Aero Modeller 730

CIBCULAB, TOW - Partlan outlino of th€ d€velopments, andan explanation of th€ varioussystems used by the leading A/2 glider fli€rs d€tailod by

Elton Drew

FIGUiE I

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WHAT lS chcolariow? Simplyth. c€picityto allow a glldar to covor ! conirollsdcircular flighr parh ai witl wirh rowtin€ailachsd, as ooool6d to convoniional

Why circulsriow, whsn in tho 66rly drysol glidsr flying th6 gr6.tost probl€moncountorad w.B lho achiovom.n! ot asrr.lghr tow ro full towlin€ hoight followedby 5 circular glld6 p!th? Tho rmwor, orrllh€r answ€rs, bocomo mor6 readilyapparsnt if con3lderation is givon to rhoprobloms €ncountsred whon llying in calm

Firut, a3 6nyono who has towcd ! srggingmodal in a tl.l c6lm w.ll lnows, th6boundary f6nc. and/or orhaostion tsnd toar.ivo much mor€ rspidly ihan r thormaland th6 flysr is ofton conlronr6d wilh thschoico of attompting ro b.ing ths modeldown on ths line, ilsslf a chancy opdrstion,or roleasing th. mod€l in desporltion, whhlh€ rssuliing flight time v6ry much in tholap ol th6 god3. Circuls. tow cln obviouslyovorcoma this problom.

S€condly, ckcul6r tow c6n bc a d6cidsdaid to tho|mal dorocti.n in rhos. condiiions,in that one can gottwo or mors'bi t68 at th€chsrry', b6cau3o if in doubt 6s to whothefor not a thermal has beon encountorod, on6can samply go round 6gain' and r6checkrhal o.nicular o6lch ofai..

Thirdly, circular low may bo amployodas a tacricEl llying 6id in that, s8suming adollnits wind dircctlon, ons can drllt th6mod6l downwind 6nd i6t€ uD a l.vour8blopo8llion to asse!6 which olihe opDo3ition'smodsl3 is hopotully going to m.rt llfi; alavourite ploy ot tho Russi.n oxponsnts.Thl3lsct icobviou6ly is part icular lyu36lul incompslitions having slaning lin6i fromwhlch orhsr comp€titoas are nsiurallyroluciant lo iow uDvvind. lt h63 obvious

Ti X.Rt

t , x+Rt

limiiations if .ll lho conbstants omploycirculsr iow and sdoprths i6cticl

A funhar bonu8 ls pfovidod by lh6capacity lhrt circular tow can ofl6r, of aBignitic6nl car6puh hsight boosl on roloaie;invalu.bla in gaining pr8cious soconds in

FtouiE IOth.t tov hook.orlaln.llr Int nd.d ... n..d ofobt l.l.t..taltht tor.

Clrc$lai tow ayrLm.1. fh.'F..nch Sw..n'lhs inr6ntlonal ws of circular low flishtp!!h lor glidsrs is by no no€ns now. Ths

Fronch hld . sylt6m in widospr€8d usossoaroond 1967, pe.haps €rrlier. Th6ir v6ryeimplo 3y!16m, re3unec!6d a d3vic€ usodm.ny yoars 6arli6r (.nd fej€ctod 6s virluallyusole8s) - ths offsor lowhook. O.igin.llyahi3 was inlend6d as, hcans olobraining astraighi iow followod by 6 circling glido;thi3 b€ing b€for6 th6 univ6rs.l adoption ot

Ths olt3sl hook'o original domiso i3 6asyto unduslsnd: In lhgory th€ momontimpossd by ths lino lonsion .ciing on rhsolts6! towhook b.lanc€d iha mom.nt du€to th. lixsd ruddor dstleclion. This is t.ueat th€ 3!art ol tho row and fo. a parliculo.aispo€d, bur rh6 situaiion rapidly chang€slAs lho mod€l approachos rhe top ol !h6l ins iho s6i up changes as shown intOUn 1. Now lh3 lino i6nsion will bs so6nto produco r rolling momsnr rarhor than thopt6vious yawing rnom.nl. H6nce thocouniaraction io tho rudd6.turning mom6ntha3 di3app66r6d; indc6d th6 rolling momonrt .nd! lo rol l ths modol in lhs sam6 dirocr ionsstho rudd6r is lurning th6 mod6l, snd thspooling olf sffoct iB thus sggravaiod.Combino this wilh lhc va.iablo slfoct ofchanging windsp66d 6nd it is roadilyapplrsnt why tho ldv€nt ot th. AuroRuddor had b6€n wclcomod with op€n

CIBCI]LAR TOWreslored by applying light iodsion to th€lin6. Reloaso ol the model is accomplishedbv a lut th€r incr€aso in l ins tonsion. Thiscompf€sses th6 spring to stop B" therebyintroducing a slighi amount ot rudderoffset just prior to th6 rolease of tho latch,and h€nc6 release of iho iow lin6 from thsmod6l. This offsst, co mbin6d with incroa$dair sp€6d dus to the high lin€ i€nsion,fesults in a calEpull€d climbing turn with ausslul 93in in al t i tud€.

