Lesson One
BernouIli’s Principle Why Planes Fly
Aviation is dependent upon the principle discovered by an eighteenth century scientist，Daniel Bernoulli. ①Bernoulli‘s pr inciple helps to explain why an airplane flies，how a wing produces lift，and how fuel flows in an aircraft carburetor．It explains the relationship between the pressure，the vo1ume and the velocity of a fluid in motion．Simply stated，the velocity of a given volume of a fluid in motion is inverly proportional to its pressure. As velocity increas，pressure decreas．Since both gas and liquids are classified as fluids．We‘ll u air to demonstrate the properties of a fluid in motion．
There are veral simple experiments that demonstrate Bernoulli‘s principle．The simplest may be conducted with only a sheet of paper．②Hold the paper up to your lips find blow across the top surface．Notice that the paper ris．The velocity of the air over the top of the paper is greater than that under the paper．A low pressure area is created above the paper and the difference in pressure caus the paper to ri．Perhaps you walk past the vacuum cleaner department of department store and you‘ll e a ping—pong ball suspended in the air above vacuum cleaner emingly without sport．The air passing around the curved surface of the ball is traveling faster than the air just be on the surfa
ce．③Again, the higher pressure outside the airflow caus the ball to remain in the center of the airflow．
Another simple experiment us a piece of paper, a straight pin and a simple wing spoo1．First，push the pin through the paper and inrt it in the hole in the spool，the pin simply keep the paper from dropping to the floor . Now, blow through the other end of the spool, a low pressure area is created as the velocity of the air increas between the spool and paper．The higher atmospheric pressure on the back of the paper holds the paper clo to the end of the spool．The experiments illustrate the fact that when the velocity of the fluid increas, the pressure decreas．Converly, when the velocity decreas, the pressure Increas How does this apply to aviation? Let's place a restriction in the path of moving air in this wind tunnel．Smoke injected into the air stream indicates the path the air takes around and through the restriction．The restriction is called a venturi，a given volume of air travels through and around the venturi in a given period of time．The entire volume must pass al1 points in the system. Let‘s cons ider 3 molecules of air to be our given volume，all 3 molecules must pass around and through the venturi and meet at the other end at the same time. Notice that the molecule passing through the venturi must follow a longer curved path, traveling at greater distance in the same period of time as the molecule that follows a straight path．That means the molecules traveling through the venturi must travel faster. According to Bernoulli‘s principle，as
抖动的近义词
the velocity increas, pressure decreas，this caus a reduced pressure in the center of the venturi，while pressure remains the same at either end．
Now let‘s look at a half ction of the venturi, does the shape remind you anything? Y ou are looking at the cross ction of an airfoil，such as a wing the primary lift producing surface of an aircraft．The same principle applies here as in the venturi，the 2 molecules of air reprent a certain volume，both molecules must reach the trailing edge of the wing ction in the same period of time，but the molecule traveling over the upper surface must travel further than the one traveling under the wing becau a curved path it must follow. Again，the molecule traveling the curved path must travel taster than the one traveling the straight line. The pressure differential between the upper and lower surfaces of a wing produces lift that keeps the aircraft aloft④We may illustrate simply the tremendous amount of force that generated by lift．Suppo the lower surface of the wing has an area of about 15，000 square inches, remembering that pressure is a force per unit area, a pressure of 15 lbs／inch²would produce a total force on the bottom of the wing of 225，000 pounds．The curved upper surface of the wing has a greater surface area than the lower surface，perhaps 20，000 square inches．However the pressure on the upper surface is less per square inch．If the pressure we‘ve reduced to 10 lbs／inch²on the upper wing surface，the total force would be reduce
