JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III HELICOPTER AERODYNAMICS Previous Question Paper (August/September - 2021 - R18)

A helicopter is a type of rotorcraft in which lift and thrust are supplied by one or more power-driven rotors. Its unique features include the ability to take off and land vertically (VTOL), hover in mid-air, and fly in any direction—forward, backward, or sideways—without requiring a runway. This is achieved through cyclic pitch control, which varies the angle of attack of rotor blades during each revolution to tilt the rotor disk, and collective pitch control, which adjusts the overall lift generated by the blades.

Key applications of helicopters include: search and rescue operations in inaccessible terrain, medical evacuation (medevac), urban transportation, aerial surveillance and law enforcement, offshore oil rig support, and heavy-lift utility tasks such as firefighting or construction.

A compound helicopter is a hybrid rotorcraft that combines the vertical flight capabilities of a conventional helicopter with auxiliary propulsion and lift-augmentation systems, such as fixed wings or additional propellers, to overcome the speed limitations of traditional rotor designs.

In a standard helicopter, the rotor blades must provide both lift and forward thrust. As forward speed increases, the advancing blade approaches the speed of sound (causing compressibility issues), while the retreating blade experiences stall due to low relative velocity. A compound helicopter addresses these issues by:

1. Offloading the Rotor: Fixed wings provide a significant portion of the lift during high-speed forward flight, allowing the rotor to operate at a reduced collective pitch, thereby delaying retreating blade stall.
2. Auxiliary Propulsion: Separate thrust-producing devices, such as pusher propellers or jet engines, provide forward thrust. This allows the main rotor to be slowed down, mitigating compressibility effects on the advancing blade tip.

These features enable compound helicopters to achieve significantly higher cruise speeds than conventional helicopters while maintaining efficient hover and low-speed flight performance.