航空专英1教案第5章 Forces in Flight.pdf

Ch.5 Stability and control5.1 Balance and trim1,balance in straight and level flightbalance 平 篌 亍 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 才 桑 纵 才 干 rudder pedal 方向舵脚蹬align, alignment 成一直线Figure 2.2-1 See-saw.Figure 2.2-2UnbalancedFigure 2.2-3RebalancedFigure 2.2-4 RebalancedBalance consists of two elements-the total forces acting theaircraft and the alignment of these forces. When the forces arebalanced 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, propeller rpm and airspeed butcan be set at a constant value by the pilot.Lift is aerodynamic force which changes with airspeed and flapextension but which can be controlled directly by the pilot changingthe angle of attack.Drag changes with angle of attack, configuration and airspeed.♦ L Lift (2,000 units)T :hrust (200 units) ； ： 「 『二"" brag (200 units)W Weight (2,000 units)Figure 2.2-7Lift balances weight.3nd thrust balances drag, in straight and level flight.In straight and level flight, lift opposes weight L=W thrustopposes drag T=D.The lift and weight will only decrease gradually as the weightdecreases with fuel burn-off. The thrust and drag will varyconsiderably depending on angle of attack and therefore airspeed.2, pitching momentLift -Nose-downCG CPWeight yLift A kWeight yFigure 2.2-8Lift-weight coupleproduces a pitching moment.Nose-upThrustW Nose-downFigure 2.2-9 Thrust-drag couple produces a pitching momentUnder most conditions of flight the CP and CG are not coincident,i.e. are not at the one point, causing nose-down pitching moment or anose-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 anose-up pitching moment.Ideally the pitching moment from the two couples shouldneutralize each other in level flight so that there is no resultantmoment tending to rotate the aircraftA LiftThrust-dragcouplenose-upLift-weightcouplenose-downWeight ▼Figure 2.2-10Lift-weight couple and thridrag couple in equilibrium.Figure 2.2-11 Vertically offset thrust lineA LiftThrusl-dragI couple(nose-upmoment)▼Lift-weigh!couple(nose-downmoment)ThrustThrust-dragcouple(Reducednose-upmoment)Weight ▼Lift-weightcouple(nose-downmoment)Figure 2.2-12 Following a loss of thrust, the lift-weight couple,3, the tailplanehorizontal 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ThrustBalancingaerodynamic forceWeight ''059 EPSFigure 2.2-13The tailplane provides the final balancing moment.The function of tailplane (or horizontal stabilizer) is to counteractthese residual pitching moments from the two main couples and todamped any oscillation in pitch, i.e. it has a stabilizing function.The tailplane usually has a symmetrical or a negatively camberedaerofoil.The moment produced by the tailplane can be varied either bymoving the elevator or by moving the entire tailplane.The moment arm of tailplane is quite long, and the aerodynamicforce provided by the tailplane needs only to be small to have asignificant pitching effect.The area of the tailplane is small compared with the mainplanes(main wings).5.2 StabilityThere are two elements of stability, static and dynamic, and for anaircraft, it is usual to separate the modes into the three axes ofmovement.There is longitudinal stability, lateral stability and directionalstability.There is an inseparable relationship between lateral anddirectional stability.5.2.1 Static stability and dynamic stability1,Static stabilityDisturbedOscillationReleasedFigure 2.2-14 Static and dynamic stabilityThe static stability of the aeroplane describes its tendency toreturn its original condition (angle of attack) after being disturbedand without any action being taken by the pilot. The strength of thetendency is the measure of its stability.2, Dynamic stabilityremove 取消， 去 除dead b e a t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 afterthe disturbing force has been removed.It is an oscillation which may stop immediately (well-damped ordead beat), continue but reduce slowly (slightly or lightly damped),continue indefinitely (undamped) or get worse (dynamicallyunstable or divergent oscillation).Imagine the aircraft is trimmed in straight and level flight, theaircraft will respond to the vertical gust by pitching down tomaintain its trimmed angle of attack.This oscillation is generally well-damped and reduces to zero in 1or 2 oscillation. The damping is provided by the air on the horizontalarea of the aircraft. The pitching oscillation is known as the shortperiod pitching oscillation (SPPO).The airspeed of the aircraft may also change and this can cause aslower (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 axesCentre ofgravity (CG)Normal(vertical axis)W A EFSFigure 2.2-15Angular motion can occur about three axes.We refer。

