帅哥
发表于 2009-3-21 00:00:20
presence of obstructions, natural or manmade. Forexample, a clearing in the woods, a city street, a road, abuilding roof, etc., can each be regarded as a confinedarea. Generally, takeoffs and landings should be madeinto the wind to obtain maximum airspeed with minimum groundspeed.There are several things to consider when operating inconfined areas. One of the most important is maintaininga clearance between the rotors and obstacles forming theconfined area. The tail rotor deserves special consideration because, in some helicopters, you cannot always seeit from the cabin. This not only applies while making theapproach, but while hovering as well. Another consideration is that wires are especially difficult to see;however, their supporting devices, such as poles ortowers, serve as an indication of their presence andapproximate height. If any wind is present, you shouldalso expect some turbulence. Something else for you to consider is the availability offorced landing areas during the planned approach. Youshould think about the possibility of flying from onealternate landing area to another throughout theapproach, while avoiding unfavorable areas. Alwaysleave yourself a way out in case the landing cannot becompleted or a go-around is necessary.APPROACHA high reconnaissance should be completed before initiating the confined area approach. Start the approachphase using the wind and speed to the best possibleadvantage. Keep in mind areas suitable for forced landing. It may be necessary to choose between anFigure 10-7. Slope takeoff.WindFigure 10-8. If the wind velocity is 10 knots or greater, youshould expect updrafts on the windward side and downdraftson the lee side of obstacles. You should plan the approachwith these factors in mind, but be ready to alter your plans ifthe wind speed or direction changes.10-8approach that is crosswind, but over an open area, andone directly into the wind, but over heavily wooded orextremely rough terrain where a safe forced landingwould be impossible. If these conditions exist, considerthe possibility of making the initial phase of theapproach crosswind over the open area and then turning into the wind for the final portion of the approach.Always operate the helicopter as close to its normalcapabilities as possible, taking into consideration thesituation at hand. In all confined area operations, withthe exception of the pinnacle operation, the angle ofdescent should be no steeper than necessary to clearany barrier in the approach path and still land on theselected spot. The angle of climb on takeoff should benormal, or not steeper than necessary to clear any barrier. Clearing a barrier by a few feet and maintainingnormal operating r.p.m., with perhaps a reserve ofpower, is better than clearing a barrier by a wide margin but with a dangerously low r.p.m. and no powerreserve.Always make the landing to a specific point and not tosome general area. This point should be located wellforward, away from the approach end of the area. Themore confined the area, the more essential it is that youland the helicopter precisely at a definite point. Keepthis point in sight during the entire final approach.When flying a helicopter near obstructions, alwaysconsider the tail rotor. A safe angle of descent over barriers must be established to ensure tail rotor clearanceof all obstructions. After coming to a hover, take careto avoid turning the tail into obstructions.TAKEOFFA confined area takeoff is considered an altitude overairspeed maneuver. Before takeoff, make a groundreconnaissance to determine the type of takeoff to beperformed, to determine the point from which the takeoff should be initiated to ensure the maximum amountof available area, and finally, how to best maneuver thehelicopter from the landing point to the proposed takeoff position.If wind conditions and available area permit, the helicopter should be brought to a hover, turned around, andhovered forward from the landing position to the takeoff position. Under certain conditions, sideward flightto the takeoff position may be necessary. If rearwardflight is required to reach the takeoff position, placereference markers in front of the helicopter in such away that a ground track can be safely followed to thetakeoff position. In addition, the takeoff marker shouldbe located so that it can be seen without hoveringbeyond it.When planning the takeoff, consider the direction ofthe wind, obstructions, and forced landing areas. Tohelp you fly up and over an obstacle, you should forman imaginary line from a point on the leading edge ofthe helicopter to the highest obstacle to be cleared. Flythis line of ascent with enough power to clear theobstacle by a safe distance. After clearing the obstacle,maintain the power setting and accelerate to the normalclimb speed. Then, reduce power to the normal climbpower setting.COMMON ERRORS1. Failure to perform, or improper performance of, ahigh or low reconnaissance.2. Flying the approach angle at too steep or too shallow an approach for the existing conditions.3. Failing to maintain proper r.p.m.4. Failure to consider emergency landing areas.5. Failure to select a specific landing spot.6. Failure to consider how wind and turbulencecould affect the approach.7. Improper takeoff and climb technique for existing conditions.PINNACLE AND RIDGELINE
帅哥
发表于 2009-3-21 00:00:35
OPERATIONSA pinnacle is an area from which the surface dropsaway steeply on all sides. A ridgeline is a long areafrom which the surface drops away steeply on one ortwo sides, such as a bluff or precipice. The absence ofobstacles does not necessarily lessen the difficulty ofpinnacle or ridgeline operations. Updrafts, downdrafts,and turbulence, together with unsuitable terrain inwhich to make a forced landing, may still presentextreme hazards.APPROACH AND LANDINGIf you need to climb to a pinnacle or ridgeline, do it onthe upwind side, when practicable, to take advantage ofany updrafts. The approach flight path should be parallel to the ridgeline and into the wind as much as possible. Load, altitude, wind conditions, and terrain featuresdetermine the angle to use in the final part of anapproach. As a general rule, the greater the winds, thesteeper the approach needs to be to avoid turbulent airand downdrafts. Groundspeed during the approach isAltitude over Airspeed—In this type of maneuver, it is more importantto gain altitude than airspeed. However, unless