L€pp at tho '1971 Championships, andagain in 1973 demonstraied a vafiationomploying 5n ailercn on ihs pon wing iipacriv6tod in coniuncrion with rh6 towrudder olfser (left-hand turn). This, Igather, 6nhanc€d the model's bshaviour inlh6 cifcular tow mode, pemilting tightor'on th6 l ine turns to be at ia in€d andel iminat inq any spiral d iv ing tendoncies.

Th6 main deterents to th€ mor6 wid€-sproad adoption ol these hooks in Britainars undoubtedly our capficious climate,coupled with ths compl€xity oftha hooks -machining lscilities were widoly smployedon lh€ or ig inals.The vagaf iss otourcl imatsresuli in very few occasions ideallv suitedFi tur. S - S. . t rd in.v i .n ' r inr .ndr! . ins '

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coktinuedfrom page 731

for uss ol the hook and iho anticipaled lowusoage would 6pp6ar to lead mosi Britishllisrs to rh€ conclusion rhat rhe efforrnsedsd to producs thess hooks would be

3. The Scandinavitn 'Spt ing and Ring'

Until rec6nrly, lho Fr€nch syslem appsarcdlo h8vs bssn lalqely supeGeded by varia-r ions on rho 'Eussian Hook' theme. Arevival ol inrerest has b6en triggercd off bythe adoption ol a new line releas€ syslemoriginal ing f rom Sc.ndinavians Buchwaldand Pydso, apparently d6rived from therather complicaled €nd cumbsfsomo Scan'dinavian spr ing. loaded claw d€vico des-ctibgd in tha 1 97 3 / 7 4 Ae rc M o dele, A nn u at.Bri6fly, rhis involvsd r€placing ths con-vsnt ional lowhook wi lh a f ing to which theclaw is clipped, s€e f&u/e 4. Tho r€leassof th€ claw is activaiod by an impulse fromtho spring provided by tha s!dd€n fsleasoof a smal l amount of s lack in the loadedrowl in6. l t cannor releass whi lst the l ine isslack, and rhus provides an absolut€lyposi t ivs l ine lock on.

Buchwald 6nd Pydso's development wassimply lo feplaca this claw by a hook 3sshown in Ftgula 5a. Ths hookjust hanss inthe ring and is virtually impossibl€ todis lodga in l l ighi , o lher than by a suddenimpuls€ as doscribed above - a brilliantlysimplo, but stfeclivs, idea. A similar Daoishdavice (Figute 5b, w6s illusrsled by BonCol€man in his Technical Repon on th€1973 World Championships (AercMod. eLNovembsr 1973, and stated, in thal repoft,io hav€ ba6n judged i l lsgal . Since rhon alwas stated lhat i rs use was al low€d in lhose

(b) D i.t' devic.

FiaurG 6 - Pivoted off.Gt

championships. l r is d i l t icul o see on whatqrounds i t could hsve been disal lowed sinceihe spr ins exrension

's l imired by a stop :- . - -_ '

ths spring extension is limired by a slop .and cannot provid€ any mor€ inJiishr ri.e (EiBg?ERsgxrension than pomined by rhe 2kg lino oFasa)l€st pull. Admiltedlv the device has a.ather'suspicious' appearanc6, but remem-b€f that ths spring merely providss lheimpulsoto dis lodgerh€ hook, and does nolimparrany sn€rgyro th6 model,

This 'Ring and Spr ing device wasenlhusiasticallv intfoduced ro me bv PetetAl lnut t at rhs 1974 Br i t ish Nat ionals, hehaving pick€d up lhe idea on one of hisffsquont oxcufsaons ro conrinenral contests.H€ h8d adapted itlo the Ffench offset hooksystem and thsrsby inrroduced a newdimension toirs use in thatatwas no longeressent ia l lo maintain l inetension;rh€ modelnow being lree lo circle wiih slack line, aswi lhtho Russian syslem.

