor vibration. The flapping motion is as follows: as the helicopter flies forward, the advancing blade flaps up. Since the main rotor is on a see saw, this forces the retreating blade down. This see saw movement serves to equalize the lifting force on the main rotor. Excessive flapping during startup and shutdown is controlled by the flap restraint assembly.
Flap restraint assembly:
The flap restraint assembly prevents excessive flapping of the main rotor during the start and shutdown of the helicopter but allows normal flapping at operating RPM.
When the main rotor is not turning, the arm assemblies are held in their rest position against the stops by the force of the springs. In this state, the cam surface at the bottom of the arm assemblies protrude below the bottom of the support and function as a stop to reduce the allowed range of flapping. As the main rotor starts to rotate, the centrifugal force on the counterweights move the arm assemblies from the vertical against the pull of the springs which cause the arm assemblies to pivot away from their rest position. As the arm assemblies pivot away from their rest position, the cam surface on the bottom moves upward and away from its stop position thereby increasing the allowed range of flapping. At main rotor operating speed, the cam on the bottom of the arm is fully retracted above the bottom surface of the support which allows the main rotor to flap at its full range.
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