Jet engine performance

In fixed-wing aircraft driven by one or more jet engines, certain aspects of performance such as thrust relate directly to the safe operation of the aircraft whereas other aspects of the engine operation such as noise and engine emissions affect the environment. In fixed-wing aircraft driven by one or more jet engines, certain aspects of performance such as thrust relate directly to the safe operation of the aircraft whereas other aspects of the engine operation such as noise and engine emissions affect the environment. The thrust, noise and emission elements of the operation of a jet engine are of vital importance in the takeoff phase of operation of the aircraft. The thrust and fuel consumption elements, and their variation with altitude, are of vital importance in the climb and cruise phases of operation of the aircraft. The behaviour of a jet engine and its effect both on the aircraft and the environment is categorised into different engineering areas or disciplines. For example, the emissions come under a group called combustion, the origin of vibrations transmitted to the airframe come under an area called rotor dynamics. So what is performance? The understanding of how a particular fuel flow produces a definite amount of thrust at a particular point in the flight envelope is called jet engine performance. Performance is the subject of a specialised discipline within aero engine design and development teams as is the understanding of noise and emissions by their respective specialists in other groups. The fundamental performance task for a single shaft turbojet is to match the operation of the compressor, turbine and propelling nozzle. For example, the way the compressor operates is determined by the flow resistances behind it, which occur in the combustor, turbine, tailpipe and propelling nozzle. Matching may be defined as designing, sizing, and manipulating the operating characteristics of the compressor, turbine and propelling nozzle. Three fundamental observations are built upon as outlined below to develop the required understanding to match the components efficiently. The flow through the compressor is the same as that through the turbine. The speeds are the same. The power produced by the turbine equals that absorbed by the compressor. In addition, the flow resistance seen by the compressor is determined by the two restrictors downstream, namely the turbine nozzle area and the propelling nozzle exit area. The above three ties between the compressor and turbine are adjusted and refined to account for the flows and powers not being equal due to, for example, compressor flow and electric and hydraulic power being diverted to the airframe. Thus the performance is understood and defined by using the practical engineering application of thermodynamics and aerodynamics.