JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech IV HELICOPTER ENGINEERING Previous Question Paper (December - 2020 - R16)

A single rotor helicopter consists of several primary components that work in unison to achieve vertical flight, control, and stability. The main components are:

1. Main Rotor System: This is the primary lifting device, consisting of rotor blades, a hub, and a pitch change mechanism. It generates the necessary lift and thrust to support the helicopter and control its attitude.
2. Fuselage: The main body of the helicopter that houses the crew, passengers, cargo, and critical systems like the engine, transmission, and avionics. It is designed to be aerodynamic to minimize drag.
3. Powerplant (Engine): Typically a turboshaft engine, it provides the mechanical power required to drive the main and tail rotors through the transmission system.
4. Transmission System: This includes the main gearbox, which reduces the high rotational speed of the engine to a lower, efficient speed suitable for the rotor blades, and provides drives to the tail rotor and accessories.
5. Tail Rotor System: Located at the end of the tail boom, it provides anti-torque force to counteract the torque generated by the main rotor and allows for directional control (yaw) via pedal inputs.
6. Landing Gear: Supports the helicopter while on the ground and absorbs the energy of landing. It can be skid-type, wheel-type, or float-type.
7. Flight Control System: Comprises the cyclic pitch control (for pitch and roll), collective pitch control (for altitude), and anti-torque pedals (for yaw control).

Sketches of a single rotor helicopter typically depict the fuselage as a central pod with a tail boom extending rearward. The main rotor assembly is mounted on top of the fuselage, driven by a transmission connected to the engine. The tail rotor is mounted at the end of the tail boom, often with a vertical stabilizer. Control linkages are shown connecting the cockpit controls to the swashplate assembly at the main rotor hub.

For a helicopter rotor blade with a lead-lag hinge, the equilibrium condition is determined by the balance of moments acting about the hinge pin. The lead-lag motion is governed by the inertial, aerodynamic, and centrifugal forces acting on the blade.

The equation of motion for the lead-lag angle ζ is given by:

Where:

• I_ζ is the mass moment of inertia of the blade about the lead-lag hinge.
• C_ζ is the damping coefficient.
• K_ζ is the spring stiffness (if present).
• M_CF is the restoring moment due to centrifugal force.
• M_A is the aerodynamic moment.

The centrifugal restoring moment is expressed as:

At equilibrium, the angular acceleration and velocity are zero (d²ζ/dt² = 0 and dζ/dt = 0). The equilibrium condition simplifies to the balance between the aerodynamic drag moment and the centrifugal restoring moment:

For a blade in steady forward flight, the aerodynamic drag moment M_A varies periodically due to the azimuthal change in velocity. The equilibrium position ζ_eq is therefore defined by the average drag moment balanced against the centrifugal force, leading to the condition where the blade oscillates about a mean lag angle determined by the rotor torque and rotational speed.