elementary gliding

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GLIDING

h i, PaulBlunchurdTT.7.USTRATIONS


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I saw it in

Instructive and entertaining gliding notes by Dr.A . E. Slater, the w orld-fam ous authority, will befound exclusively in The Aeroplane, th e onlyaeronautical journal devoting a regular weekly pageto gliding. Here yo u will find all the latest new s andpictures of outstanding interest to gliding enthu-siasts. Place a regular order with your newsagent.

T emple PressLimited, Bowling Green Lane , London, E.C.I. TERminus3636


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Problems of flight

WING FLAPSextemporised runways, heavy loadsthis combination is often imposed bymodern business and modern war. Todeal with it man has devised aircraft withvery slow stalling speeds, achieved bymeans of leading-edge slatsand trailing-edge flaps. Nature, in large cliff-dwellingbirds like the gannet, long ago faced the

edge. The bird loses flyingspeed sharply.At this point a tuft offeathers he alulaor" bastard plume " s extended fromthe leading edge. It has exactly the sameeffect on stalling characteristics as theleading-edge slats of an aircraft: at lowflying speeds it controls the airflow overtheaerofoil at a large angle of attack, and

same problem -and found the sameanswers. The gannet's six-foot wing-spangives it nearly 50m.p.h. in still air. But itnests in remote, craggy, far-northerncliffs, in crowded colonies of up to 10,000pairs. This means that with all its size,weight and speed it must land on asixpence. When it comes in to landupwind whenever possible t spreadsits tail and large webbed feet wide to actas airbrakes. At the same time thesecondary wing-feathers are presseddown, forming a broadflap at thetrailing

so delays the moment of stall until thebird is practically motionless.Precisely the same method makespossible the astonishingly low stallingspeed 8 m.p.h. f the PrestwickPioneer, which can land in as little as 50yards, and this is now being applied to a16 passenger twin engined version of thePioneer. So man has solved yet anotherof his aeronautical problems with atechnique that Nature has used forcountless ages.

Pilots who land on airfields both largeand small all over Britain value theexcellent and helpful service of the Shelland BP Aviation Service.SHELL anJ BP AVIATION SERVICE

Shell-Mexand B.P. Ltd., Shell-Mex House. Strand. London. W.C.2.Distributors in the United Kingdom for Shell, . ingl-Irnnian and fcugle Group*.


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ELEMENTARYGLIDINGA Manual for Pupil Glider Pilots

b y PAUL H. BLANCHARD

M.A.(Cantab)FormerlyChiefFlying Instructorof

TheCambridgeUniversityGlidingCluband TheSurrey (ilidingClub.

With aFOREWORD

byPHILIP A. WILLS, C.B.E.

WorldGlidingChampion, 1952-1954

Published byT H E R M A L E Q U I P M E N T , LTD .,

17 , HanoverSquare,London, W.I.


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"GLIDING"The official Magazine for

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Telephone : HYDe Park 3341


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FOREWORD' I HE number of people of all ages and sexes w ho want to learn* to glide is rapidly increasing, and th e Civil Gliding Clubsin th e United Kingdom and elsew here are doing a gre at job in

training th e rush of new members to th e sport of th e age.It is surprising that until now no one has produced a manual

for th e new pupil giv in g him all th e basic information he needs,and it is fortunate that, now one has appeared, it has been writtencompetently and well by Paul Blanchard, w ho w as for some timeChief Instructor of th e Surrey Gliding Club, with the assistance ofseveral other unimpeachable experts of th e British gliding movement.

This booklet is a " must " fo r anyone in any English-speakingcountry w ho wants to learn how to glide. Perhaps I should add thatif you, the reader, als o want to know where to glide, th e best wayis to contact The British Gliding Association, 19 , Park Lar.e,London, W.I, w ho can give all particulars.


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CONTFNTSPAGI:E i I .M I MARY P R IN C IPL IS

1 The glider and its components .. . . . . . . 92 W hat makes a glider fly .. . . .. . . ..113 The ef fec t of the controls .. . . . . 1 94 Before you leave the ground .. .. .. 27

MANO I rvRis5 The straight glide . . . . . . . . . 296 Medium turns . . . . . .. 327 Stalling .. . . . . . . . .. .. 378 The winch launch .. . . .' . .. .. 399 Cablebreaks . . . . . . . . . . . 42

10 Circuit and landing . . . , . . . 441 1 Spinning . . . . . . . . . . 501 2 Sideslipping . . . . . . . . . 531 3 Use of airbrakes .. .. .. .. .. ..55

APPI .N D IX(a) Instruments .. .. .. .. .. ..58(b) Thermals and Gusts .. .. .. . . 59(c) Moreabout lift and drag .. . . .. 60(d) Making the most of it .. .. .. .. ..63

Cover design byFrank H . Kinder


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INTRODUCTIONThis manual is designed to be a source of basic information to

which you, the pupil learning to glide, can refer in order to refreshyour memory on the flying instruction you receive.

The first part contains a sim ple account of the fundamentalprinciples concerned with th e flight of a glider. In order to be ableto fly you need not have a deta iled knowledge of aerodynamics,but you cannot expect to become an efficient pilot unless you havesome idea of the forces you are trying to control.

The later chapters are concerned with the various flyingexercises that you will be taught during early training. This doesnot in any way replace the instruction you will receive on the fieldor in the air, or necessarily indicate the order in which the instructionwill be given. Here we outline the fundamental basis ; the actualtechnique can only be acquired by constant practice under skilledsupervision.

We shall deal mainly with the exercises you will performon thecircuit. However, the manoeuvres involved in soaring and advancedgliding are based on the sam e principles, and some considerationof these principles will be repaid by increased mastery of the aircraft.A few points ofinterest to the more advanced pilot have beenincluded in the Appendix.

I am indebted to A. R. I. Austin, P. W . Helson, F. G. Irving and manyothers for assistance in preparing the text. The technical drawings were madeby A. R . I. Austin, and th e cartoons by P. Sullivan, both of the Cambridge Club.P.H.B.

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ELEMENTARY GLIDINGA pupil's manual

BY P. H. BLANCHARD


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2. W H A T M A K E S A G L I D E R FL Y ?

In this book w e are not dealing with " soaring " or " stayingup." Soaring, properly called, is th e art of using th e energy of theatmosphere in such a w ay that th e glider will remain airborne, climb,and travel across country as required. W e are only concerned herewith th e manner in which you can control the various aerodynamicforces to make the aircraft behave in th e w ay you wish. The sameprinciples apply when th e glider is soaring, if you consider its motionw ith respect to th e air. Soaring is made possible by the fact that theair around th e glider is moving upwards.Airspeed

In later paragraphs it will be seen how the movement of a gliderthrough the air produces the forces which keep it up and enable itto manoeuvre. The behaviour of th e aircraft is determined by itsrate of movement through th e mass of air in which it is travelling,i.e., by its " air speed." If a wind is blowing, this mass of air willitself be moving over th e ground but th e only effect this has on theaircraft (as long as th e wind is steady) is to alter its position withrespect to th e ground. The airspeed, which causes th e airflow thatyou feel on your face, depends only on th e manner in which th econtrols are manipulated, and is not influenced by th e wind.

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For navigational purposes w e may think in terms of the" ground speed " and take the wind into account ; here, where w eare concerned with controlling and manoeuvring the aircraft, w eneed only consider the airspeed.Aerofoils

The wings, tail planes, elevators, fin and rudder, which areknown as the " aerofoils," are constructed with " streamline"sections so that the air flows smoothly over them and sets up littleresistance. A typical wing section is something like this :

Lift and dragIf a wing is placed in a wind tunnel at a slight angle to a movingstream ofair, as shown below, the reaction due to the air acts upwards

and backwards. This total reaction can be represented by tw ocomponents at right angles :

COUPONtNT

In the diagram here w e represent each force by a line in thedirection in which the force is working. The length of the linerepresents the strength of the force involved.The upward component, called Lift, is always considered asat right angles to the airflow ; the backward component, calledDrag , is para llel to the airflow. The com bined effect of lift and dragis the total reaction.Lift is the force which supports the weight of the aircraftand so enables us to keep it in the air. In steady flight the lift isadjusted so as to be equal to the weight.

