Lesson OneBernouIli’s Principle Why Planes FlyAviation 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 inversely proportional to its pressure. As velocity increases，pressure decreases．Since both gases and liquids are classified as fluids．We’ll use air to demonstrate the properties of a fluid in motion．There are several 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 rises．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 causes the paper to rise．Perhaps you walk past the vacuum cleaner department of department store and you’ll see a ping—pong ball suspended in the air above vacuum cleaner seemingly without sport．The air passing around the curved surface of the ball is traveling faster than the air just be on the surface．③Again, the higher pressure outside the airflow causes the ball to remain in the center of the airflow．Another simple experiment uses a piece of paper, a straight pin and a simple sewing spoo1．First，push the pin through the paper and insert 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 increases between the spool and paper．The higher atmospheric pressure on the back of the paper holds the paper close to the end of the spool．These experiments illustrate the fact that when the velocity of the fluid increases, the pressure decreases．Conversely, when the velocity decreases, the pressure Increases 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，asthe velocity increases, pressure decreases，this causes a reduced pressure in the center of the venturi，while pressure remains the same at either end．Now let’s look at a half section of the venturi, does the shape remind you anything? You are looking at the cross section 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 represent a certain volume，both molecules must reach the trailing edge of the wing section in the same period of time，but the molecule traveling over the upper surface must travel further than the one traveling under the wing because 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．Suppose 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 reduced 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 seen 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 insert 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 causes fuel to flow into the venturi where fuel is mixed with air before entering the engine combustion chambers．Venturi 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 increases，its pressure decreases．As the velocity of air passing through a venturi increases，the pressure in the narrowest part of the venturi decreases．As the velocity of air increases over the upper surface of a wing，the pressure decreases，the imbalance of forces between the upper and lowar surfaces causes the wing to rise．Bernoulli’s principle is also partly responsible for fuel flow in an aircraft carburetor which uses a venturi in the primary air passage．Venturi may also be used to create a measured rate of fluid flow．Venturi-like orifices are found in aircraft hydraulic systems and in fuel controls．Words and PhrasesBernoulli’ s principle 贝努利原理carburetor 汽化器n．inversely 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sewing spool 缝纫机线轴conversely 相反地ad．restriction 限定，节流n．venturi 文氏管，细腰管n．molecule 分子n．cross section 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．Notes①Aviation is dependent upon the principle discovered by an eighteenth century scientist，DanielBernoulli．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。