Thas syslem has been used wilh somesuccess by steve Mariotr who has added arelinemsnt, ss shown in f49u.e 6. ThisDrovides rhe laciliN ro reduce rudd€r oftseras towline lension increas€s, and pfovides,it is claimed, sreaier control ovef therelease opefai ion whi ls l ro ia in jng rho abi l i tyro c i rc let ight ly wi th stackl ine.

lnlerestingly boih Poter's and Stevoshooks ur i l is€ spr ings havins much srealerexlension than those shown in rhe 74ero-Mode et Annual &awinss, and in a skelchof Buchwalds and Pydsos hook whichappearcd in Frca Flight News, March, 1974sccompanying a drawins of an A/2 design,'Styk by Prydso and Konsgard.

Aero Modeller 784

lo i r int .h. h.ppy b.nd of . i r . l . . towcn do.. not r .quir ..rt.n.iv. work.hop f..iliai.r -. ..l..tion of n..dl. fild, no. l0and 55 drillrr S.rd l0 BA t.p. and . t.p wr.nch r.prc..nt th.only tool. rcq!ir.d .ddition.l to tho.cfoond in mo.t mod.ller"

Elton Drew continues his. fascinating account of the latest

techniques employed in A/2 gl iderswith an account of his personalexperiences

Following my rctum lrom theSwedish World Champs, I de.idedto try circular tow for myself. Thisled to a very protracted series ofexperiments. Protracted in time spanrather than in efiort I might add andit must be said that, besides a lackof spare time, the weather wascertainly responsible for slowingqown progr€ss.

FiSure 7a - Aisembly

OIBOT]LAB TOWInitially, the main attribute which

attracted me to the Russian systemwas the posilive line lock-up pro.vided. The carapult height gain wasalso regarded as desirable bonus,whilst. at that time. the circular towfacility was considered as of lessimportanc€ in view of the scarcityofcalm conditions in this couDf{y.

Having no machiring facil ies ofmy own, the first requirefieD ,vas todevise a hook which coold beproduced with limited tools, All myhooks have been Droduced with theusual range of adromodelling toolssupplemented with 8 and l0 BA taps,tap holder, appropriate tapping sizedrills (No. 50 and 55 respectively), aselection of files and, of course, agood vice,

The fundarnental constructionaldifference from the Russian desigowas the replacement of the pistonand cylinder arrangement with a

- Part 2

main body fashioned from 0.2in.thick dural plate, the moving hookbeing situated in adrilled hole.

The first attempt unfortunatelywas a failure. I had tried to over-simplify the system by doing awaywith the latch, the hook simplybottoming into a slotted projectionon the hook body. This provedvirtually impossible to release fromthe line, everything working per-fectly up until this point. As soon asthe releasing tension (applied tocompress the spring and bring thehook clear of the slotted stop) wasreleased, the hook snapped backkeeping the tow ring 6rmly trapped.Filing away the slop projectionGroup of ahr.G hoot.. At top rirht itElto.'. !n.ucc...ful fir.t .tt.mot whl<hw.. .up.rce.d.d br th. c.ntrc hook-.l.in . rari.tion on th. Rq..i.n hookth.mc. lt f..tu r.. . hovini l.t.t tor linclock.on, bst 1..k. fr.ili.r for .isht.r'on-li.e'ci..l.r th.nfr.c-rlld. cir.l... Syrt.mIt dfrwn in Firurc. 7. .nd 7b. At low6rl.ft it a furth.r d.v.loDm€^t trilor.d t6.uit 'Lc.dina L.drr Init.ll.tio. - l.t.rr.pl.c.d by rina-.nd-.prid8 d€vic. rhown

@- . . . , . " . , , ""

t ia2!:_,,..fr

Author ".l6plilicd" i!..i.n hook vrri..tion

- nott hook op.r.t.. in .onisnd.

ion with convlntion.l Aqtorudd.r.toD.tor alld. turn .dju.tm.nt,

FlSure tb -Details

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-\_...l-.jcoi# | ,/- |*l_ \4- :irt":*,. ,,.",

