航空论坛 › 飞行手册 Flight Manual › 直升机飞行手册Rotorcraft flying handbook

帅哥 发表于 2009-3-20 23:43:46

ACompare the upper surface of an airfoil with the constriction in a venturi tube that is narrower in the middlethan at the ends. The upper half of the venturi tube can be replaced bylayers of undisturbed air. Thus, as air flows over theupper surface of an airfoil, the camber of the airfoilcauses an increase in the speed of the airflow. Theincreased speed of airflow results in a decrease in pressure on the upper surface of the airfoil. At the sametime, air flows along the lower surface of the airfoil,building up pressure. The combination of decreasedpressure on the upper surface and increased pressureon the lower surface results in an upward force.As angle of attack is increased, the production of lift isincreased. More upwash is created ahead of the airfoilas the leading edge stagnation point moves under theleading edge, and more downwash is created aft of thetrailing edge. Total lift now being produced is perpendicular to relative wind. In summary, the production oflift is based upon the airfoil creating circulation in theairstream (Magnus Effect) and creating differentialpressure on the airfoil (Bernoulli’s Principle).NEWTON’S THIRD LAW OF MOTIONAdditional lift is provided by the rotor blade’s lowersurface as air striking the underside is deflected down-Figure 2-10. Lift is produced when there is decreased pressure above and increased pressure below an airfoil.LiftDecreased PressureIncreased PressureIncreased VelocityDecreased PressureFigure 2-9. The upper surface of an airfoil is similar to theconstriction in a venturi tube.2-5DRAGThe force that resists the movement of a helicopterthrough the air and is produced when lift is developedis called drag. Drag always acts parallel to the relativewind. Total drag is composed of three types of drag:profile, induced, and parasite.PROFILE DRAGProfile drag develops from the frictional resistance ofthe blades passing through the air. It does not changesignificantly with the airfoil’s angle of attack, butincreases moderately when airspeed increases. Profiledrag is composed of form drag and skin friction.Form drag results from the turbulent wake caused bythe separation of airflow from the surface of a structure. The amount of drag is related to both the size andshape of the structure that protrudes into the relativewind. Skin friction is caused by surface roughness. Even

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though the surface appears smooth, it may be quiterough when viewed under a microscope. A thin layer ofair clings to the rough surface and creates small eddiesthat contribute to drag.INDUCED DRAGInduced drag is generated by the airflow circulationaround the rotor blade as it creates lift. The high-pressure area beneath the blade joins the low-pressure airabove the blade at the trailing edge and at the rotor tips.This causes a spiral, or vortex, which trails behind eachblade whenever lift is being produced. These vorticesdeflect the airstream downward in the vicinity of theblade, creating an increase in downwash. Therefore,the blade operates in an average relative wind that isinclined downward and rearward near the blade.Because the lift produced by the blade is perpendicularAircraft Yaw—The movement ofthe helicopter about its verticalaxis.THRUSTThrust, like lift, is generated by the rotation of themain rotor system. In a helicopter, thrust can be forward, rearward, sideward, or vertical. The resultant oflift and thrust determines the direction of movement ofthe helicopter.The solidity ratio is the ratio of the total rotor bladearea, which is the combined area of all the main rotorblades, to the total rotor disc area. This ratio provides ameans to measure the potential for a rotor system toprovide thrust.The tail rotor also produces thrust. The amount ofthrust is variable through the use of the antitorque pedals and is used to control the helicopter’s yaw.Figure 2-11. The load factor diagram allows you to calculatethe amount of “G” loading exerted with various angle ofbank.Load Factor - "G's"Bank Angle (in Degrees)0 10 20 30 40 50 60 70 80 909876543210Figure 2-12. It is easy to visualize the creation of form drag by examining the airflow around a flat plate. Streamlining decreasesform drag by reducing the airflow separation.2-6to the relative wind, the lift is inclined aft by the sameamount. The component of lift that is acting in a rearward direction is induced drag. As the air pressure differential increases with anincrease in angle of attack, stronger vortices form, andinduced drag increases. Since the blade’s angle ofattack is usually lower at higher airspeeds, and higherat low speeds, induced drag decreases as airspeedincreases and increases as airspeed decreases. Induceddrag is the major cause of drag at lower airspeeds.PARASITE DRAGParasite drag is present any time the helicopter is movingthrough the air. This type of drag increases with airspeed.Nonlifting components of the helicopter, such as thecabin, rotor