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CONTENTS

Basic Helicopter Flight Aerodynamics 1

Hovering = 1Transition from Hover to Forward Flight 3Translational Lift 4Transition from Forward Flight to Hover 5Power Required 7Power Available 9Forward Flight 12Dissymetry of Lift 12

Limits of Forward Speed 15Stability 17Cyclic Control Forces 18Vortex Ring 19

Control on the Ground 23

Ground Resonance 23Taxying 25Blade Sailing 26

Centre of Gravity 26

Autorotation 28

Autorotative Force 29Forward Speed 32All-up Weight 3%Altitude 35

Range and Endurance 35

Copyright

This material is for the sole use of enrolled students and may notbe reproduced without the written authority of the Principal, TOPNZ.

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AIRCRAFT ENGINEERING

HELICOPTERS ASSIGNMENT 10

BASIC HELICOPTER FLIGHT AERODYNAMICS

Hovering

when the helicopter is at rest on the ground with rotorrev/min set at the normal take—off figure, the lift resultingwithout collective pitch is negligible. In this condition, theonly effective force acting on the aircraft is that of gravityacting on the mass. The only reason that this unbalanced forcedoes not produce movement is because the ground supplies anequal and opposite reaction.

As collective pitch is applied and the rev/min kept constant,so the lift is increased and the weight is taken off the wheels.The reaction from the ground is reduced, but there is still nomovement of the aircraft. when the lift exactly balances theweight, a new state of equilibrium has been created, with theaircraft at rest and with no reaction from the ground.

As pitch is further increased, lift exceeds weight and theexcess force creates an acceleration upwards (F = Ma). Thatis, the aircraft will now climb vertically, given perfectstill-air conditions. As the fuselage starts to move, parasitedrag results and must be added to the weight. A new stage ofequilibrium will be reached at the climbing speed, whereparasite drag is equal to the excess of lift over weight.

To achieve hover, pitch is reduced until the lift againequals weight. The parasite drag then decelerates the rate ofclimb,at the same time, itself reducing to zero. A new stateof equilibrium is then reached, with lift equal to weight andthe aircraft stationary at the required height.

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This, then, represents the perfect hover, assuming nooutside interference, wind, and so on, with lift exactly balancingweight. Ideally, there should be no further movement of thecollective, cyclic, throttle, or tail rotor controls to maintaina constant position. In practice, however, small controlcorrections must constantly be made to keep an accurate hover.

If power is now reduced, the aircraft will descend as aresult of excess weight over lift. The descent will again bean acceleration until the parasite drag from the fuselage onceagain equates the forces. The descent is then at constant speed.

Ground effect: As a slowly descending helicopter nearsthe ground, its rate of descent reduces, and it may even cometo a hovering attitude ——-even though no changes to the collectiveand throttle controls are made. This phenomenon is caused byground effect.

The effect is brought about by an increased pressure areabeing created between the rotor disc and the ground as a resultof the normal downward flow of air through the disc beingslowed by the ground immediately below the rotor. The effectis sometimes called ground cushion because the impression is ofthe aircraft "sitting" on an air cushion.

The closer the rotor is to the ground, the more the airwill tend to be trapped and slowed and therefore "cushion"the aircraft. That is, the closer the aircraft is to theground, the greater will be the ground effect and therefore thelesser the power required to hover. Because ground effectdecreases with height above the ground, it is not easy tostate positively a height at which the effect will be negligible.For practical purposes, the ground cushion is taken as the rotorheight above ground equivalent to the length of one rotor bladeor one half of the rotor diameter. Thus, the larger the rotor,the thicker the ground cushion.

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The second factor affecting ground effect is the nature ofthe ground itself. Because the effect depends upon the slowedair maintaining a streamline flow, the smoother, firmer, andmore level the ground is, the greater the effect will be. Thus,a level stretch of tarmac or concrete will give maximum effect.Long grass, small bushes, or uneven ground tend to break up thesmooth flow of air and reduce the effect. Sloping ground causesan inequality of ground effect round the disc and hence sometendency for the aircraft to "slide" down the slope. A similarresult will occur if the disc is not parallel to the ground,for example, when hovering in a wind or in the transition fromthe hover to translational flight. A wind tends to displacethe "cushion" downwind of the helicopter.

Re-circulation: Some energy is lost by the spillage of airaround the tips of the blades. This can be aggravated whenhovering near the ground, particularly if someobstruction fairly near to the rotor, such as a hangar dooror a high building, which causes the air, after passing throughthe rotor, to re-circulate down through the rotor again. Thisdetracts from the ground effect and, when the obstruction is onlyon one side of the rotor, causes an inequality of lift aroundthe disc so that the aircraft tends to "creep in" towards theobstruction.

Transition from Hover to Forward Flight

This transition, which will be made nearly every time thehelicopter is flown, is usually accomplished as the helicopteris climbing from its take-off site. However, it can be amanoeuvre during flying training, and so we'll consider thetheory from the point of view of keeping the lift factorconstant.

' In Fig. l, the centre line shows the flight path. Theparallel dotted lines show that lift is equal to weight so thata constant height is maintained.

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Figure l (a) shows the helicopter in a perfect hover (stillair) within the ground cushion. The lift is shown as a combinationof power and ground effect.

To achieve forward flight from hover, you need to tiltthe disc forward with a forward movement of the cyclic stick, andso create a thrust force in the required direction. See Fig. l (b)You then need to increase the size of the useful force to keepthe lift component equal to weight. The tilting of the discalso causes some loss of ground effect, requiring yet morepower to compensate. In fact, power is normally increased tomaximum to ensure no loss of height. This can be a hazardoperationally if the power margin is small. It may be possibleto hover, as the result of ground effect, when at nearly fullpower, but the sink caused by the change of disc attitude may besuch that not enough power is left to prevent the aircraftsliding off the ground cushion and so striking the ground.

The thrust force created by tilting the disc now causesthe fuselage to accelerate in the direction of disc tilt, thatis, forwards in this case. Acceleration will continue untilthe parasite drag of the fuselage balances the thrust component—— Fig. l (c). An equilibrium state is established, and speedwill now remain constant. The speed at which this occurs willdepend on the amount the disc is tilted and whether there isenough power to provide the necessary useful force at this discattitude.