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The force shown as weight represents the total weight of theaircraft. Although the weight is distributed over the entire structureand acts vertically downwards all along it w e can add all the weight

together and show it as one straight line acting through the centreof gravity. (The centre of gravity is that point about which theaircraft, if placed on a pivot, would be perfectly balanced.)

Similarly the lift of the aerofoil is obtained from all over thewing, but the total effect may be represented as one force passingthrough the Centre of Pressure. The Centre of Pressure bears thesame relationship to lift as the centre of gravity does to weight ;however its position is not precisely fixed, as the distribution of liftover th e wing varies with the angle of attack.

Similarly, drag is shown as one straight line which representsthe resultant effect of all the drag.

Throughout this book w e have stated that the lift is equal tothe weight in steady flight. W e now se e from the disposition of th eforces above that th e weight of the aircraft is actually counterbalanced by the resultant of the lift and drag forces, i.e., by th etotal reaction. In practice, however, the lift is very nearly equal tothe total reaction, since the component of drag is small for flatgliding angles, and th e effect of drag in this connection can beignored.

By the use of the controls the pilot may destroy the equilibrium ;it will be seen later that the use of the elevators alters the attitudeof the glider with respect to the airflow, i.e., alters the angle of attack.If the angle of attack is decreased, for example, the values for liftand drag are altered and th e forces no longer balance. As there isan unbalanced force a change occurs, and this continues until anew equilibrium is set up.

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air passing over the top of the aerofoil is reduced and this exertsanupwards sucking effect. Thusboth the air passing over and theair passing under have a lifting action.

In fact the amount of lift provided by the sucking effect is greater than that provided by the pressure on the under surface.Theupward sucking effect is onlymaintained so long as the airflowover the top ofthe aerofoil remains smooth. If the flow becomesdisturbed, the effect is very greatly reduced ; the greater the dis-turbance the greate r the loss oflift. Such a turbulent airflow canbe shown diagrammatically:

It is when theair over theaerofo il is in this turbulentstate thatthe aerofoil is said to be stalled. Stalling occurs when the angle ofattack is increased beyond the"critical angle"or"sta llingangle,"or"angleofmaximumlift." Thevalueofthis angledependson thedesign of the particular aerofoil. Stalling always occurs when theangle of the aerofoil to the airflow is increased beyond this value,and it cannot occur unless the angle is so increased.We are usually concerned with the stalling of the air craft,which is dependent on the sta lling of the wing aerofo il su rfaces.Let us consider ase t of circumstances inwhich theaircraftis sta lled.The wings of an Olympia will stall at an angle ofattack ofabout 15. Supposewe are flying steadily downwards at 50m.p .h.and 6 angle ofattack. This means that the lift provided by thewings is equal to the weight of the aircraft (about 630 Ib . wt).Bygentlymoving theelevators, so as to increase theangleofattack,we disturb the balance and bring theaircraft to level flight. In thiscondition the aircraft will be flying at a slightly greater angle ofattackand a slower airspeed than previously, and the lift producedwil l still be equal to the weight of the aircraft. However, in levelflight we no longer have a component ofgravity to maintain ourfo rward motion, and the speed falls off on account of the drag.

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This would cause a steady loss of lift, and th e state of level flightcan only be maintained by increasing the angle of attack still furtherso as to keep th e lift constant. The drag is also increased by thisprocedure, so that th e speed falls off more rapidly, and finally in anattempt to maintain adequate lift th e angle of attack is brought tothe stalling point (15). At this point th e wings lose a large part oftheir lift and th e nose usually drops ; th e aircraft loses height rapidlyand is not under the normal control of th e pilot until recovery hasbeen made.When a stall is performed gently as described above, th e speedat the stall will be at a certain value known as th e " stalling speed "of the aircraft (about 34 m.p.h. for th e Olympia). The stall willalways take place if the airspeed is allowed to fall to this value.However , it is misleading to think of th e occurrence of th e stall asbeing dependent on speed. A glider may be made to stall at higherspeeds, as will be seen, and th e so-called stalling speed is merely th espeed at which th e aircraft, carrying an average load, stalls when anattempt is made to fly level by th e gentle use of the controls.Airspeed an d angle of attackIn flying th e glider w e are concerned with a number of factors,th e angle of attack of th e wings being th e most important as regardsperformance. A glider is not normally equipped with an instrume ntwhich indicates th e value of the angle of attack, so that you are taughtto interpret, from th e value of th e airspeed and the behaviour of th eaircraft, when the angle of attack is approaching that of th e stall.You are able, by using th e airspeed as a guide, to adjust the angle ofattack to the most favourable value for a particular circumstance.It is important, however, that you should realise that this relationshipis not precisely fixed, and w e shall now go into this in a little moredetail.Consider once more the gentle stall described above ; byincreasing th e angle of attack as th e airspeed fell off w e kept th e liftequal to the weight until th e stalling angle of attack w as reached.It follows that the Stalling Speed is that speed required to make thelift equal to th e weight when th e angle of attack is th e " stallingangle."

It is apparent from the above considerations that the relationbetween th e airspeed and the angle of attack will vary with th e loadcarried by the aircraft. If our glider is heavily loaded with maps,barographs, and packed lunches, and w e carry out a gentle stall,more lift is required to balance the increased weight, and th e stalling

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3 . THE EFFECT OF THE CONTROLS

Before w e go on to consider the effect of the controls it is essential to note this point : w e will describe the action of thecontrols on the aircraft without any reference to the horizon, or toany point on the earth. The controls alw ays have the same primaryeffect relative to the aircraft (unless it is stalled) no matter what itsposition may be relative to the earth.Primary effects

1 . The ailerons. These are parts of the wings and are connected as show n. They are moved up and down by moving thecontrol column (stick) from side to side, and arelinked together sothat one aileron moves down when the other moves up :

RIGHT

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Movement of the s tick to the le ft ra ises the le ft aileron anddepresses the rightaileron. Thus the le ft aileron presents a reducedangle of attack to the airflow , and therefore gives less lift ; theright aileron, hav ing b een low ered, presents an increased angle ofattack and hence gives more lift.

The result is that the aircraft ro lls to the le ft around a line drawn throughthefuselage fromnose totail (thelongitudinalaxis).T his is called amovement in the "rollingplane." Ifyou hold thes tick to the le ftyouwill continue to ro ll. If, therefore, youwant toalte r theattitudeof the aircraft in the rolling plane, youmove thes ticktooneside, w ait until therequiredattitudeisreached,andthencentralise thes tick tomaintain theattitude.When thew ings ofthe aircraft are not level with the ground,it is said to be"banked." B ankingor"apply ingbank" involves amovement in the ro lling plane and is carried out by the ailerons.Itwill be seen late r how bank in theappropriatedirection enablesthe aircraftto tu rn.

2 . The rudder.This is hinged to the trailing edge ofthe fin.

Itcanmove to therightorthe left, and is controlled by the rudder20


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bar. When the rudder bar is pushed forward by th e right foot, th erudder moves out to th e right-hand side, and therefore is reacted onby the airflow. As a result the tail of th e aircraft moves round to th eleft, the aircraft pivoting on the centre of gravity ; the nose thusmoves in the direction of th e right wing-tip. When the rudder bar ispushed to the left th e reverse happens, the nose m oving in th e direction of the left w ing-tip. The se movements are in the " yawingplane."The rudder is normally regarded as a secondary control :it is used to assist th e ailerons to perform a correct turn. However ,use is made of th e rudder in sideslips and in certain unusual conditions of flight.3 . The elevators. These form part of th e tailplane, beinghinged to it . They are raised or depressed by a backward and forwardmovement of the stick.

When the stick is moved backwards, the elevators rise and th eairflow past them applies to them a downwards force. This resultsin the tail of th e aircraft falling in relation to th e nose, or as it isusually thought of, in the nose rising in relation to th e tail. Similarly

P I T C H N C

when th e stick is moved forward th e elevators are depressed and th enose goes down. The elevators thus cause a movement in th e

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"pitching plane," the airc raft pivoting about a lateral ax is throughits centre ofgravity.This pitching movement in flight alters the angle of attack of

the w ings to the airflow , and the balance of forces is upset, causing'the flight path to change.

