航空专业英语：专英1教案第5章

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1、Ch.5 Stability and control5.1 Balance and trim1,balance in straight and level flight balance 平衡 see-saw 杠杆 bean 梁 wooden bean 木梁 suspended bean 悬吊梁fulcrum 支点 trim 配平 equilibrium 平衡 control column 操纵杆 rudder pedal 方向舵脚蹬 align, alignment 成一直线 Balance consists of two elements-the total forces acting th

2、e aircraft and the alignment of these forces. When the forces are balanced and aligned the aircraft is said to be in equilibrium. There are two types of forces-static forces, dynamic forces. Weight is a static force it can be considered constant at any time. Thrust varies with engine power, propelle

3、r rpm and airspeed but can be set at a constant value by the pilot. Lift is aerodynamic force which changes with airspeed and flap extension but which can be controlled directly by the pilot changing the angle of attack. Drag changes with angle of attack, configuration and airspeed. In straight and

4、level flight, lift opposes weight L=W thrust opposes drag T=D. The lift and weight will only decrease gradually as the weight decreases with fuel burn-off. The thrust and drag will vary considerably depending on angle of attack and therefore airspeed.2, pitching moment Under most conditions of fligh

5、t the CP and CG are not coincident, i.e. are not at the one point, causing nose-down pitching moment or a nose-up pitching moment. The different lines of action of the thrust force and drag force produce another couple, causing a nose-down pitching moment or a nose-up pitching moment.Ideally the pit

6、ching moment from the two couples should neutralize each other in level flight so that there is no resultant moment tending to rotate the aircraft 3, the tailplane horizontal stabilizer 水平安定面 function 功能,作用 counteract 平衡,中和 residual 剩余的, neutralize 抵消 coincident 一致 The function of tailplane（or horiz

7、ontal stabilizer）is to counteract these residual pitching moments from the two main couples and to damped any oscillation in pitch, i.e. it has a stabilizing function. The tailplane usually has a symmetrical or a negatively cambered aerofoil. The moment produced by the tailplane can be varied by eit

8、her by moving the elevator or by moving the entire tailplane. The moment arm of tailplane is quite long, and the aerodynamic force provided by the tailplane needs only to be small to has a significant pitching effect. The area of the tailplane is small compared with the mainplanes（main wings）.5.2 St

9、ability There are two elements of stability, static and dynamic, and for an aircraft, it is usual to separate the modes into the three axes of movement. There is longitudinal stability, lateral stability and directional stability. There is an inseparable relationship between lateral and directional

10、stability.5.2.1 Static stability and dynamic stability1,Static stability The static stability of the aeroplane describes its tendency to return its original condition（angle of attack）after being disturbed and without any action being taken by the pilot. The strength of the tendency is the measure of

11、 its stability.2, Dynamic stability remove 取消,去除 dead beat 非周期的,无振荡的 indefinitely 无限地 divergent 发散 short period 短周期, long period, phugoid 长周期, Dynamic stability is concerned with the motion of the body after the disturbing force has been removed. It is an oscillation which may stop immediately（well-

12、damped or dead beat），continue but reduce slowly（slightly or lightly damped），continue indefinitely（undamped）or get worse（dynamically unstable or divergent oscillation）.Imagine the aircraft is trimmed in straight and level flight, the aircraft will respond to the vertical gust by pitching down to main

13、tain its trimmed angle of attack.This oscillation is generally well-damped and reduces to zero in 1 or 2 oscillation. The damping is provided by the air on the horizontal area of the aircraft. The pitching oscillation is known as the short period pitching oscillation（SPPO）. The airspeed of the aircr

14、aft may also change and this can cause a slower（long period）oscillation where the aircraft leisurely（慢慢地） follows a path where the airspeed and altitude are exchanged. The slow motion is called the phugoid.3,the three reference axes We refer（归类） the motion of the aircraft to motion about each of thr

15、ee axes-each passing through the centre of gravity and each mutually perpendicular （at 90o to each other）. These are sometimes called body axes. Stability around the longitudinal axis is known as lateral stability.Stability around the lateral axis is known as longitudinal stability.Stability around