Further flying highlighted themajor disadvantage with Russianhooks as far as I was concerned.Everything worked well in very calm ,_,conditions: srraisht row. circular ; *:i .:"'kl" )tow (albeii in wide circlei) at will. :";':" jl::"'Jft J. c'followed by climbing catapulted,and smooth, easy release. With the

i \S-"\ \ -+e]

light spring load necessary toachieve thr's happy state of affairstrouble was immediatelv exoeriencedin the lightest of breezds; ai soon asthe hook comDressed due to theincreased line - tension unwantedrudder offset would come in on thestraight tow, and in some cases theIatch released unknowingly, allowingthe line to slip ofr as soon as thetension was reduced. Increasine the

i_/'

rute to combat this is anrs solution but this can only beso far beforc one encounters

problems with releasing the model incalm conditions-Arthor'. h.thod of providinS r.1....

improved the situation to the extentthat a release could occasionally beobtained, but it was obvious that themoving latch was a necessity, Hencehook No. 2 shown in the Frgrle 7,which illustrates the constructionaldetails. This had a spring loadedIatch and functioned exactly as theRussian hooks, with one exception;it lacked the facility to provideincreased rudder offset in the circulartow mode. The installation utilizedan existing fus€lage and retained theexisting rudder stop for both glideand circular tow - as mentionedpreviously, the circular tow facilitywas initially not considered to be ofgreat importance. It was some timebefore the folly of this approach wasrealised! Indeed initial results withthe new hook were sumciently en-couragrng to prompt a start on arather exotic model designed aroundsuch a hook and details ofthis modelLeadihg Lady will

^ppet l^ter ,

Attempts at circular towing withthese models soon rcvealed thedesimbility for increased rudderoffset io this mode, as I found thatfor a given ofls€t the circle with lineattached was apparently wider thanthe free gliding circle. Hence, onewas forced to covel an enomousamount oi ground if the tow circlewas to be followed, a necessity ifcontrol is to be maintained. Feasiblein flat calm on a flat site, but notentirely satisfaatory: flat calm condi.tions lreing rather rare.Author'! ..cond iu.ri.n-typc hook(c.ntr. hook ot photo on opporit. p.s€)inrt.ll.d in . h.c*'Llv.lr L.dr'fu..h8€,.howin! thc towlin. in thr.c dii.r.ntmod... At top i. rhown th. rtr.irht towD6ition. b.low th.t i. th. .i"Gol.r towmod.. .nd .t bottoh i. .hown th. ..1.r.€.Not. th.t th. timcr ha h.d to b. rc-oo.ltlon.d f u.th.. torw.rd.

Aero Modeller 786

\Ar f.r ri.ht."....n . coupl. of'.prina.nd

'itrr'hook .lTh.. on th. l.i! i. brcd on

.n.x.mDl. d.monitrr t .d by P.ac. Al ln l t t r .d. . or i r in. t .d by S..ndin.v i .n. . Onth. risht ir th. .sthor'. l.t.r, .i n plili.d, .d.ttion-a d..wn in Fitur. t. Aboy. ir rnoi... towhook, or morG co...ctly, towrina. F..hion.d tro6 dur.l pl.t.. lirhint lan..wlv. l .nd r lnr- l i t t .d to r t .nd. .d ' l lv . l r L.dy' fu. . l . t . - r . l . r r . dcvic. i . .hown

The Russians and Cze.hs hadobviously €ncountered these condi-tions, in fact many flyers employed asubsidary pennant, retained by thelatch. to indicatc if the latch hadbeen sprung inadvertently. Thiscondition is not always evident by'fe€l'alone, and is very embarassing ifundetected. as one can conturue totow with the lalch open, only lo havelhe line slip off the moment tensionis released. Presumably their sophi-sticated hooks, which enable thespring pre-load to be readily adjustedon the field, can alleviate the pro-blem, and this, combined withperhaps more gcnerally calm condi-lions make them a more practicalproposition than would appear thecase in Britain, Even so. IvanHordjsi states lhat the s€cret forsuccess is lo use the highest possiblespling loading consistent with releascteoulrements.

Gary Burroughs oi New Zealandalso produced a very well thoughtout valiation specifically designcd toovercome the problem of unwantedtum coming on during straight towin gusly conditions (F/"e Flight NewsAoril 197!). Here the rudder offs€tfoi the catipult tum was induced byrelease of the latch rather lhan bycompression of thc spring.

