Jawaharlal Nehru Technological University Hyderabad B.Tech IV HELICOPTER ENGINEERING Previous Question Paper (July/August - 2022 - R16)

Rotor systems are classified based on the method used to attach the blades to the rotor hub and the degrees of freedom allowed for blade motion. The three main types are the fully articulated, semi-rigid, and rigid (hingeless) rotor systems.

1. Fully Articulated Rotor System

This system uses three or more blades, each attached to the hub through a series of hinges that allow independent movement. Sub-classifications are typically based on the arrangement of these hinges:

• Flapping hinge: Allows the blade to move vertically to balance lift across the rotor disk.
• Lead-lag (drag) hinge: Allows the blade to move horizontally in the plane of rotation to compensate for Coriolis effect.
• Feathering hinge: Allows the blade to change its pitch angle for cyclic and collective control.

Features: Excellent maneuverability and reduced vibration, but mechanically complex and heavy due to the number of hinges and bearings.

2. Semi-Rigid Rotor System

Also known as a teetering rotor, this system typically uses two blades rigidly attached to the hub, which is then mounted to the mast via a teetering hinge. Sub-classifications include:

• Underslung: The blades are mounted below the teetering axis to reduce Coriolis forces.
• Fixed-pitch or variable-pitch variants: Depending on the control mechanism.

Features: Mechanically simple and lightweight. However, it is susceptible to mast bumping during low-G maneuvers and has limited control authority compared to articulated systems.

3. Rigid (Hingeless) Rotor System

In this system, the blades are rigidly attached to the hub, but the system relies on the inherent flexibility of the blade materials (such as composites) to accommodate flapping and lead-lag motions. Sub-classifications include:

• Flex-beam: Uses a flexible structural element to replace traditional hinges.
• Bearingless: Eliminates all mechanical bearings by using torsionally flexible materials for pitch change.

Features: Provides high control sensitivity and responsiveness because blade flapping directly translates to hub moments. It is simpler than fully articulated systems but requires advanced materials to handle high fatigue loads.