16、the normal axis is known as directional stability.Rotation around a point or axis is called angular motion; the number of degrees of rotation is called angular displacement, and the speed with which it occurs, angular velocity.The motion of an aircraft is best considered in each of the planes separa

17、tely, although the actual motion of the aircraft is a little more complex. For example ,rolling into a level turn the aircraft will not only roll but also pitch and yaw. We will consider longitudinal stability first, then directional stability and lateral stability. Roll and yaw are closely connecte

18、d.5.2.2 Longitudinal stability inbuilt 固有的,内在的, dart 飞镖, arrow 箭 To be longitudinal stable, an aircraft must have a natural or inbuilt tendency to return to the same angle of attack after any disturbance without any control input by the pilot. If the angle of attack is suddenly increased by a distur

19、bance, then force will be produced that will lower the nose and decrease the angle of attack.1，the tailplane and longitudinal stabilityIf a disturbance, such as a gust, changes the attitude of the aircraft by pitching it nose up, the tailplane will be presented to the relative airflow at a greater a

20、ngle of attack. This will cause the tailplane to produce upward, or decreased, aerodynamic force, which is different to that before the disturbance. The altered aerodynamic force gives a nose-down pitching moment, tending to return the aeroplane to its original trimmed condition. Example：the tail fi

21、n of a dart or an arrow.2，the CG and longitudinal stability The further forward the CG of the aircraft, the greater the moment arm for the tailplane, and therefore the greater the turning effect of the tailplane lift force. A forward CG leads to increased longitudinal stability and aft movement of t

22、he CG leads to reduced longitudinal stability. The more stable the airplane, the greater the control force that you must exert to control or move the airplane in manoeuvers, which can become tiring.The tailplane provides static longitudinal stability.3，design considerationTailplane design features a

23、lso contribute greatly to longitudinal stability- tailplane area, distance from the centre of gravity, aspect ratio, angle of incidence and longitudinal dihedral are considered by the designer. At high angle of attack the mainplane may shield the tailplane or cause the airflow over it to be turbulen

24、t. This will decrease longitudinal stability.5.2.3 Directional stability Directional stability of an aeroplane is its natural or inbuilt ability to recover from a disturbance in yawing plane without any control input by the pilot If the aircraft is disturbed from its straight path by the nose or tai

25、l being pushed to one side（i.e. yaw）. The vertical fin（or tail or vertical stabilizer）is simply a symmetrical aerofoil. As it is now experiencing an angle of attack, it will generate a sideways aerodynamic force which tends to take the fin back to its original position. The powerful moment（turning e

26、ffect）of the vertical fin, due to its large area and the length of its moment arm between it and centre of gravity, is what restores the nose to its original position. The greater the fin area and keel surface area behind the CG, and the greater the moment arm, the greater the directional stability

27、of the aeroplane. The fin provides directional static stability. As the yaw causes rolling moment so that behavior of the aircraft with yaw and sideslip involves both its directional stability and its lateral stability.5.2.4 Lateral stability Lateral stability is the natural or inbuilt ability of th

28、e aeroplane to recover from a disturbance in the lateral plane, i.e. rolling about the longitudinal axis without any control input by the pilot.A disturbance in roll will cause one wing to drop and the other to rise. When the aeroplane is banked, the lift vector is inclined and produces a sideslip i

29、nto the turn. As a result of this sideslip, the aeroplane is subjected to a sideways component of relative airflow. This generates forces that produces a rolling moment to restore the aeroplane to its original wings-level position.1，wing dihedral Each wing is inclined upwards from the fuselage to th

30、e wingtip, and adds to the lateral stability characteristics of the aeroplane.Positive wing dihedral increases lateral stability.As the aircraft sideslips, the lower wing, due to its dihedral, will meet the upcoming relative airflow at a greater angle of attack and will produce increased lift./The u

31、pper wing will meet the relative airflow at a lower angle of attack and will therefore produce less lift. It may be shielded somewhat by the fuselage, causing an even lower lift to be generated. The rolling moment so produced will tend to return the aircraft to its original wings-level position. Neg

32、ative dihedral, or anhedral has a destabilizing effect. In some aircraft with a high-mounted sweep wing, anhedral is used to compensate for excessive lateral stability.2，wing sweepback The wing can add to lateral stability if it has sweepback. As the aircraft sideslips following a disturbance in rol