mast, tail, and landing gear, contribute to parasite drag. Any loss of momentum by the airstream, dueto such things as openings for engine cooling, createsadditional parasite drag. Because of its rapid increasewith increasing airspeed, parasite drag is the major causeof drag at higher airspeeds. Parasite drag varies with thesquare of the velocity. Doubling the airspeed increasesthe parasite drag four times.TOTAL DRAGTotal drag for a helicopter is the sum of all three dragforces. As airspeed increases, parasitedrag increases, while induced drag decreases. Profiledrag remains relatively constant throughout the speedrange with some increase at higher airspeeds.Combining all drag forces results in a total drag curve.The low point on the total drag curve shows the airspeed at which drag is minimized. This is the pointwhere the lift-to-drag ratio is greatest and is referred toas L/Dmax. At this speed, the total lift capacity of thehelicopter, when compared to the total drag of the helicopter, is most favorable. This is important in helicopterperformance.Figure 2-14. The total drag curve represents the combinedforces of parasite, profile, and induced drag; and is plottedagainst airspeed.0 25 50 75 100 125 150SpeedDragParasiteDragProfileDragInducedDragTotal DragMinimumDrag orL/D maxL/Dmax—The maximum ratiobetween total lift (L) and the totaldrag (D). This point provides thebest glide speed. Any deviationfrom best glide speed increasesdrag and reduces the distance youcan glide.Induced DragFigure 2-13. The formation of induced drag is associated withthe downward deflection of the airstream near the rotorblade.3-1Once a helicopter leaves the ground, it is acted upon bythe four aerodynamic forces. In this chapter, we willexamine these forces as they relate to flight maneuvers.POWERED FLIGHT

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In powered flight (hovering, vertical, forward, sideward, or rearward), the total lift and thrust forces of arotor are perpendicular to the tip-path plane or plane ofrotation of the rotor.HOVERING FLIGHTFor standardization purposes, this discussion assumesa stationary hover in a no-wind condition. During hovering flight, a helicopter maintains a constant positionover a selected point, usually a few feet above theground. For a helicopter to hover, the lift and thrustproduced by the rotor system act straight up and mustequal the weight and drag, which act straight down.While hovering, you can change the amount of mainrotor thrust to maintain the desired hovering altitude.This is done by changing the angle of attack of the mainrotor blades and by varying power, as needed. In thiscase, thrust acts in the same vertical direction as lift.The weight that must be supported is the total weight of thehelicopter and its occupants. If the amount of thrust isgreater than the actual weight, the helicopter gains altitude;if thrust is less than weight, the helicopter loses altitude.The drag of a hovering helicopter is mainly induced dragincurred while the blades are producing lift. There is,however, some profile drag on the blades as they rotatethrough the air. Throughout the rest of this discussion,the term “drag” includes both induced and profile drag.An important consequence of producing thrust istorque. As stated before, for every action there is anequal and opposite reaction. Therefore, as the engineturns the main rotor system in a counterclockwisedirection, the helicopter fuselage turns clockwise. Theamount of torque is directly related to the amount ofengine power being used to turn the main rotor system.Remember, as power changes, torque changes.To counteract this torque-induced turning tendency, anantitorque rotor or tail rotor is incorporated into mosthelicopter designs. You can vary the amount of thrustproduced by the tail rotor in relation to the amount oftorque produced by the engine. As the engine suppliesmore power, the tail rotor must produce more thrust.This is done through the use of antitorque pedals.TRANSLATING TENDENCY OR DRIFTDuring hovering flight, a single main rotor helicopter tendsto drift in the same direction as antitorque rotor thrust. Thisdrifting tendency is called translating tendency. ThrustLiftWeightDragFigure 3-1. To maintain a hover at a constant altitude, enoughlift and thrust must be generated to equal the weight of thehelicopter and the drag produced by the rotor blades.Blade RotationTorqueTorqueDriftTail Rotor ThrustFigure 3-2. A tail rotor is designed to produce thrust in adirection opposite torque. The thrust produced by the tailrotor is sufficient to move the helicopter laterally.3-2greater the centrifugal force. This force gives the rotorblades their rigidity and, in turn, the strength to supportthe weight of the helicopter. The centrifugal force generated determines the maximum operating rotor r.p.m.due to structural limitations on the main rotor system.As a vertical takeoff is made, two major forces are acting at the same time—centrifugal force acting outwardand perpendicular to the rotor mast, and lift