Gluing Wood in Aircraft Manufacture

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MICROCOPY RESOLUTION TEST CH~RT MICROCOPY RESOLUTION TES1 CHART NATIONAL BUReAU or STANDenD,,196,-A NATIONAL BUREAU or SlANDARDS-1963-A ======~=~~~======~ TECHNICAL BULLIlTIN No. 205 ~ OcToBER,1930

UNITED STATES DEPARTMENT OF AGRICULTURE WASHINGTON, D. C.

GLUING WOOD IN AIRCRAFT MANUFACTURE

By T. R. ~I.'RU.\X,' Sellior IVootl Tcc1LIlO/oflist, Forest ]'roducts La·bora·tory; lJrmwh of Rescarch, l.t'orcst Scrvice

CONTENTS

Pagc Page III traduction••• ______I Oluing dilTerollt specic.~ nnd surfaccs of wood_ 34 Spccic.~ gluod••______•______(llues for nircrafL. _.' . ___ .•______2 34 Properties of nircraft glues ._ ._. ______3 Uccommelldations for gluing side·grnin Other lIloisturc·resistunt Ildhcsivcs__ w_ .. __ 4 surfnces of vnrious specie~Lk ____ "______35 Dumbility of lIircruft ~Iues __ . _..._....._ 6 'l'rcnting the wood bcfore gluing ______._ 38 Inerellsing tho durability of glued joints_ Q Oluing end·gmin surfaccs ______3S Testin~ ~lues for f1ulllity ______....______._ 10 Drying nnd conditioning glued stock ______39 'rests for cllsein gluc_ ...____ ..______10 'rests for blood·nlbumin glues ______Hecommendr.d practicc for the principal air 14 craft gluing operations ..______40 'rests for nnimal glue .._. __ ...______14 PropcHers ______l'repnrntion of glues for u"o in aircrnft ______41 15 I'lywood______42 l'repllring casein glue ______15 l'repnring nnimnlgluc. ______._._ Lnminnted spars, spar flanges, bow ends, 16 pontoon ribs, and rcinforcing blocks ___ 43 Prcparing blood-albumin gIUC>L ___ •___ •• Ji Prepllrution of wood for gluinS ______._ Senrf-type jOillt5 ______. ______45 Ji Win~ ribs______•______Moisturo contcnt ______. ______. __ _ 17 Box boollls- ___ •______•______46 J,uruber______.. ___ . k ______.. _ 19 (Jeneral assembly ______46 Hcratchcd lind tooth-planed joints _____ ._ 20 48 Shaped joints contrasted with plain Principles of cross· banded alld laminnted jOlllts______•______•______._ wood construction ______49 20 Cross· handed construction ______,-eneer______.. ____ .. _... ___ .. ___ w_ .. ____ .... _...... 20 49 Bnlanced construction ______50 Gluing technic /IS relnted to tho quality of I.aminated-wood construcLion ______gluo Joints ______•______._ ..__ ,.•••. ___ ._ 22 51 SlItisfnctory nnd unsatisfactory joint5 .._. 2'~ Olue formulas for use in aircrafL ______52 Spreading the gluc_____ ._. ______•____ _ 22 Cnsein glne formula No.lL ______52 Single nnd double sprending ___ •______23 Casein glue formula No, 4B ______54 quantity of glue 5prood. ______23 1IJood·nlbumin glue-hot press formula __ 54 Consistency of glue mi'ture____ ._. ____ _ 24 l'araformaldehyde-blood·albuminf ormuln ______•______g lu 0_ Elfectture ______of a.o;sembly time and tcmpera- •__ _ 54 Wnter·resistant animal glue formula ____ _ Pressing and cJamping_____ ..______24 Literature cited ______55 2i 56

INTRODUCTION The modem airplane requires from about 250 to 2,000 board feet or more of lumber and considerable quantities of veneer and plywood in its construction. It is estimated that 8.000 to 10:000 airplanes, chiefly of commercial type, were built in 1929, of which approxi mately 90 per cent had wing structures of wood. Even in the fast

1 A('kllowl!'d;.;nH'nt Is mn(ir to I'llrl()l1s nH'IlIhrrR of the For('st rrodl1cts Laboratory nlld of tht' War lIud Navy V('pnrtmpnts for nssl~tallce in the prl'Pllratioll of the mllllU'scrlpt and to H'vNal airt~l":Ift m:lllufncturl'rs (01' contrilmtl'd photo;.;raphs and inform:ltioll. 'Mllllltllhll'd h~' thl' U, S, l)I'pllrtlllent of AgL'icullure at Madison, Wis., in cooperation with the l'nil'ct'slty of Wisconsin. llOiH::!O-30--1 1 2 TEOIINlOAL BULLETIN !:!05, F. S. m~rT. OJ~ AGIUCULTUnE combat type of military plane woodcn 'wing structures are used. In !"ome plancs the entire wing fmnlcwork and covering is of glued wood construction. (PIs. 1 and 2.) In prope'Her's, control sllrfaces, fuselages, :wd the pontoons and huB" of seaplanes and flying boats wooel is also lIsed (l'l3, 98).a (Pls.:2 and 3.) A partial list of more detail parts, made wholly or partly of wood, includes: "'\Ying beams (solid and box types), ribs, leading and trailing edges, bow ends, braces, reiuforcing or shear blocks, and light com partments; aileron ribs, braces, and covering; rudder, stabilizer, and elevator parts; tunk compartments; propellers; center covers, stcp boards and walk ways; turtle decks; engine bearers; bulkheads; instrumcnt boards; floors; scats; cabin sil1es; fairings; doors; strut streamlining; anel lwels, ribs, partitions, braces, and covering of pontoons and hulls. (PIs. 4, 5, 6, und 7.) Glue is used not only in laminating and building up large and irregular wooden parts and in the making of plywood bllt it ulso affords the principal m('ans of fastening the various wooden parts together into the finished stl'l1ctul'e. .Joints. togetber with fast('n ings, nre, howC\'er, genemlly regarded as the weakest part of the builL-Up ('onstr·uction. Consequently they control the design in very lar~c part, despite the fact thut it is known thnt the teehnie of gluing as practiced in aiI'f'rnft is capable of considerable improvement ( "';;",0') .....0) Since nearly eyery article of g-hl(,d-wooL1 eonstruction represents un economy in the use of timber resources the Forest Products Lab oratory lUIS conducted studies upon /!Iues and ~luin/!; a large part of which has pertained to the use of glue in aircraft." The purpose of the present bulletin js to ~i\'e specific information about the gluing of wood that is directly applicable to aircraft.

GLUES FOR AIRCRAFT :Most uses of glues in aircraft require adhesives that retain a large propol·tion of their strength uncleI' moist conditions, even to the extent of remaining safe in service aiter exposure to free water. Of the various adhesives commonly llsed in woodworking, blood-albu min and casein ~lues most nearly meet these requirements.a This principle excludes Ht once, the vegetable glues made from starch, as well as fish /!lues and unh'eated animal glues, which, although admirably suited for interior use. lose their strength rapidly and almost completely when exposed for eYen short periods to a 111ois ture-satul'ated atmosphere or to free water. Only where the glued lIIemben:; ar(' thorollf!hly pl'oteeted again:::t moisture ellan/!es hy suit able coatin~s are any of the' l('s8 moisture-resistant /!llles useful in aircraft. In wooden propellers, high-/!rade animal glue has been nsecl su('('essfully. Any increase in rnoi~tllre content large enough

, Ttalic nl1111b<'I'S in P" r<'ntlH'~es I'f'f"r to LIt<'rlttllr" f'itNI. p. :'n. 'A ~('nc.l·al t11~Cus~ion of tlw lISC or glue in woodworking irlllll;;frf<,s i~ ;.:I\·en in The Gluln~ of 'Yood (::!_P. , Hescarch con!lm'h'l1 III cQop"rntloll with th(' War (nul Xn\'y Dppartmcnts and 1he XaUoual Advi"ory COllllllittl'(' (01' Al'I'onllutlc~. °Th!,s bulll'tllI Is (',,"lilll'd 10 a rJls(,lIssi

PROPERTIES OF AIRCRAFT GLUES The principal adhesives of proved value for making wood joints in aircrtd't are casein gIlleR, blood-albumin glues, and animal glues . ",1)('11 PI"OPl'rly protected llgainst large moisture changes. In Table 1 the general pl"Operties and characteristics of casein, blood-albumin, and animal glues used in aircraft are listed. On most of the points of comparison there is a lack of definite and specific knowledge, nlld as a result only general terminology can be used in describing thPll1. Furtht'rmol'c, there is lL wide vnriation nmong the glnes of the thn'c classes. Only the strongest and most durable glues of each class arl' desl"ribed.

TAIlL~~ 1.-]>rO[lcrlics Ulul oll11ract(ri.~f.ics of d);'ffercllt c/aS8C8 Of woodworking !lIIlIW Il,~('dill (I.ire!"u,l'

=ropcrt). or ('h~rn~~':~istiC 1... ___c_n5C_in_g.l_u_C___ Blood·albumin glue IAnimal glue (untreated) 1 .-.-----... ----1------Rtrcngth (dry) '.. .".... '""ry hi~h to medium ___ Medium to 10'". ______•• Ver~' high. Strt'nJ!th (W(lt nrt(lr soak· .A bon t 25 to !i0 per cell t Ahout 50 to nearly 100 ,"erylow. ing in watcr 48 hours). of dry stn~ngtb; varies per el'nt of dry wiLhgi(l('. strl'ngth. DUTllhility in 100 pl'r cent Dtlight to lDediunL ______Flight to pronounced .... FSllll!Iy slight. 'rend('ncy to stllin \\'00<1. • .I\llIrked with sOllle None, except thut dark None to very slight.. wo.lIls. glue lIlay show tp.!'cugh thin veneer. Dulling ('!Teet on tools ' __ ., :\[oderatc lo pronouDced. Slight ..______• l\[o,lerate. Sprcading--cu'l1ncity: s Extremes reported ' .• 30 to 80______•_____ 30 t.o 100...______20 to 55. Cornmoll rungo 4~"""~.f ali to 55______.______.. ,______... 25 to 35.

I Ornd~s nn'i quality only of glu~s that pass l,T, S. Govcrnment aircraft !pacifications. , Unsell ehi~Uy on joint strength tests. 3 Casein nmi nniIlllll Hlues 11m lik..l\, to deteriorate seriously if kept liquid 1Il0rc tban 1 day. • 13ased on reports from ('OIumerchil operators. • Expressed in square fect of single gluo line per pouDd of dry glun for venecr work. Apart from joint tests there is comparatively little specific in for mntion on the mechanical and physical properties of glues as mate rials of constrnction. Some data, however, are available on the ten sile strength of animal glues (2, 9, 10, 15, 1'7) and on thp relation between moistme content and relative humidity for animal and casein glues. In Figure 1 lS shown the tensile strength-moisture-content relation for an animal glue that. meets the specifications for aircrnft work. Small specimens of the glue at different moisture content were 4 TEOHNICAL BULI..ETIN 205, U. S. DEPT. OF AGRIOULTURE tested in tension. '.rhe decrease in stren~th with increase in moisture is striking. Curves shown in Figure 2 indicate the relation of rela tive humidity to moisture content of the same animal glue and of one type of a water-resistant casein glue. The casein glue absorbed an even larger percent ~~------~------age of moisture than the animal glue, but fl'om other tests wa::: found to ret It i n It greater proportion of its dry strenf,rth. 16 'Yater-resistant glues, particularly casein and blood albumin, set on wood at almost any moistme content, but animal glne will not set on 'wood Df high mois ture content, 15 per cent being about the maximum for satisfac tory results. Animal glue treated wit h formaldehyde yielding materials gives joints of about the same initial resistance in cold water as the water 4 resistant casein glues. However, tests indicate that it is les~ durable 2 under prolonged expo sure. Blood - albumin glues are less affected 16 ~4 3l 40 MOlsrv/?{ CONTENT (fII? C!IIi) 4~ by soaking in water than other water-resist FIGURE l.-'.rIw I'frl'ct of mo\sturf' cnnt~nt on tht' tf'n· ant o'lu s silo fltrem.'th ot Ull nnlmnl ~Iu ... that meets aircraft ' . '" e.. Rllecitlc:ltl"UH Form u I a s for the preparation of water resistant. casein, blood-albumin, and animal glues, which rate well in water resistance among the glues of their classes and which are suitable for use in aircraft, are given on p. 52.

OTHER MOISTURE-RESISTANT ADHESIVES Besides the glues that have been discussed there are other adhesives that compare ravombly in many respects with those now in use. An improvement in one 01' morc respects may make their usc in aircraft desirable. Some of these adhesives are discussed in the next three paragrapbs. GLUING WOOD IN AmCRAFT MANUFACTURE 5

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°O~--~/~O----~2~O----~3~O----~4~O----~5tO~--~G~O~--~"~O~---t,~--~~--~1~~ RELA TlVE HUMIPITY (!ER CENT)

FIGURE ::!.-Effpct of relntiv!' humitlity on moisture contPllt of nnlmnl anti casein glues suitable (or nlrcruft. 'J'eHts maue nt SU" .IJ'. 6 TF.cnNlCAL Bl'LLETIN ~o;:;, U. S. DEPT. OF A(JRICULTURE

VEGE'rAIlLR-I'ROTEIS (If.U1:::S Soybean and peannt meal serve as bases for adhesives; which in p-eneml propl'l'tips I'('};cmbl(' casein glucs. Some of them have II good degn>l' of watpl' resistanC'(> and are reluti vely cli(lap, but in their pt'es Nlt stage of de\'elopment they do not show so high a dry strength as the hdtl'1' quaI.ity casein glues.

l'HEXor.-.\I.IH:HYIlE COXDEXRATIOX PltODt;"CTR

By hcating It phenol with an aldehyde it is possible to produce hard, strong adhesin' subAances, which when set by hot pressing appear' to bp unafl'edcd by moisture in any form. Nonaqueous solu tions of the:.;e materials ha \'l' been prepared that can be applied to wood. Tests of plywood glued with these products indicate very Hatisfactol'y joint stl'cngths un<1l'1' all moisture conditions tried. Apparently phenol-aldeh~'<1e adlH'sives are more durable under pxtrl'rncly ullfanmtble ('OJl

Other mntel'ials, including cellulose cements, asphalts, reS1l1S, gUII1S, and 1"ubber have been tried as adhesi\'es for wood. Cellulose cements h:w(' sct \'ery slowly in wood joints in tests at the Forest Produets Laboratory and are otherwise difficult to use to obtain satisfactory adhesio·n. Likewise nsphalts, resins, and gums have been tried. out alone 01' in combination with other materiuls. but the I'esults havc not been entirel~T satisfactory. In their preseilt forms these materials are not suitable as glues for aircraft jomts, although they appear to have possibilities of development. Plywood hus been glued by the hot-pre:.;s process with un adhe;-.;ive having rubber as its basic Illatcrial. .Joint tests of such plywood ill both wet and dry con dition indicate satisfactory strength. High cost is lL present practi ('nI limitation affecting it:.; u:.;c.

DURABILITY OF AIRCRAFT GLUES None of the glues in practical use at present can be expected to fOl'm permanent joints in aircraft parts that are subject to pro longell saturation with water, such as seaplane hulls and floats, unless the glues or joints are especially treated. Even the most water-resistant blood-albumin or casein-glued joints, 'which show a strength when first saturated with water of 25 to nearly 100 per ('cnt of their dry strength, fail completely when exposed without protection for a long tinie to free water or to extremely high atmos pheric humidities. Failure in such cases is apparently caused by chemical decomposition of the glue or by its deterioration from the

7 'I.'he fOllowllll: patents r~Jate to so,'hpan and peanut-menl ndheslves: JOHl'SOl', O . •\llIlf:!'iln:. (I'. S. l'"t"nt No. 1,400.7ii7,1 P. S. Patent Office, Of!'. Gnz. 312: 132. l!):!!\; (He IG,42:!). U, S. !'ntcllt Office. OII. Gnz. :mo: :!,s9. 192G. LAUCKS, I. F., and IlA\·,DSOI'. G. \·r::m:T.\uLr: GLCB A~D l'HOI'F.SS (If' llAKIXG SAllE. (T'. S. Patents Nn~. UlSO,Ta:.! an<1 Ul!l1.G(J1.) fl. S. 1'aten t OtIice, Off. Gil?. ani: 1130. 1028. and 376: 4(JS. :w~s. OSGooll, G. II, m.m:. (U. S. Patents Xos. 1,G01.50G and 1,(jOl.507.) U. S. Patent Ofli,"" Off. GIIZ. :{iiO:Stll. l!l!W. OS(lOOD, (L II. \\,A"'rJIII'ItOCW'~" n:m:T.\nr.E PltOTEIN- U.\lHl or.Ur). (V. S. !'at"lIt No. l,70(J,GH.) U. S. 1'utl'ut OtIicc, orr. Gaz. aso: 875. 1929. GLUING WOO[J IN AUWUA:b"r l\IAN U.I!'AC'.rUHE 7 action of fungi and bacteria or pedutps both. Furthermore, unpro ti!cted water-resistant joints, which ure known to withstand n, limited number of soaking periods of several days each followed by drying out without seriously nffecting their strength, fail eventually if sub jected to :t loner series of large moisture changes by alternate wetting and drying. Under such cyclic conditions mechanical failure may be a factor in the bt'eakdown of the joints in addition to chemical decomposition or the action of fungi. Casein and blood-albumin glues, or even high-grade animal glues, do not deteriorate under conditions that produce about 12 per cent moisture content in wood, which :is close to the average founcl in a survey of aircraft stations. (Table 2.) 1.\..t such a moisture con

35 ~ , ~ J 30 II

5 I I I II I / , 0 I / , if' / / / 5 i.- /V ~ ~ I ~I ... *'~*' V- 0 I L-t-'. ,'(F.{l-' V· :~~ -'. I ~~~ 5 -, I .....,: ~t:l--i- r-r ~4-:. -I- 0 I o 10 n ~ ~ ~ w w w 90 100 RnA TIllE HUMIf)/TY I/{ ATMOSPHERE (PER cad) FIOI!IlE a.-Relntlon of the ('qulllhrium moisture content of wood to the relative humidity of the surrounding atmosphere at three temperatures tent, which is considered fairly representative, well-made joints of properly constructed members may reasonably be expected to remain permancnt with only ordinary protection. Glue failures, however, are reported to occur in parts of aircraft under unusually adverse senice conclitions, whcre the moisture content of the wood un doubtcclly exceeds the 12 pCI' ccnt avcrage considerably. Adequate data are not available for determining the exact critical moisture (~ontent at which casein or blood-albumin glues retain their strengths permancntly and above which deterioration sets in. The available data indicate, howe\,cl', that the critical range of moisture content is withi n 1:') to 20 per ('ent for wood. 'Wood subjected to relative humidities of approximately 7ii to 85 PCI' ('cnt and to a temperature of 70° F. takes 011. an equilibrium moisture content within this range. (Fig. 3. ) Under conditions where the wood retains 20 per cent or more moisture thcre is no positive assurance of the perman ence of glues without special trcatll11!nt. • 8 TECHNICAL BULLETIN 205, U. S. DEPT. OF AGRICULTUHE

l'50 "',-,~ ...,'" ~'" D .... ~ " '"~ '-. ~ " E t~ ~ "~- ~ ~ 0 ~" F ...,~IO

4 6 8 w aM. m • a TIME OFEXf'OSURE TIi 95 PER CENT RELtlTlV£ HUMIPITv(nONTHJ) FIGURE 4.-Ef'fcct of trpatmcnt of casein glues anll plywooll joints on llurnhillty: A, Uuln'utetl; B, plywl)oll spl'clml'us IlipPC11 In IIllphnlt pnint; C, v1ywnod sped nl('ns tl"l'nt(~(J with coal-tar Cl"l'osotL·; D, plywood spcl'inwns trcntc(i wllh fJ naphthol in IInsecd oil ; E, crcosotc :\(l,lcd to glue heforc milking jOints; F, fJ naphthol IIddcd to glue b~forc making joints; 0, Cl'c()sole milled to glue and Illywood specimen!; .. coatcd with aluminulll powder in spnr val'nlsh; H, fJ Dnll'hthol added to glue and plywood slJ(.'CiuMlIJ~ con ted wllh. uluminuJU I.·....wdel· in tliIlU' varnish. Joint strcngth tcsts JJlnde evcry JJlOllth 011 30 spccllucns or each treatmCllt Tech. Bul. 205, U. S. Dept. of Agriculture PLATE 1

SOME TYPES OF COMMERCIAL AIRCRAFT EMPLOYING WOOD

.\ I 12-pnSSN1!!l\f monoplane in whic'h the entire wing' structure, including the ('ovcrinJ!. is made of wood. \\"00(1 is also used in part in the cahin ('onstruction; H. 6'pnss('nger monoplane with fr:lIlll'\\'ork anti l('adin~ l'd~e of wing and "nrious parts of t.he ruseln!!c made of wood; C, 2-pas seuger seuplane with wing structure, main float nnu wing tip llouts cOllstructed oC wood. Tech. Bul. 205, U. S. Dept. of Agriculture PLATE 2

TYPICAL WOODWORKING DEPARTMENTS IN COMMERCIAL AIRCRAFT FACTORIES

.A, Building win!!~ ~nlifllly of wood (or n larJ,!c transport plnne. 'rhe Crume of the wing is cQ\*ered with plywood; II, using wood Cur turlle decks and Cuirings in Cuseluges. Tech. Bul. 205. U. S. Dept. of Agriculture PLATE 3

FLYING BOAT AND PROPELLER CONSTRUCTION .\, ("onstructing lIying boat hulls of wood; B. nlllnufnl'luring propellers of woo

WING BUILDING OPERATIONS A, )Iuking wooden wing stru",ures for hoth biplanes nnd monoplnnes; B, inspecting u wing struc lure before cO\'cringj C, co\'ering wings with cloth. GLUING WOOD IN AIRCRAPT MANUPACTURE 9

T.U1LE 2.-Moistltl"c content tou'llil in woodell- a,irplU'lIC parts 'undcr scrvicc co/utitiolls

Moisture content- Stations where Kinds of Speci· Maxi· Mini· Kind of constmction Service samples woods nlCns Average mum for mum for wcro tested tested for speci· anyone anyone taken 1 mens station station . tested' Bnd wood aud wood ------Number Number Number Per ~ent Per cent Per cent Solid nnd Inminated wood ' •.• Navy••.__ 10 10 419 12. 7 15.3 8.8 •__do __ •••. 6 4 35 13.8 17.8 9.1 ~~Ii~~~~i ilimizi,itcd-w-oo[1-3~~~ Army••... 10 4 371 11.5 14.0 9.3 10.3 11.3 8.4 PropelIer:i•••••••••...._.••... ._.do..••.. 9 ~------7b Plywood••••••••••.•' ••• ' •.•. ••.do•.•••• 7 -----_ ... _-- 39 13.9 16.7 11.7

1 Army nnd Navy stations nre considered separately. although they nre frequently located close to each other. , Gram!. averago for all stations where determinations were made; station averages were prorated on number of specimens tested. , Elcluslvo of propellers.