operational considerations dictate otherwise, the crosshatched or shaded areas of theheight/velocity diagram should be avoided.10-9more difficult to judge because visual references arefarther away than during approaches over trees or flatterrain. If a crosswind exists, remain clear of downdrafts on the leeward or downwind side of theridgeline. If the wind velocity makes the crosswindlanding hazardous, you may be able to make a low,coordinated turn into the wind just prior to terminatingthe approach. When making an approach to a pinnacle,avoid leeward turbulence and keep the helicopterwithin reach of a forced landing area as long aspossible.On landing, take advantage of the long axis of the areawhen wind conditions permit. Touchdown should bemade in the forward portion of the area. Always perform a stability check, prior to reducing r.p.m., toensure the landing gear is on firm terrain that can safelysupport the weight of the helicopter.TAKEOFFA pinnacle takeoff is an airspeed over altitude maneuver made from the ground or from a hover. Sincepinnacles and ridgelines are generally higher than theimmediate surrounding terrain, gaining airspeed on thetakeoff is more important than gaining altitude. Thehigher the airspeed, the more rapid the departure fromslopes of the pinnacle. In addition to covering unfavorable terrain rapidly, a higher airspeed affords a morefavorable glide angle and thus contributes to thechances of reaching a safe area in the event of a forcedlanding. If a suitable forced landing area is not available, a higher airspeed also permits a more effectiveflare prior to making an autorotative landing.On takeoff, as the helicopter moves out of groundeffect, maintain altitude and accelerate to normal climbairspeed. When normal climb speed is attained, establish a normal climb attitude. Never dive the helicopterdown the slope after clearing the pinnacle.COMMON ERRORS1. Failure to perform, or improper performance of, ahigh or low reconnaissance.2. Flying the approach angle at too steep or too shallow an approach for the existing conditions.3. Failure to maintain proper r.p.m.4. Failure to consider emergency landing areas.5. Failure to consider how wind and turbulencecould affect the approach and takeoff.Figure 10-9. When flying an approach to a pinnacle or ridgeline, avoid the areas where downdrafts are present, especially when excess power is limited. If you encounterdowndrafts, it may become necessary to make an immediateturn away from the pinnacle to avoid being forced into therising terrain.Airspeed over Altitude—This means that in this maneuver, obstaclesare not a factor, and it is more important to gain airspeed than altitude.10-1011-1Today helicopters are quite reliable. Howeveremergencies do occur, whether a result of mechanicalfailure or pilot error. By having a thorough knowledgeof the helicopter and its systems, you will be able tomore readily handle the situation. In addition, byknowing the conditions that can lead to anemergency, many potential accidents can be avoided.AUTOROTATIONIn a helicopter, an autorotation is a descending maneuver where the engine is disengaged from the main rotorsystem and the rotor blades are driven solely by theupward flow of air through the rotor. In other words, theengine is no longer supplying power to the main rotor.The most common reason for an autorotation is anengine failure, but autorotations can also be performedin the event of a complete tail rotor failure, since thereis virtually no torque produced in an autorotation. Ifaltitude permits, they can also be used to recover fromsettling with power. If the engine fails, the freewheeling unit automatically disengages the engine from themain rotor allowing the main rotor to rotate freely.Essentially, the freewheeling unit disengages anytimethe engine r.p.m. is less than the rotor r.p.m.At the instant of engine failure, the main rotor bladesare producing lift and thrust from their angle of attackand velocity. By immediately lowering collective pitch,which must be done in case of an engine failure, lift anddrag are reduced, and the helicopter begins an immediate descent, thus producing an upward flow of airthrough the rotor system. This upward flow of airthrough the rotor provides sufficient thrust to maintainrotor r.p.m. throughout the descent. Since the tail rotoris driven by the main rotor transmission during autorotation, heading control is maintained as in normal flight.
帅哥
发表于 2009-3-21 00:00:46
Several factors affect the rate of descent in autorotation; density altitude, gross weight, rotor r.p.m., andairspeed. Your primary control of the rate of descent isairspeed. Higher or lower airspeeds are obtained withthe cyclic pitch control just as in normal flight.In theory, you have a choice in the angle of descentvarying from a vertical descent to maximum range,which is the minimum angle of descent. Rate of descentis high at zero airspeed and decreases to a minimum atapproximately 50 to 60 knots, depending upon the particular helicopter and the factors just mentioned. As theairspeed increases beyond that which gives minimumrate of descent, the rate of descent increases again.When landing from an autorotation, the energy storedin the rotating blades is used to decrease the rate ofdescent and make a soft landing. A greater amount ofrotor energy is required to stop a helicopter with a highrate of descent than is required to stop a helicopter thatis descending more slowly. Therefore, autorotativedescents at very low or very high airspeeds are morecritical than those performed at the minimum rate ofdescent airspeed.Each type of helicopter has a specific airspeed at whicha power-off glide is most efficient. The best airspeed isthe one which combines the greatest glide range withthe slowest rate of descent. The specific airspeed issomewhat different for each type of helicopter, yetcertain factors affect all configurations in the samemanner. For specific autorotation airspeeds for a particular helicopter, refer to the FAA-approved rotorcraftflight manual.The specific airspeed for autorotations is establishedfor each type of helicopter on the basis of averageweather and wind conditions and normal loading.When the helicopter is operated with heavy loads inhigh density altitude or gusty wind conditions, bestperformance is achieved from a slightly increased airspeed in the descent. For autorotations at low densityaltitude and light loading, best performance is achievedfrom