actingupward and parallel to the mast. The result of these twoforces is that the blades assume a conical path insteadof remaining in the plane perpendicular to the mast.CORIOLIS EFFECT(LAW OF CONSERVATIONOF ANGULAR MOMENTUM)Coriolis Effect, which is sometimes referred to as conservation of angular momentum, might be compared tospinning skaters. When they extend their arms, theirrotation slows down because the center of mass movesfarther from the axis of rotation. When their arms areretracted, the rotation speeds up because the center ofmass moves closer to the axis of rotation.When a rotor blade flaps upward, the center of mass ofthat blade moves closer to the axis of rotation and bladeacceleration takes place in order to conserve angularmomentum. Conversely, when that blade flaps downward, its center of mass moves further from the axis ofBefore TakeoffDuring TakeoffLiftCentrifugalForceResultantBladeAngleFigure 3-4. Rotor blade coning occurs as the rotor bladesbegin to lift the weight of the helicopter. In a semirigid andrigid rotor system, coning results in blade bending. In anarticulated rotor system, the blades assume an upward anglethrough movement about the flapping hinges.Centrifugal Force—The apparentforce that an object moving alonga circular path exerts on the bodyconstraining the obect and thatacts outwardy away from the center of rotation.To counteract this drift, one or more of the followingfeatures may be used:• The main transmission is mounted so that the rotormast is rigged for the tip-path plane to have a builtin tilt opposite tail thrust, thus producing a smallsideward thrust.• Flight control rigging is designed so that the rotordisc is tilted slightly opposite tail rotor thrust when

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the cyclic is centered.• The cyclic pitch control system is designed so thatthe rotor disc tilts slightly opposite tail rotor thrustwhen in a hover.Counteracting translating tendency, in a helicopter with acounterclockwise main rotor system, causes the left skidto hang lower while hovering. The opposite is true forrotor systems turning clockwise when viewed from above.PENDULAR ACTIONSince the fuselage of the helicopter, with a single mainrotor, is suspended from a single point and has considerable mass, it is free to oscillate either longitudinally orlaterally in the same way as a pendulum. This pendularaction can be exaggerated by over controlling; therefore,control movements should be smooth and not exaggerated. CONINGIn order for a helicopter to generate lift, the rotor bladesmust be turning. This creates a relative wind that isopposite the direction of rotor system rotation. Therotation of the rotor system creates centrifugal force(inertia), which tends to pull the blades straight outwardfrom the main rotor hub. The faster the rotation, theHoverRearwardFlightForwardFlightFigure 3-3. Because the helicopter’s body has mass and issuspended from a single point (the rotor mast head), it tendsto act much like a pendulum.3-3rotation and blade deceleration takes place. Keep in mind that due to coning, a rotor blade will notflap below a plane passing through the rotor hub andperpendicular to the axis of rotation. The accelerationand deceleration actions of the rotor blades are absorbedby either dampers or the blade structure itself, depending upon the design of the rotor system.Two-bladed rotor systems are normally subject toCoriolis Effect to a much lesser degree than are articulated rotor systems since the blades are generally“underslung” with respect to the rotor hub, and thechange in the distance of the center of mass from theaxis of rotation is small. The huntingaction is absorbed by the blades through bending. If atwo-bladed rotor system is not “underslung,” it will besubject to Coriolis Effect comparable to that of a fullyarticulated system.GROUND EFFECTWhen hovering near the ground, a phenomenon knownas ground effect takes place. This effectusually occurs less than one rotor diameter above thesurface. As the induced airflow through the rotor disc isreduced by the surface friction, the lift vector increases.This allows a lower rotor blade angle for the sameamount of lift, which reduces induced drag. Groundeffect also restricts the generation of blade tip vortices

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due to the downward and outward airflow making alarger portion of the blade produce lift. When the helicopter gains altitude vertically, with no forward airspeed, induced airflow is no longer restricted, and theblade tip vortices increase with the decrease in outwardairflow. As a result, drag increases which means aAxis ofRotationBladeFlappingCenter of MassFigure 3-5. The tendency of a rotor blade to increase ordecrease its velocity in its plane of rotation due to massmovement is known as Coriolis Effect, named for the mathematician who made studies of forces generated by radialmovements of mass on a rotating disc.Large BladeTip VortexNo Wind HoverBlade TipVortexOUT OF GROUND EFFECT (OGE) IN GROUND EFFECT (IGE)Downwash PatternEquidistant 360°Figure 3-7. Air circulation patterns change when hovering out of ground effect (OGE) and when hovering in ground effect (IGE).This elbow moves away fromthe mast as the rotor is tilted.This elbow moves towardthe mast as the rotor is tilted.MastAxisCM CMCMCMFigure 3-6. Because of the underslung rotor, the center ofmass remains approximately the same distance from themast after the rotor is tilted.3-4higher pitch angle, and more power is needed to movethe air down through the rotor.Ground effect is at its maximum in a no-wind conditionover a firm, smooth surface. Tall grass, rough terrain,revetments, and water surfaces alter the airflow pattern,causing an increase in rotor tip vortices.GYROSCOPIC PRECESSIONThe spinning main rotor of a helicopter acts like a gyroscope. As such, it has the properties of gyroscopicaction, one of which is precession. Gyroscopic precession is the resultant action or deflection of a spinningobject when a force is applied to this object. This actionoccurs approximately 90° in the direction of rotationfrom the point where the force is applied. Let us look at a two-bladed rotor system to see howgyroscopic precession affects the movement of the tippath plane. Moving the cyclic pitch control increasesthe angle of attack of one rotor blade with the resultthat a greater lifting force is applied at that point in theplane of rotation. This same control movement simultaneously decreases the angle of attack of the otherblade the same amount, thus decreasing the lifting forceapplied at that point in the plane of rotation. The bladewith the increased angle of attack tends to flap up; theblade with the decreased angle of attack tends to flapdown. Because the rotor disk acts like a gyro, theblades reach maximum deflection at a point approximately 90° later in the plane of rotation. As shown infigure 3-9, the retreating blade angle of attack isincreased and the advancing blade angle of attack isdecreased resulting in a tipping forward of the tip-pathplane, since maximum deflection takes place 90° laterwhen the blades are at the rear and front, respectively.In a rotor system using three or more blades, the movement of the cyclic pitch control changes the angle ofattack of each blade an appropriate amount so that theend result is the same.VERTICAL FLIGHTHovering is actually an element of vertical flight.Increasing the angle of attack of the rotor blades (pitch)while their velocity remains constant generates additional vertical lift and thrust and the helicopter ascends.Decreasing the pitch causes the helicopter to descend.In a no wind condition when lift and thrust are less thanweight and drag, the helicopter descends vertically. If90°AxisUpwardForceAppliedHereReactionOccursHereNew AxisGyro TipsDown HereGyro TipsUp HereOld AxisFigure 3-8. Gyroscopic precession principle—when a force is applied to a spinning gyro, the maximum reaction occurs approximately 90° later in the direction of rotation.BladeRotationAngle of AttackDecreasedMaximumUpwardDeflectionMaximumDownwardDeflectionAngle of AttackIncreasedFigure 3-9. With a counterclockwise main rotor blade rotation, as each blade passes the 90° position on the left, themaximum increase in angle of attack occurs. As each bladepasses the 90° position to the right, the maximum decreasein angle of attack occurs. Maximum deflection takes place90° later—maximum upward deflection at the rear and maximum downward deflection at the front—and the tip-pathplane tips forward.3-5lift and thrust are greater than weight and drag, the helicopter ascends vertically. FORWARD FLIGHTIn or during forward flight, the tip-path plane is tilted forward, thus tilting the total lift-thrust force

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forward fromthe vertical. This resultant lift-thrust force can be resolvedinto two components—lift acting vertically upward andthrust acting horizontally in the direction of flight. Inaddition to lift and thrust, there is weight (the downwardacting force) and drag (the rearward acting or retardingforce of inertia and wind resistance). In straight-and-level, unaccelerated forward flight, liftequals weight and thrust equals drag (straight-and-levelflight is flight with a constant heading and at a constantaltitude). If lift exceeds weight, the helicopter climbs;if lift is less than weight, the helicopter descends. Ifthrust exceeds drag, the helicopter speeds up; if thrustis less than drag, it slows down.As the helicopter moves forward, it begins to lose altitude because of the lift that is lost as thrust is divertedforward. However, as the helicopter begins to accelerate, the rotor system becomes more efficient due to theincreased airflow. The result is excess power over thatwhich is required to hover. Continued accelerationcauses an even larger increase in airflow through therotor disc and more excess power.TRANSLATIONAL LIFTTranslational lift is present with any horizontal flow ofair across the rotor. This increased flow is most noticeable when the airspeed