INCREASING THE DURABILiTY OF GLUED JOINTS

As a result of research work at the ,Forest Products Laboratory 8 special treatments of glues and joints have been discovered that materially increase the dura.bility of glued joints under severe expo sme. In Figure 4 are shown the results of a still incompleted series of prolonged exposure tests on casein-glued plywuod. The results shown are based on two casein glues-one made in accordance with Forest Products Laboratory formula 4B (p. 54) and the other a special commercial aircraft glue. Because of the similarity of the results obtained with the two glues the data were averaged. The special treatments were of three general kinds; namely, (1) addi tion of preservative materials to the gh~~s before making the joints; (2) preservative treatment of plywood that had been glued with untreated glues; and (3) treatment of glues before making joints, followed by the application to the glued joints of aluminum powder in spar varnish, which is an effective moisture-excluding coating. In similar tests, blood-albumin glue was treated with sodium chromate and made into plywood. 'l'en per cent of sodium chromate proved to be effective in increasing the durability of the blood albumin glue. Plywood, which had been made with untreated blood-albumin glue, was also treated either with () naphthol in linseed oil or with creosote. The treatment of the plywood speci mens both with the 6 naphthol in linseed oil and with the creosote :increased the durability of the joints. In one series of tests, plywood which had been glued with blood-albumin glue and treated with creosote after gluing, was exposed for four and one-half years in a moisture-saturated atmosphere without any appreciable loss in strength. In applying special treatments to aircraft joints it is preferable to treat, as far as possible, complete assemblies, such as wing sections

• Tests were conuucted In cooperution with the Nutional Auvlsory Committee for Aeronautics. 10 TECHNICAL BULLETIN 205, U. S. DEPT. OF AGRICULTURE and floats (pis. 5 and 1, A) before covering. It is recommended thut the assembled parts be dipped in a hot bath of boiled linseed oil conta.ining 25 per cent of () naphthol O or in coal-tar creosote. Better protection will be afforded by dipping the member first in a hot bath, about 200 0 F., aud then in a cold bath, allowing the member to remain about one-half hour in each. The linseed oil and 6 na.phthol mixture is mushy at urdinary room temperatures and the "cold bath" must be kept at about 1.50 0 to obtain proper absorption and to prevent the formation of a thick layer on the snrface of the wood. The weight of the member will be increased somewhat more with the increased temperature and with the time it remains in the baths, The weight added in a treating process will depend also upon the size and shape of the members. Small pieees having large surface areas in proportion to their volume, such as the specimens used in the tests on which Figure 4 are based, will have more weight added per unit volume than large, compact part~ . The foregoing treatments are distinctly superior to the present methods of dipping, spraying, or brushing with varnish or asphaltic materials. Applying the preservatiYe to the glue alone will increase the ·weight of thE; member only slightly and, although not so effective as treating the whole mem'JeL', it may give sufficient protection for certain aircraft parts exposed under average conditions of use.

TESTING GLUES FOR QUALITY Due vigilance should be exercised to see that glues for use· in air craft conform to standard specifications or requirements. Specifica tions for glues and glue materials are valuable both as a guaranty of quality and as a means of obtaining more uniform results in use. No attempt is made in this pUblication to describe acceptance tests in detail, since the published literature pertainiug to the subject is ample and readily available 10 ([35,11,6, !87).

TESTS FOR CASEIN GLUE Joint tests constitute at present the principal means for judging the value of! casein glues for use in aircra.£t. Considerable time is required to make joint tests. Moreover, they are difficult to perform accurately, and at best yield only indirect comparisons of glue proper ties. On the other hand, joint tests, when properly carried out, afford highly important information about the value of the glue in service and its adaptability to manufacturing conditions. Block shear and plywood tests for joints have been adopted as standard by the Army and Navy. Except for certain changes as to the form of specimen and shearing tool used in the block shear test and the method of measuring water resistance in the plywood test,

• The treatment here recommended is the same as used on the plywood specimens indi cated in Fignre 4. Thp. minimulll alllount of If naphthol, which will properly protect the glued members is not known. '0 Detailed information 011 U. S. Army and Navy acceptance tests are contained in United States Arm~·. Glue, Casein, Specif. No. 98-14020-D. r; p., illus, 1(120; United stutes Army, Glul', Hi

1~IGUltFl ;'.-S(l{'cirnen lind sh~llring tool uS('d at Forest Products Laboratory for hlot"k-shenr joint t~8t. Experience hus shown that it is nece~Il'·y to conform strictly to the details shown In order to obtain comparable results Experience has shown that the following points are important in making joint tests: nLOCK SHE--\R JOI;\"T TEST

(1) Select a wood (hard maple is satisfactor~'), which glues well, is high in density, of straight grain, alHl free from defects. Condition to a uniform moisture content of about 7 per cent. (2) Glue the joints large enough that four or more specimens (fig. 5) can be cut from each joint; also test two or more joints for each glue. (3) Surface the wood pieces smoothly allt.! to It uniform thickness imme diately before gluing. (4) Follow directions carefully in preparing glue for use. (5) Spread approximately 1% ounces of wet glue evenly to each square foot of joint and apply pressure ulliforll1l~' to the surfaces when the glue is at the proper consistency. (P. 24 for d~tail:,;.) (6) Allow the glue to s('t uncIpr pressure for 15 hours or more and then condition joints to a uniform moisture content of approximately the same as 12 TECHNICAL BULLETIN 305, U. S. DEPT. OF AGnrCULTURE b('fore gluing. (Sen'n dan' conditioning time at room temperature is usually suffidellt.) en Prepare speeimens of the type shown in Figure 5, and tcst on a testing machine cquipped with u shcaring tool as shown also in Figure 5. Apply the load to tilt' 8l1ccinl('n ,:t

PLYWOOD JOINT TEST (1) V;:e 11 I'tronj! wood (reI low IJii'eh is quitc- :;;atisfaetory). (2) Heh'ct veUeer that is straight grained, comparatively fr('e from checks, smooth, uni form in thicklleHS, un!! free from other vi;;ible defects. Results obtained from specimens with cross grain through the face plies and with IJadly checked cores do not represent the full strength of thc joint and should be disregarded in testing the gualit~- of glues. (3) Use, where possible, a standard construction. Three plies, each one-sixteenth of an inch thick, glued witl) the grain of the core at right angles to the faces, arc generally USt·d. (.,1) Condition all vcneer to approximately the same mois SPECIMEN fj ture content before gluing. It h, recommended that the mois FlGrRfl G.-Specimens Rm1 lITlps \l~l'd at Forest Products Labonltory for plywooll joint test. Ex ture content be not higher than perience "U6 ~hown that. It is necessnry to con 12 nor lower than 7 ver ccnt. form strictly to the d!'hlils shown in order to (5) PrelJllre glues for use in obtain comparable r('~llits Ilccordance with manufacturer's directions. (6) GIue the pl~'wood under Cllrefully controlled conditions. (P. 35 for details.) (7) Lea\'c Ihe pllnels umle,' vressure for Ilt least 15 hours and then cnnllition t.hem to aPlll'()ximlltt'lr the same moisture content that the veneer had before gluiug. ~l'hree

11 The nnrnge hre"king strpngth r(''1UlrN1 11)' the D. R. Nu,"y Dept. Spedf. No. 52G8 (l!l24) nnd till' U. !5. Army Specif. No. U8-14(1:!O-D (I!):!;;) is 2.400 pounds per square inch. If ttl!' test Is carl'CulIv performed ~en'rnl al':llIabl(' high-grad,' cascln glu('s give joint strengths con~ldl'I'nhly In "XCl'8S of 2.400. At the Forest l'roducts Laboratory 0. Duruber haye exc~'I)ued 2,800 pounus pcr slluare Inch. GLUING WUOD IN AII!OHAFT 1\tANU1'ACTUItE 13

(I:.!) 111 ('lise a :;IIl'cilll(,1l fails with a load per square inch less than the re quired uvel'age stren!:th 12 ulld the failure occurs 50 per cent or mOl'e in the wood, the specimen shuuld lJe llisregaruell in cumputing the average. Plywood nUl(le of different thicknesses of veneer will give different values when tested in the manner shown in Figure 6. This is because of the form of specimen, its tendency to bend, and the eccentric loading which occurs under test. The test values vary with the thicklless of both the face and core plies, and the order of magnitude

VAlL COG£~ it INCHj FA.CES Vt1lilAlfl£ .'/11 TH1(!fNESS 0" ,, , ,. ,r " J I" I " ,I , ," , " r.; io" ~ I i:: I"". ""< I .!,k ~ " ~ ..," 5~i ~ . ;4 ~ Q llJreO /:',;;'/' ., .-[[JrEf) WEt "'rltH JDA/OII' I.v """TER 4~JI~f./,fJ ~.... AU rACES ii I/~CHj COIil' '·ARIA~LE. IN THIC/fNEJJ ~ ,1 I f ( \ "~ \ \ ~ iO" ... •\ ~ \ '"'>. ..\ u."

~ ~ = - Pim;c:;= S'K:f{crH - (I'fJUf{()S - PC!> - SQUARe - !.vell) - - !.'Wl:RE 7.-ElT('ct at ply thickn('ss on str(.'ngth ,'alll('s obtained in plywood test of these variations is shown in Figure 7. It is important, therefore, that a standard constrnction be used in making tests on glnes. In addition to joint tests casein glues are sometimes tested to determine their working life. Highly water-resistant casein glues, when mixed ready for use, thicken after a time, usually within a few hours, to a jelly and become unusable. The 'working life is con sidered as the period in which the glue mixture remains in a satis

u u, S, Army Spt'Clf. No. !lB-140::!O-D (1!l::!(l) IIlld C. S, :-;'l\"y lJept. Sp"eif. No. G::!G8 (I!!::!") r('tluirc 1111 Il,'('rar;(' o( :!;;O pounds ,,,'r "

fach))"y consistency fot' spreading on wood. A requirement of fonr to five hOllrs' working' life is reasonable. It must be romell1berc(l, however, that some thickening will occur during this period. A glue that does not thicken and set e\yentually to It firm jelly is almost (~ertllin to be low in water resistance, even thouB"h the mixture is used while fresh (p. 3). Hence the working-life test is valuable not only as It measure of the time during which the glue remains in It satisfactory condition for use but as It rough check on its water resistunee. It seems possible that consistency and jelly strength tests in addition to determining' the working life of casein glues may eventually be used to measUl'e certain of their mechanical properties (4), TESTS }o'OR BLOOD-ALBUMIN GLUES The blood-albumin glues used in the manufacture of aircraft plywood are mixed by combining the various ingredients nt the time of use, The aircraft manufacturer is not concerned with the testing' of blood-albumin g'lucs directly since this elass of adhesives is used ehiefly in gluing plywood, most of whieh is made in plywood manu facturing plants. Joint tests are the prineipal mcans by which blood-albmnin glues are evnluated, The plywood test described 10r casein glues (p. 12) can abo be used for blood-albumin glues,

TESTS FOR ANIIIIAL GLUE The viscosity and jelly strength of specified concentrations of solutions, tested at defilllte temperatures, are at present the prill dpal criteria of quality in animal glues. For propel' execution theso tests require carefuL control of conditions, accurate measuring ap paratus, and a tmined observer. In general, the higher the vis eosity and jelly strength, considered jomtly, the better the strength :mcl general serviceability of the glue. Either one of the two tests without the other may Yield a fictitious indication for a given glue. The adoption of standard methods and equipment will greatly simplify the te!:iting of animal glues ancllnrgcly overcome the COIi fusion which has existed in the past. The National Association of Glue :Malluiacturers has worked out and adopted It uniform system of testing and grading glues based on the viscosity and jelly strength, which ~)romises to bring about a very desirable simplification (5). The Jelly point of an animal glue is the temperature at which 11 solution of the glue changes from a liquid to II jelly. It deter mines in large part the necessity for warming the wood and is thera fore of considerable importance in the shop. As an acceptance test, however, it is doubtful whether the jelly point reveals any service quality of the glue not indicated by Viscosity and jelly strength. No standard method for determining the jelly point has yet been agreed upon, but an approximation can be easily obtained by cooling It portion of glue solution and noting the temperature at which it ceases to flow. A number of other properties of animal glues are of practical im portance. Acidity or alkalinity, tendency to foam, odor and keep ing qllality, and amount of gl'ease :ll1cl other extraneolls materials present all determine to It gl'eater Ot· less extent the fitness of It glue for high-grade joint work as required in aircraft. The first three Gf,lHNG WOOD IN AIDCnAFT 1\[ANUFACTUUE 15 of these properties can be determined comparativ(:ly easily within thl' limits requireel of It glue for joint work and should be made lL consideration in selection. The presence of ~rease and other for eign materials in small amounts is not particularly objectionable, anel the exad percentages can be determineel only by quantitative analyses, which would hardly be justifh~c1 under all conditions. The amount of moisture present should be taken into consideration in the preparation of solutions for the viscosity and jelly strength tests.

PREPARATION OF GLUES FOR USE IN AIRCRAFT Clean, cold water should be usell with the glues that are undet: consideration in this' bulletin, anel the quantity of dry glue and water should be determined by weight rather than measure or gucss. The aim in mixing glues should be to produce a solution of the propel' consistency and ft'ee fronl air bubbles, foam, and lumps of lIndissol\'c(t rnatcrial. "Machine mixing normally produces a more thoroughly mixed glue than stirring by hanel.

PREPARING CASEIN GLUE Casein glues suitable for use in aircraft are of two kinds, those prepared in dry form ready to be added to water lwd those in which the variolls materials are combined at the time of mixing with water. The first are gencrally referred to as "prepared" glues and the second as " wet-mix" glues. Both kinds are mixed without heating and in the sall\e type of equipment. Various types of mixcrs hllye been used successfully, but the dough type (pI. 8, A and B), equipped with It mechanism for turning the paddle in a double rotary motion at two or three different speeds, has been quite gcnemlly used with excellent results. The. chief requisites of a mixer for casein ~lues are (1) thorouRh agitation, preferably with clilferent speecls of the paddle, and (2) a bowl that can be readily removed from the machine for ~leaning, made of some metal that will not cOl'l'ode rapidly from the action of alkali. In aircraft repair shops in which only small quantities of glue are needed the mixing may be clone by hand, but this is recommended only where machine stiLTing is not feasible. A small mixer of the type shown in 1">late 8, A and B may be procured, equipped with two sizes of mixing bowls in which quantities of wet glue varying from 2 to 20 pounds mny be .mixed successfully. 'Vhere hand stirring is necessary a paddle or stIff spatula should be used. Egg beaters may be used but unless operated at low speed they have a tendency to stir in air and thus produce a foamy mIxture. r.rhe conditions and pro cedure recommended in succeeding paragraphs for mechanical mixers should be approximated as nearly as possible in hand mixing. A survey of aircmft-gluing operations (22) has shown that various types of mixers are in use with varying results. Drill presses, to which are attached a small, homemade paddle or stirrer are used in a number of plants. These arc llsnally run at too high a speed and canse a foamy mixture. Furthermore, the stirrer does not agitate the mixture uniformly, and lumps are apt to form while the dry glue powdeL'is being added. Other I-speee\ mixers now in usc fre qucntly permit lumps to form or whip ail' into the mixture. For these rcasons the use of a 2-specclmixer, equipped with a stirrer large 16 TECHNICAL BULLETIN 205, U. S. DEPT. OF AGRICULTURE

cnough to agitate the wl~ole 11lixtur~ uniformly ~lt speed~ o~ 100 to 120 and 50 to 60 revolubons per mmute, respectlYely, WIll Improve t.he mixing in many aircraft factories. The correct proportion of glue and water varies with the particular brand of glue und somewhat with the type of joint. Manuflwturers llsuaUy recommend the proportions of glne and water, and these should be followed unless othCl' proportions are known to give 'better results. For most prepared casein glues a ratio of 1 part of glue to 2 parts of watcr (by weight) gh'es a proper consistency for side grain joints. For wet-mix glues the proportions of 'water and other ingredients are given in the formulas in the appendix. For gluing end-grain joints some nIdation in the glue-water ratio is necessary ns described on page 45. 'Vith prepared casein glues the dry powder is simply mixed thoroughly with the water and stirred until it has dissolved. The watel' should fil'st be placed in the bowl of the mixer and the glue sprinkled or sifted in slowly with the paddle in rapid motion. Care should be used that large lumps do not form. After the dry glue hus ull been added the motion of the paddle should be slowed down, but the stirring sholll!l be continued (usually 20 to 30 minutes) until a smooth m ixtu!'e of e'-en consistency results. Variations in pro cedure are advisable for certain prepared casein glues. In such glues the glue-wat-er p!'oportions and other details are usually recom mended by the manufacturer. ",Vet-mix cllsein glues 11I'e prepared according to formulas by the addition of the separate ingredients at the time of mixing. Detailed dil'ections for mixing casein glues suitable for aircraft work are given on pages 52 and 54. The prepared glues am convenient and easy to mix, while the wet mix glues require somewhat more attention and skill in mixing. ",Yith either gIlle, howen'!', a procedlll'e can easily be acquired for mixing the va!'iolls ingredients 'which the average man can follow without difficulty. .. Most water-resistant casein glues become noticeably thicker on standing at room temperatures, but in such cases the quality of joints apparently is unaffected so long as the glues can be applied satis fllctor'ily. On the other hand, casein glues that do not set to a jelly llIltil a long time after mixing frequently become thinner; such thinning action usually indicates a deterioration of the glue, which may render its use unsafe. In general, mixtures that remain liquid for a long time do not produce joints of high water resistance, such as required in ai!'craft, even though the mixture is used while fresh. Mixers, spreaders, and other equipment usee1 with casein glues should be thoroughly cleane(l at regular intervals to prevent deteri oration of the glue and the inclusion of pieces of dry gIns in the freshly prepared mixture. A thorough cleaning every working day, before the glue hardens, is highly desirable.

