国家精品课程：“机械制造装备设计”第一章.pptVIP

* * * * * * * * * * * * * * * * * * * * * * * On the ground, motive power can be provided by pushing against the ground, to accelerate in the opposite direction, or to maintain speed against resistive forces. In flight, an aircraft can not push against the ground. An aircraft could be propelled by throwing passengers violently out of the back of the aircraft. This would not be good practice, nor would it be continuous. The ejection mechanism would apply a force on the ejecting passenger. An equal and opposite force would be applied by the ejecting passenger on the ejection mechanism. It is this force that would propel the aircraft. Jet engines work by continuously scooping up air at engine approach speed Vflight and accelerating it rearwards to eject it at a fully expanded jet speed Vjet. This can be a continuous process since air is always available within the atmosphere. The engine applies a force to the air to accelerate it, and the air applies an equal and opposite force to the engine. This force is transmitted by the engine mounts to the aircraft. Any engine which gives thrust by accelerating a body of air rearwards can be considered as a form of jet propulsion. Propeller engines are in this class. More commonly, the term jet propulsion is reserved for applications where the jet emerges from a rear nozzle at high speed. Newton’s Second Law states that force equals rate of change of momentum. Therefore thrust is given by W(Vjet - Vflight). There is some approximation in this expression if at exit from the engine nozzle the flow is not yet fully expanded to atmospheric pressure. For steady level flight thrust is equal to drag. For accelerating flight thrust is greater than drag. A simplified turbojet aeroengine. The compressors at the front consist of rotors mounted on discs and stators fitted in a casing. As the air passes through multiple stages of rotor and stator it keeps on being turned to and fro in such a way that pressure incr