a slight decrease in normal airspeed. Followingthis general procedure of fitting airspeed to existingconditions, you can achieve approximately the sameglide angle in any set of circumstances and estimate thetouchdown point.When making turns during an autorotation, generallyuse cyclic control only. Use of antitorque pedals toassist or speed the turn causes loss of airspeed anddownward pitching of the nose. When an autorotationis initiated, sufficient antitorque pedal pressure shouldbe used to maintain straight flight and prevent yawing.This pressure should not be changed to assist the turn.Use collective pitch control to manage rotor r.p.m. Ifrotor r.p.m. builds too high during an autorotation, raisethe collective sufficiently to decrease r.p.m. back to the11-2normal operating range. If the r.p.m. begins decreasing,you have to again lower the collective. Always keepthe rotor r.p.m. within the established range for yourhelicopter. During a turn, rotor r.p.m. increases due tothe increased back cyclic control pressure, whichinduces a greater airflow through the rotor system. Ther.p.m. builds rapidly and can easily exceed the maximum limit if not controlled by use of collective. Thetighter the turn and the heavier the gross weight, thehigher the r.p.m.To initiate an autorotation, other than in a low hover,lower the collective pitch control. This holds truewhether performing a practice autorotation or in theevent of an in-flight engine failure. This reduces thepitch of the main rotor blades and allows them tocontinue turning at normal r.p.m. During practiceautorotations, maintain the r.p.m. in the green arcwith the throttle while lowering collective. Once thecollective is fully lowered, reduce engine r.p.m. bydecreasing the throttle. This causes a split of theengine and rotor r.p.m. needles.STRAIGHT-IN AUTOROTATIONA straight-in autorotation implies an autorotation fromaltitude with no turns. The speed at touchdown and theresulting ground run depends on the rate and amount offlare. The greater the degree of flare and the longer it isheld, the slower the touchdown speed and the shorterthe ground run. The slower the speed desired at touchdown, the more accurate the timing and speed of theflare must be, especially in helicopters with low inertiarotor systems.TECHNIQUERefer to figure 11-1 (position 1). From level flight atthe manufacturer’s recommended airspeed, between500 to 700 feet AGL, and heading into the wind,smoothly, but firmly lower the collective pitch controlto the full down position, maintaining r.p.m. in thegreen arc with throttle. Coordinate the collective movement with proper antitorque pedal for trim, and applyaft cyclic control to maintain proper airspeed. Once thecollective is fully lowered, decrease throttle to ensure aclean split of the needles. After splitting the needles,readjust the throttle to keep engine r.p.m. abovenormal idling speed, but not high enough to causerejoining of the needles. The manufacturer oftenrecommends the proper r.p.m.At position 2, adjust attitude with cyclic control toobtain the manufacturer’s recommended autorotationor best gliding speed. Adjust collective pitch control, asnecessary, to maintain rotor r.p.m. in the green arc. Aftcyclic movements cause an increase in rotor r.p.m.,
帅哥
发表于 2009-3-21 00:01:03
which is then controlled by a small increase in collective pitch control. Avoid a large collective pitchincrease, which results in a rapid decay of rotor r.p.m.,and leads to “chasing the r.p.m.” Avoid looking straightdown in front of the aircraft. Continually cross-checkattitude, trim, rotor r.p.m., and airspeed.At approximately 40 to 100 feet above the surface, orat the altitude recommended by the manufacturer (position 3), begin the flare with aft cyclic control to reduceforward airspeed and decrease the rate of descent.Maintain heading with the antitorque pedals. Care mustbe taken in the execution of the flare so that the cycliccontrol is not moved rearward so abruptly as to causethe helicopter to climb, nor should it be moved soslowly as to not arrest the descent, which may allowthe helicopter to settle so rapidly that the tail rotorstrikes the ground. When forward motion decreases tothe desired groundspeed, which is usually the slowestpossible speed (position 4), move the cyclic controlforward to place the helicopter in the proper attitudefor landing.The altitude at this time should be approximately 8 to15 feet AGL, depending on the altitude recommendedby the manufacturer. Extreme caution should be usedto avoid an excessive nose high and tail low attitudebelow 10 feet. At this point, if a full touchdown landingis to be made, allow the helicopter to descend vertically(position 5). Increase collective pitch, as necessary, tocheck the descent and cushion the landing. Additionalantitorque pedal is required to maintain heading as collective pitch is raised due to the reduction in rotorr.p.m. and the resulting reduced effect of the tail rotor.Touch down in a level flight attitude.A power recovery can be made during training in lieuof a full touchdown landing. Refer to the section onpower recoveries for the correct technique.Figure 11-1. Straight-in autorotation.11-3After touchdown and after the helicopter has come to acomplete stop, lower the collective pitch to the fulldown position. Do not try to stop the forward groundrun with aft cyclic, as the main rotor blades can strikethe tail boom. Rather, by lowering the collectiveslightly during the ground run, more weight is placedon the undercarriage, slowing the helicopter.COMMON ERRORS1. Failing to use sufficient antitorque pedal whenpower is reduced.2. Lowering the nose too abruptly when power isreduced, thus placing the helicopter in a dive.3. Failing to maintain proper rotor r.p.m. duringthe descent.4. Application of up-collective pitch at an excessivealtitude resulting in a hard landing, loss ofheading control, and possible damage to the tailrotor and to the main rotor blade stops.5. Failing to level the helicopter.POWER RECOVERY FROM PRACTICEAUTOROTATIONA power recovery is used to terminate practiceautorotations at a point prior to actual touchdown.After the power recovery, a landing can be made or ago-around initiated.TECHNIQUEAt approximately 8 to 15 feet above the ground,depending upon the helicopter being used, begin tolevel the helicopter with forward cyclic control. Avoidexcessive nose high, tail low attitude below 10 feet.Just prior to achieving