PREPARING ANIMAL. GLUE

An animal glue which meets aircraft specifications 13 requires for each pound of dry glue about 21,4 pounds of water for stundard side

'" S(lPl'lIkntion No. ::-1-10 (1!)::!jJ of tllP U. S. Army and No. il2G4C (1925) of the U. S. Nnvy {:ave!' 11111111:11 gllll~ fur lise ill alrcrnft. :r~ CJ ~

~ :r f" f e.. > ~ ~. ~

"U r

~ 111 tn

THE STRUCTURE OF A HALF MONOPLANE WING

This part of tho wing:, CX('l'pt the a i1cl'CJII. is 1I1:1dl~of wood. 'Its l'ollstl'lH'tioli n.'qllln\s lilt' lIIukin!! of tIloll' thun l,tJIJIl j!hH,'d joints. Knw tl1l' 1I1:1Jlysrnnll piN'CS

tiC sprlJl't! nlill tho use of plywood for the sidl'S of lhe gas ('(Jlllpal'llIwnt, liw shit's of 1110 !Jox lIl'allls, lhe Illuding: ~dgt~,the welJ;j of lhe ribs, nnd Ihe g\1:-;~(\IS. Tech. Bill. 205. U. S. D.pt. of Agriclliture PLATE 6

•

WING STRUCTURE MADE LARGELY OF WOOD

",,\, wl (;HS t:luk t'OInpartuWlIl, IIJ! Sitll\S of box ill'lIl11S. (rJ ('olllpression rih weh~. nnd Id) ntlmrrous !!ll:..~N~ luadp nf piy"O(ul. In ('1I]J slrip~, \f) rih cHn~()nn\s, IUt ~I'nr ntll1~l'S, und Ih) }1!"Un's nUHll' of 'nlld spnl(-e; II, pi; II nod USc (itll (Il) ailerons, tI') spnrs, lllld te) ribs. (d) Bul\' cud laminated (rom sc\·cnll I'IC(''CS ur sprul'c. Tech. Bul. 205. U. S. Dopt. 01 Agriculture PLATE 7

WOOD IN PONTOON. FUSELAGE. AND AMPHIBIAN CONSTRUCTION

A. PonLOon h':lIJII~S mnde Inr~l'l,r of \\'0'.1<1. The ('urn't1 rihs nrc of thin laminnted <'onstruction; D, wood IS uSl'd 011 lilt:' (mtlle of (mi('lnl!('~" us n haSt' to whieh Is ntttlc'hed the co\"crin~ and various uC'('es sorh's. ~tloaI5, floors, in.:-:trulllt.mt hennis, hulk hC'uds. nud other parts nrc niso made largely of plywood; (', wood Ilud plywooli used in the construction oC un amphibian hull. Tech. Bul. 205. U. S. Dept. of Agrkulture PLATE 8

SATISFACTORY MIXING EQUIPMENT FOR CASEIN AND ANIMAL GLUES

.A, 2-spe('d, 5-qllnrt size, et(1(·trie~ {,:lsein glut' mix(lr in lI!'r in nn oir('rnfl, fIle-tory. The dry glue and W:ltl'( fUll \\lliglwtl 011 the seale nnt! t Ilf~ mixl'd ~ltw is distrihuted to the workmen in the sm(lll pal'llflinNI pall('r ('tips: B. 3-spt1(l{1 dOllJ!h-tYpp, c){'l'lrie mixer, equipped with :J and 8 flunrt howls nntl two Si71'S III pn,hlh~s Itn mhinll ",~"in ~Iue; C, two types of electric glue pots \\ iLh nutoltlullC tcmpernture l'lJutrol for uniIllui gillo. GLU[NG WOOf) IN AlltCRAl!'T 1\IANUl!'ACTUIUl 17 grain ~lliing. This pl'oportion may be varied to 1 part glue to 2 parts water whel'e a mixture of a heavier consistency is required, as 011 end-~raill surfaces or in quick pressing operations and on warm wood. On the othCL' hand, a mixture of 1 to 3 lIlay be used for sizing pUJ'poses. The following J'ecolllmelHlatiolls al'e made for the preparation and handling of animal glne for aircraft work: (1) Soak glue in clean, cold water until aU particles al'e softenedY Mix thoroughly the glue and water whell combining so as to insure uniform absorption; (2) mdt glue in It heater i'J'ollL which the solution may be removed as soon as melted or in which the temperature can be accurately contl'Olle([ (pi. 8, C); (3) keep the melted glue at a temperature between 140 0 and 1500 F.; (4) lise the glue us quickly as possible afteL' melting. Olue that has been heated for a total time of four holl"- should be diseanle(l ;1r. (5) clean thoroughly all glue rccei_'les at least once during each working day.

PREPARING BLOOD-ALBUMIN GLUES

l3lood-albumin ~Iues are prcparcd by combining the several, in gredients at the time of IIlIXlllg. Dried soluble blood albumin is gOlll'raily used. The proportions of water and other materials added to the (lI'jcd blood albumin vary ~reatly with different formulas. Two blood-glue forlllulas developed at the Forest Products Labo ratol'y, with detailed dircctions for mixing, are given on page 54. In geneml blood glllcs, used commercially, are prepared in ac cordullce with secret formulas and processes. ~Iost formulas are rcportcd to have originated in European and Asiatic countries where blood glues have been used for a longer time and more extensively than in this country. The mi:-..i;ures of most value for aircraft are of It viscous, or 110ncoagulated type, and have a comparatively low water content. The wodcing life of blood glues is extremely variable, depending chiefly upon the proportions of alkali and water added to the dried albumin. Glucs can casily be mixed that have a working life of several hout's to a day Ol' more. As with other glues made from animal material, pl'cC'autions should be taken to prevent deteriora tion from bacteria and other organisms of decay. PREPARATION of WOOD FOR GLUING

MOISTURE CONTENT 'Yood for gluing shou1r1 have the proper moisture content, uni formly c1istribuh,d throughout, and should be free of casehardening amI othel' strcs!:;c'> (31).

11 T)le tIm!' rf'qulrl'd to sortf'n the g'lllc thoroug'hly may vary from one to several bours. On!' hour IH Hulllcil'nt time for a ground glue that will pass an 8-mesb screen. Longer erlorlS are rf'qull'l',1 for eoarsI'ly ground alld OakI' glul's. Glue wblcb Is soaked tor several r.lOurs should lit' placed In It refrIgerator at Il temperature of allout 4()'0 t, GO° F. to prevent d.'t"rlorntloll. ,. Hatclws nE IIIlxl'

Any considerable change of moisture content in wood nfter glu :ing uSllally develops stresses in the glue and reduces the load that the nl(~mber will withstand in service. The ideal moistme relation to be aimed at :in gluing for maximum joiut strength, therefoL'e, is that the moisture content of the wood at the time of gluing plus the moisture added by the glue should be equal to the avemge moisture content that the member will have in service. In Table 2 ure shown the llverage maximum llnd minilllllllll11oisture-content values of wood in service in airplane parts, as deterl11ined at vllrious aircraft stations throughout the United States. 'ruble 3 shows the approximate per centages of moisture added to wood by the glue in certain types of construction. The effect of a different 1l10istl1l'e content of the wood, before and after gluing, on the joint strength of casein-glue plywood is illus tl'lltecl in Figure 8. The plywood for the wet tests wus soaked in cold water for 4B homs lind tested in the saturated condition. The dry tests were made at a moisture content of 10 to 12 per cent.

TAJlUJ 3.-l'crccntauc.~ Of moi,~tllrc (ldtlce~ to 'Wooel In ullli.ng'

~ Species IInti thickness (If- ~ :§ Construc .~ ~I------;------;------;------I~ ~ ~§ tion ~~ -;;~ f~ Fllce.q Crossbands Core I,llminlltions >. .:=:e.BS~ .!a.Q Z ~ .iVo. In. P.ct. 3 \4s-inch, yellow ______H.-inch, yallow ______Hu 48.8 birch. birch. 3 ~~2-inch, yellow ______•______%"-inch, yellow ______73' 32.5 birch. birch_ 3 _____ do______.. _ ).itl-inch, yellow ______Js 29.6 ______Ho-inch,poplar. lIondu- ______Ho-inch, Hondu %6 19.6 run nm,l\ogn.ny. tim IIlahogllny. 5 }'H-inch, yellow ~~o-inch. yellow Ho-inch, yellow ______hirch. _____poplar.do ______poplar_do______5 }1o-inch, yellow l4 30.9 hlrch. ).i.-inch, yellow ).i.-inch, yellow ).i.-inch, yellow ______% 21.2 23.1 IX7 ______:::~~3t:::::::::: :::~);~~_~~:::::::::: :::~;f~~{:·:::::::::: :::::::::::::::::::}1i-inch, Sitka l~~% 23.4 17_3 Lilminllted_ 10 ______..______• %-inch,Sl)ruce. white oak 7H 2 _____ •______I-inch, white IIsh_ 2 1.7 _8

1 Percentages calculatcd from average weights (oven clry based on volume when air dry) of various species us given in '1'1111103 of Report No. 8·l of the Nutionul Advisory Committee for Aeronauties (7) or 'rahle 2 o( U_ S. Department o( Agriculture Dulletin556 (19). In the calCUlations it is n,,,,uIIlod that all the sur plus moisture added by the gluo is ahsorbed hy the wood. This assumption is known to be somewhat in error, but it nevertheless alfords a fairly ~atisfactory basis for comparison. , A glue mixture of I part dry glue and 2 pnrts water and a spread of 40 square feet of single gluo line pet pound 01 dry glue is IlSsumed in this calculation. 3 Core consists of 3 plies. , Outor 12 plios all of some species and thicknoss. From these data it might seem that the proper moisture content of wooll at the time of gluing is an extremely v!triable quantity, depending upon the high 01' low moisture content of the finished article in service and the exact thickness !tnd other characteristics of stock being glued. In general practice, however, adjustments can not be made for all such widely varying :factors. Experience has shown that It moisture content of 5 to 10 per cent is satisfactory for GT,UING WOOD IN AIROllAFT MANUFAOTURE 19 veneer and thin Ituninations that are to be glued into plywood and larninated construction, respectively, provided all plies or lamina tions arc at approximately the same moisture content within this range. For laminations one-half inch thick and thicker, a moisture content of 10 to 12 per cent is recommended, except for propellers. Propellers are commonly coated with an effective moisture-resistant finish, so that a moisture content of about 8 per cent before gluing is recommended. ",Vood for aircraft seldom must be conditioned to a moistm'e content as low as 5 per cent or higher than 12 per cent. Under average ail'craft-manufacturing conditions t1 rt1nge of mois ture content from 7 to 12 per cent is the most easily obtained and maintained and will ordinarily prove satisfactory for most veneer and lumber. The moisture content of wood that has been properly dried for gluing mny change in stor'age or in the factory during malLufacture,

/00o~I-'--1---L-5~1-'L-l---L-/f;;o-L-L-L-L-!'/5,-L-L-L-L..J NO/JTVRe CONTeNT Or VENEER 8Ho.~E eWINC V'[R C[ld) FrOl.1ltN S.-Etl'cct of moisture content of veneer upon the strength of plywooli pUllels depending upon the relative humidity of the surrounding atmos phere. In the manufacture of certnin aircraft parts it may be nec essary to control the relative humidity in the storage room or factory to ob'tain the best results, The relation of relative humidity to the moisture content of 1',ood is shown in Figure 3. ,

LUMRER

CONDITIONING Lumber that has been dried to the approximate average moisture content desired may still show differences between varIOUS boards and between the center and the outside of the same piece. A varia tion of 1 per cent in moisture content may be expected between boards of the same species even when they are of uniform dimen sions and are exposed continuously under the same conditions of temperature and humidity. Larger differences may be adjusted by allowing the lumber to condition under suitable atmospheric tem perature and humidity until the moisture content of each board comes to equilibrium with the moisture in the surrounding air. 20 TECHNICAL BULLETIN 205, U. S. DEPT. OF AGRICULTURE

(Fig. :3.) Tho thickness of boards, kind of wood, and moisture dif ferences present all affect the time reql1ircd for toe lum1::er to como to equilibrium. MACHINING Machining lumber before it is to be glued should accomplish three important objects: (1) Elmination of defects, (2) reduction to the proper dimensions, and (3) preparation of the contact surfuees for the glue. The stock should be conditioned to the desire(l mois ture content prior to the machining. It is best to machine the stock just before gluing, so that the surfaces will not become distorted from atmospheric moisture changes. The surfaces for gluing should be smooth and true. Plies for cross-banded or laminated construction (fig. 9, A to G) should be of uniform thickness. A small variation in thickness in each piece may cause a difference of serious proportions, as, for example, when a number of similar pieces are piled in the same order as they come frol11 a planer. Planer marks, chipped or loosened grain, and other surface irregularities are objectionable. . SCRATCHED AND TOOTH-PLANED JOINTS Scratching, tooth planing, or sanding the surfaces to be glued is still practiced by many operators, who either believe that roughen ing and tearing up the surface fibers gives the glue a better chance to adhere to the wood or that the irregularities are cut away and flatter pieces, more uniform in thickness, are obtained. Investiga tions into the penetration of glue into wood and tests of the strength of well-made Joints show no benefit from such practices (934).

SHAPED JOINTS CONTRASTED WITH PLAIN JOINTS The :fact that the tongue-and-groove and other shaped joints present a larger gluing suI"face tlll111 the plain joints is often con sidered a theoretical advantage. In practice, however, this advan tage is often lost entirely or in part. Shaped joints are more diffi cult to machine than plain straight edges so as to obtain a perfect fit of the parts. Lack of contact may make the effective holding area smaller. in the shaped joint than on a flat surface, and this may actually reduce the strength. Furthermore, if proper condi tions are maintained, plane tangential or radial surfaces (fig. 9, A to, G) of most woods can be glued so as to develop the full strength of the wood, and the extra contact surface therefore becomes super fluous so fur as strength is concerned. In edge-joint gluing, however, the tongue and groove is an ad vantage in that the pieces are held in alignment during the assem bling process and there is less slipping of the parts when pressure is a:pplied. This makes possible more rapid clampil1g in many op eratIOns. When end-grain surfaces have to be joined, as illustrated in Figure 9, H and I, cutting to sharp angles and the· use of reinforc ing members are usually necessary.

VENEER Veneer is cut by three different methods-sawing. slicing, and rotary cutting. By far the largest part of all veneer'is cut by the rotary process beeause it is eheaper :tnd involves less waste in manu GTJUING WOOD IN AInCnAFT }\[ANUFACTURE 21 facture. Both rotary-cut and sliced veneers are checked somewhat on the side of the sheet next to the bolt or flitch (pI. 9, B) during the manufacturing process whereas sawed veneer is more uniform

A [ J I c

~ .. - I FIGURE n.-Types of joints: A. Plywood (ull Yen!'er) ; n. plywood (lulIlll<'r core) ; C. plywood (IulIllJer or vl'ncel' core) ; D. llent plywood (ull veneer) ; E. curved IlLlIli lllLtl'd construcnon; F. Illmlnntpd propeller hull; G. section of lnmlnated beam; H. serrate or finger joint; I, sClLrf joInt in properties on the two sides of the sheet. The amount of checking varies with (1) the kind of wood, (2) the thickness of the veneer cut, (3) the preparation of the bolt or flitch for cutting, and (4) the efliciency of the machine-cutting operation. 'Woods with small pores, such as birch, gum, and poplar, are easier to cut without 22 TEcnXICAL BULLETIN 205, U. S. DEPT. OF AGRICULTURE serious checking than woods with large cells, such as oak anu ash. Thick stoek is more liable to check in cutting than are thin sheets. Proper heat treatment of the bolt and flitches is important for best results in both rotary cutting and sI.icing. Them appeal'S to be 110 reason why plywood for aircraft should not be tnade from well-cut veneer made by any of the three methods. Complll':ltivc strength and warping tests, whieh were made on ply wood munufadured from well-eut veneer by the thn'e processes and from the Sllme logs, showed no consistent difl'erences in properties. On the other hand, badly checked "enE'er when USN) as cores in a-ply panels (pI. 9, C) is known to n'duee the joint st!'ength in the plywood and should therefore be rejeeied for aircraft work. The following charaeiel'isticH are important in selecting veneer for llSt' in aircraft: (1) rnifonnity of thi('kness in the sallie piece; (2) SI1100thlleHS and i1atneHS; (3) freedom from large c1weks or other defeds (pI. tl, B); and (-!) straight grain alld nbsel1ee of decay (pl.!),D).

GLUING TECHNIC AS RELAT.ED TO THE QUALITY OF GLUE JOINTS

SATISFACTORY AND t:NSATISFACTORY JOINTS The conditions Im'oh'ed in the gluing operation mllst be controlled so as to produec It, eontinuous film of solid glue ill the joint with Ildequate adhesion to both pieces of wood. These conditions invol\'e II, sufficient slH'ettd of the. glue on the wood surfaces and a correct balanee between the consisteney of the glue at the time of pressing and the amount of pressure used. A satisfactory joint and two com: mon types of unsatisfadory joints, resulting from different gluing ('onditions, are shown in Plate. 10. The three joints were all made from the same kind and quality of glue and wood, prepa.red in the same way. The" starved" JOInt (pI. 10, B) illustrates the result of one extreme, where the glue is too thin, and the "chilled" or " dried II joint (pI. 10, C) the opposite, where the glue is too thick, for the amounts of pressure used. A correct balance of the con sisteney of the glue and the amount of pressure used gll\'e the good joint. (PI. 10, A.) The starved and chilled joints are most com monly pro

SINGLE AND JlOODLE SPREADING In most commercial gluing operations the glue is spread on but. one of the two contact faces of the pieces being glued (single spread ing). At times, however, both contact faces are coated if the great est precaution is being taken to insure good joints. In order to determine the compal"ative yalue of the two methods a series of tests was made on plywood, a part of which was single spread with casein ~llIc and the other double spread. The results of the tests, given in Table 4, indicate that the increase in strength by double spreading is slight.

'l'AIIL~1 .J.-('OIllI)(l1"i~on of results of t(wt.~ on single a·/ul aoublc sprca-ding in gll/.;'II!!

A\'crllge' Joint strength ohtained when .Method or sprellding Tested Tested ary wet

Lb•• pcr Lb•. l,er 3q. in. .q. in. Hingle...... 369 22.S UOllhlo...... 3.. 235 ~------.-----~ .------~----~----- , Euch test "Iulle Is un uvorage or ut least .5 specimens. A more extensive test was made on thiek laminations under a widel' range of gluing conditions. The results again showed that good joints can bc~ made by either method and that no great dif ferenee in strength of joints ordinarily occurs, whether one or both eontaet faces are coatNI, if other conditions are satisfactory. How ever. undel' adverse conditions of gluing, as when the glue became 'o'er." thick before pressing, double spreading was found to be more reliable. QUANTITY OF GLUE SPREAD

'Within c('rtain limits the stren~th of the glue joint increases with tlw qlllllltity of g-Iuc spread. ThIS is illustrate~l in Figure 10. The datu al'C from plywood glued with casein glue under good gluing conditions. In other tests it was found that with less favorable '. gluing eonditions the quantity of g-lue spread affected the joint strength to 11 still grcater extent. The same general relationship holds fOL' other, glues, although the optimum amount of spread and rate of chang-e III strength vary somewhat.. A spread 6f 114 ounees of wet glue per sqaarefoot of single glue line (about 38 square feet pCI' pound of dry glue mixed 1 part glue to 2 parts water) is satisfactory for most conditions and kinds of gluing. A larger or smaller quantity may be used advantageously uncleL' CCI-tain conditions. 'Yith casein and blood·albumin glues a spread of more than 11/.1, ounces appa.rently neither lowers nor in cl'eases the joint sh-en~rth materially, whereas a smaller quantity nor mally produC'cs inferior joints. Tests have shown tha.t animal glue has It wjder range of satisfactory spread for maximum joint strength, the umount to use being determined by the gluing conditions; good 24 TIWIINICAL BULLETIN 205, U. S. DEPT. OF AGlUCULTURE joints may be obtained with spreads of 1 to 1%, ounces per squal'e foot if other conditions are adjusted accordingly. A. hl.l·ger quantity of glue is required when both contact surfaces are coated and when conditions arc such that the glue dries considerably on the wood before pressing. 'Varm wood, a long assembly time, and exposure to the air arc factors that bring about quick drying of the glue.