三类) the motion of the aircraft to motion about each ofthree axes-each passing through the centre of gravity and eachmutually perpendicular (at 90° to each other). These are sometimescalled body axes.Figure 2.2-16 Rolling about the longitudinal axis.Figure 2.2-17 Pitching about the lateral axis.Yawing planeof motionFigure 2.2-18 Yawing about the normal axis.Stability around the longitudinal axis is known as lateral stability.Stability around the lateral axis is known as longitudinal stability.Stability around the normal axis is known as directional stability.Rotation around a point or axis is called angular motion; thenumber of degrees of rotation is called angular displacement, and thespeed with which it occurs, angular velocity.The motion of an aircraft is best considered in each of the planesseparately, although the actual motion of the aircraft is a little morecomplex. For example polling into a level turn the aircraft will notonly roll but also pitch and yaw.We will consider longitudinal stability first, then directionalstability and lateral stability. Roll and yaw are closely connected.5.2.2 Longitudinal stabilityinbuilt固有的， 内在的，d a r t飞镖，arro w箭To be longitudinal stable, an aircraft must have a natural or inbuilttendency to return to the same angle of attack after any disturbancewithout any control input by the pilot.If the angle of attack is suddenly increased by a disturbance, thenforce will be produced that will lower the nose and decrease theangle of attack.1, the tailplane and longitudinal stabilityNose-downpitching momentRestoringaerodynamicFigure 2.2-20 Longitudinal stability following an 'uninvited' nose-up pitch.GustNose­downFigure 2.2-21 Longitudinal stability following an ,unmvited, nose-down pitch.If a disturbance, such as a gust, changes the attitude of the aircraftby pitching it nose up, the tailplane will be presented to the relativeairflow at a greater angle of attack. This will cause the tailplane toproduce upward, or decreased, aerodynamic force, which is differentto that before the disturbance. The altered aerodynamic force gives anose-down pitching moment, tending to return the aeroplane to itsoriginal trimmed condition.Example: the tail fin of a dart or an arrow.Figure 2222 longitudinal stability is provided by the (ail fins of a dan2, the CG and longitudinal stabilityFigure 2.2-23 At forward CG - greater longitudinal stability - longer moment arm.The further forward the CG of the aircraft, the greater the momentarm for the tailplane, and therefore the greater the turning effect ofthe tailplane lift force.A forward CG leads to increased longitudinal stability and aftmovement of the CG leads to reduced longitudinal stability.The more stable the airplane, the greater the control force that youmust exert to control or move the airplane in manoeuvers, which canbecome tiring.The tailplane provides static longitudinal stability.3, design considerationTailplane design features also contribute greatly to longitudinalstability- tailplane area, distance from the centre of gravity, aspectratio, angle of incidence and longitudinal dihedral are considered bythe designer.At high angle of attack the mainplane may shield the tailplane orcause the airflow over it to be turbulent. This will decreaselongitudinal stability.5.2.3 Directional stabilityDirectional stability of an aeroplane is its natural or inbuilt abilityto recover from a disturbance in yawing plane without any controlinput by the pilotIf the aircraft is disturbed from its straight path by the nose or tailbeing pushed to one side (i.e. yaw ).The vertical fin (or tail or vertical stabilizer) is simply asymmetrical aerofoil. As it is now experiencing an angle of attack, itwill generate a sideways aerodynamic force which tends to take thefin back to its original position.Uninvited yawFigure 2.2*24 Directional stability following a skidThe powerful moment (turning effect) of the vertical fin, due to itslarge area and the length of its moment arm between it and centre ofgravity, 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 ofthe aeroplane.The fin provides directional static stability.As the yaw causes rolling moment so that behavior of the aircraftwith yaw and sideslip involves both its directional stability and itslateral stability.5.2.4 Lateral stabilityLateral stability is the natural or inbuilt ability of the aeroplane torecover from a disturbance in the lateral plane, i.e. rolling about thelongitudinal axis without any control input by the pilot.A disturbance in roll will cause one wing to drop and the other torise. When the aeroplane is banked, the lift vector is inclined andproduces a sideslip into the turn.As a result of this sideslip, the aeroplane is subjected to asideways component of relative airflow. This generates forces thatproduces a rolling moment to restore the aeroplane to its originalwings-level