level attitude, with the nose stillslightly up, coordinate upward collective pitch controlwith an increase in the throttle to join the needles atoperating r.p.m. The throttle and collective pitch mustbe coordinated properly. If the throttle is increased toofast or too much, an engine overspeed can occur; ifthrottle is increased too slowly or too little in proportion to the increase in collective pitch, a loss of rotorr.p.m. results. Use sufficient collective pitch to stop thedescent and coordinate proper antitorque pedalpressure to maintain heading. When a landing is to bemade following the power recovery, bring the helicopter to a hover at normal hovering altitude and thendescend to a landing.If a go-around is to be made, the cyclic control shouldbe moved forward to resume forward flight. In transitioning from a practice autorotation to a go-around,exercise care to avoid an altitude-airspeed combinationthat would place the helicopter in an unsafe area of itsheight-velocity diagram.COMMON ERRORS1. Initiating recovery too late, requiring a rapid application of controls, resulting in overcontrolling.2. Failing to obtain and maintain a level attitudenear the surface.3. Failing to coordinate throttle and collective pitchproperly, resulting in either an engine overspeedor a loss of r.p.m.4. Failing to coordinate proper antitorque pedal withthe increase in powerAUTOROTATIONS WITH TURNSA turn, or a series of turns, can be made during anautorotation in order to land into the wind or avoidobstacles. The turn is usually made early so that theremainder of the autorotation is the same as a straightin autorotation. The most common types are 90° and180° autorotations. The technique below describes a180° autorotation.TECHNIQUEEstablish the aircraft on downwind at recommended
帅哥
发表于 2009-3-21 00:01:14
airspeed at 700 feet AGL, parallel to the touchdown area.In a no wind or headwind condition, establish the groundtrack approximately 200 feet away from the touchdownpoint. If a strong crosswind exists, it will be necessary tomove your downwind leg closer or farther out. Whenabeam the intended touchdown point, reducecollective, and then split the needles. Apply properantitorque pedal and cyclic to maintain proper attitude.Cross check attitude, trim, rotor r.p.m., and airspeed.After the descent and airspeed is established, roll into a180° turn. For training, you should initially roll into abank of a least 30°, but no more than 40°. Check yourairspeed and rotor r.p.m. Throughout the turn, it isimportant to maintain the proper airspeed and keep theaircraft in trim. Changes in the aircraft’s attitude andthe angle of bank cause a corresponding change in rotorr.p.m. Adjust the collective, as necessary, in the turn tomaintain rotor r.p.m. in the green arc.At the 90° point, check the progress of your turn byglancing toward your landing area. Plan the second90 degrees of turn to roll out on the centerline. If you aretoo close, decrease the bank angle; if too far out, increasethe bank angle. Keep the helicopter in trim with antitorque pedals.The turn should be completed and the helicopteraligned with the intended touchdown area prior to passing through 100 feet AGL. If the collective pitch wasincreased to control the r.p.m., it may have to belowered on roll out to prevent a decay in r.p.m. Makean immediate power recovery if the aircraft is not11-4aligned with the touchdown point, and if the rotorr.p.m. and/or airspeed is not within proper limits.From this point, complete the procedure as if it were astraight-in autorotation.POWER FAILURE IN A HOVERPower failures in a hover, also called hovering autorotations, are practiced so that you automatically makethe correct response when confronted with enginestoppage or certain other emergencies while hovering.The techniques discussed in this section refer to helicopters with a counter-clockwise rotor system and anantitorque rotor.TECHNIQUETo practice hovering autorotations, establish a normalhovering altitude for the particular helicopter beingused, considering load and atmospheric conditions.Keep the helicopter headed into the wind and holdmaximum allowable r.p.m.To simulate a power failure, firmly roll the throttle intothe spring loaded override position, if applicable. Thisdisengages the driving force of the engine from therotor, thus eliminating torque effect. As the throttle isclosed, apply proper antitorque pedal to maintain heading. Usually, a slight amount of right cyclic control isnecessary to keep the helicopter from drifting to theleft, to compensate for the loss of tail rotor thrust.However, use cyclic control, as required, to ensure avertical descent and a level attitude. Leave the collective pitch where it is on entry.Helicopters with low inertia rotor systems will begin tosettle immediately. Keep a level attitude and ensure avertical descent with cyclic control while maintainingheading with the pedals. At approximately 1 foot abovethe surface, apply upward collective pitch control, asnecessary, to slow the descent and cushion the landing.Usually the full amount of collective pitch is required.As upward collective pitch control is applied, the throttle has to be held in the closed position to prevent therotor from re-engaging.Helicopters with high inertia rotor systems will maintainaltitude momentarily after the throttle is closed. Then, asthe rotor r.p.m. decreases, the helicopter starts to settle.When the helicopter has settled to approximately 1 footabove the surface, apply upward collective pitch controlwhile holding the throttle in the closed position to slowthe descent and cushion the landing. The timing of collective pitch control application, and the rate at which itis applied, depends upon the particular helicopter beingused, its gross weight, and the existing atmospheric conditions. Cyclic control is used to maintain a level attitudeand to ensure a vertical descent. Maintain heading withantitorque pedals.When the weight of the helicopter is entirely on theskids, cease the application of upward collective. Whenthe helicopter has come to a complete stop, lower thecollective pitch to the full down position.The timing of the collective pitch is a most importantconsideration. If it is applied too soon, the remainingr.p.m. may not be sufficient to make a soft landing. Onthe other hand, if collective pitch control is applied toolate, surface contact may be made before sufficientblade pitch is available to cushion the landing.COMMON ERRORS1. Failing to use sufficient proper antitorque pedalwhen power is reduced.2. Failing to stop all sideward or backward movement prior to touchdown.3. Failing to apply up-collective pitch properly,resulting in a hard touchdown.4. Failing to touch down in a level attitude.5. Not rolling the throttle completely to idle.HEIGHT/VELOCITY DIAGRAMA height/velocity (H/V) diagram, published by themanufacturer for each model of helicopter, depicts thecritical combinations of airspeed and altitude should anengine failure occur. Operating at the altitudes and airspeeds shown within the crosshatched or shaded areasof the H/V diagram may not allow enough time for the
