chapter 2 section 2 why airplanes fly II.ppt

1、Why Airplanes Fly (II),- Drag,The Classification of Drag,Induced drag (诱导阻力) Parasite drag（寄生阻力；废阻力） Total drag （总阻力）,The total drag is the combination of the induced drag and the parasite drag,Induced drag(诱导阻力),Drag is any force acting parallel (平行) to the flight path but in the opposite direction

2、. The resultant force created by an airfoil is basically in the correct direction for lift, but not exactly. The resultant force （合力） is not perpendicular to the relative wind. Instead, it slants slightly toward the direction of the relative wind.,While lift (perpendicular to the relative wind) is c

3、reated, some drag is also induced by the creation of lift. And this component of the wings resultant force is called induced drag(诱导阻力).,Induced drag，lift and angle of attack,Induced drag is the undesirable but unavoidable by-product of lift, and increases in direct proportion to ( 正比)increases in a

4、ngle of attack. The greater the angle of attack up to the critical angle, the greater the amount of lift developed, and the greater the induced drag.,Parasite drag （寄生阻力；废阻力）,Any solid object that moves through the air must disturb and displace the air molecules along its path. The air molecules res

5、ist this disturbance, and the resistance is manifested as a drag force called parasite drag. Parasite drag is the resistance of the air produced by any part of the airplane part that does not produce lift.,The amount of parasite drag depends on a number of interacting factors, such as the size of th

6、e object, its shape, and the roughness of its surface.,Several factors affect parasite drag. When each factor is considered independently, it must be assumed that other factors remain constant. The more streamlined （流线型的）an object is, the less the parasite drag. The more dense the air the airplane m

7、oves through, the greater the parasite drag. The larger the size of the object in the airstream, the greater the parasite drag. As speed increases, the amount of parasite drag increases. If the speed is doubled, four times as much drag is produced.,The more streamlined an object is, the less the par

8、asite drag,The parasite drag is generally divided into three types: Form drag (形状阻力，构形阻力) Skin Friction drag （表面摩擦阻力 ） Interference drag （干扰阻力） Form drag is caused by the frontal area （迎风面）of the airplane components being exposed to the airstream. Skin friction drag is caused by air passing over the

9、 airplanes surfaces and increases considerably if the airplane surfaces are dirty and rough, Interference drag is caused by interference of the airflow between adjacent parts of airplane such as the intersection of wings and tail sections with the fuselage., Form drag（构形阻力） The effective size of an

10、object as it moves through the air is called its frontal area (迎风面积). You can visualize frontal area as the shadow an object would cast if a light source came from the same direction as the relative wind.,A given objects frontal area can vary depending on its attitude with respect to the relative wi

11、nd, just as the shape of its shadow depends on how it is presented to the light. A thin board placed edge first into the wind has a small frontal area compared to the same board placed broadside into the wind.,In general, the larger the frontal area, the higher the drag force. However, the shape of

12、the object also has a powerful effect on drag. A flat plate facing the wind broadside creates a very large drag force, but if that same frontal area is enclosed in a teardrop shape, the drag is reduced enormously.,This aspect of parasite drag, which depends on both frontal area and shape of the obje

13、ct, is called form drag.,Most aircraft have protrusions （凸出部分） that add unwanted but necessary frontal area, such as radio antennas（天线）, wing struts, or fixed landing gear. Aircraft designers often enclose these items in a metal or plastic shroud （覆盖物）called a fairing （整流罩）, which is shaped and orie

14、nted to reduce drag as much as possible., Skin Friction drag（表面摩擦阻力 ） The slowing of air molecules due to skin friction drag is another component of parasite drag. Smooth surfaces are obviously better than rough ones. The amount of skin friction drag is proportional to the total amount of surface ar

15、ea of the object., Interference drag（干扰阻力） When two different shapes are joined together, such as a wing and fuselage, air may not flow smoothly near the intersection, creating interference drag. Designers often add specially shaped piece of metal or plastic, called a fillet（整流片）, to blend the surfa

16、ces and reduce the interference drag.,Total drag （总阻力）,The total drag on an airplane in flight is the combination of its induced drag and parasite drag.,Total drag = parasite + induced,Factors That Influence Lift and Drag,The aerodynamic forces on an airplane are influenced by four variables: Its si

17、ze and shape. The density of the air through which it is flying. The angle of attack of its wing. The speed at which it is moving through the air.,Size and Shape,Although the manufacturer determines the basic size and shape of an airplane, the pilot does have some ability to modify shape by using th

18、e control surfaces.,Air Density,An airplane creates forces by moving air molecules around, so naturally those forces are influenced by the density of the air. Density is the number of molecules in a given volume of air.,Lift and drag increase with air density because more molecules are being affecte

19、d (all other factors being constant).,Lift and Drag versus Angle of Attack,The most direct way a pilot can control lift is through the angle of attack of the wing. Increasing the angle of attack increases lift and induced drag (all other things being equal).,Stall phenomenon,There is, however, an up

20、per limit to what the wing can do. At low angles of attack, the airflow follows the airfoil surface closely and is efficiently redirected downward. As the angle of attack increases, the airflow begins to separate from the upper surface of the airfoil at its trailing edge, and this separation produce

21、s an area of disturbed air.,The separated airflow hinders the creation of downwash and increases drag significantly. This separation continues to increase as the angle of attack increases. When enough lift-producing downwash is replaced by swirling eddies （旋转气流）of disturbed air, lift begins to decre

22、ase.,At this point the airfoil is stalled. By definition, a stall occurs when lift decreases as a result of an increase in angle of attack. The angle of attack at which the stall begins is called the critical angle of attack. (临界迎角),It must be emphasized that only angle of attack is responsible for

23、the stall process; Neither speed nor the attitude of the aircraft controls it. An airfoil stalls any time the critical angle of attack is exceeded. A stall can occur any airspeed and in any aircraft attitude. The only way to recover from a stall is to reduce the angle of attack.,Stall and wing desig

24、n,The wing of most airplanes is designed so that stalling occurs progressively rather than all at once. The designers goal is to make the inboard part of the wing stall first and the tips last.,There are two benefits to this scheme: First, the disturbed air from the stalled inboard wing strikes the

25、fuselage and tail, creating a buffet (noise and shaking) on the airframe. Because it comes early in the stall process, this important warning signal should not be ignored.,Second, the control surfaces on the outboard part of the wing (the ailerons) remain effective even while most of the wing is sta

26、lled, so the pilot has control of the airplane as long as aerodynamically possible. A wings progressive stall characteristics are built into the design by patterning different shapes of the airfoil along the span of the wing or by building the wing with its tips at a lower angle of attack than the i

27、nboard part.,Lift and Drag versus Speed,The most powerful factor in the creation of lift and drag is the speed at which the airplane moves through the air. If all other variables are held constant, lift and drag vary with the square of the speed. This means that if speed is doubled, the lift and dra

28、g forces increase four times. Conversely, if speed is halved, the aerodynamic forces become one-fourth their initial value.,In the real world of flying, of course, other variables are not constant. In fact, in steady flight (constant speed, no change in altitude), lift is held constant (equal to wei

29、ght). So let us examine the effect of speed on an airplane in level flight.,Lift and drag in level flight As an airplane changes speed, it must simultaneously change its angle of attack in order to keep lift constant. For instance, an increase in speed must be counteracted by a decrease in angle of

30、attack. But changes in speed and angle of attack also affect the various types of drag.,We know that induced drag changes with angle of attack. Therefore, in level flight, as speed increases, induced drag decreases.,Since parasite drag is a function of the number of air molecules being disturbed, it follows that parasite drag increases with speed, whether the airp