position.1, wing dihedralFigure 2.2-27Positive dihedralcorrects uninvited bank.Each wing is inclined upwards from the fuselage to the 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, willmeet the upcoming relative airflow at a greater angle of attack andwill produce increased lift.The upper wing will meet the relative airflow at a lower angle ofattack and will therefore produce less lift. It may be shieldedsomewhat by the fuselage, causing an even lower lift to begenerated.The rolling moment so produced will tend to return the aircraft toits original wings-level position.Figure 2.2-28Anhedral on a high-mounted swept wing.Negative dihedral, or anhedral has a destabilizing effect. In someaircraft with a high-mounted sweep wing, anhedral is used tocompensate for excessive lateral stability.2, wing sweepbackThe wing can add to lateral stability if it has sweepback. As theaircraft sideslips following a disturbance in roll, the lowersweepback wing generates more lift than the upper wing. This isbecause in the sideslip the lower wing presents more of its span tothe airflow and higher velocity than the upper wing and therefore thelower wing generates more lift and tends to restore the aeroplane to awing-level position.3, high keel surfaces and low CGFigure 1-31 (RighU Fin and rudderFigure 1-40 Keel surface area changes with CG position.In the sideslip that follows a disturbance in roll, a high sidewaysdrag line caused by high keel surfaces (high fin, a T-tail high on thefin, high wings, etc.) and a low CG will give a restoring momenttending to raise the lower wing and return the aircraft to theoriginal wings-level position.Figure 2.2-30High keel surfaces and a low CG correct uninvited bank.4, high-wing aroplaneIf a gust causes a wing to drop, the lift force is tilted. The resultantforces will cause the aircraft to sideslip. The airflow striking theupper keel surfaces will tend to return the aircraft to the wings-levelcondition.A high-wing aroplane increases lateral stability, it has less dihedralcompared to a mid- or -low wing design.Figure 2.2-31Pendulum stability tends5, 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 thedisturbance in roll causes.Figure 2.2-32 Roll causes sideslip, which causes yaw.The sideslip exerts a force on the side or keel surfaces of theaircraft, which, if the aircraft is directionally stable, will cause it toyaw its nose into the relative airflow. The roll has caused a yaw inthe direction of the sideslip and the aeroplane will turn further off itsoriginal heading in the direction of the lower wing.The lateral stability characteristics of the aeroplane, such asdihedral, cause the lower wing to produce increased lift and to returnthe aircraft to the wings-level position.There are two effects in conflict here:The directionally stable characteristics (large fin) want to steepenthe turn and drop the nose further.The laterally stable characteristics (dihedral) want to level thewing.spiral m o d e螺旋模态， Dutch r o l l 荷兰滚rig h t矫正，best 极力， wallow 摇摆If the first effect wins out, i.e. strong directional stability and weaklateral stability (large fin and no dihedral), then the aircraft will tendto bank further into the sideslip, towards the lower wing with nosecontinuing to drop, until the aeroplane is in a spiral dive. This iscalled spiral instability, or the spiral mode.If the lateral stability (dihedral) is stronger, the aircraft will rightitself to wings-level, and if the directional stability is weak (smallfin) the aircraft may show no tendency to turn in the direction ofsideslip, and causing the wallowing effect, Dutch roll, which is bestavoided.2) yaw followed by roll 偏航引起滚转Figure 2.2 33 Yaw causes roll and sideslip and further rollIf the aircraft is displaced in yaw, it is can cause sideslip. Thissideslip will cause the lateral stability characteristics of the aircraft'swing, such as dihedral, sweepback or high-wing. This causes arolling moment that will tend to raise the forward wing, resulting inthe aircraft rolling towards the trailing wing and away from thesideslip.The aircraft's inherent directional stability (from the fin) will tendto 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 lateralstability is good (dihedral) it can cause Dutch roll (rolling/yawingoscillation) . Often the aircraft is fitted with a yaw and/or rolldamper (a small control surface driven by a rate gyro) to stop theoscillation. It is uncomfortable for the pilot and passengers.If the directional stability is dominant (large fin) and the lateralstability not so strong, it can cause spiral instability or spiral mode.rate g y ro阻尼陀螺，dom inant占优势， 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 areabetween the wheels at all times on the ground, otherwise theFigure 2.2-34 The CG must remain