The Aircraft Engineer May 29, 1931

7/27/2019 The Aircraft Engineer May 29, 1931

1/8

M e, 29, 1931 Supplement to FLIGHT

ENGINEERINGSECTIONEdited by C. M. POULSEN

May 29, 1931CONTENTS

Hull Design of Flying Boats. By William Munro, A.M.I.Ae.E....OnSolid Rivets. By M. Laugley, A.M.I.X.A.. A.M.I.Ae.E.Technbal Literature ...

PAGE.. 33.. 36.. 40

HULL DESIGN OF FLYING BOATS,B Y W I L L I A M M U N R O . A. M. I . Ae . E .

If the hull design of a flying boat is sufficiently bad,the machine will never get in to the air to dem ons t r a teits aerodynamic qualities . In the design, therefore, of

any flying boat the character is t ics of the hull must begiven ear ly at tent ion.In the present ar t ic le it is proposed to describe brieflythe type of flying boat hull most generally used inEngland and to indicate the main reasons for its adop-t ion. The type refer red to is tha t hav ing a vee-shapedplaning bottom and two t ransverse s teps , and built ofei ther duralumin or Alclad.

General Principles.Size of Hull.The size of the hull is governed bv thedegree of seaworthiness required for the par t i cu la r de-sign being considered, since obviously a boat operat ing,

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2/8

31SUPPLEMENT TO" FLIGHT THE AIRCRAFT ENGINEER MAY 29, 1931

mcathese requirements conflict with load carrying require-ments, a compromise is generally made to suit tieparticular case and conditions of operationSeveral attempts have been made to do withoutstep in the planin g bo ttom for stru ctu ral reasons.Obviously, the break in longitudinal members occasion^by the existence of a step demand special attention ^construction, which in turn means time and COSL 8additional structur e weight. Fu rth er, the steps erwadded air resistance. Despite these serious objee ^the step is essential to satisfactory hull design- ^function of the step is to break down the water uC 'and enable the plane to " unstick " from the I t is extremely difficult to cou nteract the water m^j ^acting on a step-less hull with ordinary horizon'surface s. As far as one knows, no successfulboat has ever been built which did not have "one step. -.

4786


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MA Y 29, 1931 SSTHE AIRCRAFT ENGINEER SUPPLEMENT TOF LIG H TPlaning Bottom.-The vee shape of planing bottomis favoured because of it s super iorit y to the flat bottom

in absorbing the shock of landing, and in addition canbe made adequately s tro ng, with considerable saving inweight as compared with the flat bottom.The dead rise or angles from keel to chine are chosento give dynam ic stab ility and good take-off. Cleanrunning is obta ined by m ak ing th e fore body sectionsslightly concave.Determ ination of Lines o f Hull.To determine thelines of the hull which will ensure clean and steady run-ning and good take-off q ual itie s, m any d ifferent formulaehave been devised. Prob ably th e most reliable meth od,however, is to follow the shipbuilders' system of utilis-ing a known successful form as the b asis of the newdesign and submitting a scale model thus developed totank tests. In a new design , however, for a flying boa tof unique lay-out, w here no sim ilar ship could be usedas a basis, the m odel would be developed by tr ia l anderror, bearing in m ind the funda me ntal principlesalready stated.For a ship of conventional design the designer holdsto certain prop ortion s which can reasona bly be reliedon to give good results, since these are based on t heknown behav iour of num erou s full-scale flying boa ts.The main proportions a re :

1. Proportion of beam to displacement.2. Proportion of nose to s tep distance.3. Proportion of length to beam.4. Distance between steps.Fig. 1 shows a graph of these proportions which hasproved useful when st ar tin g new hull design. Thelines plotted are th e mean of a larg e num ber of know nsuccessful flying boats.Item 1.The maximum beam in inches is tak en as four tim esthe cube root of the displacement in lbs.*Items 2, 3 and 4.These dimensions are arrived at by using a multiplierderived from the cube root of the ratio of displace-ments. For example, if the known ship has a totaldisplacement of 6,000 lbs. and a beam of 73 inches,then the beam of a new ship of 8,000 lbs. total displace-ment would be