Remember. No matter w hat the attitude of the aircraft is inrelation to the ground, the controls alw ays have the same primaryeffect on the aircraft (unless it is sta lled). The ailerons give controlin the rolling plane, the elevators in the pitching plane and therudder in the yawing plane. These planes are re ferred to theaircraft and not to the earth.

The effect of a given control deflection depends upon the speedof the airflow over the control surfaces. At low speeds, largercontrol movements are necessary in order to produce the sameeffect on the aircraft.Th e fu rther effects of th e controls, and aileron drag

W hen the ailerons are moved, they have a secondary effectbesides that of moving the aircraft in the rolling plane. This is dueto the resistance that they offer to the airflow, i.e., to their drag.The aileron which is depressed obtains more lift due to its increasedangle of attack, but this involves more drag. This extra drag tendsto turn the aircraft in the yaw ing plane in the opposite directionto that in which the bank is applied, i.e., if the stick is moved to theleft the aircraft yaws to the right. This effect is known as " ailerondrag." Itcan be minimised in variousw ays, but is fairly pronouncedon most gliders.

Another effect follow s from the use of the ailerons. Whenthe aircraft is banked it tends to slip in towards the lower wing.This produces a sideways pressure of air on the fuselage and fin ;since there is more effective surface behind the centre of gravitythan in front, this results in a yawing movement towards the lower

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w il l therefore have m ore lift. This gre ater lift restores the aircraftto thenormal level position.2 . Stability in th e pitching p lane. This is provided by th e

tailplane. If th e attitude of the aircraft is disturbed so that th eta ilplane is displaced dow nw ard from the line of flight, it willpresent a positive angle ofattack to the airflow and have positivelift, consequently rising to the level position again. Similarly if it is displaced upwards it will have negative lift, and therefore fall backagain.

OF

3 . Stability in th e yaw in g plane. This is prov ided by th e finand the sides of the fuselage. A s the greater part of these surfacesis behind the centre of gravity , the aircraft^ possesses directionalstability , and if it is displaced in the yawing plane w ill tend to" w eathercock " back again.

A IRF LOW /

O FFLIGHT

Subsidiary controls1 . Trimm ers. Some gliders are fitted w ith a subsidiary controlon the elevator known as the " trimming tab " or " trimm er "

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which can be operated by the pilo t. This may be a spring device,but is more commonly an aerodynamic trimmer as shown ; it isoperated by a small lever which works in the same sense as thestick.

\

The elevatortr im isused to relievethepilotofworkin maintain-ing the required attitude in pitch. If, for in stance, th epilotwishestodescendmore steeply , hemoves the stick to effect thenecessarychange of attitude and then adjusts the trim until the aircraftmaintains that attitude without any need fo r pressure on the stick.

It should be noted that the tab and the control surface movein opposite directions, i.e ., when the tab is moved up itwill forcetheelevatordown,which in turndepresses thenose.

2. A ir brakes. Theair brakes whichmay be fitted to glidersare either (a) Spoilers, or(b) Dive brakes.(a) Spoile rs are lightly constructed surfaces which can be

projected from the wing into theairflowpassingover it:

They have the following effects:(i) They increase the drag, and hence steepen the angle ofglid e (a steeper angle ofglide is necessary in ordertomain ta in the sameairspeed).

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(b)

(ii) They reduce the li ft over a part of the wing : thismeans that the angle of attack m ust be increased in order toprovidetheli ft needed tobalance thew eig ht.The increased angleof attack in volv es increased drag,and th e angle of glide is again steepened.

(iii ) The fact that the angle of attack is increasedwithouta change in speed m eans that the aircraft is nearerthe stall without any indication from the airspeed,i.e ., the sta lling speed is increased by opening thespoile rs.(iv) They may produce achangein tr im , e.g ., anose-dow nm ovement ofthe aircraft.Spoilers are used to control the angle of glide w henapproachingto land, th us m aking it easier to land sa fely insmall spaces.Dive brakes norm ally consist of surfaces w hich may beprotruded both above and belo w th e wing:

Theyareofrobust constructionandmaybeused to lim itthedivin g speedof a glider to a safe m axim um. Theyarealsoused in th e same way as spoile rs to assis t in controllingthe approach. Their effects are:(i) They produce a m uch greater increase in drag thando spoilers , particularly at high speeds.(ii ) Theyreduce the lif t overa part of th ewing, necessita tin g an increased angle of attack and consequently

both greate r drag and higher sta lling speed.(ii i) They are usually designed to produce no change intr im .

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4. BEFORE YOU LEAVE THE GROUND

Preparation of th e AircraftMake quite sure that the aircraft you are about to fly has beeninspected according to the local schedule, andpassed as serv iceable.

If aparachuteis to be worn, make sure it is in good order, and thatcushions as necessary are availab le. Check that th e barograph, ifcarried, is switchedon, and that it and any oth er items of equipm entare properly sto wed.The Launchin g Place

The launchwill norm ally takeplace from thedownwind end ofth e aerodrome. Observe that the intended take-o ffpath is free fro mobstructions.W hile waiting fo r a launch, your airc raft should be parked in such a place that it does not constitu te a hazard to other aircraftcoming in to land. It should norm ally be parked with onewin g-tippoin ted in to wind and weighted down, and with the brakes open(if any). W hen the aircraft is turned in to wind th ere should besomeone in th e cockpit or stationed by th e nose ; a gust of win dcan easily blow over a pilotless glider.

Getting inBe careful where you put your hands and feet when youclimbin . Parts of the cockpit are rein forced to take your weight ; ifyou tread in th e wrong place you may damage the fuselage.W hen seated in th e cockpit with th e straps tightly fastened,you should be comfortable, able to see out properly, and able to

operate all controlsfully. One or more cushionsm ay benecessary.27


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Preparin g for take-o ffW hen you are satisfacto rily strapped in, make sure that maps,gloves and any other equip ment are within reach, and then carry

out the cockpit drill as in structed. Check the controls fo r full andfree m ovement in the correct directions. Position the elevato r tr im ,air brakes and other ancillary controls as appropriate. Ifa hood isfitted, m ake sure that it is properly locked.See that the cable is connected to th e correct launching hook,and te st the release mechanism.Do not be rushed into taking off before you are ready.

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6. MEDIUM TURNSThe fo rces in a turn

Consider what happens when you are in a car going round acorner at speed. You find yourself pre ssed against the sid e of thecar w hich is on the outs id e of the tu rn . This is due to thenaturaltendency of any m oving body in this case yourself to continueto m ove in a straight line unless an external force is applied to it.The framework and the seatof the carproduce a force onyou whichcarries you round th e corner. The car itself has also a tendencytogo straight on, but th e necessary force to cause it to go roundthecorner is provided by the friction ofthe tyres on the road.The force acting towardsth ecentre of theturn w hich is re quire dto keep a body moving in a circle is known as the "centripeta lforce."W hen an aircraft turns, the centripeta l force is provided bypart of the lift of the wings. This is done by banking the aircraftin the direction ofthe turn, when the li ft is able to provide a com -ponent to w ard s thecentre ofthe turn. Themagnitudeofthe centri-peta l force required varies w ith the rateof turn and with theairspeed.

The weight of the aircraft still has to be supported during theturn ; in ord er to do th is aswell as provide the necessary centripetalforce, the total lift must be in creased. As explained earlier, the" load facto r " is now greater thanone.W hena gentle turn is carriedout with noalteration of airspeed,the in creased li ft is provided by the in creased angle of attack. Thismeans that th e angle of attack is nearer th e stalling angle , and if agreater rateof turn is attempted, th e critical angle of attack m ay bere ached in tryin g to pro vid e the necessary li ft. This will result in astall at a comparatively hig h airspeed, as mentioned previously.

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For example, a glider with a " stalling speed " of 34 m.p.h. willstall at 48 m.p.h. in a turn at 60 bank.When carrying out steeper turns, the risk of an accidental stallshould be reduced by increasing the airspeed before beginning theturn. For a fixed rate of turn, the centripetal force required variesdirectly as th e airspeed ; the lift of the wings varies as the square ofairspeed (see Appendix).Hence an increase in speed can be used to provide the extra liftneeded.The main principles ofturning are summarised as follows :

1 . At a fixed airspeed the rate of turn and the size of theturning circle depend upon the angle of bank. This angle of bankdetermines the proportion of the total lift providing the centripetalforce. For a certain rate of turn at a given airspeed there is only oneangle of bank.