Translational Lift

As speed increases, power may be reduced as a result oftranslational lift. This is additional lift created by therotor, at given pitch and power settings, when moving forward,as a result of the increased mass flow of air now passingthrough the disc in a given time. Less power is required toproduce a given force if a large mass is given a smallacceleration compared with a small mass being given largeacceleration. If power is not reduced but level flight ismaintained by moving the stick forward, not only will the forward

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speed increase, but the rotor rev/min will also increase as aresult of the extra power available. Any further increase inspeed will require a disproportionate increase in power tocompensate for parasite drag, which rises as the square of thespeed —— Fig. l (d). 1

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FIG. l Transistion from hover to forward flight

Transition from Forward Flight to Hover

Figure 2 shows the change of rotor attitude from forwardflight condition to induce rapid deceleration called a flare.The effects of the flare are:

l. An increase in the useful force as a result ofthe increase in the angle of attack of the disc(forward movement is maintained). This iscomparable to the increase in angle of attack ofa fixed—wing aircraft in a steep turn or pullingout of a dive.

2. A reversal of the direction of the thrust component,causing rapid deceleration.

3. Some tendency to cut off the inflow of air,hence partly offsetting the effect described inl.

4. An increase in rotor rev/min. This is surprisingbecause an increase in angle of attack wouldsuggest an increase in drag and therefore adecrease in rev/min. However, the importantfactor is the relationship of the direction of totalreaction to the plane of rotation. Figure 2shows that, as a result of the flare, totalreaction has moved forward relative to the planeof rotation, thus causing an increase in rotorrev/min.

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RA — Rclaiive, airflow’ PR - Plal/H2. of roiaiion RPD — Rotor profiie dragL — LIFT‘ L TR TR - Total FQQCIIOHA — Pitch angle \

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' FIG. 2 Effect of flare

In forward powered flight, the relative airflow enters therotor disc from above the plane of rotation. During a flare,it enters from below.

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TO return to a state of hover from a cruise state, youcould just move the cyclic control back to tilt the rotor discto the hovering attitude. The aircraft would then lose speedas a result of parasite drag. However, a more rapid decelerationis usually required so the helicopter is normally flared.

As the result of the temporary increase in angle of attack,the aircraft will tend to climb unless power is reduced. Youmay also need to throttle back to prevent the rotor from over-speeding due to the increase in rev/min. However, the airinflow soon becomes stabilised, and translational lift decreasesas the aircraft decelerates rapidly as a result of both parasitedrag and reverse thrust. Power must then be increasedconsiderably as the aircraft is coming to rest so as to preventit from sinking. At this stage, you must maintain rotor rev/min.Finally, as all forward speed is lost, you need to restore the discto the correct hovering attitude to prevent the aircraft frommoving backwards and you should reduce power again, not onlybecause of the disappearance of the thrust component butalso because of the re—establishment of the ground cushion.See Fig. 3.

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NOTE:

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We have assumed throughout the transition thatthere is no wind effect. In practice, thetransitions will normally be made into wind.

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In practice, only one change of power is made, by eitherlever or hand throttle or both, while in theory, thecollective

power required for level flight can be divided into threecomponents

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FIG. 3 Transition from forward flight to hover

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changes of power must be made to maintain level flightng forward speeds.

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Rotor profile power: The component Of totalreaction acting in the plane of rotation,called rotor profile drag, must be overcomeif rotor rev/min are to be maintained. Thepower to do this is called rotor profilepower.

Induced power: TO create lift, you needto cause a flow of air through the rotor byapplying pitch, that is, giving the air aninduced velocity. The power needed tocause this airflow is called induced power.As pitch is increased, the rotor profilepower will also increase. Provided enoughpower is available to produce lift andstill maintain rotor rev/min, the helicoptershould be able to hover. The factors ofpower needed to drive the tail rotor and thecooling fan are included in the total ofrotor profile power.

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3. Parasite power: When the helicopter moves,the airflow over the fuselage meets aresistance to its passage. This is calledparasite drag, and the power needed toovercome this drag is called parasite power.

Figure H shows the variationsof these three components of powerrequired through the speed range.Although the buildup of forwardspeed causes complications ofouman

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FIG. 4 Components of power required rotor profile power is shown as

a slight curve, gradually increasing with speed.

As forward speed is increased, a greater mass of air willpass through the rotors. Because a large mass of air needsless acceleration to provide effective force than a small mass,the induced power can be decreased. In other words, as forwardspeed increases, rotor efficiency also increases. In thetransition stage, however, more induced power is needed to supplythe required thrust force and to compensate for the loss ofground effect.

Parasite drag tends to increase in ratio to V2. Forexample, if we have one unit of drag at 20 m/s (V), we would havefour units of drag at HO m/s and nine units at 60 m/s. Thus,parasite power is shown as an ever-increasing curve with speedincrease.

Figure 5 shows the total power required for level flight.The power required curve shows the initial demand for inducedpower in the transition from the hover, the subsequent decreaseas forward speed is built up, and the rapid increase ofparasite drag with higher speed.

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PowerPower requkedrequired

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Ia) Lightly loaded helicopter (b) Heavily loaded helicopterFIG. 5 Comparison of power required with power

available

Power Available

with fixed-wing, piston—engined and turbine~engined air-craft, the power available curve rises to a maximum and thenfalls or keeps rising respectively. with the helicopter'sconstant induction manifold pressure or fuel flow, rev/min,and altitude, there is no variation of power available through-out the speed range. Hence, it appears as a straight line.

More important is its relative vertical position in termsof the power required curve, that is, whether the availabilitymeets the requirement. This may be regarded as one of theessential factors in all helicopter operations (the power/weightratio). If you compare the two curves in Fig. 5, you can seethat if the power available is above the power required throughout-Fig. 5 (a)-—-thennot only are hover, transition, and full speedpossible, but a climb is also possible at any speed, the rate ofclimb being determined by the excess power and the weight of theaircraft. If, however, the power available curve cuts thepower required curve—Fig. 5 (b)-then it is not possible tohover, and you may need to make a "running" take—off or itsequivalent,that is, make use of translational lift, to compensatefar the pgwep shortage, The maximum forward speed will also bereduced.

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Overpitching: Where power may be marginal, during take-off,hover, or transition, overpitching may occur. This means thatso much collective pitch has to be applied to produce the requiredlift that not enough power is left to overcome the high rotorprofile drag. As a result, the rotor rev/min will fall in spiteof the hand throttle being fully open, and the aircraft willstart to sink. The application of more pitch would obviouslyonly aggravate the situation, and the only remedy is to lowerthe lever, keeping the throttle fully open, and accept theresultant sink until correct rev/min are recovered. You mayeven have to put the aircraft back on the ground and reduceweight.

NOTE: True overpitching occurs only when the throttleis fully open, though similar symptoms can resultfrom a rapid lever movement without adequatethrottle lead, for example, in the transitionto the hover. In this case, the remedy is toopen the throttle.

