Main Rotor System of Helicopter | Types of Main Rotor System

Main Rotor System of Helicopter | Types of Main Rotor System & Advantages

Main Rotor System

The rotating part of a helicopter that provides lift is known as the main rotor system. A mast, hub, and rotor blades constitute the rotor. The mast is a cylindrical metal shaft that extends upwards from the transmission and is powered and supported by it. The hub is the attachment point for the rotor blades at the top of the mast. The rotor blades are subsequently joined to the hub using a variety of techniques. The way the main rotor blades are mounted and move in relation to the main rotor hub is how main rotor systems are classed. Rigid, semirigid, and fully articulated are the three basic types.

Rigid Rotor System

The rigid rotor system is the most basic. The rotor blades in this arrangement are fixed to the main rotor hub and cannot slide back and forth (drag) or move up and down (flap). The forces that cause the rotor blades to bend are absorbed by the blade's flexible characteristics. The pitch of the blades, on the other hand, can be modified by rotating the feathering hinges around the spanwise axis.

Mechanically, a rigid rotor system is simpler than a fully articulated rotor system. Instead than using hinges, rotor blades bend to handle aerodynamic and mechanical loads from flapping and lead/lag forces. The blades themselves adjust for the forces that previously necessitated the use of robust hinges by flexing. Because of the huge hub moment normally created, the outcome is a rotor system with minimal latency in control response. As a result, the rigid rotor design minimises the risk of mast bumping that semirigid rotors provide.

Semirigid Rotor System

A teetering hinge at the blade attach point is used in the semirigid rotor system. The teetering hinge allows the blades to flap up and down while keeping them from sliding back and forth. When one blade flaps up, the other flaps down with this hinge.
Dissymmetry of lift, which causes flapping, is a phenomenon. An advance blade and a retreating blade are established as the plane of rotation of the rotor blades is slanted and the helicopter begins to fly ahead (on two-bladed systems). On an advancing blade, the relative windspeed is higher than on a retreating blade. This enables the advancing blade to produce more lift, forcing it to rise up or flap. The extra lift is lost when blade rotation approaches the point where the blade becomes the retreating blade, and the blade flaps downward.

Mast bumping is a problem that can occur in helicopters with semirigid rotors, causing the rotor flap stops to shear the mast. The semirigid rotor system's mechanical design demands that the blades' downward flapping must have a physical limit. Excessive rotor flapping results in mast banging. There is a maximum flapping angle for each rotor system design. The static stop will hit the mast if flapping exceeds the design threshold. The static stop is a part of the main rotor that provides strap fittings with limited movement and a contoured surface between the mast and hub. Mast damage or separation is caused by strong contact between the static stop and the mast during flight. Mast bumping is proportional to the amount of blade system flap.

Blade flapping is negligible in straight and level flight, perhaps 2° under normal flying conditions. With high forward speeds, low rotor rpm, high-density altitudes, large gross weights, and while facing turbulence, flapping angles rise slightly. Larger flapping angles can be induced by manoeuvring the aircraft in a sideslip or during low-speed flight at extreme CG locations.

Fully Articulated Rotor System

The hinges on fully articulated rotor blade systems allow the rotors to move fore and aft, as well as up and down. The Coriolis effect occurs during rotational speed variations, causing this lead-lag, drag, or hunting movement. The blades lag when they first start spinning until the centrifugal force is completely created. Once the blades have started rotating, a reduction in speed leads them to trail the main rotor hub until the forces are balanced. The blades of the rotor "hunt" due to constant changes in rotor blade speeds. Because they are positioned on the vertical drag hinge, they are free to do so in a fully articulating system.

On a fully articulated rotor design, one or more horizontal hinges allow for flapping. The feathering hinge also allows for variations in blade pitch by allowing rotation around the spanwise axis. Different versions have different dampers and stops to lessen stress and limit travel in particular directions.
There are several designs and variants on the three primary rotor systems. Engineers are constantly looking for solutions to lessen vibration and noise created by the helicopter's moving parts. Elastomeric bearings are increasingly being used in primary rotor systems to achieve this goal. These polymer bearings may distort and revert back to their original shape. As a result, they can absorb vibrations that steel bearings would ordinarily transmit. They also don't need to be lubricated on a regular basis, which cuts down on maintenance.

The soft-in-plane rotor system is a variant of the fully articulated system. This rotor can be found on a number of Bell Helicopter aircraft, including the OH-58D Kiowa Warrior. This system works similarly to the fully articulated system in that each blade may lead/lag and hunt independently of the others. The difference between a fully articulated system and a soft-in-plane system is that a composite yoke is used in the soft-in-plane system. This yoke connects the mast to the blade grips and goes between the blades and the shear bearing inside the grip. This yoke transfers some blade movement to another blade, usually opposite blades. While the flight characteristics are not clearly specified, they are fairly close.

Modern Helicopter Main Rotor System

Flexures have been designed into certain modern helicopter main rotors. Advanced composite materials are used to create hubs and hub components. They're made to absorb the pressures of blade hunting and lift asymmetry by flexing. As a result, several hinges and bearings in the typical main rotor system can be eliminated. As a result, the rotor mast is simpler and requires less maintenance because there are fewer moving parts. Elastomeric bearings are frequently used in flexure systems.