CONSISTENCY OF GLUE MIXTURE The consistency of the glue at the time of pressing is perhaps the most important factor in making satisfactory joints. However, the consistency of ....lue after beincr spread on the wood is extremely variable, (lepencfing upon such tlctors as the kind of glue, glue-water Ilroportion of the mixture, quantity of glue spread, moisture content of the wood, temperature of the glue, room, and wood, the time elaps 5ro ing between spreading and pressing, and the extent to which the reSTEO ORr . . • glne - coated sudaces • • are exposed to the air. The kind of glue and the moisture content '!SUO WCT . .. of the wood should be predetermined by con ditions of service. The other factors may be varied somewhat to o. 47 a~ 03 /.0 /./ U /J ,4 /.5 t6 " sui t the particular OWN"''' ()1',WE SPI1E~O {OtiNCES PU sauAI1E nor 0, CIVE liNE SPKoq gluing operation. FIGrll.: 10.-RpJlltion lK'tween qUllntlty of J!lul' spread and It is very difficult plywood joint stren!:th. 're~ts lIl!,dl' on 3·ply panels with ,'.-Inch birch fllces and ,'.-lOch r('d !':UlIl corps to measure the viscos gluctl with clIscln glut'. Joining pre6sure. 200 pounds ill'r squaT!' Inch; assembly time, 3 to I:! minutes ity of a glue mixture after it is spread on wood surfaces, but it is possible to judge of the consistency in an empirical way by touching the glue layer with the fingers. An animal glue, for example, should be thick enough to form short, thick strings, but not too thick to take an imprint or depression I·eadily. Between the two conditions thus defined good results are produced with a moderate amount of pressure. A.t the time of press ing a glue of the proper consistency will flow sufficiently to show a distinct line of glue at the joint edge. (PI. 10, A.) The absence of a line of ~rlue at the joint indicates a chilled or dried joint. (PI. 10, C.) If, on the other hand, the glue flows out and down over the edges of the piece!; excessively it indicates the likelihood of a starved joint. (PI. 10, B.)

EFFECT OF ASSEMBLY TIME AND TEMPERATURE "There pieces of wood are coated and expose(l freely to t.he air a much more rapid change in consistency of the glue occllrs than where the picees arc laid togl·ther as soon us the spreading (single or double) has been dOlle. The condition of free exposure is con yenielbltlly.referred to as "open assembly," and the other as "closed assem y." Tech. Dul. 205. U. S. Dept. 01 Agriculture PLATE 9 - "

\. A

,. .. ~ j 0 li' ~ \ ,'. .. ,,~,

F " " " ..~\ ,. > , r J .... '. ! ~"", 4' ".J' ;

fl .... "I Ii.

QUALITY OF VENEER AFFECTS STRENGTH OF PI_YWOOD

.\, ~tt'nn~ joints j.:hwd from stt'!li!!ht-g:nutlt~d Wt'Il-('ut ,'f,'n(,t,'r; H, '"('I1t'l:'f hndl)" c'hee,ked in (luttin!;, ~wt 5Ullahlt' for mn'mft p1r" ood; l" ply\\ood ti.'~l ~P(~dJJh.·r1~, with ('OfP of ('!ll'('kNI y(1n(l(~r Shown l!l H, f:1I1C'd hy hrl';tkin~ froUl dWt'k to ('hl'I'k awl !!:1\'l' lnw $ll'l'lIJ,!lh \'uhw:-;; 11. {·rn~$·J!mirl('d \'l'Ut'pr, whh'h shunltl hl~ rt'jl,(·tl'ci fur Hin'raft US(': E. hrokl'll plywooil r-:Pt'l'iIT1l'I1S wil h II cross graHwd fm'l' ply slIllIiar to lJ~ Thu ('ross-gnuucd brt'tlks n.~.sulll'd ill low slreuglh nlJucs. Tech. Bul. 205. U. S. I)ept. of Agriculture PLATE 10

STRONG AND WEAK JOINTS RESULTING FROM DIFFERENT GLUING CON DITIONS

...\. \\"('IHdw'd joiul. ttl'lti£' with H prn)lt.'r !"Illation h<'tWC(IJ1 pnlSSur(l Hnd ('()nsist~'n('Y of ~Iue; H. !'tanl~d Joml. n',~llla'«l from thl' nppltl'ntloT1 of pn'~,url' \\ hUe till' ~hJll wus too thin. O{'('llrS (re· fttwnt1~ wah Hntlllat nnd ntlll'r thin glm' mh:tun';.; on c't'rtniu \\·ood ... ; C. dlml~d joint. glue {'hilled l\\:t"l·:.::.....n(lly and :1I1101lnl o( pn.'s,.'nln\ w~t.; IUsutlh·illnt. to hring ahuut t'olllplelc ('oulnl't. Glue that tifll'S (\\('t'!ISI\loi.) bdol'l' pre.. ..;:ilI1g ~I\e$ the SUIIlC rt'SUlt4 GLUING WOOD IN AmCRAFT MANUFACTURE 25

The effect of the time element on the strength of casein-glue joints ullder closed-assembly conditions is illustrated in l!"igure II. The results shown are fot, eight groups of plywood panels spread with gille in succession llnd pressed together in the same press, giviJ'g different assembly times, The assembly time was shortest TO!' the last panels spI'ead. The consistency of the gb.;~ at the shortest assembly pcriods was too thin and at the longest periods }uld become too thick to give the best joint strength at the pressure used, In other tests the use of somewhat lighter pressure at early pel'io(ls and :t heaviel' pressure at luteI' periods was found to give more 11ni fOl'm joi nt stl'ellgth, but such variation is, of ('ourse, im possible ",hel'e all the panels are l)l'essed at one time, Of the gIlIcs llse(l fOl' woodworking, animal glue normally shows the largest change in consistency dUl'ing the glulI1g operation, This is due primurily to the striking effect of 600,------, cr lr coolin1:;>,in incI'easin to> the VISCOf-Hty of nn 500 nllimnl- glue solution, ? illustrated in l!~igure ~ TlST£O PRY 12, The cha nge of vis- 1JW -~....:.:------..--~.-- cosity of otheI' glues ~ • ------ fr01l1 the effect of

tcmpe1'llture is smnll T!STfP W!T in comparison, even if ~--~·~--~·~--~A~--,.~ they nre subjected to A ______the same temperature ---' changes, which is sel dom, HowewI', with nil gluN; n rnthcr intli 4.!Ct effect of tempem o 4 (J JZ 16 10 za tlll'e is the thickening ASS£I>1f3LY Tt.'If (t>11t1UTES) of the glue solution l?(Cll'nE l1.-EIl'~et of ns~embly time on the joint strength by (h'ying, High tem ot plywood glued with enseio glue. 'l'I!sts were lIIudll on :l-ply panels with f.-inch blr'ch falle>! and f.-inch pl'I'ntuI'es,especially of rl'll gUIII eorps. The joiuing pressure WIIS :!OO lJOundH the wood, dry the glue per s,!uare Inch on the wOOlI surfaces more rnpitlly than do lower temperntures. For nnimal glue on wood at high tl'llIpemtures (120° to 140° F,) this thickening from drying is the only means of bringing the glue to a proper consistency for PI'('sf;ing, lIlId the process is slow, The coolin~ of the glue is rapid on wood at relatively low temperatures (iO° F" and becomes the con trolling factor in producing a propel' consistency, the drying effect becoming relatively unimportant. 'Vith animal glue, therefore, 11 definite conb'ol of tempemture IB necessnry or, if changes in temper ature occur, other conditions must be varied to prevent or compensate f01' diffet'enees in consistency (p, 2i). Satisfactory joints are obtained with both casein and blood albumin glues when applied at room tempel'lltures ranging from about 70° to 1)0° F" and variationB within this range do not require any important change ill other cOllllitions, such as assembly time or nmoullt of prCBBl\l'e. 26 TECHNICAL BULLETIN !lOr., U. S. DEPT. OF AGRICULTURE

The approximate ranges 111 time at which average mixtures of uircl'lltt glues reach It satisfactory consisteney for pressing under

I ~ 561

481

441

401-

361 r.:-, ~ I !::? ~32 :... k ~ 2B '-> s:Ir) 241-

I 20

161- I /2

81

l- 4 • , - 0 I I , I 70 60 90 ~ 100 liD UO /30 140 151) IhO 170 TEMPERATURE (PUREES F.)

FWI.lllE 12.-Relatiol1 het\Y.~t'n t~mJlCl"lltll ..e UJIIl "I"roslly of an nnlmul /!Iuc thnt nWI-ts nln"'aft specifications. 'rite tests Wl'l"e mulle 011 11 soilltloll mixell 1 [lllrt or IIry glul' to !!!4 IUII·t" of wllt...r open and closed ass(,llIbly :lJlll <1ill'crent t(,Ill]>('/'H/III'('S arc shown in '1 able 5. GLUTNG WOOD IN AIRCRAFT MANUFACTUUE 27

~l'.\II1.E fi.-llwIIUo in asscmbly time tor tliffcrcllt UlIlC.~ 1

Temperature ot- A pproxl. 1---,-----1 nlato as- Kind otglt!~ Manner ot !lSSemblr semhling ------ll------~I Wood ~

of. 0 F. ,\[inul.. ('/lseln .••______••______('Io.ed ..______70 I 70 0-20 n(L .•.... _. __ ...______...... __ •••__ ••__ • __ Open.•______iO I iO 0 - 8 JJlood·/llItlIlIlln...... __ ...... ____ •______Closed. ______. __ • iO 70 5-20 ,\ nimni' ...... __ .._____ ....._...____..__ ....______.do....__ ....__ .____ iO iO M- 1 Do..... __ . __ ...... __ ...... do...... __ • __ ••__ .. 80 80 3 - 5 Do...... __...... __do.....____ ...... __ 90 90 10 -20 Do. 00, ...... • ...... ••• •••• __ •• Open••.••• __ ...... __ •• iO iO ~- 1 Do...... __ ...... __ • ____ "'__ ' __ •••do....__ ...... __ ... 80 , 80 1 - 2}~ I !

1 Conditions other thnn those g!\'en In the table were !lSSullled /IS tol1ows: Customnry mixtures ot the glnes. sproll!l Ii,{ ounces per squnro toot lind prcssell with 200 pounds prcssure per S(IUare inch of Joint IIre!l. , Anillllli ~Iu" oflhe lIIInimulII proputies required ill t::. B. Army Specit. No. 3-140 (1927) lind U. S. Navy Dept. Bpeclt. No. 5!!04C (IU25). The foregoing combinations of assemblies and temperatures are gi ,'ell as a guide and llIay be varied, provided othcl' conditions are changed. For example, mixing a glue solution thieker than custom nry would lower the time limits between which the joints can safely bo rll·essed. A l)re!lsure of more than 200 pounds per square incl1 would extpnd somewhat tho maximum time limit, while a pressure less than 200 pounds per square inch would lowel' the minimum time limit. In general, longer assembly times are advisable with thin glue mixtures, hcavy spreads, and high pressures, and short~r assembly timN; may be used successfully with thicker mixtures of glue and lighter spreads and pressures. In gluing small parts at room temperatures it is ordinarily not difficult to apply the pressure when the ~.due has a proper consistency for modenttc prcssurc. In lal'ger operations, as in gluing propellers or plywood, the lapse of time varies considerably, and consequently the consistency of the glue spread first and that spread last in the same cOllstnlCtion or stack of panels is not the saml. In such cases n !let of C'onditions must be selccted that gives a sufficient time range (p.41). Templ'l'ature, aside from its effect upon consistency and the result ing strength of the joint, also influences the rate at which the glue scts and at which the joint gains strength under pressure (p. 32).

PRESSING AND CLAMPING Pressure on the joint during the early setting of the glue is required for best results in practically all types and forms of gluing. The functions of pressure are to spread the glue out into a continuous film between the wood layers, to force air fro\':1 the joint, to bring the wood surfaces into intimate contact with the glue, and to hold them in this position during the setting of the glue.

~rETHODS OF APPLYING PRESSCRE The methods employed in applying pressure to glue joints in air eraft manu factUl'e range from the insertion of brads and screws to the use of powerful hydraulic and electric power presses, Figure 13 illustrates some of the means used for applying pressure by hand. 28 l'(~CHNTCAL BULLETIN 205, U. S. DI~I'T. Ol!' AGmCULTUnI~

In a IUI'go numbcl' of joints in the wiilg structure the insertion of bl'llds and small nails is the mothod cOllImonly used. (Fig. 13, A.) The spring clamp (fig. 13, B) is used SOIlIC in gluing small, Barrow

,------, o 0 o :------1 ,."- ..... J ______... ______.1 ___ A c

E F

~-h~g:2E=~I~----~ C; l!'IGl'RfJ 13.-nnnd deylces used in applying pr()ssurc to glue joints: Ao BrndR or nlllls; B. spring clump; Co eccentric clump; Do wood cillnip; Eo C clamp; l!', jllck SCfPW; G, l1ur clump joints, and thc eccentric clamp (fig. 13, C) in applying pressure to the flanges of box beams when gluing the reinforcing blocks in place. GLUrNG WOOD IN AlHCHAJfT l\IANUj<'ACTUHE 29 ",Vood ('lamps and C clamps (fig. 13, D anel E) are extensively lIsed ingilling spars, spar flanges, reinfOl'cing blocks, and bow ends. (Pis. 11 and 1~.) The lal'ger laminated JllC'mbers, such as spars and propellers, are IIsually pressed under jackscrews (fig. la, F) mounted on fmll1t's (pI. l~, A) where several are used on the same :ioints. The bat' clamp (fig. 1;~, G) is useful when gluing pieces edge to edge. 'rhe amount of pressllre that can be a.pplierl with (lifferent types of: ('«(IIi plllellt va ri(,s ('II(lrm(Hlsly. The alllount of pressure applied by a single brad lIIay be 0111" a matter of poul1lls whereas seveml b~lJIs lIIay be applil'd '"with the jackscrews. Tests made on some de viN'S of the tYPl'S illustrated in Figure li~ have provided the data shown in Table ti, whieh lIlay sen'e as a guide in the use of similal" pJ'('!;slIre l'qllipJll('nt. "\VheJ'(' the !;('J'ews of damps and presses rlitfel" 'from thost' illm;tratc

'PAIIU: U.-I'/'('.~,~/I/'C till Ill. un IW/,CW ('[I1I1/PS (/1111. 011l('/' (lc-vi!.'C.~ U8c(li/t UII/-ilIU

F T,ength I'itch of Dinm· Total Cocll· Equlplllent tested !l1)~I~1 Of~;I~;er screw of~~~~W loud 1 ~:r~uo~ \ ------.-,-;::::::::;; Inch,., ~-- --;::;;::: POllnd. ---- Jllckscrew•••••••••____ ...... _.. 1 liO. 0 37 !.; Hi. 33,81iO ' O. 1978 1)0...... _...... _..... 11iO.0 18 ~:. l~i6 16,31i0 ' .1997 Do...... __ ...... 1 liD. 0 31 J.i! 2H. 16.720 '.2000 DO••_...... 11-10.0 7 ~~ }1l i,1iOO '.2498 Ccllllllp...... __ ...... 'iO.n HI H; ~ir. 1,.';85 '.20 no...... __ ...... ' flU. Ii 3 ~!I 916 2,700 '.20 Do...... 'iO.2 2 ~~ 1).10 1,3iO '.23 lInrl'llllll(J ...... _...... 'in.n 21" lli % 2,810 '.20 Wood clalllp •••••• __ ...... ' il. 3 h J.i i % i20 '.20

1 l\leasure.1 in tcst. ! nm+K) .. , ('lIlculllted from tho formu!;. FL=JVR1II C'vlil-li? • which IS applicable to square·thread screws. Where F=forl-o nppliccl to 10\'er arm in pounds. [,= len~th of lo\'er arIn in inches. lI'=load in pOllnds. Rm=meun rndlus of sore'\' in Inches• .f=t'ocflkienl of friction. ".=3.1·1II!. Dm=lIIean dlnmetcr of srrew in inches. K=pitch of sere"' (single thread) or leacl (multiple thrend). • AsslIllled from reslllts of prevlolls tests. , Screws with V'Iype thrends; 11ellt-o calCUlation from formula is only approximate. To s('('ure the best results in gluing, the pressure should be dis t.ributed uniformly OWl' the joillt area. High pressure at certain points and low pressure at others results in weak spots or areas in the joint. Bloeks and ('auIs are frequently used between the clamp or press and the layers being glued to llistribnte the load from the point of eontact to other parts not directly nnder the load. This is partieularly nee('ssal'Y where thin plies are glued. Obviously such lllC'mbC'rs must be true and uniform in dimension or they do not fulfill their purpose. Other pl'ilwipal eauses of unequal pressure OIl joints are: (1) IlTegular surfaces of the wood pieces being 30 ~I'BCnNICAI~ nULL1~TIN 205, U. S. DEPT. OJ!' AGnICULTURE

glued; (2) unequal thicknesses or width of stock; (3) improper spacing of the pressure-bearing members; and ('l) deflection and otht'l' lInpel'fections in press, clamps, or other pressing equipment. 'Yhere heavy loads are applied, as in gluing propellers and ply wood, large deflections of the press members may, and frequently do, o('('ur. This is most often true with heavy screws of the type shown as Figure 13, Ii', and where I beams anel retaining clamps arc used with buncHes of v,eneered panels for retention of preSSUI'e after re moval from the press, (PI. 13, A.) The presses should be designed rigid enough and the retaining I beams should be stiff enough and I;paeec1 sutIicientiy close so that undue deflection ,,-ill not Occur. Likewise, the platens and other lal'ge bearing surfaces of large ])resses should be smooth, true, and parallel. The load ol'iginally applied to glue. joints tends to drop us the glue squeezes out fl'om between the wood layers and distributes itself in the joiut. ('al'e should be used that the correct alllount of pres 4reO'r-----______.-______--,

Ii'OOM I..'/D WOOD Ar 70'F

o o /If/II/MIIM VAwls

4ro 6CO .200 400 600 .fa,NlNe PJ1tS5I1KE (POIINOS PER SOl/ARE IIlCF9

l<'Wl'lUl 14,-Rplntlon hptwppn prpssurp nnd joint "tr('ngth, Tpsts wpre mnde on yellow hlreh nrul nnlrnnl glue. Shaded pOI'tion of un'mgc points In<1lcntcs per c\'ntagc of woo<1 [ailun' den'loppd In tpijtlng Joints. (A large perccntage of wood failure Indlcat('s that the joints tHC well made) sure is maintnined. It is therefore necessary to continue adjusting the clamps or other devices for a short time after the original appli cation of the load. A:.roUNT OF PRESSURE .A light preSSlll'e should be used with a thin glue, a heavy pres sure with a thick glue, and cOlTesponding variations in pressure should be made with glues of intermediate consistencies. In Figure 14 are shown the results of joint tests made on I-inch stock with animal glue, which illustrates the relation between the amount of 1)1"('Sl;lIl'e and the resulting average joint strength. From these ancl other cluta the ('urye of Figure 15 was derived, showing pressures recommended by the Forest Products Laboratorv for different con sistencies of gltle, 'While it is possible. to make strong joints with pressures of less than 50 pounds per' square inch, such pressures are generally not GLUING woon IN AlRCnAl!'T }\fANUIIAo~r.unE 31 feasible under commerci:tl conditions. The successful usc of light pressures presupposes that the wood surfaces are free from warping and other irregularities-It condition that seldom prevails. Pres

600

500

100

FIGt:REl la.-Joining prpssures rl)COllllnCmlecl for glues of varying consistencies. Low viscosity, represented by thinly mixed, warm animal glues aUlI thinly mixed hlood IIlbumln glm's; meclhun viscosity, l'l'prL'6ented by thickly mixed blno!l-lIlbumin glues, slightly coolecl IInlmlll glues, and thin casein glues; high viscosity. repre sented by thick clIseln glues, sl'mljelIlerl IInimal glues, nnd blood-albumin glues; I'('ry high I'iscosity. represented uy tlrmly jellied IInlmal glues sures in excess of 200 pounds pel' square inch may crush certain woods, and pressures as high as 400 pounds per square inch are applicable only to the strongest species of wood. In general, ex 32 TEC;EINICAL BULLETIN 205, U. S. DEPT. OF AGRICULTURE

pel'imcntal and practical work has justified the use of a pressure about of 200 pounds per square inch and a medium-high viscosity of glue where maximum stren!,rth of joint is r-equired. The occurrence of an occasional weak joint or part of which a joint, may cause failure in service, is of great significance to the builder of aircraft. The selection and control of gluing conditions so as to avoid weak spots in the joints is of even greater importance than the average strength of the joints. Figure 14 shows the of effect the amount of pressure on minimum value~ in joint tests made with animal glue. Figure 16 shows the gluing conditions under which low stren6rth joints are most· likely to occur wh~n gluing with casein glue.16

~ ~ ~ :;;~ ~ t:l 12oo~7}-'----~7}------~9}------~~- ~ tl ~ :~>------<. ·------{f O~r_------7------~------~~5~~

FWl!llm IG.-Occnrrf'nce of low joint strength in relation to gluing conditions with CllHCill I:lu('. Figures ill circles r('present the ring llulJlhcr of low-strength joints occur wi tho each combination and pressure 8lld assembly time. Twenty-four species of wood were glued in these experiments, under 16 combinations of gluing conditions. Ten test specimens were made from each. species under each. gluin~ condition which ~ade a total of 160 speClmens for each speCIes ana 3,840 for the 24 dIfferent woods. The lowest 10 per cent of the strength values of each species were separated and arranged under the conditions producing excluding, them, however, the values where failure was completely in the wood. The largest number of weak joints occurred with long assemblies and low pressures, and the smallest number with the short assemblies and high pressures.