wiihin the area bounded by the wheels.Centre ofgravityaeroplne will tip over - forwards or backwards.TrackApply left rudderto counleracl thiscrosswind andmaintain the desiredtrack along the runway.Rudder deflectedto counteract yawWeathercockingtendencyWindFigure 2.2-35 Weathercocking.u， /5.3 ControlThe control surfaces are the means by which the pilot overcomethe static stability of the aircraft and causes a change in flight path ora change in trimmed conditions.Figure 2.2-36The primary control surfaces - elevator, ailerons and rudder.Usually there are three sets of primary control system and threesets of control surfaces:• the elevator for longitudinal control and balance in pitch, operatedby fore and aft movement of the control wheel or column;• the ailerons for lateral control and balance in roll, operated byrotation of the control wheel or sideways movement of the controlcolumn;• the rudder for directional control and balance in yaw, operated bythe rudder pedals.Ideally each set of control surfaces should produce a momentabout only one axis but, in practice, moments about other axes areoften produced as well, e.g. aileron deflection to start a roll may alsocause adverse yaw.The deflection of the control surfaces changes the airflow and thepressure distribution over the whole aerofoil and not just over thecontrol surface itself.The effect is to change the lift produced by the total aerofoil­control surface combination.An aeroplane with too much stability designed into it has limitedcontrollability. The designer must achieve a reasonable balancebetween stability and controllability.For instance, a passenger aircraft would require more stability,UpelevatorDownwardNosedownUpward aerodynamicControl column force / —jforward / TFigure 2.2-37 The elevator is the primary pitching control029ADownelevatorwhereas a fighter would benefit from greater controllability andmanoeuvrability.5.3.1 Pitch control1, Elevator, Control column丁 backNose % 一 _ ~ ^75^aThe primary control of angle of attack is the elevator.The pilot moves the elevator by fore-and-aft movement of thecontrol column.When the control column is moved forward, the elevators movedownwards, changing the overall shape of the tail plane-elevatoraerofoil section so that it provides an altered aerodynamic force.The effect is to create a pitching moment about the CG of theaircraft that moves the nose down.When the control column is pulled back, the elevator moves upand 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 between it and the CG. The force generatedby the tailplane-elevator combination depends on their relative sizeand shape, the tailplane basically contributing to stability and theelevator to control.The larger the relative size of the elevator, the more the control.To retain satisfactory handling characteristics and elevatoreffectiveness throughout the desired speed range ,the position of theCG must be kept within the prescribed range.The forward allowable limit of the CG is determined by theamount of pitch control available from the elevator.The aft limit of the CG is determined by the requirement ofadequate longitudinal stability.Steady flight at a low speed and a high angle of attack will requiresignificant up - elevator, and backward pressure on the controlcolumn, to keep the nose up.Steady elevator deflection at different speeds.、At a high cruise speed there will need to be a steady downdeflection of the elevator to keep the nose down and maintain a lowangle of attack, hence a steady forward pressure on the controlcolumn.2, The stabilator or all-flying tailFigure 2.2-40 Separate tailplane plus moving elevator (left), and stabilator or all-flymg tail (right).Some designers choose to combine the tailplane and elevator intothe one surface and have the whole tail-plane movable- known as theall moving tail, the flying tail or the slab tail.When the control column is moved the entire 'slab' moves.5.3.2 Roll control1, AileronsThe primary control in roll is the ailerons. The ailerons areusually positioned on the outboard trailing edge of the mainplanes.The ailerons act in opposing senses, one goes up as the other goesdown, so that the lift generated by one wing increases and the liftgenerated by the other wing decreases.A resultant rolling moment is exerted on the aeroplane.The magnitude of this rolling moment depends on the momentarm and the magnitude of the differing lift forces.• The downgoing aileron is on the upgoing wing.