I did have intentions of producinga similarly functioDing hook to my'Rled plate' method for lhe LeadingLady model. However, before I gotaround to it, a coll€agu€, Ray Inke.,a machinist by profession, ploduceda beautifully engineered adaptationof BurroughsdcsiSnand incorporated

l||rtdl.tion of th..lrcul.r tor d.vi..lllcrtr.t.d on p.a. 788 (rnd to b. drrwnn.tt month), Fitt In .D.rtur. pr.vioq.lterovid.d tor Ru..hn.Vt. hook cxp.ri-m..tr-r.. .lio ..rld ol thr@ Dhoto.tn9h. o. p.a. 715.

i t in a model of similar ' layout andfeatures. His experiences were notsufficiently €ncouraginS to persuademe to proceed wilh my version!Themain problem was again the un-wanled rclease of the latch in gustyconditions, though this was a con-siderable improvement on the origi-nal system. Ray also found that tlrcdesired catapult turn with smoothrelease was not as easy to achi€ve aswith a convenlional Russiao hookdue to the activation of the rudderoffs€t occurring only after releaseof the latch.

Thus, so far, the hooks did notme€t the main rcquirements I wasseeking, in particular an absolutelypositive line lock-on facility, and myinterest in circular low considerablywaned until revived by Pet€rAllnutt'spreviously described revelation ofthe '-Rirg .rrd Sp.rhg' hook system.

I inunediately made up a springhook device based on Peter's andequipped a model with a simpleoffs€t ring attached via a swivel. Thefirst flight revealed an inunedialeproblem. Never having seen thcsystem in action I relied on the verbaldescription of th€ rcleas€ technique -

t

'Hold a yard of slack line antl tfteropplyhg re sio to the lhv releasethe slack su,lde ly'.'fhis I did and thehook, which on the ground appearsto perch so precariously in the ring,stayed firmly in place. Seeminglynothing I could do would release themodel and it had to b€ brought down,line still attached. Thc size and ra(eof spring was at first suspect and awhole range of springs were experimented with without success. Onlyknowlcdgp that others were using thcsystem prevented me abandoning thcidca. However, just as I was gettingvcry despo[dent, the 'secret' wasdiscovered by accidcnt wherl the lincsnagged on the winch. Thc resultant'ping'when i t sl ipped off the winchreleased the model. obviously 'sud-denly'meant verl suddenlyl I laterlearnt that the Amcricans had iaggedthe system'lhe Danish l*a g hook',obviorlsly very apt.

This lesson learned, I deviscd asystem for inducing this suddcnimpulse easily and without too muchwear and tear on the fingers. A smallthumb Dlate was altached to thewinch hlndle and thc lower part of

continued o page 788

't ss.z

CIISOULAR TOWcontinued tom page 786Th..uthor ' r l . t . r t . i r .u l . r tow d.v. lopm.nt. ut i l i r i .a rpr i t r t - .^d-r ina r .1. . .6. Yhirhook ov.rcom.. l id l t . t ioni o l .h. of , . . t hook.. .d unl ike th. Rutr i .n hook.. .n b.q.Gd in turtint conditlon. wl.hout utrw.nt.d G.r.Cslr turn cominr in und.r hirhaorliD. t.n.ion, or itr.dvcrt nt l.t.h r.l..G-.cG S.ot.mb.r i..u. for d...ilt.

the towline knotted at intervals, Thisenabled the necessary line slack tobe trappcd and held easily by thumbDressute and instantaneous releaseeasily achieved (see Figrre 8). lt wasat this time that a considerablesimplification of the spring hookconstruction was introduced. Thespring extension limil was nowsimply achieved by tying the towlineto either end of the spring with therequired slack passing through thespring, lhus doing away with thenecessity of the wire extension forthe stop - see Figrrre 9.

Subsequent flying with the offsettow ring system left some doubts asto whether it was the best approachto circular tow. Simple, undoubtedly;but it had. for me. three limitations.First; release by the helper could be

somewhat hairy and a potentialhazard as initially the model yawsrather violently. Secondly; whenflying in a fluctualing breeze I foundthe behaviour at the too of the linesomewhat eratic and uiroredictable.Last and perhaps most important,towing in a straight line with thcmodel at or near the top of lhe line,whilst not impossible, was notaltogether easy. This is a decidedlimitation as one often dcsires tomake off for some more desirablepatch ofsky, usually at high specd.

to be co tinuelltn p".t t ot iiilTiiii.lliipr. or er.or!unfo.tunrrely ..oD.d unch.cl.<|, Th...refefu.d b Fisur€ 2. whcre th. fo.mul.w. i t len R:Gd.3rhould hzver.rdR:Cd4,

ANDTAND.nd !pr in8 1. . ! th3 3hould h.v. re.d l in. , norl in. ADolot i . r for th. <onlu. ion c.ured,