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发表于 2006-10-10 19:18:09
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来自： 中国北京
不得已把八九年前刚上学时候的入门英语教材找出来了...瀑布汗-_-...
手工录入的啊,累死我了,希望对楼主有用
The theory of flight is based on aerodynamics which is the study of objects in notion through the air and the forces that produce or change such motion.Mechanics and electricians who work on the aircraft need not know every physical detail,but they must understand the relationship between atmosphere,the aircraft,and the forces acting on it in flight.This knowledge is necessary in order to make decisions afficting flight safety.understanding why the aircraft is designed with a special type of primary and secondary control system and why the surfaces must be smooth is important when maintaining today's complex aircraft.
When air flows over the upper surface of an airfoil,its speed or velocity increases.At the same,the pressure decreases and a low pressure area is formed.The pressure on the lower surface of the airfoil is higher than on the upper surface.The difference in pressure between the upper and the lower surfaces of the wing causes an upward force which is called lift.
--Any form that is designed to create lift is called an airfoil.
in order to understand how an aircraft flies and why is stays in the air you should know the four forces that act on the aircraft in flight:
(1) Lift,the force that acts on the aircraft in an upward direction.
(2) Gravity,the force that pulls the aircraft to the earth.
(3) Thrust,the force that moves the aircraft forward.
(4) Drag,the force that is caused by the resistance of air an that exerts a braking action.
These for forces are in balance when the aircraft is in level (unaccelerated) flight.
To understand the principle of lift,a knowledge of the wing is essential.The front edge of the wing is called the leading edge.The rear edge of the wing is called the trailing edge.The surface on the top of the wing is curved or cambered.
A thick wing with a high degree of camber produces more lift than a thin wing with a low degree of camber,but it also causes more drag.
For horizontal flight,lift and gravity must be equal.If the lift decreases,the aircraft loses altitude.In other words,lift is required to counteract the gravitational force.
The force which moves the aircraft forward is called thrust.Thrust is produced by the engines when a mass of air moves backwards at high velocity.This causes a forward reaction that moves the aircraft.
The relative motion of the air over an object that produces lift also produces drag.Drag is the resestance of the air to objects moving through it.
In order to maintain a steady speed,thrust and drag must remain equal.If the R.P.M. of the engine is increased,thrust becomed greater than drag,and the speed of the aircraft increases.If you want to double the spped,you need four times the thrust.
In order to get more thrust,you need more fuel,which also increases the weight of the aircraft.
Today's aircraft designers have to keep in mind all these details when constructing new aircraft types.It is their job to decide if a higher speed is more important than a high lift,or vice versa.They also modify and improve the wings because the quality of the aerodynamic design influences the concept of a new airplane.The aim is maximum efficiency an high subsonic speed,but also to construct a wing which is able to fly at speeds slow enough for take-off and landing.For this reason aircraft designers developed the winglets(or vortex diffusers),which can reduce drag in cruise by about 10% and improve climb performance and stability.
The so-called supercritical wing is a wing that can be flown much faster(up to Mach 0.95 for example) before a shock wave develops.The supercritical airfoil has a relatively flat upper surface.Air above the wing therefore doesn't accelerate as much as when flowing over an airfoil that has more camber.
These are only two examples of technical development to improve aerodynamic efficiency and reduce the fuelconsumption of today's modern airplanes. |
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