The same principles apply to thrust generation, though not for destructive purposes. The rate of change of momentum determines the forces associate with, as in this example, a collision. If on the other hand, the truck is brought to a sudden halt, such as hitting a concrete wall, the damage can be far severe. If the same slow moving heavy truck is brought to a gradual halt, the damage caused is lower. Theoretically, the lightweight car mentioned in this example, if travelling fast enough, can inflict as much damage as a slow moving heavy truck. Larger the momentum, more are the forces associated with a collision. Mass and speed determine momentum: larger of either, or both, will result in a greater momentum. A car travelling at a higher speed than an equally lightweight but slower car will inflict a greater damage in a collision. Watch this video to understand how a jet engine works:Ī heavy truck travelling at the same speed as a light car will inflict a greater damage in a collision. This allows the creation of an engine that marries the best of both worlds: the advantages of a jet engine (such as high reliability, low failure rates, the ability to operate at high altitudes, and the high energy conversion efficiencies), with the benefits of methods that deliver high propulsive efficiency. The hot exhaust contributes to little (turbofan), or no thrust (turboshaft, turboprop). The energy extracted from the hot gases, via an extra set of turbines, drive either the fan, the propeller, or the rotor attached to a shaft, leaving very little energy in the hot gases that leave the turbines. In all three derivatives of the “pure” jet engine, the actual jet engine is relegated to the compressor, combustion chamber, and the turbines that drive the compressor. In a turboshaft engine, the energy in the shaft is, much like a turboprop engine, passed through a reduction gearbox that drives a large rotor, such as that in a helicopter, which moves a very large mass of air at a slower speed. The high torque and low speed drives a large propeller, which moves a larger mass of air at a slow speed. In a turboprop engine, the turbines turn a shaft, which moves to a reduction gearbox that turns slower but with greater torque. The fan moves a large mass of air, at a good speed. In a turbofan engine, the energy of the hot expanding gases is used to drive a set of turbines, which turn a large ducted fan at the front of the engine. In all other derivatives of a turbojet, such as a turbofan, turboprop, and turboshaft, the propulsive efficiency is determined by the mechanism that extracts energy from the turbines in the engine, and how that is used to move a mass of air. This is achieved by accelerating a small mass of air to high speeds. For a given size, a “pure” jet engine such as a turbojet can deliver significantly more thrust than other forms of subsonic air breathing propulsions. In the case of turbojet engines, the mechanical energy generating thrust is the hot expanding gases that exit from the engine at a high speed. Propulsive efficiency deals with how effectively the extracted mechanical work is used to generate thrust. For all forms of jet engines, including turbojets, turbofans, turboprops, and turboshafts, this is the energy conversion that takes place inside the core of the engine, which includes the compressor, the combustion chamber, and the turbines that extract mechanical energy from the hot, expanding gases. Thermal efficiency deals with how efficiently the engine extracts mechanical work from a unit mass of fuel that is burnt. In this article, we disregard mechanical losses in transmissions, gearboxes, and all forms of inter-mechanical conversions. The fuel efficiency of an aircraft is dependent upon three factors: the drag contributed by the airframe, as parasitic drag from the fuselage, wings, horizontal and vertical tail planes, and induced drag from aerodynamic effects such as wing tip vortices the drag contributed by the engine, by virtue of its shape and size, and effects due to the exhaust gases and third and most importantly, the efficiency of the engine itself.įrom a high level, there are two factors that determine the efficiency of an engine: The thermal efficiency, and the propulsive efficiency.
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