The factors governing power available and power required are

l. Altitude, temperature, and humidity: Thesethree factors can be dealt with under oneheading because the basic problem is one ofair density. The reason for applying pitchto the rotor is to accelerate a mass flow ofair through the rotor. A given pitch settingat constant rev/min will, however, acceleratea given column of air, and the mass flowwill therefore depend on the density

mass = volume X density

Any reduction in air density will thereforereduce its mass flow and the resulting lift,so requiring an increase in pitch and,consequently, power to balance the weight.An increase in altitude, temperature, orhumidity will, in each case, cause a decreasein density and therefore in the performanceof the rotor.

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Each of these factors also affects the engineperformance resulting in a decrease in thepower available at the same time that thepower required is increasing. The humidityused is the specific humidity, that is,the actual amount of water vapour present,as opposed to relative humidity.

2. Wind effect: The effect of moving the rotordisc through the air is to increase the massairflow through the rotor and so decrease thepower needed. Wind blowing through therotors will have much the same effect asrotation of the rotors.

3. A11—up weight: Because more pitch must beapplied to supply an increase in lift (assumingconstant rev/min), the factor most affectingpower required is all—up weight. Thus, theratio of weight to power is of great importancein all helicopter operations.

SUMMARY

Ground effect, or ground cushion, is said to extendvertically upward to a rotor height above ground ofhalf the rotor diameter.

Power may be reduced once translational lift is gained.

Parasite drag increases very rapidly with increasingair speed.

In forward—powered flight, the relative airflowenters the rotor disc from above the plane ofrotation. when flared it enters from below.

PRACTICE EXERCISE A

State whether each of the following statements is trueor false.

l. In a hover in still air, lift = weight, andthrust = drag.

2. The type of ground surface has a great influenceon the strength of the ground cushion.

3. Translational lift will increase parasite drag,and so more power will be needed.

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4. Induced power required increases with forwardspeed.

5. When the power available is just enough forhover, a transition to level flight is safelypossible.

6. An increase in AUW will lower the maximum forwardspeed.

7. Air density affects power available but notpower required.

8. Wind effect increases the mass airflow throughthe rotor.

9. In level flight, the relative airflow entersthe rotor from below the plane of rotation.

10. Ground effect extends vertically to a rotorheight above ground of one rotor diameter, andthe ground cushion extends to one half-rotordiameter.

(Answers on page 39)

Forward Flight

As soon as forward speed is gained, the effect of dissymetryof lift is felt. This effect, which has an important bearingon the cyclic control of the helicopter, also imposes a limit onthe forward speed.

Dissymetry of Lift

When the aircraft is moving forward, a rotor blade in the180° of rotation from the tail cone to the nose is said to beadvancing, and this side of the rotor disc is called the advancinggigg. From the nose back to the tail cone, the blade is said tobe retreating around the retreating iide of the disc. If a

two-bladed rotor is considered, the maximum effect of the forwardspeed will be experienced with the blades athwartships (Pig. 6).On the advancing blade, the relative airflow is the sum of the

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effects of rotational velocity and forward speed (V1 + V2).On the retreating blade, it is the difference between the two

(V1 — V2). Because airflow affects lift, then, given equal pitch,the advancing blade has more lift than the retreating blade, andthe disc therefore tends to roll to the retreating side. Theformulas are

Lift (advancing blade) = CL kc (V1 + V2)2 S

Lift (retreating blade) = CL tp (V1 - V2)2 S

This is known as dissymmetry of lift.

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T/FIG. 6 Relative airflow on advancing and retreating blades

In fact, the disc would not roll to the retreating sidebecause the blades have been given freedom to move about eithera gimbal ring (two-bladed rotor) or flapping hinges (three ormore bladed rotor). As soon as the dissymmetry of lift occurs,it causes flapping to take place, the advancing blade flappingup with increase in lift, and the retreating blade downwardsdue to decreased lift. This movement about the flapping hingecauses a further element of relative airflow either up or down,thus altering the angle of attack to compensate for the effectof V2 and so restoring equality of lift (Fig. 7). The flappingcaused by dissymmetry of lift will also cause a change of disc

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attitude, which will be maximum 90° on from the athwartshipsposition (phase lag), that is, fore and aft, the advancing bladerising in front and the retreating blade falling at the tail. Thistilting back of the disc as a result of forward speed is calledflap—back. See Fig. 8. ,

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FIG; 7 Change of angle of attack due to flapping

If flap-back is allowed to occur, the direction of usefulforce will change, as will the components of lift and thrust,andequilibrium in forward flight would be impossible. To maintaina steady forward flight attitude, you therefore need to preventflap—back from occurring because of V2.

The equality of lift between advancing and retreating bladesis achieved by a cyclic pitch change. When the cyclic controlis moved forward to begin forward flight, you must move thestick further forward as soon as the effect of V2 becomes apparentThis means that pitch is reduced where (V1 + V2) is maximum andincreased where (V1 - V2) is minimum.

' Thus, an equality of lift is effectively maintainedthroughout, so preventing any flapping from taking place. Thereis now an angular difference between the plane of the controlorbit and that of the rotor disc, which corresponds to the angleof flap—back of the disc when flapping was allowed to occur.See Fig. 8. This means that the correction for V2 has been madeby feathering and not by flapping.

If the speed is such that the forward limit of cyclic stickis reached, then it is impossible to prevent flap-back resultingfrom any further increase in speed. This could represent alimitation of forward speed.

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‘ FIG. 8 Flapback

Limits of Forward Speed ‘

The factors limiting the maximum forward speed of thehelicopter are

1. Limit of cyclic control,

2. Reversal of airflow,

3. Stalling of the retreating blade,

H. Compressibility,

5. All-up weight, and

6. Altitude.

Limit of cyclic control: When the forward limit of the

cyclic control is reached, it is no longer possible to counteractflap-back, and so any further increase in forward speed isprevented. Offset flapping hinges, a stabiliser at the tail,and a delta hinge effect incorporated in the pitch operating arm,have all been used to overcome cyclic control limits, with theresult that, under normal conditions, the cyclic control isunlikely to run out of movement before other factors impose theirown limits.

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gr “ Reversal of airflow: In“'1 ewe considering dissymmetry of lift,

we compared rotational speed (V1)and forward speed (V2). However,

' this comparison is true only at, any one station along the blade

96 _ £2? _ W@i because V1 varies along the span,whereas V2 is constant for the

Reversed ‘awflow whole blade. Towards the root of

I the retreating blade, the relativeairflow (V1 — V2) can become verysmall, and at a high V2, a negative

imoo quantity. The area of the bladeFIG 9 Reversal of airflow affected in this fashion spreads

from the root as V2 increases.Figure 9 shows the exaggeratedeffect.