• The upgoing aileron is on the downgoing wing.2, Adverse aileron yawDeflecting an aileron down causes an effective increase in camberof that wing and an increase in the effective angle of attack. The liftfrom that wing increases, but unfortunately so does the drag. As theother aileron rises, the effective camber of that wing is decreased andits angle of attack is less, therefore lift from that wing decreases, asdoes the drag.Roll rightSame amount ofaileron deflectionRear viewFigure 2.2-42 Downward aileron has increased drag - adverse yaw.The differing lift force causes the aircraft to bank one way, butthe differential aileron drag causes it to yaw the other way./' 一 隔 I' . ^ 一 j Increased deflectionof upgoing aileronDifferential aileronsFigure 2.2-43 Differential ailerons equalise aileron drag, reducing adverse yaw.Adverse aileron yaw can be reduced by good design incorporatingdifferential ailerons, Frise ailerons, or coupling the rudder to theailerons.LiftFigure 2.2-44 Frise-type ailerons equalise aileron drag and reduce adverse yaw・ Differential ailerons （ 差动） are designed to minimize adverseaileron yaw by increasing the drag on the downgoing wing on theinside of the turn. This is achieved by deflecting the upward aileronthrough a greater angle than the downward aileron.・ Frise ailerons increase the drag of the descending wing on theinside of the turn. As the aileron goes up, its nose protrudes into theairstream beneath the wing causing increased drag on the downgoingwing.On the other way, the wing is rising, the nose of the downgoingaileron does not protrude into the airstream, so cause no extra drag.Frise-type ailerons may also be designed to operate differentially, toincorporate the benefit of differential ailerons.• Coupled ailerons and rudder cause the rudder to moveautomatically and yaw the aeroplane into bank, opposing the adverseyaw from the ailerons.The primary effect of rudder is to yaw the aeroplane, and thesecondary effect is to roll it. The primary effect of ailerons is to rollthe aeroplane, and the secondary effect is to yaw it.3, Roll is followed by yaw 滚转引起偏航The secondary effect of ailerons is to cause yaw.When the aeroplane is banked using the ailerons, the aeroplanewill slip. As a result of the sideslip, the airflow will strike the side ofthe aeroplane and the large keel surface (rear fuselage and fin ),which are mainly behind the CG, cause the nose of the aeroplane toyaw in the direction of bank.Figure 2.2-46 Roll is followed by yaw.5.3.3 Yaw control1, RudderFigure 2.2-55 Left rudder pressure - nose yaws left.The primary control in the yawing is the rudder. The rudder ishinged to the rear of the fin (or vertical stabilizer). It is controlledfrom the cockpit by the rudder pedals to the rudder bar.By pushing the left pedal, the rudder will move left. This alters thefin- rudder aerofoil section, and sideways lift is created that sendsthe tail to the right and yaws the aeroplane to the left about thenormal axis. With left rudder applied, the aeroplane yaws left,hinge 较接，cockpit 座舱2, Yaw is followed by roll 偏航引起滚转The secondary effect of rudder is roll. The primary effect ofrudder is to yaw the aeroplane. Having yawed the aeroplane, thefurther effect of rudder is to cause a roll.3, Slipstream effectAs the slipstream corkscrews （ 螺旋形前进）around the fuselage,it strikes one side of the fln/rudder at a different angle to the other./------*<.■­. ClockwiseLeft yaw tendencyRight rudder required to balance slipstream effectFigure 2.2-57The slipstream strikes one side of the rudder.If the slipstream over the fin and rudder changes, then the rudderdeflection must be changed to balance it.4, Rudder in crosswind take-off and landing 带侧风起飞' 着陆中的方向舵take-off 起飞，landing 着陆， approach 进近，crab 偏航，crab-wise偏航方向，touchdown 接地， 触地crosswind 侧风Figure 2.2-58 Crosswind take-offIn ground operation, any crosswind will hit the side of fin andtend to weathercock the aircraft into wind.The rudder must be used to stop the aircraft yawing into wind andkeep it tracking straight along the runway.Align the aeroplane with therunway centreline using therudder just prior to touchdownWindWind0B3A FCSFigure 2.2-59 Crosswind landing.On approach to land, the most common technique is to crab theaircraft into wind so that it is flying in balance (i.e. directly into therelative wind and with the rudder ball centered) and trackingsomewhat 'crab-wise' along extended centerline of the runway.Just prior to touchdown the aircraft is yawed with the rudder, sothat when the wheels touch, they are aligned in the direction of therunway.Another technique in a crosswind landing is the sideslippingapproach.Near the ground, you would yaw the aeroplane straight (with therudder) so that it is aligned with the centerline. Unless thewheels touch almost immediately, the wind will cause the aeroplaneto drift towards the side of the runway. To avoid this, you wouldlower (using the ailerons) the into-wind wing sufficiently to stop theaircraft drifting off the centerline prior to touchdown.The aeroplane is now sideslipping and fly a litter bit out ofbalance.5, Rudder powerWhile the rudder must be sufficiently powerful to handle theabove requirements satisfactorily, it must not be too powerful. Givenmaximum deflection by the pilot it should not cause structuraldamage.5.4 Other control devicesOthers control devices include:• modified aileron design• spoilers• slots• leading edge strips・ speed brakes・ vortex generators• strakes• tabs副翼修型设计扰流片， 阻力板缝前缘条减速板涡流发生器侧板调整片1, Modified aileron design and spoilersTo reduce adverse aileron yaw, there are several techniquesemployed:Figure 2.2-47 Left aileron down.Figure 2.2-48 Right aileron up further than down.