2 . As the angle of bank is increased the wing loading becomesgreater. This involves an increased stalling speed and an increasedrate of sink.T he use of the controls in a turn

In carrying out a correct turn we must use the controls so thatthe lift component providing the centripetal force always pointstowards the centre of the circle we are following. The aircraft musttherefore be banked to th e amount appropriate for the desired rateof turn, and also moved in the pitching and yawing planes by theelevators and rudder respectively.

The amounts of pitch and yaw are regulated so as to producethe required flight path. The relative amounts needed vary with thesteepness of the turn. In a well designed glider a large part of theyawing moment re quired is provided by th e weathercock stability.T he use of the ailerons. Ina turn the ailerons are used to keepthe bank constant at the amount appropriate fo r th e required rate of

turn (e.g., 20 30 of bank for a medium turn).You m ay hear of effects which tend to alter the angle of bank

during a turn. These are of no great importance, but are brieflygiven here for co m pleteness :1 . The outer wing in a turn is travelling faster than the innerwing, as it is further away from the centreof the circle. This resultsin greater lift on the outer wing, which tends to increase the angle

of bank.2. Relative to the air, a glider descends as it turns ; after the33


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side of your face opposite to th e direction of th e turn. The aircraftis in fact being made to travel broadside through th e air. This isan inefficient condition which will cause unnecessary loss of heightand will not get you round th e turn easily. On th e other hand, ifyou attempt a turn without using any rudder, th e aircraft will" slip " inwards when it is banked over ; this effect is aggravatedby aileron drag, as mentioned earlier, but slightly reduced by th ew eathercock stability. Y ouwill th en feel a tendency to slide inw ardson your seat, and you will feel th e draught on the side of your facewhich is in the direction of th e turn.This slipping and skidding is thus due to th e incorrect coordination of the rudder movement with the movement of th e othercontrols. You can correct a skid either by using less rudderor morebank. However, you will find it easier to use the rudder fo r any suchcorrections of slip or skid, w hile using th e ailerons to keep th e bankconstant and th e elevators to keep th e speed correct. Ifyou areskidding outwards, you have on to o much rudder ; if you areslipping inw ards, you have not enough.How to carry out a turnThe actual manoeuvres involved in a turn should be carried outin three parts, namely, going in , staying in and comin g out. Beforestarting any turn always havea good look round. Youmay be turningacross th e path of another aircraft.

1 . Going in : look round. Then, to go into a turn to th eleft, move the stick gently to th e left until you have applied thedegree of bank which you think will be appropriate ; then centraliseth e stick and hold that bankconstant throughout th e turn by smallcorrections as necessary. At th e same time as you apply the bank,apply a little left rudder. As th e aircraft banks, keep the airspeedcorrect with th e elevators by keeping the nose in its proper positionrelative to the horizon.2 . Staying in : keep th e angle ofbank constant by use of th eailerons, and keep th e nose moving smoothly round th e horizon

by use of th e ele vators and rudder. Ifth e nose rises or fa lls, correctwith th e elevators ; if you find yourself slipping or skidding, correctwith th e rudder. Look round frequently.3 . Coming out : move the stick in th e direction opposite to

that of th e bank, until the a ircraft assumes a lev el position. Atth e same time as you apply the opposite bank, apply a little oppositerudder to prevent skidding. When th e aircraft straightens up on th e

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new course, centralise the rudder and stick. Use th e elevators tokeep th e airspeed correct.Airspeed in a turnYou will remember that th e stalling speed of th e aircraft isincreased during a turn. You will probably be taught to fly at anormal cruising speed w hich provides an adequate margin for gentleturns, in which case no alteration of speed is necessary. For steeperturns (above 30 of bank) it is advisable to fly a little faster thanusual ; th e nose must then be kept in a lower position on th ehorizon by th e appropriate adjustment of th e elevators.Faul t s in a turn

The faults occurring in turns may take the following forms :1 . Failure to keep the bank c onstant.2 . Movement of the nose up or down with respect to th ehorizon.3. Skidding or slipping.A fault can be caused in more than one way. For instance,

if you are slipping in during a turn, it may be due to too much bankor too little rudder. However, if each control is considered as havingits ow n function, as outlined above, any fault can easily be corrected.In this case, if you find th e angle of bank and the position of th enose to be correct, you must correct such a slip by applying morerudder. This application of rudder will cause th e nose to dropunless a little more elevator is used, so that for a smooth turn allthree controls must be used in close harmony. This appears to be aformidable procedure when set down on paper, but co-ordinationis soon achieved by practice.T o summa ri se :

For an accurate turn :1 . Keep th e bank correct with th e ailerons.2 . Keep th e airspeed correct with th e elevators, by holding th enose in the right position.3. Counteract any slipping or skidding with th e rudder. Ifyou are skidding, use less rudder ; if you are slipping,use more.

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7 . STALLINGIn an earl ie r chapter we explained the m eaning of the term

" stalling," and outlined one set ofcircum stancesinwhichan aircraftm ight be stalled.The speed range of a glider is usually quite small, and you arenorm ally fly ing within 10 m .p .h . or so ofthe sta lling speed . It isth erefore important for you to be able to recognise the approach

ofa sta ll , and ifyou should stall inadvertently , to recover w ith them inim um loss ofheight.Itwil l be rem em bere d that an aircraft stalls when the angle of

attackof its win gs is brought past the stalling angle. This happensin steady flig ht at the " sta lling speed " ; under increased loadingit occursatsome higher speed. Thus an aircraft maybeaccidenta llysta lled:(a) If the speed is allowed to drop to the " sta lling speed"in straight flight.(b) If the loading is increased (e.g., a turn is commenced)when flying only slightly above th e stalling speed.A sta ll is onty a hazard if it occurs at a low altitu de, or if it isallo w ed to develo p into a spin.

How to detect the approach of a stallIf a stall is perform ed, th e symptoms of its approach are asfollows:1 . Hig h position of th e nose.2. Absence of noise (due to th e slow airspeed).3 . In effectiveness of th e controls, particularly th e ailero ns.4 . In creased rate of descent (th is may show on instrum ents).W hen th e aircraft is properly sta lled, height is lost very rapidlyand the nose usually dro ps.5 . On some gliders a slight shudder or "buffetin g" ofth e tailmay be noticed.

RecoveryTo some extent the recovery from the sta ll is auto m atic, as theangle of attack of the win gs is reduced by the droppingof th e nose*However, th is should be hastened by a gentle forw ard m ovem ent

of thestick. Ifth estickis moved forw ard too farduring the recovery ,an unnecessarily steep dive results , and considerable height may belo st. Ifone of the wings should dropwhen you are carrying out astall , no attempt should be m ade to pick it up with aileron. As37


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already pointed out, th e ailerons are ineffective in a stall, and use ofthem may even make things w orse. The correct procedure is to keep straight by use of hard opposite rudder, and to carry out therecovery from th e stall in th e normal manner. After flying speed hasbeen regained, th e ailerons can be used to complete th e levellingof the wings.A consideration of th e mode of action of the ailerons willshow w hy th ey cannot be used at the stall. The lo w ered aileronnormally lifts a wing because it is presentin g an increased angle ofattack to th e airflow. If, how ever, the w ing is already at th e point ofstall, lo w ering an aileron will merely stall it more.