J ' 8 000 _ . ,A / . . X 73 inches.V 6,000Keel heights above datum, chine heights , chine half-breadths and all other linear dimensions would simi-larly be propo rtioned up from th e know n hu ll bymultiplying these by/JV 6,000The wing and power loading of the two ships should beapproximately equal.A set of lines draw n u p from th e figures obta ine d bythis method will give a shape of hull possessing verynearly the same cha rac teris tics as th e orig ina l. Beforestarting constructional work, however, it is advisable> have tank te sts mad e. The m ethod of propo rtion-ing En should be looked on a s a quick me ans of ar riv in g. re -sonably good lines which can be cleaned up byinvestigation in the tank.A sst of hull linos is shown in F ig . 2. The up pertract-are shown is of the simplest form, and would,ftf ?? arse> **e modified for the particular requirementshav I d e B i g n i n h a n d - T h e planing bottom lines shown8 h >en proved successful in pra ctic e, altho ugh a morePronounced turn-down at the chine would probablyn even cleaner running.$' 3 Sives the tables of offsets for the lines shownig 2.B "f Curve for Planing Tiottnm.In the layout ofra vee planing bottom, a master curve can be

p^*PlM>f Developments" l.y Major R. B. Penny i n " The Journal' ronutioai Society," September, 1927.

MULL OFFSETS.XoIFP' /3*/ i

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4/8


5/8

MA Y 29, 1931 THE AIRCRAFT ENGINEER SUPPLEMENT TOFLIGHTpermissible shear stress of the material, i.e., x /, .Its bearing strength in a plate is given by the thick-ness of plate (t) multiplied by the diameter of the shankid) and the maximum permissible bearing stress of thematerial, i.e., t X d X /(,.

RADIUS. -A-HNECESSARY

FLATDiagram

forTable 2.

Assuming that both rivet and plate are of the sameor similar materials, we get a relation between thesetwo strengths such that, if made equal, i.e.,

X f t d fthe collapse will be simultaneous. This gives us themost economical size of rivet for any given thickness ofplate. In duralumin, taking / , = 16 tons per sq. in.and /& = 32 tons per sq. in.,

KXd*X 16= t yd x 32

412-56^ = 32-t.d12-56d = 32t

d = 2-55 t

fitting. The appropriate sizes mayfollows: TABLE 3.

be tabulated as

ThicknessofPlate.

22 s.w.g.20 18 16 ..14 ..12 10

Diameter of Rivet.

Alum. AlloyandMild Steel.

V -in.dia.8 " "

i

Stainless andH.T. Steel.

i8

i

It will be noticed that thicknesses below 22 s.w.g.have not been considered. Tests June shown* that therelationship breaks down. This is an important point,as a very considerable amount of aircraft riveting isdone in structures, such as strip steel spars, which arefrequently in thicknesses down to 28 s.w.g. Mr. Rad-cliffe is led to the conclusion that the bearing strengthof the rivet need not be considered in these cases, thecriteria of strength being the shear value of the rivetand the bearing and buckling values of the plate. Thisbeing so, he concludes that rivets in single shear are

F16.I F IG . 2 FIG 3

FIG. FIG.5 F1G.6

FIG.8Similarlyinmild steel where/, = 18 tons per eq. in.and fb = 38

d ^stainless steel rivets (D.T.D. 24A)

kigh-tensile steeld= 3-18 *.

C l this it appears that the most economical size ofiactDlatl fv,aPProxima*ly three times the thickness of the


6/8

SUPPLEMENT TOFLIGHT MAY 29, 1931THE AIRCRAFT ENGINEERIn fittings where only one or two r ivets are used, thesefigures should be reduced by 30 per cent.

Dla.

In .r*AiAAjV

Or rawSectionArea,Bq. in.

0-O0S10-00690 01230-01920 02760 04910-07670-1104

TABLE 4.Single Shear S trengtht.Alum. AlloyT).T.l).110/ , - 35,850ib . per sq. in.

I,b.I l l2474416899901,7602,7503,060

Mild Steel andStainless SteelJ).T.J). 24Af. = 40.320lb . per sq. in.L b .1252784957751,1121,9753,0904,450

H.T. Steel/ . = 74,000lb . per sq. in.

L b .2S05109101,4202,0403,6805,670S.175Nat*. For doub le shear mu ltiply these values by 2.

T A B L E 5.Bearing Strengths ofRimts inPlates above, 002 2- in . (24 s.w.g.).Aluminium Alioy, Spec. L . 3 . / t = 70,000 Ib. per sq. in.Dl.ofRivet.

In.A5 ?JftiI'Vi

22R.w.g.0 028in .

122183245307868

20s.w.e.0 086in .

167236815895474680

18R.W.g .0-04Rin .

2103154205256308401,050

16s.w.g.0 064in .14s.w.g.0 080in .