帅哥
发表于 2009-3-21 00:01:26
critical transition from powered flight to autorotation.An engine failure in a climb after takeoff occurring insection A of the diagram is most critical. During aclimb, a helicopter is operating at higher power settingsand blade angle of attack. An engine failure at this pointcauses a rapid rotor r.p.m. decay because the upwardmovement of the helicopter must be stopped, then adescent established in order to drive the rotor. Time isalso needed to stabilize, then increase the r.p.m. to thenormal operating range. The rate of descent must reacha value that is normal for the airspeed at the moment.Since altitude is insufficient for this sequence, you endup with decaying r.p.m., an increasing sink rate, nodeceleration lift, little translational lift, and littleresponse to the application of collective pitch to cushion the landing.It should be noted that, once a steady state autorotationhas been established, the H/V diagram no longerapplies. An engine failure while descending throughsection A of the diagram, is less critical, provided a safelanding area is available.11-5You should avoid the low altitude, high airspeed portionof the diagram (section B), because your recognition of anengine failure will most likely coincide with, or shortlyoccur after, ground contact. Even if you detect an enginefailure, there may not be sufficient time to rotate thehelicopter from a nose low, high airspeed attitude to onesuitable for slowing, then landing. Additionally, thealtitude loss that occurs during recognition of engine failure and rotation to a landing attitude, may not leaveenough altitude to prevent the tail skid from hitting theground during the landing maneuver.Basically, if the helicopter represented by this H/V diagram is above 445 feet AGL, you have enough time andaltitude to enter a steady state autorotation, regardlessof your airspeed. If the helicopter is hovering at 5 feetAGL (or less) in normal conditions and the engine fails,a safe hovering autorotation can be made. Betweenapproximately 5 feet and 445 feet AGL, however, thetransition to autorotation depends on the altitude andairspeed of the helicopter. Therefore, you shouldalways be familiar with the height/velocity diagram forthe particular model of helicopter you are flying.THE EFFECT OF WEIGHT VERSUSDENSITY ALTITUDEThe height/velocity diagram depicts altitude and airspeed situations from which a successful autorotationcan be made. The time required, and therefore, altitudenecessary to attain a steady state autorotative descent,is dependent on the weight of the helicopter and thedensity altitude. For this reason, the H/V diagram forsome helicopter models is valid only when the helicopter is operated in accordance with the gross weight vs.density altitude chart. Where appropriate, this chart isfound in the rotorcraft flight manual for the particularhelicopter. Figure 11-3. Assuming a density altitude of 5,500 feet, theheight/velocity diagram in figure 11-2 would be valid up to agross weight of approximately 1,700 pounds. This is found byentering the graph at a density altitude of 5,500 feet (point A),then moving horizontally to the solid line (point B). Moving vertically to the bottom of the graph (point C), you find that with theexisting density altitude, the maximum gross weight underwhich the height/velocity diagram is applicable is 1,700 pounds.The gross weight vs. density altitude chart is notintended as a restriction to gross weight, but as an advisory to the autorotative capability of the helicopterduring takeoff and climb. You must realize, however,that at gross weights above those recommended by thegross weight vs. density altitude chart, the H/V diagramis not restrictive enough.VORTEX RING STATE (SETTLING WITHPOWER)Vortex ring state describes an aerodynamic conditionwhere a helicopter may be in a vertical descent with upto maximum power applied, and little or no cyclicauthority. The term “settling with power” comes fromthe fact that helicopter keeps settling even though fullengine power is applied.In a normal out-of-ground-effect hover, the helicopteris able to remain stationary by propelling a large massof air down through the main rotor. Some of the air isrecirculated near the tips of the blades, curling up fromthe bottom of the rotor system and rejoining the air500450400350300250200150100500ABSmooth Hard Surface.Avoid Operation inShaded Areas.INDICATED AIRSPEED KNOTS(CORRECTED FOR INSTRUMENT ERROR)HEIGHT ABOVE SURFACE - FEET0 10 20 30 40 50 60 70 80 90 100 110 120A BC7,0006,0005,0004,0003,0001,500 1,600 1,700 1,800 1,900GROSS WEIGHT – POUNDSDENSITY ALTITUDE – FEETFigure 11-2. By carefully studying the height/velocitydiagram, you will be able to avoid the combinations of altitude and airspeed that may not allow you sufficient time oraltitude to enter a stabilized autorotative descent. You mightwant to refer to this diagram during the remainder of the
帅哥
发表于 2009-3-21 00:01:44