reaches approximately 16 to 24knots. As the helicopter accelerates through this speed,the rotor moves out of its vortices and is in relativelyundisturbed air. The airflow is also now more horizontal,which reduces induced flow and drag with a corresponding increase in angle of attack and lift. The additionallift available at this speed is referred to as “effectivetranslational lift” (ETL). When a single-rotor helicopter flies through translationallift, the air flowing through the main rotor and over thetail rotor becomes less turbulent and more aerodynamically efficient. As the tail rotor efficiency improves,more thrust is produced causing the aircraft to yaw leftin a counterclockwise rotor system. It will be necessaryto use right torque pedal to correct for this tendency ontakeoff. Also, if no corrections are made, the nose risesor pitches up, and rolls to the right. This is caused bycombined effects of dissymmetry of lift and transverseflow effect, and is corrected with cyclic control.ResultantResultantLiftThrustHelicopterMovementWeightDragFigure 3-11. To transition into forward flight, some of the vertical thrust must be vectored horizontally. You initiate this byforward movement of the cyclic control.No Recirculationof AirMore HorizontalFlow of AirReducedInduced FlowIncreasesAngle of AttackTail Rotor Operates inRelatively Clean Air16 to 24KnotsFigure 3-12. Effective translational lift is easily recognized inactual flight by a transient induced aerodynamic vibrationand increased performance of the helicopter.ThrustLiftWeightDragVertical AscentFigure 3-10. To ascend vertically, more lift and thrust must begenerated to overcome the forces of weight and the drag.3-6Translational lift is also present in a stationary hover ifthe wind speed is approximately 16 to 24 knots. In normal operations, always utilize the benefit of translationallift, especially if maximum performance is needed.INDUCED FLOWAs the rotor blades rotate they generate what is calledrotational relative wind. This airflow is characterizedas flowing parallel and opposite the rotor’s plane ofrotation and striking perpendicular to the rotor blade’sleading edge. This rotational relative wind is used togenerate lift. As rotor blades produce lift, air is accelerated over the foil and projected downward. Anytime ahelicopter is producing lift, it moves large masses of airvertically and down through the rotor system. Thisdownwash or induced flow can significantly changethe efficiency of the rotor system. Rotational relativewind combines with induced flow to form the resultantrelative wind. As induced flow increases, resultant relative wind becomes less horizontal. Since angle ofattack is determined by measuring the differencebetween the chord line and the resultant relative wind,as the resultant relative wind becomes less horizontal,angle of attack decreases. TRANSVERSE FLOW EFFECTAs the helicopter accelerates in forward flight, inducedflow drops to near zero at the forward disc area andincreases at the aft disc area. This increases the angleof attack at the front disc area causing the rotor blade toflap up, and reduces angle of attack at the aft disc areacausing the rotor blade to flap down. Because the rotoracts like a gyro, maximum displacement occurs 90° inthe direction of rotation. The result is a tendency forthe helicopter to roll slightly to the right as it acceler-ates through approximately 20 knots or if the headwindis approximately 20 knots.You can recognize transverse flow effect because ofincreased vibrations of the helicopter at airspeeds just

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below effective translational lift on takeoff and afterpassing through effective translational lift during landing. To counteract transverse flow effect, a cyclic inputneeds to be made.DISSYMMETRY OF LIFTWhen the helicopter moves through the air, the relativeairflow through the main rotor disc is different on theadvancing side than on the retreating side. The relativewind encountered by the advancing blade is increasedby the forward speed of the helicopter, while the relative wind speed acting on the retreating blade isreduced by the helicopter’s forward airspeed.Therefore, as a result of the relative wind speed, theadvancing blade side of the rotor disc produces morelift than the retreating blade side. This situation isdefined as dissymmetry of lift. If this condition was allowed to exist, a helicopter witha counterclockwise main rotor blade rotation would rollto the left because of the difference in lift. In reality, themain rotor blades flap and feather automatically toequalize lift across the rotor disc. Articulated rotor systems, usually with three or more blades, incorporate ahorizontal hinge (flapping hinge) to allow the individual rotor blades to move, or flap up and down as theyrotate. A semirigid rotor system (two blades) utilizes ateetering hinge, which allows the blades to flap as aunit. When one blade flaps up, the other flaps down.Figure 3-13. A helicopter in forward flight, or hovering