DURATION OF PRESSURE Joints should be retained under pressure at least until they have a sufficient strength to withstand the internal stresses tending to sepa-

J. ~rhe glUt' mixture contnlned nbout 10 per No. olB cent Ipsa water tban shown in formula (p. 54) ant! was slightly thicker than an average casein glue. Tech. Bul. 205. U. S. Dept. of Agriruiture PLATE 11

.~ :: '".:: ~ gf ~ ~ ~ 3 '"~

'r, :>

~ " ~ on -5 'To :5 :0'" :;: -5 z 0 '" f .5 q; :: c: ~ I1J ~ Il. '" 0 ~.~ Ij Z ~~ :J ,,~ ..J 19 -'" c: " q; ~ Il. (JJ ~.f' X " 0 ~ m _. « ;l ''lj 3 ~ .;.: :; :0 ;;

~ ~ E ~ "5 :> " '"::'" ] -n i" -, Tech. Bul. 205. U. S. Dept. or Agriculture PLATE 12

SPAR CONSTRUCTION

.\. \n :1in'r:lft hf'tory prf';;:~ 10:u1l',1 with 1:ITJlluatplt ~Jlllrs: of -";Oil,t (-011<:11"\1('1 ion. Two pi(l{'p~ of sprut't.' art· g-ltHifillll!lltiwr In fOl'rn OJh' ""n!'. f'n\~slJn' J)t'r~qU:ln'lU< I. orjoint Hl'pn I~ frnm 12.; 10 I.W IIOllIlds; B. g,hnn~ l\\lI\{HH'jH~ hind;..... hl thl' Ihn!!I'" lIf fl htl\ :-.par Pn':-.I;:un', IIflP1:.-(\ hy 1It(\nJl~ or \\nod d:ung..:. lS !(I:o;utl1('H'ut' alit! flol prnpl'rl~ dlslrtlmlt'l} for IIlnkilll! llw IlI''''' joiII1S; C\ n box $purshow. JIll! IawUlahld 1I'H1g,,~. ;,~ply rpmfllff:lJlJ! hlur'k, lIud a-ply \ (lJ)t,I()r $Idl.'s. 'fJJt! IJlyw(IOU !:!ides \\(~n.' llJ'll.~l'd to the rralllt.'wnrk: of IIw ~p:lr hy rhe use of brads oulj-'. PLATE 13 T cell Bul. 205. ll. S. Dept. of Agr;"Jiture

POWER PRESSES USED IN GLUING PLYWOOD A, Jlydmulit-, ('uhl prtlS.s sho\dllJ.: stH('k of palwls with I Iwnlll5 nl1ll fl1tniuin),! dumps in IlhlC'c; n, hplrnuli{'. hoI \11:01<' press liSt'll ill till' IIllllllJfllclllrc of IJlood-lllhulIlill gilll'tl plywood. GI~UING WOOD IN MHCltAFT J\IANUJ!'ACTUHE 33

I'ate the wood pieces. It is safe to assume that under fU\'orable g-luing conditions this stag-e will be reached in fl'om two to SHen hours, acconling to the thickness and absorpti\'e power of the wood. A pressing pcriod beyond the. minimllm is advi:·mble as It precau tionary meaSUL'e where. operating conditions permit. The rate at whieh the joints gain strengtb is the principal factoL' l1etcl'lnining the length of time ill the pn~ss. Joints int'rcase in ,. l-ltn'ngth mainly as It result of drying of the g-lue layer, and drying is in tUI'll alreeted by several fadors. The quickest release of pressure is possible when a fast-setting- glue. a thin spread, and warm, dry, thick layers of wooll are used in a warlll rOOlll.

18a~r------'

• • o

o SITI

LECEND .-AN/NAL ClIfE O-CrlSEIN eWE

~~-L--~-L--2~-L--3~-L--4~-L--5~-L--+6--~~7--~~8--~~9 JOINr ME (HOVIfS)

1!'Wl'ItI-l 17,-St!,('n~th·tlml· "plllt iun fur 11111"':1) lind ,'IlHelll !!Inc joint's. 'r~Rt.S were 11111(1<' 011 '}.·illch hlll'd IIIllpi,' IIl1d Sitkll HprUCl' 11K soon liS )losslull' n((,'r ~"ints were Itlk('11 from pl·I\~S. Endl value is un H\'cragc or at lemit !.!.J test Hl)(!CJUlcnS from o or ilion' Join ls Figure 17, showing the inerease of strength of joints with time of pressing, is bascl1 011 tests of animal and casein glues. The gluing was clone under eonditions favol'llble to the production of strong joints. The rates of inerl'ase ill strcngth of the two glues are ap proximately equal whl'1l the glues are used ill It room and on wood at a temperature of about 70° F. Animal-gille joints made in a warlJler room Hnd 011 Iwate

sents the final strength of the joints. A further conditioning, ex tending in all over a 7-day period, permitted an equalization of JllOistlll'e which resultetl in all a "erage joint strength on sugar ma pIe of approximately 3,000 pounds per square il1('h. In othC'l' tests of the sallle series, joints~ which were pressetl for 2 hours and then allowed to season for 22 hours, proYed as strong as those that had oeen pressed for 2-1: hours. Sugar-maple joints pressed for only one-half hour al1(1 seasoned for 24: hours, although of fair average strength, showed greater variation and would not be so dependable whel'e unifol'lll maximulll strength 1S importa nt. In gluing '"eneer the endy setting of the glue is about as quick as in gluing thick joints, but beyond this the yeneer joint apl)e:1rs to increase in strength mOl'e slowly" The thin layers of 'WoO( do not continue to absorb the moisture as rapidly from the glue as the thicker stock, and a longel: pressing period is therefore reqHi reel. Plywoo(l is usually left under preHHUI'e j'ol' 1H to 2-1: hours, but this is determined 1ll0l'e by routine of opemtion than by the rate of setting of the glue, Pressing plywood beyond ;) to 7 hOllrs appears to be unnecessary if it has been properl,v glued, although the customary longe'l' periods are not harmflll amI insure an additional margin of safety.

'With most bloocl-albumin glues hot-plate presses are required in making joints" (PI. 13, B.) The glue must be bronght to a mini mum telllpemture of about 1(iO) F. to render the" joint ,Yater resistant. Heat anoekets in the panels during the pressing proeess. The time required in pressillg "aries with the temperature of the platens and thickness and kind of the material being pressed. The nriation in time in actual practice is from 2 01' 3 minutes to as much as 30 mi nutes. Casein glues for use ill ail'craft are usually pressed cold. Ex periments ha,"e shown, ho\\"e'"el', that the quality and water resist ance of casein-glue plywood can be SUbstantially improved by the application of heat. Hot pressing after the joints had set but before they had dried resulted in a somewhat greater improvement than hot pressing at the time of gluing" ,"Vith some brands of casein glues the wet strength of the resulting joint was increased by 35 to 40 per cent. GLUING DIFFERENT SPECIES l\ND SURFACES OF WOOD

SPECIES GLUED A very large part of tIl(' wood used in aircraft is Sitka spruce which, fortunately, is cOlllparati,"ely NiSY to glue, Ash, basswood, black walnut, mahogany, hard maple, white oak, and yellow poplar are also used and glued to some extent. Spruce does not require so GLUING woon IN AInC'I1A'FT l\[ANUFACTUTIE 35

('art-flll conlrol of gllling conditions as the otl1<'1' species in order to make joinls of maximulll str('ngth. A::;h, black walnut, birch, hard mapll', and whilt' oak will \"l·quirc very careful control of the gluing operatioll to obtain the llpst results. Gilling It·sl's ha\'(' bl'PIl llladp at the FOI,'pst Pl"Oducts Laboratory on about ,1O !lift'pl'en! spC'cips, inelu!ling those used most in ait'craft ('onstruetion. The woods wprt' glued with animal, casein, and vege tnbl(' glues IIndl'r a, wicle range of pl'('ssures and assembly times 17 and the joints\yprc tc·"ted in Slll'al' in thc manner illustrated in Fig III'C 5. The joints whi('h showed no lllar\wd or gelleml vm'i al ion in strpngth W('I'(' taken as a ba;;is fm' comparing the gllling C'haradl'rislit's of the (lilft'n'nt speeies of ,,·ood. Frolll these te>its definite' r('('onlllt('IHlatiollS ha\"{~ bl'en made for the gluing of each spl'('i('s.JH The rpslilts obta inNl with animal and c:tsein glues al'e appli('nbll' to airl'raft gllling. In Figul'e IH the a\'(~rage joint strengths of 31 specieB, which have bC'('n gItH'd ill ni r('ra ft 01' whi('h frolll tlwir pI'operti('s and abundance Illay wal'l'Hnt ('onsiti('ration for ail'('ratt purposes, are plotted againsi; tIlt' s}Jl'C'ifie ~a"ity of tlte wood tested. 'fhe strength of wood is known to ,'1\I'Y .in a genel'al way with its density, and from Figure IH the sail\(' I'plntioll is s('en to exist for glul.'d joints, e,'en where comp\(>t\. wood failllI'l" (lid not occ'uI' in testing.19 The amount of w(Jod failul'e o(,(,Lll'l'ing during tpst oYer the ~lue-liJl(~ area was gener ally in I'e,'el'se ol'der to the strength of the points and the specific gmvity of the ,,'oods. 1<'01.' example, Sitka spruce joints showed 1~I':l(,ti('a\ly 100 pl'I' eellt :l,'emge wood failure (0 per cent of glue-line iatllll'l'), I1talto~ally about tlO pCI.' c(,llt, alltl ash and birch each about fj() pl'l' cellt.

RECOJIIMENDATIONS FOR GI,l'ING SIDE-GRAIN SURFACES OF VARIOUS SPECIES

WITII C,\SEIN GLUE

For gluin~ si(le-~l'ain SUd:lCt'5 with casein ~lne the variou>i species listed ill Figlll'(' l~ an' divided into two gronps according to the conditions applieuble, as follows: Species GllI.illg COIIIIU;OIl8 recommended

(:II0UI' 1

('('dar.PiI', whitl' \H'l't(>\'n______I'(>(L------1 . Gl11(' of medium consistency, spread 1;4, to 1% 1I!'lIllock, westcl'n______OIllICl'i-i per squure foot', pressure from 100 to Pint', llOl'tl.lPl.'n WhHl'------'11GO vounds per square inch, and assembly Pille. W('};tl'l'Il Yl'lIow ______tillle 0 to 20 Illinutes closell, or 0 to 8 millutes

It!.'! \wood______•_____ ~_____ oll!.'!I. Hpl'Ucl', Hitku ______

IT PI'''~~lIn'" of 100 1.0 400 Jlotlll(l~ pl'l' Sqlllll'C! hwh lind lIs"pm!>!y timps of 1 to 2r. millutps wprp ll:-'tl(1. ~ill{'p .toO pounds prl'SSlIrp would lin Vt,' eruBlwU: (0;1.)1111' of till' wfluker woods, the hl~h ..st "I'l'~KUI'" us"d WIIS tht' IIIlIxllllulII thlll til" woO(l wOllld ~tllnll without IlIjUl'Y· 1~ 1"01' It lI1on~ ('umph-t.(. di:-;l~ll~si()n (If r('l~ults Hnd rPl'oUlUlf'JHlntlol18 for the gluiug of the .1[) ~llt'ci .. ~ ~I't' Thl' Ulllill" 01' Wootl l~\!· "'111 01(' type nr SP~('IIII"11 1"t'11 in It'slinl; the J!1tH'tl joillts (fig. ri) the unIt stress III tht· J!11It' is hi1(III'I' tlllIn In til(' wOlltl I"'~IIIIH' thl' sht nl'in~ lo"d is horlll' hy a larJ!,'r IIrea of wood thall or g-hl{' litH'. 1'~l1rthpl'mort". tIll' ~h'f':-;s lIlay 1)(1 '"PI'Y ('lose to tlH~ ultimute l-t l"('nJ!th Of til(' w"o,I, nml r'" I'lIllIu'.. lIlay ()('('III' t'l11"f1y or ('ntll'I'ly in the J!11Il' litH,'. ]"01' Ihp:-l(' rt':l~nlJ~ n hl1!h pP1'('P1l1n1!1' of ;.tIll!' fnihll'" (low Pt'I'('t'ntH~(I wood faillll'I'\) HIlly occur III 1(~Hl untl ~1I11 th,> stn'ng! II or the jIlillt I,l' US high lIS that u( tht! wuo(1 !tHo'lf, 36 ~rE('JIN[C'AL IHr[,LE'L'IN 205, U. S. DEPT. OF AOP.WULTURB

NpcCics GIlLing e(}/lliilioll8 recollllllc/Hied GitOU1' 2 Ash, whit(·______Bns:,;\I'oOlL ______J~eech ______111\'ch, yl'llow______Ct'!Iur, AllIska______('/If>1'rY. hlaek______Cotto!I WI 'Ill 1______1';1111, AIIIPI'icllll. ______JoJlm, l'IICk______• Douglas fil'______.. Glue of thick cOIli';ii';tPIlCY, i';prend n4. to ll1:! GUlli, hluC'k______OUllCPS Jll'1" HqwlI'e foot. PI"('Ssure 150 to 200 GUlli, I'('d (hl'lIl"t) ------1)('lIl1l1s Ilt'l' squlIl'e illC/I, nud aHsclIIlJly time GUill, I'ed (Sllp)______u to 1!! llIilllltps cJo:ietl, OL' 0 to G millutes 10ckoryGum. !:ullehL______.. ------. • ______._ - ('PCII, MnhognIIY ______, __ , "" l\llIgllOli f1.______,\llIpl(', S01'l___. ______•.___ MUrlll', hal'tL______Oak, 1:('11 ______. __ Ollk, Whitl'______l'ov1a I', ,\'l'lIu\I" ___ • ______. ____ _ SyCUIIIOI'l'______"'alnut, ulack______OC'casional wenk joints in I'ed gum (hcartwood), black cherry, and hickory are obtained C\'cn \\'llPn glued lIndt'l' the conditions recolll 1U(,Il(Ied. r.r.his tl'oublc can be largely a \'oided by treating the wood bdol'~ gluing in til(' manner drscl'ihed on page :-l8. Sitka spruce, wliic'h is lIsed ('xtrnsi\'ely in alrcl'aft, is Ycry satisfactorily glued lIndcl' the ('onditions l'ccommC'IHletl fOl' Group 2 woods us well as those fo!' Group 1, in whiC'h it is listed, On thc other hand. whitc ash, birch, or hnrd mnplc is less satisfactorily glueclunder some of the conditions l'e('ol1llllC'llciNl for Group 1 woods and should, there lore, be glued under t.he more restricted conditions of Group 2.

WITn ""'DIAL GLeE The principal specics of woods glul'd with animal glue in aircraft con;;tl'lIdioll may be placed ill a· sing-Ie group in so far as recom mendations fOI' gluing- practices are concerned. The following spl'eiC's ineIucll' til(' woods USl'd for propellcrs and a few others I'(·qlliring similar gluing technic: Ash: yellow birch, black eh('I'l'Y, red gum. Illagnolia. mahogany~~O soft maple, hard maple, re(l oak, white ouk. Hnt! blade walnut. Gluing conditions r('commcndec1 for tiH'SC species of wood are ;;1Iown in Tabl(' 7.

'" 'I'h... 1!111fn l! ronllillons Tl'(JlIircd for mrrhog'any ar!' lrsR rigid Own for the other sppcles listed, tout it glues sntisfnctol'i1r under the conditions recommended. GLUiNG WOOl) L.,{ AUtCBAF'r )[ANUL'AC'XUllJ,; 37

'!';\IIU; 7.-GllliIlY cOllditiollS r('('(}III1I1(,II(]('(~ 'with a/lill/at yluc {()r 1Good.~ 1IS('(l b! aircraft

('Inset! I'rc.ssurc nsse.mhlyl (lll1e-wnt('r I prOI""linlls, hy ' ...·Ight limo

1./,.. wrsq. hr. J1illulc. 1,~)-200 7>r 1 1 to ~~,.; ..... Ir.o-:!\lO 5 ilo_,. ' a - IW-~~)() )3 -15 1>0. ' 1fiO-:"'OO 10 -IS 1)0••_, '

I A ~llIc whi!'lIIllI'('I.S tho rcrillircmcllts of U. 1;. Army SpccillcaUoli No. a-HO, lIud NiI")- Department Spt't"illctllion No. 52().tC. ''''0,,,1 (11"("'5 aro IlIld (ogethl" liS soon liS 51"('11<1 wit,h ~11I~. 3 Wdghl of wet gille IIIhluro IIpplled Iwr s'lunrc fooL of glll(·lIn~. •u J600 .-AN/NA~ ewt: O-CASEIN ewE

I-ASH, IVHIT! 2-dASSJl'COO J -BEECII 4 -IlIRell. YELLCIY S-CECA", AOS!(,l 6 -ClCAR, WESTERN REO 7-CII!RRY, tJLACK 6 -(orriJI/II0CO tI' 9 -COUClM FIR IO-E.LH, "NElfICAN II-ELN, ROC/( d' iZ· fIR I Will r£ 2800 - Ij-eUM. BLACK 14-&UN, REO (HEARTJI'OOiP 15-CtlM,l?E{) ~APII'OOO) ..".. 16-CUM. TUPELO g lfOC 17-HlMLCCK, WESTERN 18-1//C!(ORY ~ 17-NAClIOLIA ZO-/'I,4HOCAIIY ~ 14tC ZI-MAPLE, SOFT Zl-NAPLE, HARD ~ 23-011/(, l?EO ... U-OAK, WHIT! ~ :.Joo 25-PIf'/f,NORTHERII /l'1I1T! }j ., 26-PItI!, WESTeRN ynwiY • ~'S'" 27-pgP1AR, rUlCIY It·/?Et'J)'CCP !"-lo'" tC'OC Z~-SP!lUc!. SITKA ~~ JO- SYCAMOf{! :;'" > j/· WALNUT, 8LACK ~ 16JC ~ lere

1M)

0.E5 0.70 0.75 C.:5 cwo N5 0.50 055 MO SPECIFIC C.i'AVITY l"I'C sl)('cies should not be usc'il whC'I'C' Illaxinllllll strength of joint i:-; of /)I'illlllry importance IIlllc!;!; trcatcd in thc nramler described in Table 8.

TREATING TilE WOOD lll';YORE GLUING

'Vith wootls that arc natul'lIlI." ditlicult to gillt' 0\' wl1('I'('. ('onditions art' not the' most i'avomb/c' to tilt' PI'()(illC'tion of strong joints it is pos sible to bring about strongL'I' joints by trcating the wootl before gilling. 'Yith allimal glill', ('allstic soda. proved to be one of the most t'fliec tive of sC'v('ml tr'pating lJwl"C'!'ials iTied out ('xpP"ill1entaJly. The wood sudaces to be' joilH'd wen' bl'llRhed with a 10 PPl' tPnt solution of eaus tic soda, \vipl'd afi<'l' a i('\\' Illinuh's to n'lllore any dissol\'(~clll1atel'ial or t'xc('ss of' til(' solw'lIt. allowed to dl-Y, 1111(1 tlleri. g-Illcd IInder !;tancl :t1'd g-llling practice. The data in Table' 8, (ll'rived 1'1'0111 joint tests of yellow birC'h (Iwal-twoml) !!Iu('d with animal ~,dl\(" indicate the im prO\ ~mellt resulting from thc callstic-sod:L treatment.