33、l, the lower sweepback wing generates more lift than the upper wing. This is because in the sideslip the lower wing presents more of its span to the airflow and higher velocity than the upper wing and therefore the lower wing generates more lift and tends to restore the aeroplane to a wing-level pos

34、ition.3，high keel surfaces and low CG In the sideslip that follows a disturbance in roll, a high sideways drag line caused by high keel surfaces（high fin, a T-tail high on the fin, high wings, etc.）and a low CG will give a restoring moment tending to raise the lower wing and return the aircraft to t

35、he original wings-level position. 4，high-wing aroplaneIf a gust causes a wing to drop, the lift force is tilted. The resultant forces will cause the aircraft to sideslip. The airflow striking the upper keel surfaces will tend to return the aircraft to the wings-level condition.A high-wing aroplane i

36、ncreases lateral stability, it has less dihedral compared to a mid- or low wing design.5，lateral and directional stability together1) roll followed by yaw 滚转引起偏航 For lateral stability, it is essential to have the sideslip that the disturbance in roll causes.The sideslip exerts a force on the side or

37、 keel surfaces of the aircraft, which, if the aircraft is directionally stable, will cause it to yaw its nose into the relative airflow. The roll has caused a yaw in the direction of the sideslip and the aeroplane will turn further off its original heading in the direction of the lower wing.The late

38、ral stability characteristics of the aeroplane, such as dihedral, cause the lower wing to produce increased lift and to return the aircraft to the wings-level position.There are two effects in conflict here：The directionally stable characteristics（large fin）want to steepen the turn and drop the nose

39、 further.The laterally stable characteristics（dihedral）want to level the wing. spiral mode 螺旋模态, Dutch roll 荷兰滚 right 矫正, best 极力, wallow 摇摆If the first effect wins out, i.e. strong directional stability and weak lateral stability（large fin and no dihedral）, then the aircraft will tend to bank furth

40、er into the sideslip, towards the lower wing with nose continuing to drop, until the aeroplane is in a spiral dive. This is called spiral instability, or the spiral mode.If the lateral stability（dihedral）is stronger, the aircraft will right itself to wings-level, and if the directional stability is

41、weak（small fin）the aircraft may show no tendency to turn in the direction of sideslip, and causing the wallowing effect, Dutch roll, which is best avoided.2） yaw followed by roll 偏航引起滚转If the aircraft is displaced in yaw, it is can cause sideslip. This sideslip will cause the lateral stability chara

42、cteristics of the aircrafts wing, such as dihedral, sweepback or high-wing. This causes a rolling moment that will tend to raise the forward wing, resulting in the aircraft rolling towards the trailing wing and away from the sideslip.The aircrafts inherent directional stability（from the fin）will ten

43、d to weathercock or yaw the aircraft in the direction of sideslip.3）stability characteristics and aeroplane controlIf the directional stability is poor（small fin）and the lateral stability is good（dihedral）it can cause Dutch roll（rolling/yawing oscillation）. Often the aircraft is fitted with a yaw an

44、d/or roll damper（a small control surface driven by a rate gyro）to stop the oscillation. It is uncomfortable for the pilot and passengers.If the directional spiral is dominant（large fin）and the lateral stability not so strong, it can cause spiral instability or spiral mode. rate gyro 阻尼陀螺, dominant 占

45、优势,占主导地位6, Stability on the groundtip over 翻倒, taxing 滑行, ground loop 地转skid 空转, brake 刹车, wheel 车轮runway 跑道 The centre of gravity（CG）must lie somewhere in the area between the wheels at all times on the ground, otherwise the aeroplne will tip over - forwards or backwards. 5.3 Control The control su

46、rfaces are the means by which the pilot overcome the static stability of the aircraft and causes a change in flight path or a change in trimmed conditions. Usually there are three sets of primary control system and three sets of control surfaces： the elevator for longitudinal control and balance in

47、pitch, operated by fore and aft movement of the control wheel or column; the ailerons for lateral control and balance in roll, operated by rotation of the control wheel or sideways movement of the control column; the rudder for directional control and balance in yaw，operated by the rudder pedals.Ide