with a headwind or crosswind, has more molecules of air entering the aftportion of the rotor blade. Therefore, the angle of attack is less and the induced flow is greater at the rear of the rotor disc.ResultantRelativeWindResultantRelativeWind10 to 20KnotsAABBInducedFlowInducedFlowAngle ofAttackAngle ofAttackRotational Relative Wind Rotational Relative Wind3-7As shown in figure 3-15, as the rotor blade reaches theadvancing side of the rotor disc (A), it reaches its maximum upflap velocity. When the blade flaps upward,the angle between the chord line and the resultant relative wind decreases. This decreases the angle of attack,which reduces the amount of lift produced by the blade.At position (C) the rotor blade is now at its maximumdownflapping velocity. Due to downflapping, the anglebetween the chord line and the resultant relative windincreases. This increases the angle of attack and thusthe amount of lift produced by the blade.The combination of blade flapping and slow relative windacting on the retreating blade normally limits the maximum forward speed of a helicopter. At a high forwardspeed, the retreating blade stalls because of a high angle ofattack and slow relative wind speed. This situation iscalled retreating blade stall and is evidenced by a nosepitch up, vibration, and a rolling tendency—usually to theleft in helicopters with counterclockwise blade rotation.You can avoid retreating blade stall by not exceedingthe never-exceed speed. This speed is designated VNEand is usually indicated on a placard and marked on theairspeed indicator by a red line.During aerodynamic flapping of the rotor blades as theycompensate for dissymmetry of lift, the advancing bladeRelative WindRelative WindDirectionof FlightAdvancingSideBlade TipSpeed PlusHelicopterSpeed(400 KTS)Blade TipSpeed MinusHelicopterSpeed(200 KTS)RetreatingSideForward Flight100 KTSBladeRotationFigure 3-14. The blade tip speed of this helicopter is approximately 300 knots. If the helicopter is moving forward at 100knots, the relative wind speed on the advancing side is 400knots. On the retreating side, it is only 200 knots. This difference in speed causes a dissymmetry of lift.Figure 3-15. The combined upward flapping (reduced lift) of the advancing blade and downward flapping (increased lift) of theretreating blade equalizes lift across the main rotor disc counteracting dissymmetry of lift.Direction of RotationChordLineResultant RWChordLineResultant RWChordLineDownflapVelocityResultantRWChordLineResultantRWUpflap VelocityAngle of Attack at9 O'Clock PositionAngle of Attack at3 O'Clock PositionAngle of Attack overTailAngle of Attack overNoseABCDDAB

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CRW = Relative Wind= Angle of AttackVNE—The speed beyond which an aircraft should never beoperated. VNE can change with altitude, density altitude, andweight.3-8ward. Drag now acts forward with the lift componentstraight up and weight straight down. TURNING FLIGHTIn forward flight, the rotor disc is tilted forward, whichalso tilts the total lift-thrust force of the rotor disc forward. When the helicopter is banked, the rotor disc istilted sideward resulting in lift being separated into twocomponents. Lift acting upward and opposing weight iscalled the vertical component of lift. Lift acting horizontally and opposing inertia (centrifugal force) is thehorizontal component of lift (centripetal force).As the angle of bank increases, the total lift force is tiltedmore toward the horizontal, thus causing the rate of turnto increase because more lift is acting horizontally. Sincethe resultant lifting force acts more horizontally, theeffect of lift acting vertically is deceased. To compensate for this decreased vertical lift, the angle of attack ofthe rotor blades must be increased in order to maintainaltitude. The steeper the angle of bank, the greater theangle of attack of the rotor blades required to maintainaltitude. Thus, with an increase in bank and a greaterangle of attack, the resultant lifting force increases andthe rate of turn is faster.AUTOROTATIONAutorotation is the state of flight where the main rotorsystem is being turned by the action of relative windCentripetal Force—The forceopposite centrifugal force andattracts a body toward its axis ofrotation.ResultantLiftThrustDragResultantWeightHelicopterMovementFigure 3-18. Forces acting on the helicopter during rearwardflight.achieves maximum upflapping displacement over thenose and maximum downflapping displacement over thetail. This causes the tip-path plane to tilt to the rear and isreferred to as blowback. Figure 3-16 shows how the rotordisc was originally oriented with the front down following the initial cyclic input, but as airspeed is gained andflapping eliminates dissymmetry of lift, the front of thedisc comes up, and the back of the disc goes down. Thisreorientation of the rotor disc changes the direction inwhich total rotor thrust acts so that the helicopter’s forward speed slows, but can be corrected with cyclic input.SIDEWARD FLIGHTIn sideward