In addition, th e effect of aileron drag will be to cause a yawin th e direction of bank, and this occurring at the stall may giv e ris eto a spin (see later). Y ou must therefore resist th e natural tendencyto use th e ailerons under such circumsta nces. The rudder acts bychecking the tendency of th e banked aircraft to yaw inwards, andproduces a yaw in th e other direction, which may be regarded asspeeding up th e wing which has dropped (which is on the outside ofth e yawing turn) ; th is wing is thus unstalled, regains its lift andrises again.How to carry out a practice stall an d recovery

1 . At an adequate height, take a look round, especially under-neath you.2 . Level th e wings, and then ra is e th e nose slightly above thehorizon. Hold it in th is position by a steady backward m ovement

of th e stick as th e airspeed falls off. Note how quiet everythingseems, and how movements of the controls have very little effect.When th e sta lling angle ofattack is reached, th e nose dropsalthough th e stick is being held rig ht back.If a wing should drop, keep straight by using hard oppositerudder.3 . To recover, ease th e stick gently forw ard. W hen th e speedsta rts to build up, ease out of th e shallow dive.On some types of gliders , if a stall is performed very gentlyth e aircraft may " mush " downw ards without th e nose dropping,with th e stick held back. Height is lost rapidly and the nose must bedepressed in order to recover control. In general, the higher theposition of th e nose at th e stall th e more it will drop.By carrying out a stall durin g a steep turn, th e effect of increasedload factor on th e stalling speed may be observed. This is best donein conjunction w ith spinning practic e (see la ter).

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slightly forward will slow th e aircraft down somewhat , as well aseasing the force on the wings ; this is because th e aircraft is madeto travel on a shorter arc. Similarly, a normal launch will appeartoo slow unless th e aircraft is climbing at th e normal attitude. Ifhowever, th e winch is being driven to o slowly, this cannot be counteracted by climbing at a steeper angle ; this only causes th e aircraftto " mush," which puts more load on th e winch and leaves the aircraftin an awkward position if th e winch should fail completely.O n some aircraft a pitching oscillation (" bucking") maysometimes be noticed towards the top of the launch, particularly ifthe launching speed is rather fast. Bucking can usually be checkedby easing th e stick slightly forward.The cable is normally released when no more height is beinggained. This will occur shortly before the glider arrives over thewinch. As the winch cannot always be seen during the launch, thepilot must decide when to release from other indications. Aprominent feature to one side of th e winch may be observed ; th erate of climb may be judged by using instruments, or by th e positionof th e nose on the horizon. Lowering the nose of th e aircraft priorto release will assist releasing by reducing th e load on th e hook andcable.

If at any time during th e launch the speed should rise above thespecified safe launching speed, the cable should be released withoutdelay to avoid straining th e aircraft, and subsequent action carriedout as in th e case of a cable break (see later).If any launch takes place too fast or too slowly, th e pilot should

report this on landing to th e instructor or winch driver.H ow to take-off and cl imb

1 . Carry out th e preparations for flight detailed on page 2 8 .2 . Make sure that th e take-off path is clear, and that no one

is standing in front of any part of th e aircraft ; then tell th e signallerthat you are ready to be launched. The signaller should check thatthere are no aircraft approaching to land at th e time.

3. During th e ground run, keep th e wings level with th e aileronsand keep straight with th e rudder ; coarse use of th e controls isnecessary because of th e slow speed. Use the elevator to keep th eglider running forward in a level attitude, with neither th e nose northe tail on th e ground.

4 . The glider will usually take itself off w hen sufficient speed isreached. Keep th e initial climb gentle by appropriate use of th eelevator (the actual amount and direction of th e control movement4 0


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required depends upon the aircraft, the position of the hook andthe speed of the launch). Keep the wings level with the ailerons.

5 . At a safe height (100-ft. or so) steepen the clim b to the optim um angle. This steepening should be done gently to avoid strainingthe cable.

6 . Watch the ground to see how the launch is progressing.When just short of the w inch, release by pulling the knob (pull ittwice to m ake sure). The release may be made rather less violentby putting the nose down prior to letting go of the cable.Occasionally it is necessary to launch slightly across wind.This means that if the wings are kept level the aircraft will driftduring the launch until it is downwind of the winch. This will givemaximum height of launch under these conditions, but it may involvedropping the cable in an inconvenient place. If so, it is necessary tosacrifice some height and to maintain the original path by usingslight bank into w ind.

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The dow nwind leg is used to make major adjustments, th ecircuit being made smaller or larger by edging in or out accordingto th e height available.The second turn across w ind will normally be made a shortdis tance downwind of the landing point. This distance will varyaccording to the height available and th e strength of th e wind.The glider should at all times be kept within easy gliding dis ta nce

of th e field. If th e aircraft is very low on th e dow nwind leg, th eturn must be made early, before reaching the downwind boundary.This will involve landing some distance into th e field, but it is betterto do this than to risk having to carry out th e final turn at a lowaltitude.

During th e second crossw ind turn, th e pilot must again considerhis position in relation to th e la nding ground. He may then finishth e turn so that th e" crosswind " leg carries him nearerth e boundaryof th e field, or fu rther away, or directly across wind, according toth e height available and the strength of th e wind.

In a moderate or strong wind, allowance will have to be madefor drift on this leg by heading somewhat into wind. The amount ofdrift experienced is a good guide to th e strength of th e wind. Theflight path on th e final leg (into wind) is steeper according to thestrength of th e wind, and this must be allowed fo r in ju dging th eapproach. During the approach, th e pilot should ensure that hehas ample air speed ; it is always advisable, when near the ground,to fly rather faster than usual.In a perfect circuit, th e final turn into w ind is made as the

glider approaches th e line of the landing run ; how ever, th e pilotcan turn in and land at any time on the crosswind leg if he thinksthat he is low. Ifth e approach has been made too hig h, height maybe lost by th e use of th e airbrakes, or by sideslipping. As an alternative, if local conditions permit, th e pilot may, if he thinks he is to ohigh, extend his crosswind leg, and th en turn back and fly in th eopposite direction. Several beats to and fro may be carried out ifnecessary, until enough height has been lost to enable th e aircraftto turn in and la nd. The turns at th e end ofsuch beats should alwaysbe made into wind ; otherwise th e pilot will lose sight of th e la ndingpoint and, if th e wind is strong, the glider may be blow n a long waydow nwind.

Beats should be of a sensible length, th e turns at th eend should be made gently, and the final turn into w ind should becompleted no lower than the usual height.45


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Under certain circumstances, e.g., th e presence of numerousother aircraft on the circuit, it maybe undesirable to loseheight in th ismanner.

CROSSW/KD xiFTbO v

L OW

/N(V TOO LOW)

The landingThe final turn into w ind should be completed at a good heigh t(100 ft. ormore). The remainder of th e approach should be madesteadily at the correct speed to w ard s an unobstructed part of thelanding ground. As th e glider gets close to th e ground, the glidepath is gently flattened outso that th e aircraft is flyingjust above th eground and paralle l to it. In thiscondition th e airspeed will steadilydecrease ; in order to prevent th e aircra ft from h itting th e ground

too early, the angle of attack is in creased by a steady backwardmovem ent of the stick at such a rate that the loss of lift due to thereduction of airspeed is balanced by th e increased lift due to th ehigher angle of attack. This brings the ta il of th e aircraft nearer theground, and when th e aircraft reaches th e correct attitude fo rlanding it should be allowed to sink on to th e ground, touching mainwheel and tail skid togeth er. After landing, th e glider is keptstraight and level by coarse use of th e controls until it has come torest. The pilot should stay in th e cockpit unti l help arrives.Faults

If th e stick is moved back too slowly, the aircraft will touchdow n too fast in a nose-down attitude, and may bounce. If it ismoved back too quickly, the aircraft may climb and subsequentlystall well above th e ground. If th e lastpart of th eapproach is made46


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too slowly, th e aircraft m ay stall during rounding-out and landheavily, or it m ay touch down with the tail skid first, due to thecoarse elevator movement needed to flatten out.W i n d gradient

The wind near the ground is slowed up by friction with theground, trees and other obstacles. This effect is known as " windgradient" ; it is of importance when the wind is m oderate orstrong and is most marked from the ground level up to about 30 ft.W hen the surface wind is , say, 20 m.p.h., the wind at 30 ft. might wellbe 30 m.p.h.

When a glider is flying in a steady wind, the wind has no effecton the flight, apart from altering the position of the glider withrespect to the ground. However, when the glider is flown rapidlythrough a region of varying wind speed, the inertia of the aircraftcauses the airspeed to be affected for a time, until the aircraft hassettled down in the new conditions. In the case of a glider approaching to land into wind through the wind gradient, the sudden dropin wind speed causes the airspeed to fall off. If th is is not taken intoaccount, the aircraft m ay be stalled inadvertently. Hence, whencoming in to land in a strong wind, the pilot should put the nosedown during the last stages of the approach, thus ensuring' plentyof speed. In any case, the approach in a strong wind should bemade at a higher speed than usual in order to provide adequatecontrol in turbulent air.