TAIII,'" ii,-R(',~u/(,~ of COl/8th' ,Yrl(/a. (/,('(//'/II('/I./' on joinl,~ of 1/ellow birch (heart. 'l{"OOIl) III/d, llIlIIIWI !/II/c'

f:hl'nr ('ondition of spl~cimcn stn'ngth oCjoint

Lb,., prr 8f}. in. j.J CC7It Wood unf,rrllt.cd (good Joint {·onditlons}••••••••••.•••••••••__ ._._.__ •.•••• cr 2,700 Gil O,f~j \\'00<1 untrcutcd (stun'cd joint condIUonr.)...... __ ...... _...... J, UIO "'00<1 treated with 10 !wr cent ClllIstic,sodn solution (stun'ed joint concli. 6 .66 tions)''''•••••••••••••••••• ,._•••••_•.•..••••••••.•.•.•••....••.•••••.•. 2,7iO 88 .62

I Elich kst ,'uillo shown T('prl'scnts lit lenst, 10 slwcirnens, , Specific cm"it)' bused on on'n·dry weight lIud '-OltlltHI at 7 per~... ,t 1Il0istllf" content,

Other species. which showed an impro\'cl11t'nt ill joint strength whcn treated before g-Iuillg with animal gllle were: Black c1wITY, red gUII\ (heartwood), red gum (sapwood), hard lIIaple, red oak, and ,,,hite oak. 'Vith caseill gluC', hydl'ated lime (10 g-rams acltlecl to 90 grams of watel') guve slightly bettel' l'('slilts than caustic soda whell used as It surface tI'eatlllcnt for blaek chen-y, hickory, and red gum. Hickory, which is diflicult to glue with casein glue, produccd appr'eciably bette!' joints after treatuH'lIt with eithl'l' lime water or caustic soda.

GLUING END·GRAIN SURFACES

TI1(~ methods. PL'Hcticc's. n'slilts () f ic'sts Oil joi nts, and I'PCOmlllclllla tiolls which have thus far bel'll prcseuted I'elate more specifically to the gluing of !;ide-grain surfaces of wood, Such surfaces are in GLUING woon IN AmcnAFT 1\rANUFACTURE 39

\,01\,('(1 exe1nsin'[y in plywooll amI laminatea constt'uctions. (Fig. H, A to 0,) .Joints bet WPCIl side-gmin surfaces in most species can bc made as strong in ShCiIL' parallel to the gmin, tension across the gmin, OL' cleavage as the wood its(,lf. The highest stresses developed ill thesc joints do not exceell 3,000 or at the most 4,000 pounds pel' sflual'o illeh. Thegllling of eI1l1-gl'llin surfaces, on the other hand, is not accol11 pl!slH'<1 with the· same degr~e of success. ,Stmight .end-grain butt )Oll]ts are rarl'ly attpmpted III any type of constructLOIl, and where wood .is subjected to tension stJ'csses plu'allel to the gmin, joints of this tYP(1 can not be dep('Illil'd. IIpOIl to develop more than a small part of the strength of the wood. Most North Alill'riean species of wooa are capable of withstand ing (j,OOO to 20.000 pounds pel' square inch in tension parullel to the gmin. Tests made in gluing straight end-gl'llin sUl~fllces have shown thnt such joints are cl'I'lltic and rllrely exceed about 3,000 01' ,~,()()O pounds pCI' sqllare inch in strength. Their strength is limih'd ~y sl'v~'ntl factors inclu~ling: (1) ~tructur<: of th~ wood, (2) pcnetl'lltlOn of the glue, (3) ILir bubbles 111 openlIlgs of the wood, (4) quality of gille, (5) consistency of glue, (6) application of glue, (7) amount and dllmtion of pressure, and (8) shrinkage of glue. Since factors 1, :I, IUlIl 8 are largely beyond operative control, any impro\'('llIent in strength Illust be brought about primarily throli'J'h the otill'r lac-tors. High-grade glue, insuring ma:\;mum tensile strcngth (fig-. 1), should be choscn. In general, a thick consistency of glue solution gives bctter results than a thin one, so that the pro portioll of watl'r in the mixture should be somewhat less than for side-gmin gluing. ,\Tith c\'cn the 1110sb C:1rci'1I1 gluing of straight butt joints not more than about 2;') p('r (,l'lIt of the tensile strength of the woo(l parallel to the grain has bel'll thtained in tests. It is evident, there fore, that in order to obtain n tensile strength of the various species that is grcat('r than 2:') per cent of the tensile strength of the wood, a sC'nd, serrated (fig. 0, II and I) j 01' other form of joint must be llsed instead of plain end gluing. 'Vhere it is l1ecessary to elongate nwmbl'I's, guch as longcrons or beams, some such form of joint is ]'('l'OlllnH'IHle(l. The plain sead! (fig. 0, I) is perhaps the easiest to gille and involvcs fewer machining difJiculties than the many angle forllls (fig. \), II). DRYING AND CONDITIONING GLUED STOCK The gluing operation atllls moisture to the wood. Glue that has s('t in joints contains only a part of the water added at the time of mixing, the remaindcr beillg absorbed by the wood or removed by e\'aporntion. The absorbed moisture 111ust be allowed to dry out Ol' to distribute itself through the wood in order to insure the full strcngth of the joint and to reduce the tendency of the glued member to warp. Table 3 (p. 18) shows the percentages of moisture added to differ ent cOllstl'u('tiolls .in gluing, cnleulatcd for It specified glue mixture and sprend. N () allown 1l('('S ha\'l' Ix'en made for surface evaporation, bllt: the cOlllputed percentages are checked closely by actual deteI'- IIIi lIatiomi. 40 'l'ECJTNIC,\L BULT;ETIN 205, U. S. DEPT. OF AGnICULTURE

III />1'(1)('111'1':; (fig, 0, If) al1(l similar eonstrllctions, gluer} from thick lalllillatio!ls, the. Illoistul'o from the glue need lIot be eliminated but may simply be allowed to dist!'.ibute itself throughout the con stl'llction. Even cornplete equalization would require a verr long time. li'or birch und white oak in luminations about three-fol1l'ths of an inch thick alld under' :t\'el'llge room temperatures, a condition ing period of 7 to 10 days is sldlicient. For quicker-dryi1lg woods, such as spruee, which permit :t more rapid distl'ibution, a 2-day period should sllmee under most conditions. However, where con strllC'tiollS nre glued from laminntions one-fourth of an inch OL' in less thiekness they will normally eontain too mnch moistme and should bn dried for one to three weeks or longer depending their upon thir,knl'ss anrl width. A spruce bow eud, for example (fig. 9, E), will IIsIIall,Y dl'y sulliciently at room conditions during 7 10 days, to but R thick lal1lillated spal' flange would require two or tiu'l'P timps ns long, In plywood it is R<1visable to dry out by artificial methods a part, nt least; of the moistlll'P nddecl ill gllling, nntil the construction reaehed has the an.'l'nge serviee moisture content, Drying plywood to nn excessively low moisture content materially increases warping, openillg of joints, ulld other defects, It is therefore recommended that for the gt'It('!'a1 reqnil'ements of aircl'llft, plywood be dried to un avt'rage moistlll'e content of 10 to 12 per eent. This is somewhat 10WCI' than the IUrel'nge found in sCl'viee (,fable 2) but is higher than the moistllre ('outent in most fabricating shops in winter, Tests ha\re shown that it is possible to dry panels three-sixteenths of an iJ1('h or less in thickness very easily in 8 to 1G hours under rela tively mild conditions, Satisfactory results may be obtained with I'atllt'1' a wi(1e I'll !l/!e or drying tempemtures, such as 700 to 1400 F" if propel' relative humidities are used, Tablr !) shows sen~!'al combinations of temperatures and relative hllmidities, whieh will pro(lu('c a moistl1l'e ('ontent of 8 to 12 cent per jn ))1)'woo(l panels within a reasonable drying period, If ex posed for an indefinite prJ'ioel to the cOlHlitions outlined in Table 0, howevel', the plywood ",ill ('ome to a moisture content somewhat lower than those indicated, (Fig, 3,)

TAnr,E O,-OOIll7Jill(/!i01l-8 of frl/lPcraflircs amT. rela.five ll1t1l1iditiclJ' ,~II·if(/ble fO'/' produoillu (['//H/istlirc cOlltrllt of 8, 10, or 12 Jler CCllt in. ply/coo(l, p(mds

nclnti\'u humidities Cor usc with dilTerent temperatures ]\roisture c.>ntent ,l,\~ired, lX'r cent 70· F. 80· .F, 00· F. 100· F, 110· F, 120· F, 140· F. ------1------ 8______Prr cent Per f",i Per cellt Per CflIt Per cent Per 10______cent Per cent • 30 :11 :12 33 35 :17 41 J 2______4:1 H 45 46 48 55 50 Ii3 50 5; 58 Ii\) 61 65

RECOMMENDED PRACTICE FOR THE PRINCIPAL AIRCRAFT GLUING OPERATIONS The following-pages giv(' a SUmllllll'y of information and specific l'p('omm(,IHlations ollilllporbwt detailH ill the prineipal olwrations the in gliling of: (1) Pl'ojlellel's; (2) plywood; (iI) laminated spurs, GLUING WOOD IN AITICnAFT MANUFACTURE 4! spar flanges, bow elHls, pontoon ribs, and rei nforeing blocks; (4) SClll'r-type joints; (G) wing ribs; «(j) box beams; and (7) geneml nssembly. Necessary prerequisites in all gluing operations are properly dried and machined stock, and equipment that spreads the glue and applies pressure uniformly over the joint. These requirements must be met or weak joints will result despite the most careful control of other conditions. PROPELLERS Propellers (pI. 3, B) are usually made in special propeller plants. Theil' gluing and shaping requires equipment not usually a,vailable in nircnd't factories. :Mm;t propellers are glued with animal glue. Casein glues are used only to It snlall extent, since their water resistance for this purpose is of little vnlue. Birch, white onk, black wltlnut, and mahogany nre the principlll species of wood used. A pI'opeHer-gluing operatlOn requires several minutes for com pletion which, combined with the difficulty of gluing the species usecl, necessitntes cawful control of the gluing conditions. Since nnimal glue is largely used in propellers, most of the following details lire applicable to animal glue:

GLUING ROO~[ The gluing room should preferably be separated from the rest of the fnetory. A room tl'mperature of 70° to 75° F. is recommended, but gluing may be succl'ssfully done at room temperatures of 85° to 00°. 1Io\\,e\-er, such high temperatures are uncomfortable for the workmen and are unnecessary. GLUE Glul'S conforming to the rl'quil'ements given in United States Army and Navy Specifications Nos. 3-140 and G2G4C, respectively, al'e recommended. GLUE-WATER PROPORTION One part dry glue to 211..1, parts watcr (by weight) should be used for a glue that 11ll'l'ts the minimum requirements of the foregoing speC'ificlltions, (See pl'ecNling paragraph and p. 16.)

~lOIS1'liltE CO:\,TENT OF WOOD A moistnrc content of thc wood at the time of gluing of approxi matd~r 8 1)('1' cl'nt (p. II» is recommended.

TEl[P~;RATUl!E OF THE WOOD Wooc1 that is at approximately room temperature should be warmed at 8GO to noo F. for about one hour before applying the glue. Higlwr tl'mpemtul'es and shorter he!tting periods can also be used, but higher temperatures arc more apt to lead to trouble.

OLt'E SPREAD Onc and onc-fourth

spread, 2:1 per cent 1I10re glue should be used.) A more uniforlll sprea,d of glue is obtained by the use of mechanical spreaders than by hand spl'ellc1in9" The glue-coated wood should be laid together as soon as spread ~ closed assembly). (P. 24.)

ASSE~mr.Y 'rUlE A closed assembly time of 8 to 15 minutes or 10 to 18 minutes with wood warmed at 8iiO or 90° F., respectively, is recommended. l:>rcssure should be applied not sooner than 15 or 18 minutes ufter the first i:lIninution is coated with glue, but not later than 8 or 10 minutes after the last lamination is spread. (Taule 5,)

A pressure of 150 to 200 pounds per square inch of single-joint urea should be upplied for a minimum time of 5 lioUl·S. Pressure distributing blocks, cut from trimmings from the laminutions being glued, and cauIs should be used to equalize the load over the joint Ill·eas. (Pp. 27 to 29.)

CO::'>Dl'rroNING The glued blocks should be conditioned at room temperatures for at least two days before rough carving and five additional days before finish cal'ving (p. (39).

PLYWOOD Aircraft plywood is usually manufactured in plywood plants where large glue spreaders and power presses (pI. 13, A and B), not usually found in aircmft factories, are available. Either blood-albumin or casein glues are successfully used in the manufacture of aircraft plywood. 'Vhile a number of species of wood are permitted in aircraft-plywood specifications 21 the list of 'woods acblUlly used in the past has consisted almost entirely of busRwood, birch, mahogany, Sitka spruce, and yellow poplar. Methods and practices that produce high-grade, wat€r-resistant plywood have been developed (.J, 20). The hot-press method is used with blood-albumin glues and either the hot-press or the cold press method with casein glues (p. 34). The following is a sum mary of recommended conditions for gluing aircraft plywood:

Gr,UING ROOM Ordinary room temperatnres an

Highly water-resistant types of blood-albumin or casein glues (p. 2) are recommended.

:!1 The SJlec~es p,:rmittcd .i!l pl~'w~o'l In ~~l(, 1;;, fl; Arll}.Y and Nnvy 'mcclfien~lon~ are s;lven In U. fl. ArmJ, I lJwnod. AlJclal.t, Specl!' No. 8_-6-A, I p.• !!Ius" 19_8, and III U. S. Navy, Plywood (Alrcrnft Usc), Dept. Speci£. No, ::l9P1:3, G p" lJIus, 1929. GLUING WOOD IN AmCItAFT MANUFACTURE 43

A glue mixture of comparatively thick consistency (1 part by weight of dry glue to 1.8 to 2 parts of water) is recommended for most prepat'cd casein glues. Spruce glues weU with a somewhat thinner mixture than required for hardwoods (p. 35).

QUALITY Q<' YENEER Smooth, tightly cut veneer that is free from bad cross grain and rot (p. 20) i::i recommended.

TEMPER..-I.Tt:RE OF WOOD 'Vooll that is of ordinary room temperature is satisfactory.

GLUE SPltEAD One and one-fourth ounces of glue mixture per square foot of joint area should be applied evenly with machine spreaders, single spread (p.23). ASSKMnLY TIME FOI' blood-albumin glue an assembly ti.me of 5 to 20 minutes (closed assembly) is recommended. For casein glne an assembly time of 3 to 15 minutes (closed assembly) is recommended. On spruce wood the assembly time may safely be extended to 18 or 20 minutes (p. 35).

PltESSUItE A pressure of 150 (for spruce and basswood) to 200 (for birch) pounds per flquare inch of panel area should be applied in the cold press proC'ess for It minimum time of 5 hours. For the hot process, the pressing time depends upon thickness and number of panels pressed (p.34). DRYING The glued panels should be dried to a moisture content of about 12 per cent (p.3V).

LAMINATED SPARS, SPAR FLANGES, BOW ENDS, PONTOON RIBS, AND P..EINFORCING BLOCKS In laminating spars, spar flanges, bow ends, pontoon ribs, and reill iOl'('ing blocks sHcral minutes may be rcquired from thc time the firtit glue is sprcad on the wood until pressure is applied. Further IIlore, the largc glue line areas of most of these members require large presses (pI. 1~, A) or the usc of many hand clamps (pI. 12, B) In order to obtain adequate pressure. The gluing operations are somewhat simplified, however, in that spruce is the principal and often the only wood used, and the gluing is clone ,yith casein glues. The following conditions are l'ecomIll

OLUlNl1 ItOOM Ordinary room temperatures and shop conditions are satisfactory.

GLUE Glue conforming to rnited States Army and XU\'y Specifications Nos. 08-14020-D and 52G8, respecti\'cly, is recollunelllled.

GJ.l'~;-\\'XrElt l'l:OI~OltTIONS .A glue mixture of medium ('onsistcl1ey (by weight 1 part of dry glue to 2 parts of water) is reconllllended fOl' most prepared glues (p, 15).

:\LOIS'lTUE t'ONn;N'l' OF WOOll .A. moistlll'l1 content of the wood nt the time of gluing of 7 to 12 pel' cent, depending upou thickness of individual laminations and other factors (p, 17 and 'I'able 3), is recouullellded.

",Yood that is at ordinary room temperature IS satisfactory.

OLl'I:: Sl'lt~;.\[) One and one-fourth Olll1CeS of glue mixture should be spread uni formly per sqllare foot of joiut un'a, (If both sides of joint arc spl'cad, 2:> per (,Put 1I101'e glue should Lt, used.) .Machine spreading is preferred. The glue-coated pieces should be laid together as soon as spread (p. 24). .\SSE:\wr.Y 'rUlE Pressure should be applied withiu 20 minutes after glu(' js first spread on the wood. ",Yhere it is impractical to lay the pieces to gether as SOOn as c(JatNI with ghw, the maximum assembly time should not exceed 8 minutes. (Table 5,)

l'ItESSURE A pressllre of 100 to HiO pounds per square inch of single joint area should be applied for It llIi niIllUlll time of 4 hours (pp. 30 to 32).

COSDITIO:>1ING The gluctl m~mbers should be conditioned for at least two clays at room temperature before machining. ",Vhere indivic1uallaminations are one-fourth of an inch 01' less in thickness the conditioning period should bc one to three weeks, depending upon thickncss and width of lami.nated mcmber (p, (0). In laminating hardwoods with casein glue, a glue mixture of thicker consistency (about 10 per cent less watcr than for spruee) should bc used with an asst'll1uly timc of 12 or (j minutes for closed and opcu ilssPllIulieH, I'l'speetin>(y, amI with a pressure of 150 to 200 pounds per square inch applied for' at least 5 hours (p. 35). GLUING WOOD IN AmCRAFT }\[ANUFACTURE 45

SCARF-TYPE JOINTS Scarf and finger, or Rel'rate, joints (fig. 0, H and I) are fre quently made in building long pi{'ces, such as in laminations for beams and propellers. The gluing surfaces are partly end-grain wood. The following procedure is recommended for gluing scad and otiWl' end-gmill sudaces in aircraft: The end-grain sudaces should be sized with a glue mixture some what thinnel' than that used £Ol' regular gluing. A sizing mixture of 1 part glue to 3 parts water is recommended £01' animal ancl pl'epared easein glues that meet aircl'Uft requirements. The sizing eoat should be allowl'd to dry on the wood surfaces. The mixture for the final gluing RllOlIl

TABU) lO.-.'?loJlc.v Gf .~(·("·r joillt.~ r(,('(}l11m.rlHkll. fOl' .~('I:('rl1l .~J1c('ic.~ of wood.~

Species Slop" Specip~, I Slope Rt'll glllll ___ ••• ______._. _____ I in S. He" oak______111115. YcUow pOl.lnr ______1)0. White oak______j1.in 15. Sltk" SprtlL'C_. _____ ._._____ 1 ill 10 (~stillmte(\).1 Hlllck waillut______. Do. lIonllllnllllllahogany__ .- ••• I inlO. 'White U5h------.------i 1 ill 15 (estimated).' YeUow birCh. __ •__ ._.__ ••_ •• 11 ill 12

I In l<'st~, """h) on plnln s,"lrf wing-h

WING RIBS Wing ribs ure glued with casein glue. The prmcipal species of wood uSl'd are Sitka !>prnce~ mahogan)', yellow poplar, and basswood. The gluing of the rib!> involn~s the making of muny joints of small size and is almo~t entirely a hand operation. The ribs are assembled in jigs or forms, the contact surface!> of the pieces are couted with glue, and brads are commonly inserted at the joints. This procedure is tediou!>, tiInt:' ("on!>uming, and the gluing conditions are usually not the best (reB). In rib-gluing opemtions no\\- ('ommonly used, three conditions are upt tu be llllfa\"oraull' to the proLiuetion of strong joints: (1) 11' l'egular glue !>pr('uds; (2) too long an as!>elllbly time on some joints; :mcl (::~) inadt'Ir·ips. (Figo 19, A and B.) Similar equipment may be IIsl'ful for' ("oating };tifl"l'nC'rs, diagonals. and otll('l" P!lrts at the contact points. Pressurl' forms an' also used !>u("cessfully for gluing Clip strips to tIl(' wpbs and gluing diagonals and stiffeners in place. These Illl'thods wi II doul)tIess i!1(Tl'ase as models and types of aircraft be("onl(' more standardized and as the size of manufacturing opera tiom; in("l"l'as('. The u::,e of such mechanical appliances for spread ing and ])!·l's.sing tends to reduce the assembly tin1(' and will re~,;ult in more p("onornieal production by gluing the ribs in multiples. Most wood surfac-l's that IU'e glued in rib ('onstruc-tion are side grain, t1wrefore the gluing conditions r(>('ornmended for gluing Sitka spruce with easein glul' into laminated construction (p. 43) are recommended for the gluinrr of wing ,·ibs. How(·wr. in gluing the cap strips to the w{'us or ribs thl're may be ill\'ol\"ed the gluing of a. certain amount of end-grain wood. The use of plywood in the web results in the end groains bC'ing l'x.,losl'd on approximately one-half the area of the {'dg('s. This condition 1H'c-{'ssitates bringing a larger area.of the cap strip into ("on/aet with the plywood web, which can be done by set ting the w('b into a grooo\"l' in the eap strip (fig. 19, A). or by dividiug the eap strip and gluing a pieee to l'ach side of the web (fig. 19, B). Beeausl' of til(' difJiculty involved in bringing about perfect contact in the grOOVl' tyP(' of joint, the divided cap-strip type is easier to glue to obtnin a perfect joint.