Stalling of the retreating blade: Probably the most importantfactor limiting forward speed is the stalling of the retreatingblade by the cyclic pitch applied to prevent flap-back. Becausethis cyclic pitch change is added to the collective pitch alreadyapplied to provide lift, it is easy to reach the critical angleof the blade and so cause it to stall.

The symptoms and effects of this stall are similar to thoseof the usual fixed-wing stall, that is, judder, loss of lift,and an increase in drag. No two helicopters react to bladestall in exactly the same way. Usually, the onset of vibrationand erratic cyclic control forces signals the start of thecondition. As stalling continues to move inward from the tiparea, vibration increases, followed by a partial loss of controland a nose-up pitching tendency. Severe stalling may resultin large rolling tendencies and complete loss of control.Retreating blade stall imposes a limiting V2 on all helicoptersfor any given conditions.

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Compressibilitg: The upper limit for rotor rev/min isreached when the advancing blade tip speed reaches thecompressibility region, with vibration and loss of efficiency.

To provide a given amount of lift, either a small rotor mayrotate quickly, or a large rotor rotate slowly, the latter beingnormally the more efficient. In either case, the tip speed willbe similar, and compressibility is usually inevitable when theforward speed (VQ reaches about 200 knots.

All—up weight: The higher the all-up weight of the helicopter,the greater must be the collective pitch applied to lift thatweight. The greater the collective pitch applied, the lesscyclic pitch can be used before the stall angle on the retreatingblade is reached. That is, the higher the all-up weight, thelower the limiting speed V2.

Altitude: As the rotor rev/min must remain more or lessconstant, the helicopter will lift less as the altitude increasesbecause of the reduced air density, and so a greater pitch angleis needed to lift a given weight. Thus, the higher the altitude,the greater the collective pitch for a given weight and the lowerthe limiting speed at which the critical angle will be reached,that is, the lower the IAS for retreating blade stall.

For the same reasons, the cyclic control movement ataltitude becomes less effective in terms of disc movement. Tocounteract the flap-back arising from a given forward speed (IAS)at increased altitude, you need to make a larger forward controlmovement. Consequently, the limit of forward control will bereached at a lower limiting speed.

Stability

If an aircraft in flight tends to return to its originalposition after being disturbed, it is said to be stable. If itremains in its new position, it has neutral stability, and if itdeparts farther and farther from its original patch, it isunstable. The helicopter is unstable in flight so far as changesof discattitude are concerned because a change of attitude (flare)

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of the disc causes a change in the useful force in both magnitudeand direction. This will, in turn, have a further effect on thedisc. In general, the discattitude must be controlled by thepilot at all times to prevent the helicopter from going out ofcontrol.

Cyclic Control Forces

In forward flight, a lateral force, arising from the reactionof the control orbit to the forces acting in the pitch operatingarms whenever the control orbit is displaced from the perfecthover attitude, can cause the aircraft to roll.

Forward flight implies that the control orbit is tiltedforward, and so the maximum forces being exerted in the pitchoperating arms to cause a pitch change are more or less on the lateralaxis of the aircraft. Because there is a downwards force on theadvancing side and an upwards force on the retreating side, theequal and opposite reaction from the control orbit will tend totilt it towards the retreating side, which would cause the air-craft to roll in that direction. To prevent this, the pilotwould need to push the cyclic control in the opposite direction.See Pig. l0.

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In aircraft with manualcontrol, this load can berelieved with an adjustablespring loading in the controlsystem to hold the control columnin a given position. The amountof spring loading needed torelieve the load will vary withthe forward speed. Another wayof preventing these loads being .felt by the pilot is to useirreversibles in the fore andaft and lateral control runs.

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In aircraftfelt by the pilotthe servo jacks.failure, however,to roll violently

with servo controls, the load is not normallybecause the forces cannot be fed back throughIn the event of a hydraulic system supplythe load is felt suddenly and the helicopter tendsto the side of the retreating blade. One

helicopter maker used lock—load valves on the servos to preventthose feedback forces being felt, butrelied on the pilot reducing speed toacceptable. To prevent or reduce thefailure, a duplicate servo system may

different hydraulic system.

Vortex Ring

Another name for vortex ring isnames are freely used to describe the

other helicopter makerswhere the forces becameeffects of a hydraulicbe provided and powered by a

settling with power. Both

same condition.

In normal powered flight, there is an induced flow of airdownwards through the rotor. In the event of a fuselage movementnormal to the rotor disc, it is possiblerelative to the disc directly opposedtherefore causes a very confused patternThis movement of the air on rotor blades

to set up an airflowto the induced flow, which

of flow round the rotor.at high angles of attack

will stall the blades at the hub. This stalling can move outwardalong the blade as the rate of descent increases.

In particular, a turbulent vortex called a vortex ring, iscreated around the periphery of the disc. See Fig. ll. Thecombination of conditions in which a vortex ring is likely tooccur are during

l. Powered flight with induced flow through therotor,

2. Movement of the aircraft causing a relativeflow normal to the disc from the oppositedirection, and

3. Relatively still air conditions.

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The first example in which.. A the conditions could be met is

\ a vertical descent with power._‘:f4;}t Vortex ring is unlikely to occur

§;_ ‘~”* ‘ \::? in normal conditions until the

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C_:\(’\ rate of descent exceeds 300 ft/min.

As a safety margin, a rate of250 ft/min should not be exceededin a vertical descent in still air.

FIG. ll_ Vortex ring

The second example is in a fairly steep flare, where theaircraft is also being allowed to descend. If power is applied inthis condition, vortex ring can result.

Another example is when power is applied in recovery from anautorotation. If the path of movement is normal to the disc, forexample, during descent in autorotation, and power is appliedwithout a change of disc attitude, then vortex ring can result.

In all cases, the question of wind must be considered. Ahead wind of 10 knots or more will normally be enough to preventa vortex state from being reached. Remember, however, that itis possible to lose this headwind component quickly, for example,descending into a clearing or below high buildings when a vortexstate may result if the rate of descent is too rapid.

Conversely, it is possible to be descending downwind withground speed clearly apparent, but with the rotor descendingin relatively still air and therefore subject to vortex ring.