d to 200，000 pounds．The difference in pressure between the lower and upper wing surfaces is 25，000 pounds．The greater force acting on the bottom wing surface，this is an extremely over simplified explanation of the production of lift on the wing．However，the basic idea is sound．⑤There are other factors to consider，such as the shape of the airfoil，the angle of attack，and lift and drag coefficients as well as the speed of the airfoil through the air．We‘ve en how Bernoulli‘s principle creates lift on an aircraft wing whether in powered or unpowered flight．⑥But，the principle also applies to any important function in aircraft engines，specifically the fuel flow through an aircraft carburetor．Let‘s return for a moment to the venturi in the wind tunnel，you recall that the pressure in the restriction of the venturi was lower than the pressure outside the venturi．A venturi is found in the primary air passage of float—type aircraft carburetor. As air is drawn into the engine by the pistons，pressure is reduced in throat of the carburetor．Let‘s inrt a tube in the narrowest part of the venturi．and attach the tube to a fuel supply which is open to atmospheric pressure．The difference in pressure between atmospheric pressure on the fuel supply and pressure in the venturi caus fuel to flow into the venturi where fuel is mixed with air before entering the engine combustion chambers．V enturi or orifices may be found in other parts of the aircraft，in a hydraulic line or a fuel control，a measured orifice produces a measured pressure differential between two sides of the line，causing fluid to flow at a pre—determined rate．
Let‘s review we‘ve learnt about the relationship of pressure and velocity of a fluid in motion．Bernoulli‘s principle says that as the velocity of fluid increas，its pressure decreas．As the velocity of air passing through a venturi increas，the pressure in the narrowest part of the venturi decreas．As the velocity of air increas over the upper surface of a wing，the pressure decreas，the imbalance of forces between the upper and lowar surfaces caus the wing to ri．Bernoulli‘s principle is also partly responsible for fuel flow in an aircraft carburetor which us a venturi in the primary air passage．V enturi may also be ud to create a measured rate of fluid flow．V enturi-like orifices are found in aircraft hydraulic systems and in fuel controls．
Words and Phras
Bernoulli‘ s principle 贝努利原理题西林壁教案
长颈鹿用英语怎么说carburetor 汽化器n．
inverly proportional 反比于
lip 唇n．
throat 咽喉，狭口n．
property 性质n．
vacuum cleaner 真空吸尘器
suspend 悬挂v．
curved surface 弯曲表面
wing spool 缝纫机线轴
converly 相反地ad．
restriction 限定，节流n．
venturi 文氏管，细腰管n．
molecule 分子n．
cross ction of an airfoil 翼型剖面
sound 完好的，合理的a．
angle of attack 攻角，迎角
piston 活塞n．
orifice 小孔n．
lift and drag coefficients 升力和阻力系数
hydraulic 液压的a．
reinforce 加强v.
concept 概念，基本原理n．
Notes
①Aviation is dependent upon the principle discovered by an eighteenth century scientist，Daniel
Bernoulli．航空学与十八世纪科学家丹诺尔·贝努利所发现的原理密切相关。
to be dependent upon意为“依靠于…”“取决于…”。discovered by…是过去分词引导的定语修饰the principle。
②The simplest may be conducted with only a sheet of paper．可用一张纸来进行最简单的试
验。
to be conducted with意为“由…处理”“由…进行”。a sheet of paper意为“一张纸”，paper 是不可数名词，所以一张纸必须用a sheet of来表示，同样的用法还有许多。
③The air passing around the curved surface of the ball is traveling faster than the air just be on the surface．绕球曲面流过的气流比即将到达球表面的气流速度要快。
passing around the curved surface是现代分词引起的定语，修饰the air；faster than…意为“比…快”，比较的内容前后要一致。
④The pressure differential between the upper and lower surfaces of a wing produces lift that keeps the aircraft aloft．机翼上下表面的压差产生了升力，使飞机保持在空中。
⑤However the basic idea is sound．但基本思想是正确的。sound在这里是形容词。
⑥We‘ve en how Bernoulli‘s principle creates lift on an aircraft wing whether in powered or unpowered flight．我们已经看到了，不论是有动力飞行还是无动力飞行，贝努利原理是如何在机翼上产生升力的。
whether…or…意为“不管…还是…”。
Grammar
用it作形式主语的各种结构
常见形式有：
请看下面例句：天麻片怎么吃
It is often inconvenient to combine vectors by the geometric method．
用几何方法来合成矢量通常是不便当的。
It is worthwhile to u an ultra—high vacuum system for such purpo．
把超高真空装置用于这种场合是有价值的。
If a body moves from one position to another，it is said to have had displacement．
如果物体从一处移到另一处，就认为它有了位移。
It is common practice to build rockets in pieces．
通常的做法是把火箭分级制造。
It takes sunlight over eight minutes to travel to the earth．
太阳光传播到地球要8分多钟。
It requires a long—term practice to be a good pilot．
要成为优秀的飞行员需要长时间的实践。杜甫之七言绝句
It makes steel weak in strength to rust．
生锈使钢的强度下降。
Exercis
怎么制作南瓜饼1. Answer the following questions：
1) What relationship is described mainly by Bernoulli‘s principle?
2) How the paper be moved when you hold the paper up to your lips and blow across the top
旧手机搬家到新手机surface? and why?
3) In which ction of the venturi the pressure is lowest when the air stream pass the tube?
天津寝园4) Why the air traveling over the upper surface must travel further than the air traveling under
the wing?
5) List some factors that must be consider to calculate the lift except the pressure difference．
6) What is the function of the measured orifice in a hydraulic line or a fuel contr01 7
7) Why the powered flight come into being? (The lift can be created by just the wings
according to Bernoulli‘s principle．)
2. Choo the correct answer：
1) Which one is not correct?