BOX BEAMS The construction of box beams consists of gluing diagonals, or spacers, and reinforcing, or fittiJi~ blocks, between the flanges and of gluing plywood sheets to the srdes. (PIs. 11 and 12, Band C.) SPnt("(' is the prl',"ailing wood for all parts, except the plywood, which commonly has either spruce or mahogany face plies. "ohel"l' rO('infol"c-ing blocks, sllch as 1n Figure 19, and Plate o C, a, 12. B, ar (' glul'd into box beams at fitting points, the grain of the bloeks is approximately parallC'1 to the grain of the flanges or it may

be at an allgle of us lllueh as 30° 0 'Vhel'e the grain of the blocks is G.LUING WOOD IN AlltcnAFT :MANUlfACTURE 47 at an ang1e of a;; much It;; i30" ot' more the proecdUl'e recommended fot" glui ng end-gm in joi nts (p. as) shollId be followed. Likewise, diuphragHlslIsed to ;;tiffen a b(':lIll'or hollow mcmb('I", u;; shown in li'igurc 11>, C, v, and E, a, and in Plate 11 involve the gluing of cnd

~r=====S A

I/'Ii \ ,I'

.'!. , ..'/1' I()

------.-_ .. -----,. ------~

F FIIH'Rfl l!l.-SpedaJ joints mn<1~ 1n aircraft: A, S<.'dion of win~ ribs; B. win~ rib.\)rlllll Ilf'l<('lllhly; C. £INllils of hox·bl'lIl11 construction; D. cross HC('t\on ,,[ Jllmlnlltl'i1 I hpllms; g, bullt,ul> laminated construction (airship); lind F, wrnppt.'d \"eIlCf..'t" mt!'rnlwr

grain to side-grain ,yooc1 wholly 01' in part, and the gluing directions for end-grain surfac('s should be followed. In gluing reinforcing blocks or spacers into box beams, clamps or other pJ'es~mre .dt>\'iC'cl:l are ordinarily used. (PI. 12, B.) The clamps should be of such ;;ize and so spacrlt as to apply the proper anioullt of pressure per square inch of glue line area. 48 ~mcnNlCAL BULLETIN !:l05, U. S. DEPT. O.l!' AOHrcuLTufm

In gluing the plywood sides to the framework of box beams, both surfaces are side gl'llin. (Fig~ 19, U and C and pI. 12, C,) Never theless the joints are subjected to reillti\'ely severe internal due stresses to u difference- in shrinkage with any moisture change of the cl'oss-banded fact'S and the solid wood flanges. Allowable streRses working for sllch joints should be based 011 plywood rather l:unirlllted than wood values, l\laxilllu1l1 str:ength of such joints is im l)ortant, lind the r'ecollunendations fur gluing laminated members should be ('arpfully followed (p. 4;3). III cnrrent practice is pressure often applied by the insertioll of bmcis and in some instances by means of pl'esses and clamps, 1Vhethel' brads lire uf:ied 01.' not. desiruble it is that pressure be applit'd in a, press 01' by mellnS of hand suitable dumps. Becll\(, ~ of the size of the joint Ilreas, however, it is recommended that jacks(,rews (fig, 13, :F) or power presses be used, where l)/'udicable, If brads are used, the pn'ssure may be applied dther before or,' after their ins!'L'tion. If applied afterwards the time elapsiI1" betwe('n spreflljing the glue and pressing shonld not exceed ~o rnin~tes, If pressecPfil'st, it is best to allow the glue to set for at least 01' () 5 hours bdorc inserting the brads, but the joint need not be undt'I' pl'l'ssure all that time, It is recommended that the prcssure be applied for at least one-half hour,

GENERAL ASSEMBLY In wing ass('mLly the ribs are fastened to the beams (pI. 15) and other: parts and in some cases all or a part of the wing structure covered is with plywood, In most of the joints, side-grain faces are in ('ontact, und recommendutions for gluing laminated constructions should be followed (p, 43), The grain of adjacent pieces is fre quently at right angles, ho\\,e\'er, and severe internal stresses occur on such joints with chan!!es in moisture content of the wood, hence the necessity for ('areful gluing. In gluin/! the cap strips of the ribs to the beams, precautions must be used to insure all adequate amollnt of glue uniformly applied the to joiut arca, Pr!'ssllre should also be applied to these joints by Jlwuns of clamps. This should be done even though brads or screwS are ust·d, In the gluing of shear blocks to the beam and wing rib (shown in iig. 19, B. (!i), the grain of th!' blocks is a('ross 01' diagonal to the face graill of both the rib and the beam, bringing into contact side grain faces in It similar way as in plywood, The pieces are glued usually and nailed with brads, but the only pressure that is applied to the joints in gluing results from the insertion of the brads. This procedure is not likely to bring about complete contact with .casein glue, owing to illsufficient andllonunifol'm preSSllre, and the stren6rth of the joints might therefore be expected to be el'ratic. reliable A mOl'e method is to apply pressure with small clamps and allow the glue to set. This may be done either immediately before or after the insertion of brads. \Vhere regular clamping is not feasible a, momentary ltpplication of the, proper amount of pressure aiter insert ing tlJ() bl'llds will be bettel' than 110 damping nt nIl. Reinfor'cing blo('ks of hardwoods, sllch as nsh, bir'('h, maple, or ouk, 01' of plywood are frequently glued to solid or laminated spruce T«", But. 2();, l .5. DellI. (,r A~n

WINGS GLUED UNDER PRESSURE

.\t \lL \\llod \\ HlI! stt'lIf'lun' ;l.!'st~mhh'(1 wlthnnt the ('ustomnn· hrnds. Pressure was HI)pJicd with d:unps or ..::wul"r dt~\'tnls tn :111 joinl$; 11, win)! rlh glued in 0 jig un:111 ~lrl]J:: of llll~ W!IlJ.! nh WVrt' l!huld lJlit!('r pr{'sSlIrt'• 'nH.~ lUlldred odd joints hI"t \H'PIl !-:Us..'t'lS ~\JlI nlhl"r P:lrts of tlw rih W['fP gltwtl und,')' pn's!,II'"4' IlJlPlied by illscrtin!! hrads ait hough ~Ul'll jlJltlts alford all Ul'ponuUlIY for ltpplYlUg I he Jlr"~titHC by power or dump de\'ices, Tcch. 13ul. 205. U. S. Dopl. o( Agriculture PLATE 15

WING ASSEMBLY

~\t nllJilu! Ow nh, lO solId :-\pnl(·(' :-opnr:-: or n Inpl:ulc \\,jn~. PrrssllfC'. is npplied only hy Hwans of !In,,ls. II, ~l\lflll\~ tIl :\s,;~l\lhl\' 1111111"11\>11\""" BIt!. Joints which hold the rills nud Ill.'l.ms tllgethe~ nre highly slrc..'

PIUNCIPLES OF CROSS-HANDED AND LAMINATED WOOD CONSTRUCTION

The llIakin~ of sntisfactol'y ~llled joints of any type depends not only on ~o()d gluing but also on propel' design. The lwineiples or rules that ~on'I'n all glued-wood construction arc based largely on the avoiclancl' of inll'rnal stresses 01' the balancing of these stresses when necessarily pn·sent.

CROSS-BANDED CONSTRUCTION

Chan~es in the moisture content ofwooc1 indu('e or relieye internal stl't'sses. In (Toss-banded ('onstl'Uctions (fi~, 9) thesl' stresses are ehidl), the result of the arrangeml'llt of the plies at different angles. Adjael'nt plies tend to shrink or swell across the grain under moisture changes; and since each ply, by reason of its small longitudinal shrillking or swelling, restl'ains thc adjoining ply or plies, large intcrnal stresses are set lip in each layer. If the plywood is drying, all plies tend to shrink in the direetion across the ~rain. Thus all plil's arc put. in tension Iwrpendieulnr to thc ~rain and in compression parallel to the ~rain. There is, in fa(·t, It she:lI,jng force at work on thl' ~Iuc joints. In plywood that is thus stressed from drying, moistening the plics tends first to relieve the stresses und, if con tinued far enou~h, to introducc the exactly opposite forces. Moisten in~ and (lrying, then·fore, aitel'llate in their action, each at first cor I'ccting and finally I'l'\'ersing the effects of the other.

::! A IInl((1'<' ,'('npI'r /!llIlng operntlon is 1'lIlploy('(l In a. Pacific coast nll'cmft factory in thl' constrllt'tioll of 1I1111l0plnll!'s, 'I'he fus!'la/!", which is n 1Il0IlnCO((lIC t>'pl', is Sh:1PNl and /!hu'd III a ('()II('rl't(· furll1 IIHln/! ('''lIlpr('~So'!I all' to Ilpply prpSSlII'C, SIII!;ip-ply \'~nel'r is first phH'o'll ill (\11' fOI'II1, II ('unt of caso'in /!h,l' applipd, and a sprollll ply is tb(~n forcI'd Intu position ill 1I1l' 1'01'111 IlY " 1m/! of COlllPI"'SSo'!I ail', '1'he !;,'aillS o( thl' two plies art! Intel ut I'ight HIl1!'I('s. 'l'hh; «·on~trl1(·tioll I:; l'l'}HII'tl'11 t"O 1!in,' j.!rpat stl't'lIgth aud J)osith"c all/!llnlt'llt. with I'xtrl'lIIl'h' Ii!!ht \\,o·i/.:ht (IS). ,\ 8i III i1:11' tYIlt' of "Ollstl'lIClioll has bl'en lISPt} in pontooill" :1Iul flyihg~hoHt hulls; tll(' velH'PI" wllh'h is glupd winl marine glue and fustelled to tbe fmllll'wll,'I, with Ill'll(!>; 0" slIIull nails, is applied US a cuvering. 50 TEOilNICAL llliLLB'.rlN 205, U. S. DEPT. OF AOUJOUV.rIJHE

BALANCED CONSTRUCTION Thl'. stresses in cross-banded construction must be approximately bltianC'ed in the panPl it!iclf, 01' there will be a str'ong telldt'ncy towal'{l di!ilortion and wllrping, The necessary balance of force!i is provided by using an odd numbPI' of plies and so arrangin~ them that for any gi\'t'll ply thpI'e is II similar, opposite, und parallel ply equally distant frOIl1 thl' ('ore or ('entml ply. As already explained, jf two IJli('s nrc ..IUNI toc'ether with the "min at a. ricrht anlrle eaell 1)ly tends to dIstort"n the otlwr whell any nmoi!ihu'e ehangen o('('urs,n' and. cup ' ping is the result. By lI!iing It third ply with the gl'Uin p:u'allel to that of the otlwl' outside, 01' fuee ply, a balaneed (>fred is produced. The corresponding plie!i !ihould not only oe parallel to each other lind perppnd icular to adjacent plies, Lut they should I)e of similar propprties (7), Obtaining !i,YlI1n1l'trieally placed !iimilar plies il1\-ol\'es a con!iidel'a tion of: (I.) Ply thi('kl1l'!is, (2) kind of wood with pal'titulal' I'efer ('IH'C to (a) !ihrinkage and (0) density, (;3) 1ll0i!itlll'(' eontent of wood Ht time of "Iuin", and (,~) an"le 01' relative direction of "rain of 1Jli l's, rt t"" t"" . p It i!i pO!i!iible to get a balanced cOIl!>truetion without having a strietly parnllel-an to t.hat of the fir'st pair, This principle applies only to fh'e 01' a largt'I' odd numbel.' of plies, In some air('raft parts, how evel., fl'('('(lom from warping tendencies is of less importance than other lIl('chanieal ehal'HC'teristi('s. and more or less complete depar tures froll1 the principle of balanced construction become necessary. The amount of inh>rlHtl stress on the ,!rlue joints of cross-banded ('on!itl'uction is dptel'mined ("hiefly by the thickness of thp plies and till' den!iit.v alld shr.inkagp of the woods involved, Thick layel's and woods of hi~h d~nsity and high shrinkage pl'opelties develop the largest stl'('!i!il'S, Tabl(' 11 contains shl'ink:ige and density values for It IlUrnl)('I' of wood!i which are now glued for ain'rait together with 8e\'l'l'a] woods whose properties or abundance indicate that they may (wentually be used, In plywood the face plies. being restrained on only one side. l'xer:t a relatively large sh'css on the outer glue joints. For this reason face plies should, in geneml, be thinnel' than the in terior plies, One-eighth inch is about the maximum thickness of faee plies that cl1n be held secm'ely in place under severe moisture changes, and whel'l' considerable strength of joint is required thin ner plies must be used. GLUING WOOD IN AmCnAFT MANUI,'ACTUnE 51

1'ARI~1il 11.-ShrillkllyC anlt tIC1!,~ity of 1cood,v glllelI. for airora.[t. 1

1 ShrInkllge 1 Shrlnknge -,-- Den· .. - Den· Spo~les Species ,rlln.\ . sity 1 'I'nn. \ . sltyl genllnl Rlldlnl gominl Rudml . ------:'--I--·~ ------I Ptr ant Per cent Mnho~nny (Swielonlll SilO' Per twl Per alii O. ·16 Red onk...... S. ~ 4.0 0.03 c1es) •••••••_•••••_•..•.•, ~. 7 a. ,I .64 Northorn white pino..... 0,0 :!.:I • :10 Whlto ush (seconll growth).. 8,7 5.3 .-11 Yellow hirch...... U.2 7.2 .6:1 Wostorn yelluw pino __ ... 0.3 3.0 .69 Yelluw IlOplnr ' ...... ' 7.1 4.0 .41 Whlteollk...... , 0.0 5.:1 IIllLrkcherry ...... __ 7.1 :1.7 .51 Cottonwood, OIlS!ern...... 9.2 3.9 .4:1 .57 Su~urll1nt>leM_~ « __ .. ~ .. ~ .. _ {t5 4.9 .n:! lJlnck wnlnlll .... ,...... 7. I 5 'J Sllk.l spnll·.L...._.•. _. _.. 7.5 ".3 •as B.L'SWUO(I...... \I. a 6.0 .as 51' .American elm".~ .... __ .. _.. _.. _~ 9.5 4.2 .5l '''UllOI()gt1UL~~ ..... ~_~ .. w..... 7.ti ·1.2 .50 Retl 1:11111...... O. U 5.2 .49 SyclLllltJre_ ...... ' ...." 7.0 5.1 .11:1 ])ulI~hL~ lIr (wesl COIL~L) ... , 7.8 5.0 .51 Beech...... 10.6 4.8 nod IIIl1plo ....'...... 8 'I 4.0 .501

I Dlltn (r0111 U. S. Dept. ,\gr. 'l'e!'!l. Btli. 158 eX('tlllt (ur 11111hoglltly (IS). 2 Shriflka~o from ..-::rt.~11 to o\'CIHlry conditions oXJ)ro.~ed in l}(~rcontngo of dimensions when green. Lonei tudlnlll shrInkll~o o( norl\llll woo

LAlIIINATED.WOOD CONSTRUCTION Propellers are an important example of laminated construction in airplune work. In lllmi natNI construction (fig. 9, E, F, G) the gmin of plies is approximate'ly pumllel, and the shrinkage of all plil's combined is similal' to that of solid wood . . Howciv('I', no two pieccs of wood are exactly alike in properties. ",Vhcll gilll'(l togdhcl', ('\,cn 'with thc grain parllllcl, laminations with ditl'l.'rl'nt shrinkage properties dc"elop stresses at the glued joints, which wl'alwn till' lIll'mber and eausl' it to change shape. In most laminated construction, and especially in pl'opellers, it is of the highest importanee thllt such strcsOiCS he llYoided. 'rho construction of laminated-wood products free from internal s~resses ~n \·ol.\'{·s the pI'incil?le of combi~ing p~cces l~f similar p.roper ttes, whleh, III tUI'l1, necessItates a, ('onSlderatlOn of (1) the kllld of wood, (2) the plane of cutting and direction of grain of pieces, and (3) the moisture content of pI ies. Pieces of wood of the same specil's are 11101'e apt to be alike than pieces of dif£cl'l'nt species. Variations in wood affecting its use in laminatl'll construction al'e mainly shrinkage (Table 11) and ratc of change. in moisturc content. A joint made of woods that have about the same rate of moisture change will be freer from strt'f;ses than ()Ill' whieh induc\('s both quic'k and slow changing woods. Con tral·.)' to the cll'eet l1oh'(l in (,ross-bandcd construction, diifl'rencl.'s in density of ndjal'l·nt laminatiom;, whell not accompanied by (liffer ences III shl'inkage pI'opel'tics, do not induce scrious inlc\'I1al stresses. 52 TECHNICAL BULLETIN 205, n. S. DEI'T. OF AGRIOULTURE Conti)ining' high and low dcn~ity laminutions of thc salllc specics, provided till'.)' IIrc othpL'\\'isc alikc, into thc same laminatcd COI1l;h'lIC tion dot's not U PPClIl' to ('allsc internal sh'csscs of large magnitude. Laminated specimcns under tpst ha,"c shown little weakening or tendency to warp froIII this caUSe (14). Thel'c is n ciiirPl'cnce in shrinkage bctwepn quartered anel flat-sawn material of thc Samc spccips, Although this difference is gl'elltet' some in woods than in othcrs, an a vcmgc of 11)0 species of wood shows n tangential shrinkagc of about 8 per cpnt and a mdial shrinlmge of ubout 4y:! ))PI' Cl'nt bctwe('n thc oven-ell.Y and satul'ated conditions (/0). COlllbining qu:u,t(,lwl and flat-sawn matel'ial is a disturbing fU('tOI' in ccdain dasses 01' luminat('d construction, such as propellers, but IlIUY he r('latiYl,ly unimportant in others. The (Iir('ction of thc gl'llin of adjacl'nt laminations affects the stl'l'ngth und shape of glu{·(1 membel's, Cro"s grain in a lamination I'P!mlts in It ('(II'lain amount {)f longitudinal shrinlmge of that par ticulal' lamination, ~\\'hel'eas thp. longitudinal slu·jnlmge of strnight ~rairH'(l material i>i vpry >illlall or negligible. Consequently a con struction madp of cI'oss-grained and straight-grained laminations is 1'('I'y lih:ely to warp, . StrN;sPS in InminntNl nwmhers l'Psulting from cliffe/'ences of mois tur(' conh'nt in th(' laminations l1a\"(' bel'TI found to disappear gradu Idly and without elanger of later recurl'ence if the glued block is kept Jor 11 long filliP in till' SHnl('. atmospheric condition, because the melll bel' cheC'ks OJ.' ehang'('s shape more 01.' INis to relieve the sh'e>;sed con dition of the fiu(lrs. If, however, the original "tresses are the result of cOlllbining qUHI·tprpd and flat-sawn material thl'y may die out durilJg a long conditioning, but as soon as the atmospheric conditions ehallge the stTPS:-;PS will be S(lt up nnew. In' gClJPral, ilH'n, it should be apparent. that for laminated-wood articl(ls in ",hic'h high strcngth and accurate shapc are required all plain-sawN1 01' all qunrtt'l'-sawed mai<·rial of the same wood or simi lar woods should be used, and all pieces "hould be at It moistul'(' uniform eont('nt wl1('n giued, Ii'oJ" the manuJacture of articles where sl ight ('hnn~N; in fOJ'1ll are of no consequence, these precautions are of the Unitl'd SI:ltpS:

GLUE FOR~IULAS FOR USE IN AIRCRAFT The following wniPI'-I'psistant glue formulas, with directions mixing, for wcrp

CASEIN GLUE FORMULA NO. 11:::r Inl;r~r1h'nts Parts by weight 'yatcrCns~iu______]00 220-230 Hylll'lltcd .Iillll'______"'at!!f______20- 30 ______100 Silicllte of soda (wlltl'r glmis)______('npP(II' chlol'illl'______70 \\'utl'r______". ______-______2 3 30- 50