If an aircraft is turning into wind at all steeply as it approachesthe ground under conditions of pronounced wind gradient, thelower wing will be affected by the gradient before the upper, andconsequently lose some of its lift. This may make it difficult torecover from the turn, and is one of the reasons for avoiding turnsclose to the ground.S u m m a r y of m an oe u vr e s on the circuit

1 . Take off and climb as described on page 40.2 . After release, settle down to a glide at normal speed, and

then turn across wind.3 . If you think you are low, turn downwind immediately ;if you have plenty of height, continue across wind before makingthe tu rn.4. O n the downwind leg, keep looking at the landing place.If you are low, edge in towards it ; if you have plenty of height,edge out a bit.

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Just before touching down, rudder should be applied in thedirection of drift :AT iNSTflNf OFTOUCH T>OirtN

This counteracts any strain the touch-dow n might cause theundercarriage, by swinging the aircraft in line with the direction ofmotion ; if this is done at the correct moment, the aircraft willtouch down before the flight path has altered.As the aircraft touches down, the into-wind wing-tip should belowered slightly, and eventually placed on the ground.

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Lack ofcare on the part of a pilot may bring the aircraft to acondition when an accidental spin is possible. If the aircraft isundershooting, the pilot m ay be tempted to fly too slowly ; he mayuse too m uch rudder in the final tu rn , being relu ctant to put onbank close to the ground. As the nose drops he tries to hold it upby using the elevators ; thus a stall occurs when the aircraft isyawing and conditions are ideal fo r a spin.H ow to recover from a spin

At th e start of a spin the nose drops away sharply and theaircraft starts to rotate. It must be emphasised that the normalinstinctive use of the controls at this point will only aggravate thespin. If the ailerons are used to try and level the wings, the down-going wing is further stalled, and the yaw is m ade worse by theeffect of aileron drag ; the use of the elevators will not keep the noseup if the aircraft is stalled.

It is extremely important, therefore, that the pilot should learnto recognise an incipient spin (the start of a spin) and be able totake the proper action to prevent a full spin developing. Whenspinning, or to correct an incipient spin, the correct use of the controlsis as follows :

1 . Apply full opposite rudder. This tends to check the rotation.Som e force may be necessary.2. A fter applying the rudder, m ove the stick fo rward steadilyuntil th e spinning stops. This unstalls the wings and allows thespeed to build up.3 . W hen the spinning stops, centralise the rudder and ease outof the resultant dive, levelling the wings with ailerons once thespeed starts to build up.Note.f the rudder is kept applied after the rotation stops,a spin in the other direction may result.Do notmove the stick further forw ard than is necessary , or anexcessively steep divewill result.The ailerons should be kept centraluntil the aircraft is unstalledand is gathering speed.

Pra ctice spinsAs mentioned above, it is important that you should be ableto recognise the start of a spin and take the appropriate correctiveaction. This involves practice recoveries from spins and incipient

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spins. These may be performed dual quite safely if adequate heightis available.1 . Make sure that you have sufficient height.2 . Have a good look round, especially beneath you.3 . Commence a spin, e.g., by performing a slow turn with

too much rudder, holding the nose high and using opposite aileronto stop the bank from increasing.

4. To recover : apply full opposite rudder, then ease the stickforward. When the spinning stops, centralise the rudder and easegently out of the dive, levelling the wings with th e ailerons.

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12 . SIDESLIPPINGDuring a sideslip the aircraft is made to travel through the air

partly broadside on, so that the line of flight is at an angle to theheading of the nose. This results in inefficient performance ; theangle and rate of descent are increased without a correspondingincrease in airspeed. iy/

A sideslip may be used to correct for overshooting on theapproach to land. It may be employed when in straight flight or ina turn.The force providing the sideways movement of the aircraft isderived from the lift and the weight.

The steeper the angle of bank, the greater will be the sidewayscomponent of the motion. However, the sideslip is an unnaturalcondition of flight, both the lateral stability and the weathercockstability trying to prevent it. The lateral stability tends to level thew ings again, and the weathercock stability tends to convert thesideslip into a turn.Therefore , in order to keep the aircraft in a straight sideslip,the aileron control must be used to maintain the bank and opposite

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rudder must be used to prevent a turn. The degree to which a glidercan be sideslipped straight is limited by the rudder controlavailable.On most gliders the rudder is comparatively weak and full ruddermust be used at quite a small angle of bank. If the angle of sideslipis increased further, a turn cannot be prevented.

During a sideslip the position of the nose is rather higher thanin normal flight at the same speed. W hen recovering from thesideslip, therefore, the nose must be put w ell down to avoid the airspeed falling off. Because of this effect, and also since the aileroncontrol on most gliders is rather sluggish, the recovery from thesideslip must be carried out at a reasonable height.

Sideslipping on a turn.Aslipping turn is an effective methodof losing height, particularly on a glider with com paratively poorrudder control. In thismanoeuvre a turn is performed with excessivebank and opposite rudder, thus producing insufficient rate of turnfor the angle of bank. The glider may be sideslipped quite steeply ;the yaw producedby the w eathercock stability isonly partly counter-acted by the opposite rudder and the turn is allowed to take place.H o w to carry out a sideslipT h e straight sideslip

1 . Having made the final turn into wind, bank the glider tothe left or right and apply opposite rudder, swinging the nose roundso that the resulting sideways path is in the direction ofthe landingpoint.2 . Keep the speed at the normal approach speed ; the nosewill be slightly higher than in a straight glide.3 . To obtain a greater rate of descent, increase the angle ofbank. Notice that more opposite rudder is needed to overcome thetendency to yaw. The limit is reached (for a straight sideslip)when full rudder has been applied.4. Recover in good time : allow the nose to swing round toits original heading, level the wings and centralise the rudder. Putthe nose well down to m aintain the airspeed.

The sl ipping turnStart the final turn into w ind rather higher and closer than usual.Increase the angle of bank, and put on top rudder to cause the slip.

Recover when facing into win dThe slipping turn can be converted into a normal gliding turnat any time if sufficient height has been lost. If you are still too highat the end of the turn, carry on into a straight sideslip.

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If th e final part of the approach and hold-off are made withthe brakes fully open, the following points should be considered :1 . Since the approach is steeper, the alteration in angle requiredto level out is greater than usual, and the round-out should thereforebe sta rted earlier.2 . Deceleration will be more rapid.3 . The stalling speed will be higher.4. The wind gradient effect will be more noticeable. Thus,

if the levelling out is started to o late or with too little speed, apremature stall and heavy landing is likely, or the abrupt use of theelevators may lead to a " tail-first " landing. These effects are muchmore noticeable with dive brakes than spoilers, and it is advisablewhen using dive brakes, unless the final approach has been made at afast speed, to close the brakes partly or com pletely as the gliderapproaches the ground.W hen actually holding off, any alterations in the position ofthe brakes will cause a marked alteration of the flight path of th eaircraft, unless the attitude is corrected by using the stick. Theproper co-ordination requires some practice ; until he is quiteaccustomed to the effect of the brakes, the pilot should keep theair brake lever in a constant position once he is below about 50 ft.After touching down, the brakes should be opened fully.T h e a p p roach with th e a ir brakes

1 . M ake the final crosswind leg so that you would overshootif you did not use the brakes.2 . When you are sure you are going to overshoot, open the

brakes to steepen the angle of glide. Keep the speed slightly fasterthan for a normal approach.3 . Adjust the air brakes as necessary to control the approach.4 . On an aircraft fitted w ith dive brakes, until you are thoroughlyfamiliar with them, clos e or partly close them well before the checkand hold-off, and hold them in a constant position until you arefirmly on the ground. W hen using spoilers, it will not be necessary,usually, to close them, but again they should be kept in a constant

position during the last part of the approach.5 . When you are on the ground, open the brakes fully.Use of dive brakes in an e me r ge n c y

As mentioned on page 26, div e brakes (not spoilers) are designedto keep the speed of the glider within safe limits. If at any timethrough bad aerobatics or through losing control in cloud, there is

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A PPENDIX AInstrumentsThe usual instruments carried in a sailplane are as follow s :1 . The airspeed indicator : This instrument records the speed

of the airflow past th e aircraft (owing to the effect of wind this isnot the speed over the ground). It is operated by th e pressure ofthe air on an open tub e (the pitot tube) fixed on the aircraft andpointing fo rwards.2. The altimeter : This instrument is operated by the fall inair pressure with increasing altitude, and is calibrated to record this

directly as feet of height. It registers the height of the aircraftabove sea level or aerodrome level, or any other zero accordingto the w ay the pilot has set the instrument. A s the pressure in thecockpit does not always correspond with the actual ex ternal pressure,the altimeter is usually connected to a static head w hich is positionednear the pitot tube, and consists of a tube in line w ith the axis ofth e aircraft, w ith its forward end closed and with small holes dow nthe sides ; this is designed so that th e pressure inside the tube isequal to the static pressure outside it.