discussion on the height/velocity diagram.11-6entering the rotor from the top. This phenomenon iscommon to all airfoils and is known as tip vortices. Tipvortices consume engine power but produce no usefullift. As long as the tip vortices are small, their onlyeffect is a small loss in rotor efficiency. However, whenthe helicopter begins to descend vertically, it settlesinto its own downwash, which greatly enlarges the tipvortices. In this vortex ring state, most of the powerdeveloped by the engine is wasted in accelerating theair in a doughnut pattern around the rotor.In addition, the helicopter may descend at a rate thatexceeds the normal downward induced-flow rate of theinner blade sections. As a result, the airflow of the innerblade sections is upward relative to the disc. This produces a secondary vortex ring in addition to the normaltip-vortices. The secondary vortex ring is generatedabout the point on the blade where the airflow changesfrom up to down. The result is an unsteady turbulentflow over a large area of the disc. Rotor efficiency islost even though power is still being supplied from theengine. A fully developed vortex ring state is characterized byan unstable condition where the helicopter experiencesuncommanded pitch and roll oscillations, has little orno cyclic authority, and achieves a descent rate, which,if allowed to develop, may approach 6,000 feet perminute. It is accompanied by increased levels ofvibration.A vortex ring state may be entered during any maneuver that places the main rotor in a condition of highupflow and low forward airspeed. This condition issometimes seen during quick-stop type maneuvers orduring recoveries from autorotations. The followingcombination of conditions are likely to cause settling ina vortex ring state:1. A vertical or nearly vertical descent of at least300 feet per minute. (Actual critical rate dependson the gross weight, r.p.m., density altitude, andother pertinent factors.)2. The rotor system must be using some of the available engine power (from 20 to 100 percent).3. The horizontal velocity must be slower thaneffective translational lift.Some of the situations that are conducive to a settlingwith power condition are: attempting to hover out ofground effect at altitudes above the hovering ceiling ofthe helicopter; attempting to hover out of ground effectwithout maintaining precise altitude control; or downwind and steep power approaches in which airspeed ispermitted to drop to nearly zero.When recovering from a settling with power condition,the tendency on the part of the pilot is to first try to stopthe descent by increasing collective pitch. However,this only results in increasing the stalled area of therotor, thus increasing the rate of descent. Since inboardportions of the blades are stalled, cyclic control islimited. Recovery is accomplished by increasingforward speed, and/or partially lowering collectivepitch. In a fully developed vortex ring state, the onlyrecovery may be to enter autorotation to break thevortex ring state. When cyclic authority is regained,you can then increase forward airspeed.For settling with power demonstrations and training inrecognition of vortex ring state conditions, all maneuvers should be performed at an elevation of at least1,500 feet AGL.To enter the maneuver, reduce power below hoverpower. Hold altitude with aft cyclic until theairspeed approaches 20 knots. Then allow the sinkrate to increase to 300 feet per minute or more as theattitude is adjusted to obtain an airspeed of less than10 knots. When the aircraft begins to shudder, theapplication of additional up collective increases thevibration and sink rate.Recovery should be initiated at the first sign of vortex ring state by applying forward cyclic to increaseairspeed and simultaneously reducing collective.The recovery is complete when the aircraft passesthrough effective translational lift and a normalclimb is established.RETREATING BLADE STALLIn forward flight, the relative airflow through themain rotor disc is different on the advancing andretreating side. The relative airflow over the advancing side is higher due to the forward speed of theFigure 11-4. Vortex ring state.11-7helicopter, while the relative airflow on the retreating side is lower. This dissymmetry of lift increasesas forward speed increases.To generate the same amount of lift across the rotordisc, the advancing blade flaps up while the retreating blade flaps down. This causes the angle of attackto decrease on the advancing blade, which reduceslift, and increase on the retreating blade, whichincreases lift. As the forward speed increases, atsome point the low blade speed on the retreatingblade, together with its high angle of attack, causes aloss of lift (stall).Retreating blade stall is a major factor in limiting ahelicopter’s top forward speed (VNE) and can be feltdeveloping by a low frequency vibration, pitchingup of the nose, and a roll in the direction of theretreating blade. High weight, low rotor r.p.m., highdensity altitude, turbulence and/or steep, abruptturns are all conducive to retreating blade stall athigh forward airspeeds. As altitude is increased,higher blade angles are required to maintain lift at a
帅哥
发表于 2009-3-21 00:02:04
given airspeed. Thus, retreating blade stall isencountered at a lower forward airspeed at altitude.Most manufacturers publish charts and graphs showing a VNE decrease with altitude.When recovering from a retreating blade stall condition, moving the cyclic aft only worsens the stallas aft cyclic produces a flare effect, thus increasingangles of attack. Pushing forward on the cyclicalso deepens the stall as the angle of attack on theretreating blade is increased. Correct recovery fromretreating blade stall requires the collective to belowered first, which reduces blade angles and thusangle of attack. Aft cyclic can then be used to slowthe helicopter.GROUND RESONANCEGround resonance is an aerodynamic phenomenonassociated with fully-articulated rotor systems. Itdevelops when the rotor blades move out of phasewith each other and cause the rotor disc to becomeunbalanced. This condition can cause a helicopter toself-destruct in a matter of seconds. However, forthis condition to occur, the helicopter must be incontact with the ground.If you allow your helicopter to touch down firmly onone corner (wheel type landing gear is mostconducive for this) the shock is transmitted to themain rotor system. This may cause the blades tomove out of their normal relationship with eachother. This movement occurs along the drag hinge.Figure 11-5. Hard contact with the ground can send a shockwave to the main rotor head, resulting in the blades of athree-bladed rotor system moving from their normal 120°relationship to each other. This could result in something like122°, 122°, and 116° between blades. When one of the otherlanding gear strikes the surface, the unbalanced conditioncould be further aggravated.If the r.p.m. is low, the corrective action to stop groundresonance is to close the throttle immediately and fullylower the collective to place the blades in low pitch. If ther.p.m. is in the normal operating range, you should fly thehelicopter off the ground, and allow the blades to automatically realign themselves. You can then make a normaltouchdown. If you