• differential ailerons・ Frise ailerons・ spoilers, which are plate-like surface that are raised from the uppersurface of the wing to increase drag on the side. They may be used inconjunction with the raised aileron or instead of it. Thus the ailerongoes down on one side and the spoiler is raised on the other.・ aileron/rudder interconnect• manual rudder input2, SlotsThey are usually incorporated within the wing leading edge aheadof the aileron to retain aileron effectiveness even at the point of stallMost are fixed and exposed to the airflow at high angle of attack.Some are created by a movable plate which pops out （ 突 出）at highangle of attack due to the reduced state pressure.3, Leading edge strips/slatsFigure 2.2-49 Leading-edge strip.Strips and other shapes, including rope （ 钢索）,are add to theleading edge of the wing to encourage attachment of the airflow athigh angle of attack. In this way, the ailerons remain effective.4, Speed brakesFigure 2.2-50Speed brake (retracted) on a Mooney aircraft.Speed brakes are flat plate surface which slide out or hingeupward from the upper surface of the wing.Some aircrafts have combined spoilers and speed brakes.5, Vortex generatorsVortex generators (VGs) are finger-like protrusions on the uppersurface of the wing. They are small plates set at an angle to the flowwhich generate a small vortex. They may even be aerofoil shaped.Figure 2.2-51 Vortex generatorsThe intent is to use the vortex to stir the boundary layer andpromote an earlier transition to turbulent mixed flow. Turbulent flowremains attached for a greater distance over the aerofoil, andturbulent attached flow is better than turbulent separated flow.The VGs stir the boundary layer and effectively reduce the highdrag of separated flow.Figure 2.2-52 Ventral fin on a Robin 2160.6, StrakesQuite common on aircraft are extensions to lower leading edge offin to increase the chord and strength of the fin (dorsal fin) , underthe fuselage (ventral fin ).Figure 2.2-53 Strakes under a TBM 700.All of these design adjustments are made to improve the controland stability at various parts of the flight envelope.dorsal 脊 dorsal fin 背鳍，ventral fin 腹鳍7, Wingtip shaping, fences, end plates and winglets 翼梢形状，翼刀，端板和翼梢小翼These devices improve lift and reduce induced drag by reducingthe spanwise flow of the air.Figure 2.2-54 Wingtip tank on a Cessna 310.8, Tabs1) fixed tab 固定调整片A fixed tab is a small metal tab that can bend to a set position onthe ground only. It is fixed in flight.It is usual on the ailerons and rudders of small aircraft.2) trim tabs 配平调整片Figure 2.2-66 Elevator trim tab.An aircraft is in trim in pitch, roll or yaw, when it maintains asteady attitude without the pilot having to exert any pressure on thecontrol column.The function of the trim tab is to reduce the control surface forceto zero for that condition of flight, so that the aeroplane willmaintain it 'hands-off'.In most light aircraft trim system are mechanically operated by atrim wheel.3) balance tab 补偿调整片1. Back pressureon control ,column 〃Main aerodynamicforce from horizontalstabiliser and elevator3. Balance tabgoes downFigure 2.2-67 The balance tab4. Creates small aerodynamicforce which helps holdelevator up « • “ ” 冬On conventional tailplane it is quite common to have a balance tabincorporated as part of the elevator.If the pilot exerts back pressure on the control column, theelevator is raised and the balance tab goes down. The elevatorbalance tab unit generates a small upward aerodynamic force thatacts to hold the elevator up, thereby reducing the control loadrequired of the pilot.The balance tab acts automatically as the elevator moves.4) anti- balance tab 反补偿调整片Figure 2.2-68 Anti-balance tab.The anti- balance tab moves in the same direction as the tailplanebut moves further. The deflection at the trailing edge of the surfaceincreases the stick force.The increased load, that the pilot has to apply, to manoeurve theaircraft, prevents overstressing the structure.5) servo tab 伺服调整片A servo tab is variation of the balance tab where the pilot controlis connected, not to the main control surface, but to the tab. As thecontrol input moves the servo tab into the airflow, the aerodynamicforces generated drive