3 . The com pass : This instrument shows the heading of theaircraft with respect to the magnetic north. In England the magneticnorth is about 10 west of true north. The compass is subject tovarious errors when the aircraft turns or varies its speed.

4 . The turn and sideslip indicators : These tw o instruments areoften included on the same dial face, but are operated independently.The turn indicator is usually worked by an electrically drivengyro, and the needle moves to the right or left according to the

turning m ovement ofthe aircraft.The sideslip indicato r records sl ip and skid, and it m ay takethe form of a bubble or ball in a curved tube of l iquid (like a spiritlevel), or a pendulum type needle. A w eathercock vane or a pieceof string m ountedon the front of the fuselage fulfi l the same function,but are rather over-sensitive.

5 . The variometer : This instrument record s the verticalmovement of the aircraft w ith great sensitivity , and is the chief aidto therm al soaring. It is operated by the change of pressure accompanying a climb or descent. In th e type usually found in Britishsailplanes, these changes cause air to flow into or out of an insulatedcontainer (the "capacity ") ; this m ovement of air is arranged toraise a green or red ball as appropriate.58


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APPENDIX BThermals

A glider must always descend relative to the air. The gliderpilot must find rising currents of air in order to remain airborne.Such ris in g currents are usually "thermals," due to local heating,or " hill lift,"where the ordinary horizonta l wind is forced upwardsby a hill. Other ty pes of lift (frontal, wave, etc .) occur more rarely.Thermals are fairly localised, and are usually accompanied byareas of descending air ("down draughts "). A pilot flying throughan area where such sm all-scale air movements are occurring willfee l these movements as " turbulence." The aircraft will be bumpedabout to some extent, sometimes in temporary defiance of theposition ofthe controls.In such "rough air," im proved control will result if the speedis in creased a little (say 5 m.p.h.) but it is not necessary to correctfor every little deviation from thenormal attitude. Sm all deviationswill often cancel each otherout ; la rge deviations shouldbecorrectedby leisurely use of the controls so that the aircraft does not wallo wabout toomuch. If the pilot succeeds in remaining in oneparticula rportion of rising air, the turbulence will be minimised.Gusts

Rough air at altitude does not constitute a hazard unle ss theglider is going too fast (w hen the structure m ay be strained). Nearthe ground thermal turbulence is not usually suffic iently marked toaffect the handling of the aircraft ; however, " gust tu rbulence "may be encountered at low levels in conditio ns of strong win d, andth is may be dangerous unless anticipated. Gusts are small andvio lent vertical and horizontal air currents caused by the strongwind striking against obstacles on the ground. It is particularly marked when the ground is rough, in the leeof obstacles (buildings,trees, etc.) and in the " curl over" on the lee side of hills. Suchareas should be avoided as landing places ; the approachin a strongw ind should be made at a slightly higher speed than usual, and thefinal leg into wind should be started at a good height.

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APPENDIX CM o r e about lift and drag

You have seen that th e value of th e lift and drag forces on awing depend upon the angle of attack and the airspeed. The densityof th e air is also involved, and the full relationship is given by theformulae :

Lift=CI y ip V 2 SDrag=CD ip V 2 SWhere :

C^ is the lift coefficient, which depends on th e shape of theaerofoil, and varies with th e angle of attack.C D is the drag coefficient which similarly depends on the shapeof th e aerofoil and th e angle of attack.

p is the density of th e air.V is th e velocity of the airflow.S is the surface area of th e aerofoil.It w as mentioned earlier that when in a steady glide the values

in th e expression for lift are such that th e lift force is equal to theall-up weight of th e aircraft (for an Olympia this might be 6 30 lb.).If th e speed is increased th e aircraft is automatically flown at asmaller angle of attack (reducing th e value of Cy so that th ebalance is maintained. If w e wish to carry out a manoeuvre such as aturn in which extra lift is required, w e may obtain this lift either byincreasing th e angle of attack (up to the stalling angle) or by increasing th e speed, or by doing both.

So far w e have only considered specifically th e drag caused byth e wings (w e defined this as th e component of th e total reactionwhich is parallel to th e airflow). However the fuselage, tail unit,struts and other bits and pieces all tend to resist travelling throughth e air, and produce " parasite " drag. The total drag of the aircraftis made up of th e wing drag together with th e parasite drag. Thesetw o main types may be subdivided into induced drag, profile drag,skin friction, etc. To give an idea of th e magnitude of the forcesinvolved, the total drag force on an Olympia at best gliding speed isabout 2 5 lb. wt., of which about 6 lb. w t. is parasite drag.Lift and drag coefficients

For most purposes w e are concerned with th e lift and drag of thewhole aircraft , i.e., we wish to take into account th e fuselage dragand any other effects. W e must therefore use th e values of C^ and

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C D appropriate to the whole aircraft. These can be found by w ind-tunnel experiments on a model, or m ay be calculated from the resultsof flight tests. If the values found at various angles of attack areplotted, the curves are of the type shown ; these were obtained fo rthe Olympia.

o\-L L JU

OU

16OF ATTACK

o

LLo\-zL L J

U

o


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7.The angle of attack of the Olympia in normal flight is aboutC^ is therefore 0 .62 ; th e airspeed then has th e appropriate value(about 45 m.p.h.) such that th e lift force is equal to the weight.Lift/Drag ratio

From the graphs w e can find the ratio lift/drag at variousangles of attack. The curve of L/D (=C^/CD ) for th e Olympia isgiven below.

R A T I O-2o

o /6O F

It w as shown on page 1 5 that the angle of glide (in still air) isflattest when the ratio L/D is a maximum. Observe that this occursat an angle of attack of about 7 ; at normal loading this correspondsto an airspeed of about 45 m.p.h.

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A PPE NDIX D

T/5EC

M aking th e mo s t o fitW e here include a few notes showing how you may best utiliseth e performance of the glider under various conditions. The subjectis treated in a theoretical w ay for the benefit of the technicallym inded. The results in general are of interest to any pilot wishingto make the most of his aircraft, but the values derived are not verycri tical. For instance, it is possible to calculate the "correct"airspeed at w hich to fly between thermals on a given cross-country .However, provided that the sp eed is within about 5 m .p.h. of the

optimum, other factors (e.g., ability to find the best partof a thermal)are of much greater importance.T he Polar Curve

W e can carry out flight tests to determine the rate of sink(usually m easured in feet per second) at different airspeeds. The

o

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A-o


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graph obtained fo r a given glider is known as its "polar curve,"and is ofthe type shown ; this curve was obtained for the Olym piaat 630 Ib. all up weight.This graph gives a m easure of the " penetration " of the aircraft ,which is the ability to increase its airspeed without an undue increase

in sink. Thus the Olympia at 45 m.p.h. sinks at 2 .6 ft./sec. and at60 m .p.h. sinks at 4 .2 ft./sec. ; this corresponds to fairly goodpenetration. Compare a training glider such as th e Cadet, whichat 35 m.p.h. has a sink of 3.5 ft./sec. and at 60 m .p.h. of1 2 ft./sec.