The effects of vortex ring are similar to those of a fixed-wing aircraft in a stalled condition. Initially, the disturbedflow may result in buffeting and vibration, but these maydisappear in a complete vortex state. There will be a considerableloss of lift, which will result in the aircraft accelerating inits original direction of movement, and a large increase indrag, which will cause loss of rotor rev/min and possible loss

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_ 21 _

of directional control. Finally, the high rate of descent mayhave enough weathercock effect on the tail cone to cause sharpnose—down pitching, which will, in fact, destroy the vortexcondition.

As the vortex condition arises from the confused patternof airflow, the introduction of a new component of airflow willhelp to remove the vortex state. This is most easily achievedby a change of disc attitude, that is, by a cyclic controlmovement. In practice, the control column is normally movedforward to induce translational flight, but if this is impossiblea lateral or backward movement can be used. As soon as thedisc attitude is altered, full power may be applied to minimiseloss of height. Power should never be increased before thecontrol column is moved because this may aggravate the vortexcondition and so increase the rate of descent.

It would also be possible to change the flow by loweringthe lever and autorotating, but the loss of height in effectingrecovery in this way would be considerable. Correct recoveryfrom a full vortex state will require approximately 300 feet.Thus, vortex ring below this height is very dangerous, although,in practice, it is at such heights that the danger is mostlikely to arise, for example, descending into confined space,"quick stop", recovery from autorotation, and so on. It maybe compared with the stall on the approach at low altitude inthe fixed-wing aircraft.

1

SUMMARY‘

Dissymetry of lift occurs imediately a horizontalairflow passes across the rotor disc.

Dissymetry of lift is countered by blade flappingplus cyclic feathering, which restores equality oflift.

Flap-back, which occurs because of blade flapping,is controlled by moving the cyclic control columnforward.

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Forward speed is limited by

l. Range of movement of the cyclic control,

2. Reversal of airflow in the rotor disc,

3. Retreating blade stall, V

4. Compressibility at the rotor blade tips,

5. All—up weight, and

6. Altitude.

The helicopter is unstable in flight and must becontrolled at all times.

A vortex ring state is an unsafe condition of flight.

PRACTICE EXERCISE B

State whether each of the following statements is trueor false.

l. An advancing blade of a helicopter in forwardflight experiences a decreased airflow overits surfaces.

2. Reversal of airflow occurs on the advancinghalf of the rotor disc.

3. An upward—flapping blade has a decreased angleof attack because of the flapping motion.

4. Blade stall does not affect the limit of theforward—flight speed.

5. Compressibility at the advancing blade tipwill limit the rotor rev/min.

6. Because the air is less dense at altitude, thehelicopter will experience less drag andwill have a higher limiting speed.

7. Because the helicopter is "suspended" from itsrotor, it has natural stability.

8. Vortex ring state is likely to occur if therate of descent is more than 300 ft/min.

9. Flap—back is corrected by a slight increase incollective pitch.

sss/s/10

...23...

10. When the all-up weight of the helicopter isincreased, the limiting speed is reduced.

Lanswers on page 40)

CONTROL ON THE GROUND

When the helicopter is on the ground with its rotors turningseveral serious problems can arise. Chief among them, forhelicopters with articulated rotor heads, is ground resonance,which, if left unchecked, will cause the complete destructionof the helicopter in a few action—packed seconds.

Ground Resonance

Ground resonance is a severe low-frequency vibrationresulting from a forced or self-induced vibration of a massin contact with the ground. In the case of the helicopter (themass), the vibration can originate either as a disturbance inthe rotor transmitted to an undercarriage in contact with theground, or in the undercarriage itself due to mislanding,rough ground, and so on. In either case, an element of sympathymust exist between the original vibration and the natural frequencyof vibration of the other system.

Rotor vibration can arise from any basic unbalance of therotor, for example, blades of unequal weight or with theircentres of gravity unequal distances from the centre of therotor, or blades producing unequal lift or with their centresof lift unequal distances from the centre of the rotor. However,before being mounted on the helicopter, a set of blades isusually balanced, and so the most likely cause of rotor vibrationis faulty drag dampers. Drag dampers are incorporated tocontrol the rate of movement about the drag hinges. If theyare set incorrectly, the blades will move about the drag hingesat different rates and so cause blade unbalance. See Fig. 12.

555/3/10

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uusumcen roac: moment ARM NIL I3 :4‘ uunuucso roac: uouzur AIM

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(<1) (b)' FIG. l2 Vibration from disc unbalance

A likely cause of rotor vibration is mishandling of thecyclic control by the pilot. "Stirring the stick" while thewheels are on or very near the ground should therefore beavoided. A similar effect can result from an inexperiencedpilot trying to be too careful about his landing and onlysucceeding in touching first one wheel and then the other, sosetting up a "padding" of the undercarriage.

It is easier for a vibration to be set up in the under-carriage, when there is no weight on the wheels, that is, whenthe collective pitch is quite high. This state should beavoided as much as possible by lowering the collective leveras soon as the wheels touch on landing, and by making a smoothprogressive increase in pitch when taking off, to take theaircraft well clear of the ground.

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_ 35 _

The nature of the ground that the helicopter is resting oncan influence ground resonance. For example, on landing, onewheel may slip into a concealed hole or rut, and even thissmall movement might set up the required initial vibration.Forward movement over rough ground would naturally increase therisk of vibration, and landing across sloping ground can alsohave the same effect, particularly if the pilot is unaware ofthe slope when landing.

Because the sympathetic frequency of vibration of therotor and the undercarriage is an essential feature of groundresonance, designers choose undercarriage systems that minimisethe possibility of such a sympathy being set up. If oleoextensions and type pressures are kept to the correct figures,ground resonance becomes less likely.

In the event of resonance occurring, the best recoveryaction is to take off immediately to hover, where the vibrationshould die out quite rapidly. To allow for immediate take-off,keep the rotor rev/min up to the take-off figure all the timethere is any possibility of ground resonance. Should it beimpossible to take off, then the sympathy between rotor andundercarriage should be destroyed by reducing the rotor rev/minas quickly as possible, that is,collective down, throttle closed,switch off, rotor brake on.

Taxying

The thrust for-taxying is provided by the main rotor, with

the lift component kept to a minimum to avoid ground resonance.Aim to have the best thrust/lift ratio without havingan exaggerated forward attitude of the disc and the possibilityof the blades striking their lower stops. Keep rotor rev/minat the flight figure and steer by using the wheel brakes.Keep taxying speed down to a walking pace, and avoid roughground.

