The net thrust formula for a jet engine, such as a turbojet or turbofan, can be derived based on the principle of conservation of momentum and the mass flow rate of air and fuel through the engine.
where:
- Ë™ is the mass flow rate of air (and fuel, if applicable) through the engine (kg/s),
- is the velocity of the exhaust gases relative to the engine (m/s),
- is the velocity of the aircraft relative to the surrounding air (m/s),
- is the pressure of the exhaust gases at the nozzle exit (Pa),
- is the ambient (free stream) air pressure (Pa),
- is the cross-sectional area of the engine exhaust nozzle (m2).
Net thrust, fundamentally, represents the force generated by a jet engine, crucial for propelling aircraft forward. This force is the outcome of the engine’s operation, harnessing the principles of Newton’s third law of motion, which states that for every action, there’s an equal and opposite reaction. Initially, air and possibly fuel are ingested and accelerated through the engine, then expelled at a higher velocity. This process creates a forward-directed force, or thrust, propelling the aircraft.
Moreover, the net thrust calculation incorporates both the momentum and pressure differences between the engine’s exhaust and the ambient air. Specifically, it considers the mass flow rate of air (and fuel) through the engine, the exhaust gases’ velocity relative to the engine, and the aircraft’s velocity. Additionally, it accounts for the exhaust pressure at the nozzle’s exit compared to the ambient air pressure, alongside the exhaust nozzle’s cross-sectional area. These components are pivotal in determining the engine’s efficiency and effectiveness in generating forward motion.
Furthermore, optimizing net thrust is vital for achieving desired performance characteristics, such as takeoff, climb, and cruise speeds. Engineers meticulously design and tune jet engines to maximize thrust while minimizing fuel consumption and emissions. This optimization process involves a delicate balance, ensuring the engine operates effectively across various flight conditions.