Good penetration is obviously an advantage when travellingacross country, as it enables the distance between thermals to becovered comparatively rapidly and with little loss of height.The polar curve has the following properties :1 . If a line is drawn from the origin to any point on the curve,then the slope ofthe line is equal to the gliding angle in still air atthespeed corresponding to thatpoint. (The slope must be calculatedusing the sam e units on eachaxis.)2 . The tangent to the curve from the origin touches the curveat the point corresponding to the flattest gliding angle (instill air).3 . The topmost poin t of the curve gives the airspeed at whichthe sink is a minimum.Referring to the curve for the Olympia, we see that th espeed fo r " best" gliding angle (in still air) is about 45 m.p.h., at

630 Ib. loading. However, it m akes very little difference to the rateofsink or the range which speed is used.

A t other loadings, the appropriate speeds will be slightlyaltered. A t lighter loadings the speeds to fly are decreased, but theeffect is small.T he effect of wi n d a n d downdraughts

The " best" airspeed to fly at is affected by air m ovements.W hen trying to cover gro und against awind, theairspeed fo r flattestgliding angle is greater than in still air. To take an extreme case,if an Olym pia was flown at 45 m .p.h. into a wind of 45 m.p.h.,it would descend vertically ; clearly a greater distance is covered

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by flying faster. The correct speed atwhich to fly is obtained fromthepolarcurve as follows :

3-1^KF T ,

5-1

f o 20 3o Ao -sro GoK\PM

Along the airspeed axis lay off the amount of the headwindcomponent ; from the point reached, draw a new tangent to thepolar curve ; the point where it touches the curve corresponds tothe " best" airspeed for those conditions.A rough rule of thumb indicates that the airspeed should beincreased above 45 m.p.h. by an amount equal to one third of theheadwind component in m.p .h. Thus for a 12 m.p.h. headwind,the " bes t " airspeed for the Olympia is 49 m.p.h.Note that however much the airspeed is increased, the glidingangle in aheadwind can never beas good as 1 in 25. The slope ofthe ta ngent gives the actual gliding angle.The correct speed fo r flying in a tailwind can be found in asimila r manner (by laying off along the speed axis in the oppositedirection) but the effect on the bes t speed is slight ; for maximumrange in a tailwind it is quite good enough to fly at the speed forminim um sink (40 m.p .h. fo r the Olympia). For cross-countryflying it is usual (see la ter) to travel ratherfaster than this.W hen fly ing through a downdraught the sink is increased,and the gliding angleis less than 1 in 25. It therefore pays to increasethe speedto someextent soas to passthrough thearea more quickly .The optimumspeed canbe obtained from the polar curve as follows:markoff the value of the downdraughtalong the sink axis, upwards

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from the origin ; from th is point draw the tangent to the curve.The point w here it to uches corresponds to th e sp eed for flattestgliding angle.

O F S INK ft rBfsr / "

0

2 .t>lf Kr7/5c

A rough rule indicates that th e sp eed should be increased by3 m.p.h . above 45 m.p.h. for each 1 ft./sec. increase in sink ; thusin a 5 ft./sec. downdraught, the Olympia shouldbe flownat 60 m.p.h .Flying for speed

In keepingw ith the modern trend, gliders arebeing flown faster.It is recognised that long cross-country flights are often limitedby th e duration of therm al activity ; th e pilot should aim to achieveth e highest ground speed possible under the conditions. Toencourage this outlook, glider " races " have recently been intro-duced into com petition flying in th is country.

W hen travelling across country, th e glider pilot gains heigh tby circling in a thermal, then flies straight in the required direction,losing height. H e counts on finding another thermal before he istoo low. The race technique involves fly ing betw een thermals at aspeed faster than that fo r best gliding angle. It is assumed thatthermals are sufficiently plentiful to make th e use of the flattestgliding angle unnecessary . The correct speed at w hich to fly a68


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given aircraft depends on the average strength of th e thermals onthat day, and is independent of the strength and direction of th ewind.

The appropriate speed at which to fly can be determined fromth e polar curve as follows :From th e origin, mark off upwards the average rate of climbin thermals for th e day in question, as registered on the variometer.From th e point reached, draw the tangent to the polar curve. Thepoint where this touches the curve corresponds to the optimumairspeed at which to travel between thermals, in order to travel acrosscountry as fast as possible. When circling in thermals, the speed ofminimum sink is used. The point where th e tangent cuts the axisof airspeed gives the average airspeed made good. The resultingground speed depends on th e wind.Thus, in th e case of the Olympia, for an average rate of climbin thermals of 5 ft . sec. (variometer reading) th e optimum airspeedfor rapid progress is 60 m.p.h. and the highest average airspeedpossible is 33 m.p.h. This indicates that, with 5 ft./sec. thermalsand a headwind of 33 m.p.h. positively no progress can be made(without coming down). This point is often not appreciated.

It should here be noted that one's impression of the averagethermal strength on a given day is usually grossly inaccurate. Forthe calculations we are concerned with th e average rate of ascentwhen circling at minimum sinking speed, and this must include the70


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i>

A O P 5

ON

o

I

XV

JLO

SINK

in itial fumblin g to rind the centreofthe thermal, and the fadingoutat the top. This usually results in a rateofascent which is a quarterto a halfofwhatone would think from thevariometerreadingwhenthe glider is properly positioned in the thermal. Thus rapid speedacross country is dependent on the elimination of these periods ofhesitationtoa muchgreaterextentthanon theexact speedat whichthe glider flies betw een thermals. For an Olympia in England, abetween-therm al speed of50-60 m.p .h . according to the conditionsis normally quite good enough.

W hen flying through a downdraught, the optimum speed tofly is faster than that determined as above. A combination ofthisconstructionwith that on page 66can beused tofind theright speed.Turning in thermals

Accurate calculation of the turning characteristics ofgliders ispractically im possib le . However, a few points may be mentionedhere:

1 . Thermals are usually quite small, and the glider must bebanked sufficiently to give the optimum radius ofturn.

2 . The greater the angle ofbankat a given airspeed, thesmallerthe radius of turn, but the loading, the stalling speed, and the rateofs ink are all increased. Acompromise dependingon thedistributionof li ft in the thermal must be achieved. 25 -45 of bank is the usualsort of ramie. M ore than 50' is seldom advisable.

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3 . At a given angle of bank, the radius of turn depends on theairspeed. In a thermal a glider will normally fly at the minimumsinking speed. This speed depends on the load factor, but fo r anOlympia is about 20% greater than the stalling speed for the givenangle of bank.

4 . The Cobb-Slater variometer is affected by the increasedloading in a turn, and does not read accurately. If therefore, youput on m ore bank when circling in a thermal, and the variometershows no change in the rate of climb, you are in fact, going up morequickly than before.Summary of Appendix D (For the non-technically minded).

1 . If you want to prolong a flight as much as possible, or toclimb in a thermal quickly, you must fly at the condition fo rminimum sink. The appropriate speed depends on the loading,but fo r an Olympia is about 2 0 0 greater than the speed at the stall.

2 . When looking for thermals or trying to reach the aerodrome,you m ust fly at the t v best gliding angle." The best gliding speeddepends on the loading, but for the Olympia in still air is about 30%greater than the speed at the stall. (See paragraph four of thissummary.)

3 . There is very little difference in performance w hen flying ateither of these speeds, i.e., the speed to fly is not very critical in thisrange.

4 . The best gliding speed is affected by headwinds and down-draughts. For an Olympia the speed should be increased above45 m.p.h. (a) by an amount equal to one-third of the headwindcomponent, and (b) by 3 m.p.h. fo r each 1 ft. sec . increase ofsink above the normal value. When flying in lift , or downwind, thespeed should theoretically be reduced slightly (40-45 m.p.h. is stillquite good enough).

5 . When in a hurry and thermals are plentiful, it pays to flyslightly faster than the " best gliding speed," according to thestrength of the thermals. For rapid progress an Olympia should beflown at 50-60 m .p.h. according to the conditions.

6 . When circling in thermals the angle ofbankmust be adjustedaccording to the distribution of lift (25-45 of bank is the usualsort of range). Theaircraft should be flown at the minimum sinkingspeed. This is in creased in a turn. (See paragraph one of thesummary.)

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