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_26_

Blade Sailing

Blade sailing occurs when the rotor is either starting upor slowing down in strong, gusty winds. In this event, adissymmetry of lift is experienced between advancing andretreating blades similar to the effect in forward flight. Theadvancing blade flaps up at the front and the retreating bladedown at the rear. If this motion becomes exaggerated (particularlyif it becomes in phase with the natural frequency of vibrationof the blade), it can result in damage to the fuselage. Inextreme cases, blades have been known to strike the ground.The flapping can be countered to some extent by a small forwardmovement of the stick, but be careful, particularly in gustyconditions, because a sudden reduction of wind might cause theblade to flap down in front to a dangerous extent.

Because the blades pass nearest to the fuselage whencrossing the tail cone and the blade will be at its lowestpoint downwind, it may be advisable to turn the aircraft H5°out of wind, so that the blades pass their lowest point wellclear of the tail cone.

Most helicopters incorporate droop stops, which are heldout of position by centrifugal reaction above approximately100 rotor rev/min but fall into position below this figureand restrict the downward droop of the blades. Many helicoptersalso have flapping restrainers, which prevent the blades fromflapping up or down below a fixed rotor rev/min. However,even with droop stops and flapping restrainers, be careful whenengaging and disengaging rotors in a high or blustery wind.

CENTRE OF GRAVITY (c.e.)

The centre of gravity of an aircraft is the point throughwhich the total of weight forces act. It is normally calculatedby reference to the moments of the various weight forces arounda given datum, which in the case of the helicopter, is usuallythe centre line of the rotor. The position of the c.g. is then

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_ 27 _

quoted as so many inches or centimetres fore or aft of the datum.In the helicopter, the relationship between the useful force andthe weight will affect the behaviour of the aircraft in all stagesof flight.

So far, we have assumed that the centre line of the rotorpasses through the c.g. and that the useful force directlyopposes weight. If, however, the c.g. lies either fore or aftof the datum, then the resulting couple will cause the aircraftto adopt a nose-up or nose-down attitude. The disc has to bemaintained in its correct position in space by movement of thecyclic stick until the line of useful force passes through thenew c.g. and a new state of balance is reached. See Fig. I3.

. /ll

,’ I C.¢;¢_//65 t

/ ,/ 53. . ‘\_,

(a) forward (b) aftFIG. 13 Fuselage attitude with extremes of c.g.

Two results follow from this. First, the range of movementof c.g. may be limited by the amount of cyclic stick control.Secondly,an incorrect position of c.g. will limitinanoeuvrabilityin a given direction, for example, limitation of forward speed.

Centre-of-gravity Limits

The theoretical limit of movement of c.g. will be governedby the extremes of disc attitude because it must be possiblefor the line of useful force to pass through the c.g. ifcontrol is to be maintained. See Pig. lH. In the case of thetwin—rotor helicopter, the range lies midway between the rotorsand is longer than in the single-rotor type.

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The limit increases with the distance between the rotor headand the c.g. That is, the lower the load and the higher therotor head, the wider the limit. A similar effect is achievedby the use of offset flapping hinges, which give a moreeffective disc response in terms of stick movement and thereforeincrease the c.g. limits. The practical range laid down by thedesigner is naturally much less than the theoretical limit inorder to ensure manoeuvrability and is specified for each typeof aircraft in terms of the datum.

*. ,. ... I

\ .§\ \ /_ . -\ i.» \ /

I \ /*_ I. \ ///,

\'* V“

(a) Lateral range (b) Fore and aft range‘ .

FIG. 14 Limits of c.g.

AUTOROTATION

Autorotation is the condition of flight where the rotor isbeing driven by aerodynamic forces derived from an induced upwardsairflow through the rotor as a result of the aircraft descendingwith no power applied to the rotor shaft. It is the safetyfactor in the event of engine failure and is similar to theability of the fixed-wing aircraft to glide by maintaining agiven airflow over the aerofoils.

In the event of an engine failure, the rotor profile dragmust be reduced as rapidly as possible, and the angle of attackmust be adjusted in terms of the new relative airflow caused bythe aircraft descending. Both these requirements are met bylowering the collective pitch lever to its low pitch stopsimmediately power is lost. Further alterations to the collectivepitch will have to be made once autorotative rev/min have beenestablished as the helicopter descends.

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Autorotative Force

The autorotative force is the component of total reactionacting forward in the plane of rotation,which opposes rotorprofile drag. It depends on the direction of total reactionrelative to the perpendicular to the plane of rotation. SeeFig. l5. .

?3E>'P-I01>-_-_-lf"'Q Z\ TOTAL \\ REACTION \\‘ \

\\\ \ .\ \‘ a AUTO-ROTATIVEOw . 1- \ FORCE

aumva MRFL , ROTOR “ \R _ 9*’ PROHLE Hm““ \ DRAG “E 0? \’*° ‘v new mo \ ‘M ~a \pflig \ n \9 \

V‘ \ \ \\Yg$\ {Na

vfi“(a) normal flight (b) autorotative flight

FIG. 15 Autorotative force

The angle of attack (the angle formed between the chordof the blade and the relative airflow) determines the directionof the total reaction. On a rotor blade, it depends on

l. The rate of descent of the helicopter,

2. The forward speed of the helicopter,

3. The rotational speed of the blade, and

H. The collective pitch applied.

Figure l6 shows these four factors.

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Pfichl _ angleL inflow

\ \ angle gpqle

Plane of rotation if T‘\

ok mow

9“”\OkNe descent

<_Rotational speed Forward

speed

' FIG. 16 Angle of attack in autorotation

Three of these factors are common to the whole blade, butthe fourth, the rotational speed, varies with the span, witha consequent variation in angle of attack. Thus, the auto-rotative characteristics will alter and must be summed up for thewhole blade in order to arrive at its overall autorotativeperformance. There is usually an autorotative section of theblade corresponding to the area of highest L/D ratio, the forcefrom which will balance the rotor profile drag from the remainderof the blade and so maintain constant rev/min. Figure 17compares the forces acting at three different stations alongthe span of a blade.

- TOTAL|_|r1' "2 _REACTl0I1 ‘~

Pm: \_ -ANGLE - ~,. 12> musmm: or ROTATION \Lo“ an: or,1 nu? usscenr

ROTATIORAL svzzo FORWARD ‘mg-qQ1A'(|yf_5955“ secnoa

E

‘-/DRAT0 CU U’ O

. AUTO-ROTATIVE.rl--- react at

‘A ANGLE OF ATTACK(B) \E.~ (C) ‘-

iZ____!

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‘FIG. l7 Comparison of forces acting at three different stations along thespan of a blade _

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M An alternative method of deciding the autorotative sectionis by comparing the angles between lift and the perpendicularto the plane of rotation LA, and lift and total reaction LB.

e See Fig. 18. The effect on rotor rev/min is as follows.l. If LA is greater than LB, then rotor rev/min

will increase.