flight, the tip-path plane is tilted in the direction that flight is desired. This tilts the total lift-thrustvector sideward. In this case, the vertical or lift component is still straight up and weight straight down, but thehorizontal or thrust component now acts sideward withdrag acting to the opposite side. REARWARD FLIGHTFor rearward flight, the tip-path plane is tilted rearward, which, in turn, tilts the lift-thrust vector rear-HelicopterMovementWeightDragResultantThrustLiftFigure 3-17. Forces acting on the helicopter during sidewardflight.Figure 3-16. To compensate for blowback, you must movethe cyclic forward. Blowback is more pronounced with higherairspeeds.3-9rather than engine power. It is the means by which ahelicopter can be landed safely in the event of anengine failure. In this case, you are using altitude aspotential energy and converting it to kinetic energy during the descent and touchdown. All helicopters musthave this capability in order to be certified.Autorotation is permitted mechanically because of afreewheeling unit, which allows the main rotor to con-tinue turning even if the engine is not running. In normal powered flight, air is drawn into the main rotor system from above and exhausted downward. Duringautorotation, airflow enters the rotor disc from belowas the helicopter descends. AUTOROTATION (VERTICAL FLIGHT)Most autorotations are performed with forward speed.For simplicity, the following aerodynamic explanationis based on a vertical autorotative descent (no forwardspeed) in still air. Under these conditions, the forcesthat cause the blades to turn are similar for all bladesregardless of their position in the plane of rotation.Therefore, dissymmetry of lift resulting from helicopter airspeed is not a factor.During vertical autorotation, the rotor disc is dividedinto three regions as illustrated in figure 3-21—theFigure 3-20. During an autorotation, the upward flow of relative wind permits the main rotor blades to rotate at their normalspeed. In effect, the blades are “gliding” in their rotational plane.Figure 3-21. Blade regions in vertical autorotation descent.Figure 3-19. The horizontal component of lift accelerates thehelicopter toward the center of the turn.Centripetal Force(Horizontal Component of Lift)VerticalComponentof LiftBankAngle

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ResultantLiftWeightCentrifugalForce (Inertia)Normal Powered Flight AutorotationDirectionofFlightDirectionofFlightStallRegion 25%DrivenRegion 30% DrivingRegion 45%3-10driven region, the driving region, and the stall region.Figure 3-22 shows four blade sections that illustrateforce vectors. Part A is the driven region, B and D arepoints of equilibrium, part C is the driving region, andpart E is the stall region. Force vectors are different ineach region because rotational relative wind is slowernear the blade root and increases continually towardthe blade tip. Also, blade twist gives a more positiveangle of attack in the driving region than in the drivenregion. The combination of the inflow up through therotor with rotational relative wind produces differentcombinations of aerodynamic force at every pointalong the blade.The driven region, also called the propeller region, isnearest the blade tips. Normally, it consists of about 30Figure 3-22. Force vectors in vertical autorotation descent.B & RotationalRelative WindLiftTAFTAFTotalAerodynamicForce Aftof Axis ofRotationTotalAerodynamicForce Forwardof Axis ofRotationAngle ofAttack 2° DragChord LineInflow UpThrough RotorResultantRelative WindEquilibriumDragInflowTAFAngle ofAttack 6°Drag DrivingRegionInflowAxis ofRotationAngle ofAttack 24°(Blade is Stalled)TAFDragStallRegionInflowDrivenRangeCDrivenRegionDragPoint ofEquilibriumPoint ofEquilibriumDrivingRegionStallRegionDragAutorotative ForceADCEEDBALiftLiftLift3-11percent of the radius. In the driven region, part A of figure 3-22, the total aerodynamic force acts behind theaxis of rotation, resulting in a overall drag force. Thedriven region produces some lift, but that lift is offsetby drag. The overall result is a deceleration in the rotation of the blade. The size of this region varies with theblade pitch, rate of descent, and rotor r.p.m. Whenchanging autorotative r.p.m., blade pitch, or rate ofdescent, the size of the driven region in relation to theother regions also changes.There are two points of equilibrium on the blade—onebetween the driven region and the driving region, andone between the driving region and the stall region. Atpoints of equilibrium, total aerodynamic force isaligned with the axis of rotation. Lift and drag are produced, but the total effect produces neither accelerationnor deceleration.The driving region, or autorotative region, normallylies between 25 to 70 percent of the blade radius. PartC of figure 3-22 shows the driving region of the blade,which produces the forces needed to turn the bladesduring autorotation. Total aerodynamic force in thedriving region is inclined slightly forward of the axis ofrotation, producing a continual acceleration force. This