lift off and allow the helicopter tofirmly re-contact the surface before the blades arerealigned, a second shock could move the blades againand aggravate the already unbalanced condition. Thiscould lead to a violent, uncontrollable oscillation.This situation does not occur in rigid or semirigid rotorsystems, because there is no drag hinge. In addition,skid type landing gear are not as prone to groundresonance as wheel type gear.DYNAMIC ROLLOVERA helicopter is susceptible to a lateral rolling tendency,called dynamic rollover, when lifting off the surface.For dynamic rollover to occur, some factor has to firstcause the helicopter to roll or pivot around a skid, orlanding gear wheel, until its critical rollover angle isreached. Then, beyond this point, main rotor thrust continues the roll and recovery is impossible. If the criticalrollover angle is exceeded, the helicopter rolls on itsside regardless of the cyclic corrections made.Dynamic rollover begins when the helicopter starts topivot around its skid or wheel. This can occur for avariety of reasons, including the failure to remove atiedown or skid securing device, or if the skid or wheel122° 116°122°11-8contacts a fixed object while hovering sideward, or ifthe gear is stuck in ice, soft asphalt, or mud. Dynamicrollover may also occur if you do not use the properlanding or takeoff technique or while performing slopeoperations. Whatever the cause, if the gear or skidbecomes a pivot point, dynamic rollover is possible ifyou do not use the proper corrective technique.Once started, dynamic rollover cannot be stopped byapplication of opposite cyclic control alone. For example, the right skid contacts an object and becomes thepivot point while the helicopter starts rolling to theright. Even with full left cyclic applied, the main rotorthrust vector and its moment follows the aircraft as itcontinues rolling to the right. Quickly applying downcollective is the most effective way to stop dynamicrollover from developing. Dynamic rollover can occurin both skid and wheel equipped helicopters, and alltypes of rotor systems.CRITICAL CONDITIONSCertain conditions reduce the critical rollover angle,thus increasing the possibility for dynamic rollover andreducing the chance for recovery. The rate of rollingmotion is also a consideration, because as the roll rateincreases, the critical rollover angle at which recoveryis still possible, is reduced. Other critical conditionsinclude operating at high gross weights with thrust (lift)approximately equal to the weight.Refer to figure 11-6. The following conditions aremost critical for helicopters with counter-clockwiserotor rotation:1. right side skid/wheel down, since translating tendency adds to the rollover force.2. right lateral center of gravity.3. crosswinds from the left.4. left yaw inputs.For helicopters with clockwise rotor rotation, the opposite would be true.CYCLIC TRIMWhen maneuvering with one skid or wheel on theground, care must be taken to keep the helicopter cycliccontrol properly trimmed. For example, if a slow takeoff is attempted and the cyclic is not positioned andtrimmed to account for translating tendency, the critical
帅哥
发表于 2009-3-21 00:02:20
recovery angle may be exceeded in less than two seconds. Control can be maintained if you maintain propercyclic position and trim, and not allow the helicopter’sroll and pitch rates to become too great. You should flyyour helicopter into the air smoothly while keepingmovements of pitch, roll, and yaw small, and not allowany untrimmed cyclic pressures.NORMAL TAKEOFFS AND LANDINGSDynamic rollover is possible even during normal takeoffs and landings on relative level ground, if one wheelor skid is on the ground and thrust (lift) is approximately equal to the weight of the helicopter. If thetakeoff or landing is not performed properly, a roll ratecould develop around the wheel or skid that is on theground. When taking off or landing, perform themaneuver smoothly and trim the cyclic so that no pitchor roll movement rates build up, especially the roll rate.If the bank angle starts to increase to an angle ofapproximately 5 to 8°, and full corrective cyclic doesnot reduce the angle, the collective should be reducedto diminish the unstable rolling condition.SLOPE TAKEOFFS AND LANDINGSDuring slope operations, excessive application of cycliccontrol into the slope, together with excessive collectivepitch control, can result in the downslope skid risingsufficiently to exceed lateral cyclic control limits, and anupslope rolling motion can occur. Pivot PointBank AngleWeightTipPathPlaneNeutralCyclicTipPathPlaneFullLeftCyclicCrosswindTailRotorThrustMainRotorThrustFigure 11-6. Forces acting on a helicopter with right skid onthe ground.TailRotorThrustSlopeHorizontalAreaofCriticalRolloverFull Opposite Cyclic Limitto Prevent Rolling MotionFigure 11-7. Upslope rolling motion.11-9When performing slope takeoff and landing maneuvers, follow the published procedures and keep the rollrates small. Slowly raise the downslope skid or wheelto bring the helicopter level, and then lift off. Duringlanding, first touch down on the upslope skid or wheel,then slowly lower the downslope skid or wheel usingcombined movements of cyclic and collective. If thehelicopter rolls approximately 5 to 8° to the upslopeside, decrease collective to correct the bank angle andreturn to level attitude, then start the landing procedureagain.USE OF COLLECTIVEThe collective is more effective in controlling the rollingmotion than lateral cyclic, because it reduces the mainrotor thrust (lift). A smooth, moderate collective reduction, at a rate less than approximately full up to full downin two seconds, is adequate to stop the rolling motion.Take care, however, not to dump collective at too high arate, as this may cause a main rotor blade to strike thefuselage. Additionally, if the helicopter is on a slope andthe roll starts to the upslope side, reducing collective toofast may create a high roll rate in the opposite direction.When the upslope skid/wheel hits the ground, thedynamics of the motion can cause the helicopter tobounce off the upslope skid/wheel, and the inertia cancause the helicopter to roll about the downslope groundcontact point and over on its side. The collective should not be pulled suddenly to get airborne, as a large and abrupt rolling moment in theopposite direction could occur. Excessive applicationof collective can result in the upslope skid rising