2- If LA is equal to LB, then rotor rev/min willbe constant.

3. If LA is less than LB, then rotor rev/min willdecrease.

Z CD -'lF7‘!

UFT T-FL: T‘ [_A=lNFLOW ANGLE/ = AN eta OF ATTA ox-LAg1~-/ PITCH

‘ | / - 2.'/ tan LB. L

x HTCH / ’,,*'DRAGifPLANE OF ROTATTON lNFLOW\ ml\pAg§

vetI~““E'

FIG. 18 Comparison of angles between lift, total reaction, and the perpendicu—lar to the plane of rotation

The blades are rigged to give normal rev/min under givenconditions of flight, namely, a known all—up weight giving acertain rate of descent, a given forward speed, and collectivepitch setting. Under these conditions, the autorotative sectionwill correspond to that shown in Fig. l7 (b) and I9 (a). This isnot the most efficient autorotative state, which would beachieved with the autorotative section closer to the tip of theblade, but it provides a safety factor. In the event ofexternal forces tending to slow the blades down, the angle of

T attack is increased, hence, the autorotative section moves out,speeding up the blade to the original rev/min. Should the

. blades tend to speed up, angle of attack is decreased, auto-rotative efficiency decreases, and rev/min return to normal.

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. I RPM nsoucao(m rsasoamucsI I I MPROVED

<a> ~~@R~~"n/-'~AUTORDTATIVE

SECTION

/\ C3 \/

¢ AN6L.£ OFATTACK

FIG. l9 Variation of autorotative rev/min with collective control lever

This increased efficiency can be achieved by increasing thecollective pitch. In practice, the pilot can control the rotorperformance with the collective control. However, this islimited by the danger of reaching the peak of autorotative _efficiency beyond which rev/min will fall off-rapidly. Minimumpermissible rev/min in autorotation are therefore governed bythis factor, subject to possible coning angle limitation and anadded safety factor. -

Forward Speed

As in powered flight, where the transition into forwardflight produces translational lift and enables the power to bereduced, so in autorotation, increased mass flow through theblades improves performance, and results in a reduction inrate of descent. The graph of rate of descent in terms offorward speed compares very much with the power curve in levelflight, the initial gain being offset by the sharp increase inparasite drag as speed increases. See Fig. 20.

There is a small increase in rate of descent initiallyas a result of reducing the effective disc area to the relativeairflow when tilting the disc forward. The optimum speeds forminimum rate of descent and maximum distance over the groundcan then be found from the graph shown in Fig. 21. In thelatter case,

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Rate of descentTan 9 = velocity

u

Thus, when minimum, it gives a maximum ratio of speed torate of descent, which is equivalent to distance covered fromany given height.

RATE OFDESCENT

RATE BF amzcr or°5$°5"T PARASITE DRAG

ISPEED FORI IMAX. DIST

EFFECT OF £9mcnusso mss now _ SPEED FOR Mm‘

V FQRWARD 5PEE_D RATE OF DESBENT V 533;?“

FIG. 20 Effect of forward speed on FIG. 21 The optimum speeds forrate of descent minimum rate of descent

and maximum distance

In theory, when using either of these speeds, maximumperformance would be obtained by setting the lower figure ofrotor rev/min, but in practice, the higher figure of rev/minis maintained at the lower speed. Additional speed can, tosome extent, be converted into rev/min by the use of the flare.This relatively large change of disc attitude has already beendiscussed when dealing with transitions in powered flight andthe basic effects remain the same:

l. An increase in useful force as a result ofincreased angle of attack;

2. Change in inflow (in the autorotative case,this is increased as a result of increasingthe effective disc area);

3. Reversal of thrust component; and

H. Increase in rotor rev/min.

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...3I.I_

The first two factors result in an increase in lift andtherefore decrease the rate of descent to some extent. Theamount by which rate of descent decreases varies considerablywith aircraft type. The reversal of thrust causes a rapiddecrease in forward speed just before touchdown. The increasein rev/min appear surprising in view of the increase in angle ofattack and lift, which implies an increase in drag. However, theimportant factor is the relationship of total reaction to the iplane of rotation as shown in Fig. 22. Because total reactionmoves forward relative to the perpendicular to the plane of

' rotation, the autorotative force is increased, causing a risein rev/min.

TO T4 lLR. EAC T]

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P.etAi\\I‘5- INFLOW _ /

._»-

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_.'____,.-no--_

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_ >>> /~ /

Blade Section in FIare(a) Blade section in autorotation (b) Blade section in flare

FIG. 22 Increase of rev/min in flare

AII—up weight

The effect on autorotative performance of an increase inall-up weight is to increase the rate of descent, thusincreasing the mass flow and so causing the rev/min to rise.This increase in rev/min must be controlled by increasing thecollective pitch, which will restore the descent to a normalrate.

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_ 35 _

Altitude

The problem of autorotation at altitude is the reduceddensity. As in level flight, because the rev/min have to remainconstant, lift will be reduced and therefore the rate of descentincreased. The more important factor is the considerableincrease in rev/min due to the decrease in drag, which must becontrolled by use of the collective lever.

If a helicopter base is sited at a high altitude, you mayneed to re-rig the collective control and to reset the collectivepitch/low pitch stops to a higher pitch angle to get efficientflight at that altitude. If this is done, the low pitch stopsmast be reset before a flight is made to a lower altitude toensure that normal autorotative rev/min are available at thatlower altitude.

NOTE: If autorotation is being continued from highaltitude to sea level, rev/min are controlledby a gradual lowering of the lever as densityincreases.

Safety Height

Should the engine fail during hover, there will be a lossof height of approximately 300 feet before a full autorotativeairflow can be established.

Allowing another lOO feet to make a safe landing, it isunsafe to be hovering below MOO feet, except that, up toabout IO feet from the ground, a safe landing should be madesimply by cushioning the impact by raising thecollectivelever. Forward speed will help to establish the inflow, sothis safety height can be reduced as speed is increased, untilat approximately H5 knots, you should be able to make a safelanding from any altitude. Don't, however, fly too low athigh speeds because, in the event of an engine failure, theaircraft might strike the ground before the speed could bereduced by flaring.

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_ 35 _

400'

HEGHT ‘O 45 K. B 5 K.

FORWARD SPEED

FIG. 23 ‘Safety height and speed

RANGE AND ENDURANCE

The factors influencing range and endurance are similarto those for fixed-wing aircraft.