帅哥 发表于 2009-3-20 23:45:56

inclination supplies thrust, which tends to acceleratethe rotation of the blade. Driving region size varieswith blade pitch setting, rate of descent, and rotor r.p.m.By controlling the size of this region you can adjustautorotative r.p.m. For example, if the collective pitchis raised, the pitch angle increases in all regions. Thiscauses the point of equilibrium to move inboard alongthe blade’s span, thus increasing the size of the drivenregion. The stall region also becomes larger while thedriving region becomes smaller. Reducing the size ofthe driving region causes the acceleration force of thedriving region and r.p.m. to decrease.The inner 25 percent of the rotor blade is referred to asthe stall region and operates above its maximum angleof attack (stall angle) causing drag which tends to slowrotation of the blade. Part E of figure 3-22 depicts thestall region.A constant rotor r.p.m. is achieved by adjusting the collective pitch so blade acceleration forces from the driving region are balanced with the deceleration forcesfrom the driven and stall regions.AUTOROTATION (FORWARD FLIGHT)Autorotative force in forward flight is produced inexactly the same manner as when the helicopter isdescending vertically in still air. However, because forward speed changes the inflow of air up through therotor disc, all three regions move outboard along theblade span on the retreating side of the disc where angleof attack is larger, as shown in figure 3-23. With lowerangles of attack on the advancing side blade, more ofthat blade falls in the driven region. On the retreatingside, more of the blade is in the stall region. A smallsection near the root experiences a reversed flow, therefore the size of the driven region on the retreating sideis reduced.Figure 3-23. Blade regions in forward autorotation descent.ForwardDrivenRegionDrivingRegionRetreatingSideStallRegionAdvancingSide3-124-1Note: In this chapter, it is assumed that the helicopter hasa counterclockwise main rotor blade rotation as viewedfrom above. If flying a helicopter with a clockwise rotation, you will need to reverse left and right references,particularly in the areas of rotor blade pitch change, antitorque pedal movement, and tail rotor thrust.There are four basic controls used during flight. Theyare the collective pitch control, the throttle, the cyclicpitch control, and the antitorque pedals.COLLECTIVE PITCH CONTROLThe collective pitch control, located on the left side ofthe pilot’s seat, changes the pitch angle of all main rotorblades simultaneously, or collectively, as the nameimplies. As the collective pitch control is raised, thereis a simultaneous and equal increase in pitch angle ofall main rotor blades; as it is lowered, there is a simultaneous and equal decrease in pitch angle. This is donethrough a series of mechanical linkages and the amountof movement in the collective lever determines theamount of blade pitch change. Anadjustable friction control helps prevent inadvertentcollective pitch movement.Changing the pitch angle on the blades changes theangle of attack on each blade. With a change in angleof attack comes a change in drag, which affects thespeed or r.p.m. of the main rotor. As the pitch angleincreases, angle of attack increases, drag increases,and rotor r.p.m. decreases. Decreasing pitch angledecreases both angle of attack and drag, while rotorr.p.m. increases. In order to maintain a constant rotorr.p.m., which is essential in helicopter operations, aproportionate change in power is required to compensate for the change in drag. This is accomplishedwith the throttle control or a correlator and/or governor, which automatically adjusts engine power.THROTTLE CONTROLThe function of the throttle is to regulate engine r.p.m.If the correlator or governor system does not maintainthe desired r.p.m. when the collective is raised or lowered, or if those systems are not installed, the throttleFigure 4-1. Raising the collective pitch control increases the pitch angle the same amount on all blades.4-2has to be moved manually with the twist grip in orderto maintain r.p.m. Twisting the throttle outboardincreases r.p.m.; twisting it inboard decreases r.p.m.COLLECTIVE PITCH / THROTTLECOORDINATIONWhen the collective pitch is raised, the load on theengine is increased in order to maintain desired r.p.m.The load is measured by a manifold pressure gaugein piston helicopters or by a torque gauge in turbinehelicopters.In piston helicopters, the collective pitch is the primarycontrol for manifold pressure, and the throttle is the primary control for r.p.m. However, the collective pitchcontrol also influences r.p.m., and the throttle alsoinfluences manifold pressure; therefore, each is considered to be a secondary control of the other’s function.Both the tachometer (r.p.m. indicator) and the manifoldpressure gauge must be analyzed to determine whichcontrol to use. Figure 4-3 illustrates this relationship.

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