the main control surface in the oppositedirection, causing the desired manoeurve.Servo tabs were used on large transport aircraft as the forcerequired to move the surface was beyond the reasonable strength ofthe pilot.5.5 Stability versus controlversus ［ 拉丁语］ …对. . .trade-off 折衷， 权衡 hand-off松手， 请勿动手， 不许动手1, Stability versus controlDo not confuse stability with controllabilityStability is the tendency of the aeroplane to return its originalcondition after being disturbed and without any action being takenby the pilot.Controllability refers to the ease with which the pilot canmanoeuvre the aircraft and hence overcome the stability.There is a significant trade-off between stability andcontrollability. A high degree of stability makes the aircraft resistantto change and thereby tends to reduce the controllability i.e. goodstability makes it harder for the pilot to control and manoeuvre theaeroplane.An aeroplane is in a state of equilibrium when the sum of all theforces on it is zero and the sum of all the turning moments on it iszero.The aircraft is in trim if all the moments in pitch, roll and yaw arezero.An unstable aircraft is difficult to fly because the pilot mustcontinually interfere by applying control forces. A stable aircraft canalmost fly “hand-off' and require only guidance （ 制导）rather thansecond-to- second （ 畤 日 寺 亥U亥U ） control inputs.2, Control response 操纵响应The size and shape of the control surface and its moment aboutthe centre of gravity are of great importance in its effectiveness.Since the size and shape are fixed by the designer and CG onlymoves small distances, these can be considered constant. Thevariables in control effectiveness are airspeed and control surfacedeflection angle (V, 5 ).The aerodynamic forces vary with the dynamic pressure(l/2pV2) .very effective not very effective 84“ sFigure 2.2-61 Controls are more effective with increased airflow.Doubling the airspeed quadruples the effect of the same controlsurface deflection.At low airspeed, achieving a desired change in attitude requires amuch great control surface deflection, at higher airspeed, controlsare more effective.1) slipstream increases rudder and tailplane responseAt low airspeed, but with high power set, the slipstream may flowstrongly over the tail section, making the elevator and the ruddermore effective than at the same speed with no power on. Theailerons are not affected by the slipstream.oeSAEPSFigure 2.2-62 Propwash (slipstream).2) control loads felt by the pilotHinge moment at the control surface.When a control surface is deflected, the aerodynamic forcesproduced by the moving control surface itself opposes its deflection.This causes a moment to act on the control surface about its hingeline trying to return the control surface to its original faired position,and the pilot must overcome this to maintain the desired position.The pilot feels this as stick force.Figure 2.2-64 Inset hinge balance (left) and horn balance (right)aerodynamic balance 气动求卜偿 overbalance 过床卜偿inset hinge 内置较链 horn balance 角补偿balance tab 补 偿 调 整 片stick force 杆力An aerodynamic balance on a control reduces stick load on thepilot.The designer provides an inset hinge, a horn balance or a balancetab to use the aerodynamic forces produced by the deflected controlsurface to partially balance or reduce the moment, i.e. aerodynamicbalance of a control surface is designed to reduce the control forcesrequired from the pilot. The designer, however, must be careful notto overbalance the controls, otherwise the pilot will lose all sense offeel.3, Mass balancing 质 量 平 衡 （ 配平）Centre of gravityFigure 2.2-65External mass balance to avoid control flutter.A mass balance prevents flutter. A high speed control surfaceshave a tendency to flutter. Flutter is a vibration or oscillation thatresults from the changes in pressure distribution over the surface asits angle of attack is altered and when the centre of gravity is behindthe hinge line.To avoid this tendency to flutter, the designer needs to alter themass distribution of the surface to balance the surface so that thecentre of gravity is on, or close to the hinge line.Often control surfaces are fabric （ 纤维织4勿 ） covered on anotherwise all-metal aircraft to reduce mass and hence flutter.4, Summary of controlSummary of ControlsThe primary aerodynamic controls are the elevator, ailerons and rudder.The other pri­mary control is the throttle or thrust levers.ControlPlaneAxisDirect EffectIndirect Effectelevatorpitchlateralpitch attitude/ angle of attackairspeed/ altitude changeaileronsrolllongitudinalrollyawrudderyawnormalyawrollthrottlepitch/yawlateral/normalrate-of climb/ descent/airspeedpitch/yawflightpath changeairspeed/altitudeFigure 2.2-70Aileron control systemFigure 2.2-71 Typical light aircraft flight control systems.。