48、ally each set of control surfaces should produce a moment about only one axis but, in practice, moments about other axes are often produced as well, e.g. aileron deflection to start a roll may also cause adverse yaw.The deflection of the control surfaces changes the airflow and the pressure distribu

49、tion over the whole aerofoil and not just over the control surface itself.The effect is to change the lift produced by the total aerofoil- control surface combination. An aeroplane with too much stability designed into it has limited controllability. The designer must achieve a reasonable balance be

50、tween stability and controllability.For instance, a passenger aircraft would require more stability, whereas a fighter would benefit from greater controllability and manoeuvrability.5.3.1 Pitch control1，Elevator The primary control of angle of attack is the elevator. The pilot moves the elevator by

51、fore-and-aft movement of the control column. When the control column is moved forward, the elevators move downwards, changing the overall shape of the tail plane-elevator aerofoil section so that it provides an altered aerodynamic force. The effect is to create a pitching moment about the CG of the

52、aircraft that moves the nose down. When the control column is pulled back, the elevator moves up and an altered force is produced by tail plane-elevator aerofoil, causing the nose of the aircraft to pitch up. The strength of the tail moment depends on the force it produces and the length of the arm

53、between it and the CG. The force generated by the tailplane-elevator combination depends on their relative size and shape, the tailplane basically contributing to stability and the elevator to control. The larger the relative size of the elevator, the more the control. To retain satisfactory handlin

54、g characteristics and elevator effectiveness throughout the desired speed range ,the position of the CG must be kept within the prescribed range. The forward allowable limit of the CG is determined by the amount of pitch control available from the elevator. The aft limit of the CG is determined by t

55、he requirement of adequate longitudinal stability. Steady flight at a low speed and a high angle of attack will require significant up - elevator, and backward pressure on the control column, to keep the nose up. At a high cruise speed there will need to be a steady down deflection of the elevator t

56、o keep the nose down and maintain a low angle of attack, hence a steady forward pressure on the control column.2，The stabilator or all-flying tail Some designers choose to combine the tailplane and elevator into the one surface and have the whole tail-plane movable- known as the all moving tail, the

57、 flying tail or the slab tail.When the control column is moved the entire slab moves. 5.3.2 Roll control1，Ailerons The primary control in roll is the ailerons. The ailerons are usually positioned on the outboard trailing edge of the mainplanes. The ailerons act in opposing senses, one goes up as the

58、 other goes down, so that the lift generated by one wing increases and the lift generated by the other wing decreases.A resultant rolling moment is exerted on the aeroplane.The magnitude of this rolling moment depends on the moment arm and the magnitude of the differing lift forces. The downgoing ai

59、leron is on the upgoing wing. The upgoing aileron is on the downgoing wing.2，Adverse aileron yaw Deflecting an aileron down causes an effective increase in camber of that wing and an increase in the effective angle of attack. The lift from that wing increases, but unfortunately so does the drag. As

60、the other aileron rises, the effective camber of that wing is decreased and its angle of attack is less, therefore lift from that wing decreases, as does the drag. The differing lift force cause the aircraft to bank one way, but the differential aileron drag causes it to yaw the other way. Adverse a

61、ileron yaw can be reduced by good design incorporating differential ailerons, Frise ailerons, or coupling the rudder to the ailerons. Differential ailerons（差动） are designed to minimize adverse aileron yaw by increasing the drag on the downgoing wing on the inside of the turn. This is achieved by def

62、lecting the upward aileron through a greater angle than the downward aileron. Frise ailerons increase the drag of the descending wing on the inside of the turn. As the aileron goes up, its nose protrudes into the airstream beneath the wing causing increased drag on the downgoing wing. On the other w

63、ay, the wing is rising, the nose of the downgoing aileron does not protrude into the airstream, so cause no extra drag. Frise-type ailerons may also be designed to operate differentially, to incorporate the benefit of differential ailerons. Coupled ailerons and rudder cause the rudder to move automatically and yaw the aeroplane into bank, opposing the adverse yaw from the ailerons. The primary effect of rudder is to yaw the aeroplane, and the secondary effect is to roll it.