sufficiently to exceed lateral cyclic control limits. Thismovement may be uncontrollable. If the helicopterdevelops a roll rate with one skid/wheel on the ground,the helicopter can roll over on its side.PRECAUTIONSThe following lists several areas to help you avoiddynamic rollover.1. Always practice hovering autorotations into thewind, but never when the wind is gusty or over10 knots.2. When hovering close to fences, sprinklers,bushes, runway/taxi lights, or other obstacles thatcould catch a skid, use extreme caution.3. Always use a two-step liftoff. Pull in just enoughcollective pitch control to be light on the skidsand feel for equilibrium, then gently lift thehelicopter into the air.4. When practicing hovering maneuvers close tothe ground, make sure you hover high enough tohave adequate skid clearance with any obstacles, especially when practicing sideways orrearward flight.5. When the wind is coming from the upslope direction, less lateral cyclic control will be available.6. Tailwind conditions should be avoided whenconducting slope operations.7. When the left skid/wheel is upslope, less lateralcyclic control is available due to the translatingtendency of the tail rotor. (This is true forcounter-rotating rotor systems)8. If passengers or cargo are loaded or unloaded, thelateral cyclic requirement changes.9. If the helicopter utilizes interconnecting fuel linesthat allow fuel to automatically transfer from oneside of the helicopter to the other, the gravitationalflow of fuel to the downslope tank could change
帅哥
发表于 2009-3-21 00:02:33
the center of gravity, resulting in a differentamount of cyclic control application to obtain thesame lateral result.10. Do not allow the cyclic limits to be reached. If thecyclic control limit is reached, further lowering ofthe collective may cause mast bumping. If thisoccurs, return to a hover and select a landing pointwith a lesser degree of slope.11. During a takeoff from a slope, if the upslopeskid/wheel starts to leave the ground before thedownslope skid/wheel, smoothly and gentlyTailRotorThrustSlopeHorizontalAreaofCriticalRolloverFull Opposite Cyclic Limitto Prevent Rolling MotionFFigure 11-8. Downslope rolling motion.11-10lower the collective and check to see if thedownslope skid/wheel is caught on something.Under these conditions vertical ascent is the onlyacceptable method of liftoff.12. During flight operations on a floating platform, ifthe platform is pitching/rolling while attempting toland or takeoff, the result could be dynamic rollover.LOW G CONDITIONS AND MASTBUMPINGFor cyclic control, small helicopters depend primarilyon tilting the main rotor thrust vector to producecontrol moments about the aircraft center of gravity(CG), causing the helicopter to roll or pitch in thedesired direction. Pushing the cyclic control forwardabruptly from either straight-and-level flight or after aclimb can put the helicopter into a low G (weightless)flight condition. In forward flight, when a push-over isperformed, the angle of attack and thrust of the rotor isreduced, causing a low G or weightless flight condition. During the low G condition, the lateral cyclic haslittle, if any, effect because the rotor thrust has beenreduced. Also, in a counter-clockwise rotor system (aclockwise system would be the reverse), there is nomain rotor thrust component to the left to counteractthe tail rotor thrust to the right, and since the tail rotoris above the CG, the tail rotor thrust causes the helicopter to roll rapidly to the right, If you attempt to stop theright roll by applying full left cyclic before regainingmain rotor thrust, the rotor can exceed its flappinglimits and cause structural failure of the rotor shaft dueto mast bumping, or it may allow a blade to contact theairframe. Since a low G condition could have disastrous results,the best way to prevent it from happening is to avoid theconditions where it might occur. This means avoidingturbulence as much as possible. If you do encounterturbulence, slow your forward airspeed and make smallcontrol inputs. If turbulence becomes excessive,consider making a precautionary landing. To help prevent turbulence induced inputs, make sure your cyclicarm is properly supported. One way to accomplish thisis to brace your arm against your leg. Even if you arenot in turbulent conditions, you should avoid abruptmovement of the cyclic and collective.If you do find yourself in a low G condition, whichcan be recognized by a feeling of weightlessnessand an uncontrolled roll to the right, you should immediately and smoothly apply aft cyclic. Do not attemptto correct the rolling action with lateral cyclic. Byapplying aft cyclic, you will load the rotor system,which in turn produces thrust. Once thrust is restored,left cyclic control becomes effective, and you can rollthe helicopter to a level attitude.LOW ROTOR RPM AND BLADE STALLAs mentioned earlier, low rotor r.p.m. during anautorotation might result in a less than successfulmaneuver. However, if you let rotor r.p.m. decay to thepoint where all the rotor blades stall, the result is usually fatal, especially when it occurs at altitude. Thedanger of low rotor r.p.m. and blade stall is greatest insmall helicopters with low blade inertia. It can occurin a number of ways, such as simply rolling the throttle the wrong way, pulling more collective pitch thanpower available, or when operating at a high densityaltitude.When the rotor r.p.m. drops, the blades try to maintainthe same amount of lift by increasing pitch. As thepitch increases, drag increases, which requires morepower to keep the blades turning at the proper r.p.m.When power is no longer available to maintain r.p.m.,and therefore lift, the helicopter begins to descend.This changes the relative wind and further increasesthe angle of attack. At some point the blades will stallunless r.p.m. is restored. If all blades stall, it is almostimpossible to get smooth air flowing across theblades.Even though there is a safety factor built into most helicopters, anytime your rotor r.p.m. falls below the greenarc, and you have power, simultaneously add throttleand lower the collective. If you are in forward flight,gently applying aft cyclic loads up the rotor system andhelps increase rotor r.p.m. If you are without power,immediately lower the collective and apply aft cyclic.RECOVERY FROM LOW ROTOR RPMUnder certain conditions of high weight, high temperature, or high density altitude, you might get into a