Work = force X distance

work.1 Distance =force

To obtain maximum distance, if work is constant, forcemust be minimal. The speed (V) to achieve this is obtainedfrom the power curve, Fig. 24.

Because

/ .

POWEITan 6 = ————~r-— ...velocity

and

Power = force X velocity .. (2)

Combining (1) and (2), we get

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-37..

force X velocityTan 6 = velocity

= Force

Thus, when tan 6 is a minimum (tangent to the curve), forcewill be minimum and we have the best speed to give the greatestrange.

V; = Max endurome speedVa = Max range speed

ed

rzqur

Power

‘D(:5_..._..-- <_--.... N 5peed

FIG. 24 A power curve (exaggerated)

To get the greatest endurance, the work available must bespread over the longest possible time.

workPOWGI = .

tlmé ‘

. . work. Time = ——————power

If the work is regarded as constant, power must be minimalto ensure maximum time. The forward speed to obtain this isimmediately under the lowest point of the power curve.

In Fig. 2H, VI shows the speed for greatest range, and Ve,the speed for maximum endurance.

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SUMMARY

Ground resonance is a vibration caused by theinteraction of an unbalanced articulated rotor headand its undercarriage.

Blade sailing is controlled by flapping restrainers.

Autorotation is the helicopter’s equivalent of afixed—wing aircraft's gliding.

Autorotation rev/min increase with altitude.

PRACTICE EXERCISE C

State whether each of the following statements is trueor false.

l. Faulty dampers often give rise to blade sailing.

2. Ground resonance can rapidly lead to the destructionof the helicopter.

3. The position of the centre of gravity will have noinfluence on the effectiveness of the cyclic pitchcontrol.

4. Autorotation is to the helicopter as spinning isto a fixed-wing aircraft.

5. If the collective pitch control is raised when theengine power decreases, the rotor rev/min willincrease.

6. An increase in forward speed in autorotationresults in a reduced rate of descent.

7. Flaring results in a marked decrease in rotor rev/min.

8. Autorotation rev/min decrease with decreasingaltitude.

9. Oleo leg extensions have no effect on ground resonance

10. The higher the AUW, the greater the rate of descentand the higher the autorotation rev/min. f

(Answers on page 40)

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_ 39 _

ANSWERS T0 PRACTICE EXERCISES

EXERCISE A

Statements 2, 6, and 8 are True.

1. False: In a hover in still air, there is nodrag other than rotor profile drag because neitherthe aircraft nor the air surrounding it aremoving.

3. False: Greater efficiency is had from the rotorat translational speed due to the increasedmass of air flowing through the rotor. Thismeans that less power is needed, althoughparasite drag will have increased by a smallamount.

H. False: The increased mass of air flowingthrough the rotor means that induced powerrequired will decrease as forward speedincreases.

5. False: To move forward, the helicopter needssome thrust. If this thrust is taken fromthe power available, that is just enough forhover, the helicopter will descend.

7. False: When the air density decreases, the massflow of air through the rotor disc decreases andso the lift also decreases. To maintain thelift, the collective pitch must be increased,which calls for more power.

9. False: In all powered flight, the relativeairflow enters the rotor from above the planeof rotation.

10. False: Ground effect, which is another namefor ground cushion, is taken to extend to arotor height above ground of one half of therotor diameter.

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EXERCISE B

Statements 3, 5, 8, and 10 are True. e

1. False: An advancing blade experiences anincreased airflow. That is, it has V

F rotor+ Vfimwmrd flight over its surfaces.

2. False: Reversal of airflow occurs on theretreating half of the rotor disc.

H. False: Stall of the retreating blade becauseof the cyclic pitch used to prevent flap-backplaces an upper limit to the forward speed.

6. False: Because of the decreased air density,an increase in collective pitch will be neededto maintain lift. This will cause the stallingangle of the retreating blade to occur at alower forward speed.

7. False: Any change in the rotor disc attitudeproduces an immediate change in the useful forcein both size and direction. This has a furthereffect on the disc, and so the helicopter isunstable in flight.

9. False: A slight increase in collective pitchwill affect all blades in exactly the sameway. A forward movement of the cyclic controlwill correct flap-back.

EXERCISE C

Statements 2, 6, 8, and 10 are True.

l. False: Faulty dampers cause the blades to moveerratically about their drag hinges to giverotor imbalance. A low rotor rev/min in a gustingwind will cause blade sailing.

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3.

4.

5.

7.

9.

_ ul _

False: If the c. g. is too far forward or toofar aft, the cyclic control will run out ofaft—and—forward movement sooner than normal.An incorrect c. g. position thus reduces theeffectiveness of the cyclic control.

False: Gliding is the fixed—wing equivalentof helicopter autorotation.

False: Raising the collective controlincreases the pitch on all of the blades. Ifthis is done with decreasing engine power, therotor rev/min will quickly decrease.

False: The increased angle of attack in theflare causes an increase in the autorotativeforce and this, in turn, increases the rotorrev/min.

False: Incorrect oleo leg extensions willallow the fuselage to rock from side to sidein unison with an out-of-balance rotor head.This rocking motion is ground resonance.

TEST PAPER 10

Make a sketch of a helicopter blade section inautorotation showing

(a

(b)

(c

(d)

(e

(f)

Thethethe

The

The

The

The

The

planeplaneblade section,

pitch angle,

inflow angle,

of rotation and a perpendicular toof rotation at the trailing edge of

angle of attack,

total reaction,

lift vector,

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(g)

(h)

With

_ ug -

The rotor profile drag vector, and

The autorotative force vector.

the aid of a sketch, describe the airflow throughthe rotors in a vortex ring state. Explain how thehelicopter can enter and recover from this state andwhy it is a hazard to flight.

The power needed by a helicopter for horizontal flightcan be considered in three parts. Name these parts,and state the use of each power vector.

What

What

What

(a)

(b)

(a)

(b)

(c)

is flap-back, and how is it controlled?

is translational lift, and why does it occur?

are the effects of a c.g. position that is

Outside of the forward limit, and

Outside of the aft limit.

What is airflow reversal?

When does it occur, and

What will it do to the helicopter when it becomeslarge?

What is ground resonance? Does it affect all typesof helicopter? When is it most likely to occur, andwhatmaintenance work can be done to lessen the possibility

must be done when it does occur? What

of ground resonance occurring?

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‘SSW

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-143-

What factor has the most effect in limitin th fg e orwardspeed of the helicopter? What will happen if thisfacto ' ' 'r is ignored? List two other factors that alsolimit the forward speed.

0

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