::I Till!'! forlllllln, dl'Y"iopf'tl at til!' "0n'N t No, I'l"O,ill~t$ r.nhomtory. iB C!OI·erl't! lIy U, S. Pab'nt l,.t;;f;,S·I:!, II'lIlc'h is II 1"11 lin IIII' for til!' frc',' IIS<' of I!I"rl'J-Jltll,I:-1, thi' (IPoI,le of tllc' Unltr,1 States, S., /Iud C"cll'~:lIlnlH:II, ['. K, l'HUC'~:RN OF !Hn:s. (U, 8, J'ull'lIt ll.\:-1n',IC'rUIIIXG W.\'l'~:III'''OO~· AIJII~;" ~Q, l,t::iU,S.J:!,) 0, S, l'utc'lIl

GLUING WOOD IN AntCHA].'T l\IANU].'ACTUHE 53 , The 220 to 230 parts of water added to the casein is approximately the right amount to use with Argentine (naturally soured) casein; but if a, different casein is lIsed, tile 'water requirement wi111ie some where between 150 and 250 parts by weight. The formulu presupposes that a high-calcium lime will be used. A lime of lower grade may be used, but a proportionately larger ul\1ount of it will have to be added unless one is willing to sacrifice the water resistance of the glue. It is suggested that the usel' try 25 parts of lime to begin with. If this dOl'S not give good results, the amount can be varied within the limits speeifiecl. The density of the silicate of soda. used should be about 40° Dalllno with It Hilica-soda ratio of :hOIlL 1 to 3.25. Copper sulphate can be substituted for copper chloride. The casein amI water are placetl in the bm\"! of the llllXlllg ma chine, and the paddle is mad-e to rotate, stilTing the mixture until uIl tho watel· has been aU8orl>ed [md all the casein moistened. By allowing the casein to soak beforehand :l\>r a while it is more readily dissolved in the mixing process. In 11 separate container the hyd ruted lime is mixed with water. This mixtul'O is stirred vigorously at first, but just before it is added to the casein it should be stirred with It gentle rotary motion, j uHt enough to kccp the finest parti9les of lime in ~uspenHion. Pour thit; milk of lime quickly into the casein. "Then casein amI lime ure first combined they form large lumps, which are balls of casein coated with the partly dissolved casein. These ualls break up readily under the adioll of the mixer, becom ing smaller and smaller and finally disappearing. The solution, in the meantime, it; becoming thin und fluid, ",Vithin the first minute after adding the lime it is ,,'ell to stop the paddle and scrape the sides aml bottom of the can, and then stir again. If a deposit of casein remains unactec1 on, it may caUHC more lumps later. ",Vhen about two minutes have elapsed since the lime and casein were first put together, it will be noticed that the glue has begun to thicken lL little. The sodium silicate must be nel(led now, or else the :;lue will becomc too thic!{. Disregarding lumps in the casein, if they arc but few', pour in the sodium silicate. The glue will become e"en thicker momcntarily, but it will soon change to It thin, smooth, and fluid consistency. The stirr'ing should continue until the glue is free from all lumps. This should not take more than 15 or 20 minuteH, counting from the time the lime was added. If the glue is now a little too thick, It slllall amount of watu lllay be added. If the glue is too thin, how ever, it wiil be neeessal')' to diseard the entire batch anel start over a~ain, using a smallel· propol'tion of 'water. The copper salt may UC alhled at different times during the mixing process with apparently the sallle final results. 1£ added as It pow der or a solution to the caHein before soakin~, it is likely to huve It corrosi\"e action on the metal container, and for this reason it is advisable to add it later during the mixing process. If added at first, howe,·er, it should be thoroughly mixed with the caHein before the addition of the lime. It may be placed in solution and cOllven j(·ntly stirn·(J into the llloislened (,[\Hci11 immNliately before adding the lime or after all the other inf!l"l'dients haye becn combined and the mixture rendeL'cd smooth and uniform in consistency. ",Yhen 54 TBOHNlCAIJ BULTJBTIN 205, U. S. DBPT. Ol!' AGUIOULTUUE

added at the end of the mixing J?Cl'iod, it should be poured into the glue in n, thin stream and the mixture stirred vigorously. Stirring shoilid be continued until any lumps that may have formed at first by coagulation, when glue and copper solution mix, are broken up und a smooth, violet-colored glue is obtained. Glue pl'eparecl by formula No. 11 has pr'ovecl to be exceptionally strong and durable in aircraft construction, even under wet or clamp conditions. CASEIN GLUE FORMULA NO. 4B 2' Formula No. 11, as above specified but without the copper-chloride solution, represents an earliel' stage of development, known as for mullt 413. The mixing is the same as for formula No. 11, except fOl' tho omission of the copper-chloride solution. The glue has a medium consistency, excellent working properties, and a good work ing life. It. falls somewhat short of formula No. 11 in water-resist illg pr'operties, however,

BLOOD-ALBUMIN GLUE-HOT-PRESS FORMULA Z Ingl'e(llentH Parts hy weight Blood IIlhumin (UO pel' cent soluhility) -----______100 'YIt t'C't'______------______170 Allllllollium llydl'!Jxide (Sllccitie gravity 0.00) -----______4 Hydl'nted 11111IL______3 L 'Vuter______10 Cold watt'r should be poured over the blood albumin and the mlx ture allowed to stand for an hour or two without stirring. The soaked blood albumin can then be put into solution with a small amount of stirring. After the blood albumin is in solution the am monia, is added while the mixture is being stirred slowly. Slow stirring is nec.'essary to prevent foamy glue. The lime is then com lJined with the smaller amount of water to form milk of lime. The milk of lime is added and agitation should be continued for a, few minutes. Care should be exercised ill the use of the lime, inasmuch as It small excess will cause the mixture to thicken and become It jellylike mass. The glue should be of medium consistency when mixed lind should remain suitable for use for several hOlil'S. The exa('t proportions of blood albumin and water may be varied as required to produce a glne of greater or less consistency 01' to suit It blood albumin of different solubility from that specified.

PARAFORMALDEHYDE-BLOOD_ALBUMIN GLUE FORMULA "" Ingredients Purts by wclght Blool1 albumin (00 pel' cent soluhilit.y) ---______100 '\Vuh:>l'______140-201} Amm!JniulU hydroxhle (svecifh: gruvity OJ10) --______5lh Paraformllldehyd(' ------"-______15

,. ThlH fOl'mulo, dCI'plopl'd lit thf' Forest Pro(lucts Lnboratol'Y. Is covrred by U. S. Plltent No. l:.!Hl:3!)U, which is uvailabl(' fol' the free tlSI.! I)f the pl'oplfo of the ['nitl'cl Stall'S, n"'I"I'fat)f,IXo, S. 1'1{()(:f;"~S IW ~IAxn'A(.·rl1lt1XG' 1\'A'l'lmI'IIOOP ADlmHl\'EH. (IT. S. Plltcnt No. 1:.!!)lanli,) £1. S. l'ntl'nt Ollie!!, O.I. Gu... :.!58: :lr.~, l!JW, '" 'J:hIH formulu, 11"",'loped "t thl.! FOl'I'st I'I'nduct" I.uhol'lltol'Y, Is covf'rf'c1 hI' U. S. Patent No. l;{!Wunn, which Is uI'ailllble for thl' fr.... US" 01' thl.! jJ('ople ul' the i'nited S'"t('S. fl Il I1XII1U, S. n. Uf.I·" AXD MAI1l'I'A('TUIIII1U HA~JfJ. (r'. S. Patl'nt No. 1:l!W5fJII.) u. S. Pateut Ollle(', 011', Onz. :.!i1 : ~8. '.l!.l20. C'Il'I'hls fOl'mulll. dl'I'clopl'cl at the I"on'st J'l"I)clucts Labol'utol')', Is ctI\'erf'd h,' F. S. Puttmt No. 1,150541, which iH 1ll'1I1111hle for the "1'I'e \lSI.! of the pI'o"l" nf the UnIted States. LIXD.lnlll, A. C. UUJO[)-ALnt'MIN GLUE, (U. S. Patent No. U50il41.) U. S. Patent Oruce, 011'. Gaz, 311; 060, 1023. GLUING WOOD IN AIRCTIAFT l\;£ANUFACTUllE 55

The blood albumin is covered with the water and the mixture is allowed to stand for an hour or two, them stirred slowly. The am monium hydl'Oxi,de is next added with more stirring. Then the para formaldehyde is sifted in, and the mixture is stirred constantly at a fairly high speed. Paraformaldehyde should not be poured in so rapidly as to form lumps nor so slowly that the mixture will thicken and coagulate before the required amount has been added. The mixture thickens considerably and usually reaches a consist ency where stirring is difficult or impossible. However, the thick ened mass 'will become fluid again in a short time at ordinary tem veratures and 'will return to a good working consistency in about an hour. It willl'emain in this condition for () or 8 hours, but when the liqnid finally sets and dries, as in a glue joint, it forms a hard and insoluble film. This glue may be used in either hot or cold presses. When cold pressed, however, it has only moderate strength, and for that reason is not to be depended upon in aircraft construction where maximum strength is required. If hot pressed, it is high in strength and very water resistant.

WATER-RESISTANT ANIMAL GLUE FORMULA:n lngl'edlcnts Parts by weight Animal glue______100 1Vllter______225 Oxalic acill ______5.5 Pllrafllrmuldehyllo ______..______10 The glue is soaked in the water until the granules or flakes have been softened. It is then melted at about 140° F. after which the temperature is allowed to full to between 105° and 115° . The oxalic acid, in small crystals, and the paraformaldehyde, ground to a fine white powder, are then mixed together and added to the glue. The mixture is stirred until all of the oxalic acid has gone into solution, after which it is ready for use. The paraformaldehyde does not readily dissolve in the glue, and much of it remains as a finely divided solid during the working life of the glue mixture. A certain amount of agitation is, therefore, necessary to keep it evenly dis tributed throughout the mixture. The paraformalc1ehyde should b(> fine enough to pass through a 50-mesh sieve. The paraformaldehyde for use in this formula should be of the slow-reacting type. A fast-reacting paraformaldehyde appreciably ~hOl-tens the working life of the glue mixture. If kept at a temperature not exceeding 115° F., the glue will remain in a fluid condition for 8 to 10 hours from the time of incor porating the paraformalc1ehyc1e and oxalic acid, after which it will set to a tough, firm jelly whic11 can not be melted again. It is important to avoid heating the /rlue mixture much above 115° if a long "'orking life is required. Organic decomposition of the glue will not seriously affect the quality of the glue at this temperature, since the chemicals used in its preparation act as preservatives.

:n This formula, de'\"cloppd ot the Forpst Proc1ucts Lohorntory. is covered by U. S. Patent No. 1i120;;. which is 1l\"llilnble for the fr£'c us!.' of the people of the United States. HltUBEHKY. C. E., and Bl

130th oxalic acid alid paraiormaldehyc1e are poiBonouB materials and should therefore be handled with care. This glue combines the desirable characteristics of an animal glue with a relatively high-water resistance. However, the water resist ance varies directly with the temperature of the water to which the glued article is Bubjected. At room temperatures the glue is highly watel' resistant, but at 140 0 F. it is very low in water resistance. The joints should be conditioned for about two weeks at room tem perature to allow the glue to reach its maximum water resistance. LITERATURE CITED (1) .IhLEN, R. ",V., find 'I'm-Ax, rl'. n. 1m!>. (If.U!;:S USEII IN "\lm'LANEl l'AltTS. Nut!. AdYisory COlli. Aeronuut. Ann. Hpt. (l!lln) G: aS7-40S, illus. (Tech. Hilt. GG.) (2) BATEMAN, E., and TOWN, G. G. J!J2:J. THE IfYGnOSCOl'J:CI'f'Y OF Junm GLUES .I\"'D ~'nEl roLATION OF TENSIT.El S'I'ltENO'I'lI O~' GLUEl TO ITS MOISTURE CONTmNT. Inllus. und Engin. Chclll. 1G: 371-37G, iIlus. (::) BunusE, n. 1!)2ii. EI'~'Et'T OF GLUING CONDITIONS OX TIlE STltEXGTlI OF THE GLUE .TOINT nr PI.y\\·oon. 'lii 11., iIlu!;. l\1nlli;;oll, 'Vis. (TheSis, Unh'erfiity of 'Yiseonsin. ) (4) BIW\\'NE, F. L., alld DuousE, D. 1[128. THEl CONSfSTENC'Y OE' CASEIN GLUEl. 'Yl'isl'r, H. B .. Editor, Colloid Symposium l\1onog-raph v. 5 Jl. 229--242, illus. Ncw York. (5) DEnEAlTKI~LAElt, P. L., r'OwELr., J. R, and BAIIL]'[ANN, E. 1!'. 1!J30. I:;T.ANU.U(J) ]'fE'I'HOIlS (HEVISEU) FOR DI!.'TERlIUNINO VISC(JSI'f'Y AND .JELLY STI!ENGTH OF GLUE. Indus. Ilnd Eligill. ChclIl. Anul. Ed. 2: 348 :Jiil, ilIus. (6) DOUGLAS, 'V. D., uwl PE'fTIFOIt, C. B. 1!l2!>. TIlE 'l;J<:STING OF AUlIESI\'ES FOR TIMBER. .Tour. Hoy. Aeronuut. Soc. B;l : !>2-128, (7) Ef.],[P;NIlOI~', A. 1!l21. nATA ON TUEl DESIGN OF J'f,YWOOn FOR AIRCItAFJ'. Nnt!. Ad\'isory COIll. ACI·ollaut. Ann. Hp!". (1920) 6: 10D-122, illm;. (T('ch. Hpt. 84.) (8) FllARY, H. D. 1!>21. MI<:Itrrs OF rnFFEREX~' SPLICES FOit AJRPL.\XEl WING I1EAMS. Jour. Soc. Aut:olllo/iYe Engin. !l: 1:3:3-1:38, illus. (9) OUJ•. A. II. ]I)IG. -\ STUIlY OF V.\fl!OUS TESTS UPON GLUE, PARTICULARLY THE TENSILEl STItENOTII, Indus. Ilnd Eng-in, Chem. 7: 102-106, illus. (10)

1!l2G. A CO]'ll'AnfSON (lE' VAnIOUS TF..8TS UPON GLUE, PARTICULArtr.Y ITS TEN SfLEl Sl'ItEXGTH. IndUfl. und Eng-in. Chem. 17: 2fl7-298. (11) [ClnEAT RI!1'I'AIN]. DEI'AnnfJ<:x'r OJ' RcmNTTFfC ANn INIlUSTHIAT. RESEARCH. H)~G. lI[ECDANH'AL TEATS OF ADHESIYES FOR TIMBER BY THE IWYAL AIRCRAFT ~JHTABLISJIlIn:NT. [Gt. Brit.] Dcpt. Hci. und Indus. Hesellrch, Adhe;;;ivcs He;;;eurch COlli. Hilt. 2: 90-121, iIlus. (12) IIAJ:nECKEI:, ;1'. l!'. 1[)28. WOOl) ANn VENEERS IN TlIEl lI[ANUFACTUREl OF AIRPLANES. SO. Lum uel'lIlun ]:33(1734) : lii9--1Gl, iIlus. (13) ]1)29. YENEERS EMPLOYED IX :r.r.\SS AIRPLANE CONSTRUCTION. Veneers 2:J (:3) : 18-20, illus. (14) HEDr, A. L., KNAUHH, A. C., und REUTTER. L. 192;1. INTERNAr. STREHSES IN LAlIUNATED CON STnUC'I'IO X. Nutl. Advisory Com. AeronllUt. Anll, Rpt. (1!J22) 8: 327~,!82, illus. (Tech. Rpt. 145. ) (lii) HOI'I', G. ID~t). IJETElRlIrINA'I'ION OF TUEl TENSILEl STRJ<:NGTH OF GLUE. Jour. Indus. Hllll IDligili. Clll'lIl. 12: :JiiG-:JG8, mus. GT~UING WOOD IN A:mCnAl~T :MA.c~UFACTURE 57

(Hi) KOEnU:Jt, A. 1\)'24. TIJIil l'IWI'E1tTIES AND TSES Vb' WOOD. ::154 p., illus. New York ami London. (17) McBAIN, .T. 'V., and 1101'KIXH, D. G. 192r.i. ON AllHESIVES AND ADllESlVE ACTION. Jour. Phys. Chem. 29: 188 204, illu~. (18) MAItKWAllUT, TJ.•T. 1m:0. COll[I'.\ltATIYE STitENGTII rIWPEItTlES OF WOODS GIWWN IN THE UNITED STATES. U. S. D<,pt. Agr. Tech. Bul. 158, 39 p., illus. (19) NEWLIN, J. A., and 'YILSON, 'r. H. C. IU17. J.U:CU.\NICAL I'IWl'lmTlES OF WOODS G1WWN IN THI! UNlTEl) STATES. U. S. Dt'llt. Agr. Bul. 550, 47 p., illus. (20) Sl'oxsr..F.It, O. L. 1!)!!). WNI'Elt PROOFING l'ANELS. F.\CTOItS AFFECTING 'lIIE WATEIt RESISTANCE OP i'T.YWOOD. Hardwood Uec. 47 (8): Z~-24, 26. (21) TH~::J.EN, n. ]U21). KILN IlItYlNG IIANollOOK. U. S. Dept. Agr. nul. 1186, 64 p., illus. I'" (Ht'\'iHNI. ) (22) TRUAX, T. H. V·~' ]!l~i>. OI.UlNO I'ltACTIC~, AT AIRCItAFT 1oIAl"Uf'ACTURING PLANTS. Nat\. All viMry COlli. Aeronuut, 'l'l'ch. Nut(' 291., 11 I'., illus.

1!)28. GLUING WOOl) IN .\mCItAFT WORK. Amer. Soc. Mt·ch. Engin. Trans. (Apr. 50-16) 4!1-50: Ul-G-!. illus. (24) ]929. THE GI.UING OF WOOD. U. K DI'Pt. Agr. nul. 1500, 78 p., mus. (2r.i) --- BUOWNE, ]~. L., ami BnousE, D. 1929. SIClNIFIC'ANCE OF lIfECIIANrCAL WOOlhTOINT TESTS FOR THE SELECTION OF WOOUWOIIKING GI.UEB. Indus;. !lud Engin. Chell!. 21: 74-19, illus. (2(i) 1.'NlTEI) HTNrES DEI'.\ItTlIIE"T OF AGltlCULTURE, FOIIEST SERnCE. FOREST l'ROIlCC'TS LABOItATOny. 1919. WOOII lN AlItCRAF.T CONSTRUCTION. 149 p., iIlus. Reprinted from Nnvy Dept., llur. AerOllllut. Aircraft Design Data No. 12. (27) 1\)28. MANUM. FOR THE I NSI'E(,TfON OF AIRCItAFT WOOD AND GL1.:E Fon THE Ul".lTED STATt."S NAVY. Navy Dept., Bur. Aeronaut. SD-:U, 152 p., jIIu~. J (21)) UN1TE11 ~TA'l'ES 'VAlt DEI'AUnIElNT, Am CORPs. r· 1920. 'l'1I1il AllU'LANEI. ;;4 p" ilIus. Washingtoll, D. C,. ORGAXIZ,\TJOS OF THE rXITED STATES DEPARTMENT OF AGRICULTURE WilEN TItIS ,'rBUCATrON W,\S LAST PRINTED

S('f]r(lclljf of ..IVri{'lIitllr('. __ Al<..ik/allt S('C'rdl1rfl J)rreNor Ilf }kiC'IIU/lc' Work __ .. ______..\. F. "'0011". J)in'('/or of RC!lulatory I\"orl: .•• ______\\'ALTElt G. CA~II'IIELL. ))/I'("dor of J:.r/('II.viol/ Work .___ • ('. \\'. "·AltIlt:ltTOX. D,n'dor of 1"1"8(11;IIC[ al/d UIIJ I:;.J'/ell,~ion 11'01'1.:______('. B. SMITH, Chief. J.,ibrary ------______CL.mInEL H. BARNETT, Librarian.

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Forest Service------______-. H. Y. STUART, Ohief. Brul/(:It. Of H(>scarch ______". EARLE II. CLAPP, AS8'isla.llt For ester, in Charge. Ji'orC8t Pr(}cll/(·/.~ Lauo/'{//orJl______CARLILE P. "'JNSLOW, Director Section flf Wood Prl'NcITuliml ___ . Gf:()RGE l\I. Ilux'r, Ser~ior Clwmillf, in Clwrge. G8

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