JNTU B.Tech III Year AEROSPACE PROPULSION I Previous Question Paper (September - 2021 - R13)

i) Gross thrust represents the total momentum thrust produced by the engine nozzle without accounting for the momentum of the incoming air. It is defined as the product of the mass flow rate of the exhaust gases and their exit velocity, plus the pressure thrust term. The correlation is given by:

where m_e is the mass flow rate of exhaust, V_e is the exit velocity, p_e and p_a are the exit and ambient pressures, and A_e is the nozzle exit area. It is significant because it represents the raw force generated by the engine's internal thermodynamic cycle before subtracting the ram drag of the inlet.

ii) Thrust-to-weight ratio (T/W) is a dimensionless parameter indicating the engine's capability to overcome its own weight and provide excess force for acceleration or climb. It is defined as:

where F is the net thrust and m_e g is the engine weight. A higher ratio is critical for high-performance aircraft as it directly dictates climb rate, maneuverability, and the ability to achieve supersonic flight. It serves as a primary metric for engine design efficiency regarding power density.

The primary advantage of a turboshaft engine over a turbojet engine is the extraction of nearly all available energy from the exhaust gases to drive a shaft, rather than relying on high-velocity exhaust for propulsion. Key advantages include:

• High Torque Output: Turboshaft engines are designed to drive rotors or propellers, providing high torque at lower speeds, which is essential for vertical takeoff and landing (VTOL) aircraft like helicopters.
• Efficiency at Low Speeds: Unlike turbojets which are inefficient at low flight speeds, turboshafts are optimized for stationary or low-speed operations where propeller or rotor efficiency is superior to jet exhaust.
• Versatility: The power produced is mechanical rather than propulsive, allowing the engine to be used for diverse applications including marine propulsion, industrial power generation, and heavy-lift rotorcraft.