12s.w.g.0 104in .109.W.R.0 128in .

8B .W.g .0 160in .

Grea ter tha n doub le shear values"~4Hr56 070 084 01,1201,400

| ' of alnmin iumallojy rivets.~ 7 0 0 ~ | I875 1,1371,050 1,3651,400 1,820

1,660 1,7502,1002,2762,730

1,6822,2402,8003,8602,8003,5004,200

T A B L E 6.

Di.OfRivet.

in.A*AAfti

Mild Steel22S.W.g.0-028in .

175262350438526

20s.w.g.0 036in .

825387450563676900

, S 3 and/.=

18s.w.g.0 048in .

4506007509001,2001,500-~ -

StainleBB Steel D.T.D. 28100,000 lb.16s.w.g.0-064in .

Greater" 80 01,0001,2001,6002,0002,400

14s.w.g.0 080in .

than1,000"1,2501,5002,0002,5008,000

12s.w.g.0 104in .

doubleM.S. an

1,9502,6008,2503,900

D

10s.w.g.0-128in .

shear vd 8.S.

3,2004,0004,800

8B.w.g.0-160i n .

alues ofrivets.

4,0006,0006,000T A B L E 7.5 percent. Nickel Steel Sheet, S 4./ - 161,000 Ib.

Di.OfRivet.if f

kAAiAi

22g.w.g.0-028282421568706846

20s.w.g.0 0863825437249061,0891,448

18s.w.g.0-0484847259651,2081,4501,9302,415

16s.w.g.0-064

9651,2871,6101,9302,5803,2203,860

14s.w.g.0 0 8 0reater t

1,6102,0152,4153,2204,0254,830

12s.w.g.0 104handou

of

2,6208,1404,1805,2306,280

10s.w.g.0-128blesbeaH.T.r iv

3,8655,1506,4407,730

8s.w.g.0-160r value*ets.

6,4408,0509,660

The method of stressing a joint where the loading iseccentric is not so frequently explained, yet such jointsare very usual on the wing and fuselage structuresof aircraft .Imag ine a control pulley standing off a member andsupported from it, as shown diagrammat ical ly in Fig. 10,The vertical reaction of control wire load is 1,000 lb.acting parallel to the centre l ine of the member at adistance of 5 in.

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^ F I G . I O .

lbs

An examinat ion of the diagram shows the centroidof the r ivets to be at the point G. There are two statesof loading on the r i v e t s : 1,000(1) Direct and equal to lb. per r ivet .

6(2) Resisting the couple, 1,000 X 5 lb.- in. , in whicheach r ivet takes a load appropr iate to itsposition in re la t ion to G.The couple is, in effect, a bending moment resisted bythe moment of iner t ia of the system of rivets, i.e., a y2, where " a " is the cross-sectional area of a rivetand " y " its distance from the centroid G.The distance of r ivets A, B, E, F from G is (byPythagoras)v / l - 6 * X 1-251in. = V/2-25 + 1-5625 = \ / 3 - 8 1 2 5 in.and the distances of rivets C and D = 1-26 in. Eliminat-ing "a, " we have

Sy = (4 x 3-8125) + (2 x 1-251)= (4 x 3-8125) + (2 x 1-5625)= 15-25 + 3-125= 18-375 in.'

RIVET'A' RIVET'B' \

RIVET'CThese are the most usual mater ia ls for a i r c r a f t fit-t ings . Par t s made to other specifications may be in ter -polated. Stainless steel sheet to Spec. D.T.D. 60A isst ronger than the mate r i a l of S.S. r ivets to Spec.D.T.D . 24A. Table 6 should, therefore, be used.

Design of Joints.The design of r iveted joints is simple, and is given inmost elementary text-books on applied mechanics, suchas " Applied Mechanics for E n g i n e e r s , " by D u n c a n .

RIVET'D'

W V E T 'E ' FIG.


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3929, 1931 THE AIRCRAFT ENGINEER SUPPLEMENT TOF LIGHTThen the load due to the couple, on th e four ou ternvets A. B, E, and F, is

1,000 X o lb. in. X V L 3 ^ 1 2 5 ! 1 1 - ib.18-378 in.2= 532 lb. each.

And the corresponding loa d on the two inne r rivets C and I). 1-25 in.i, 1,000 > o l b. i n. X j g ^ g ^ , l b .

= 340 lb. each.These loads act at right angles to the lines joiningthe rivet centres to G respec tively. The couple -resistingload on each rivet is then combined with the direct load1.000 , . . , -of lb. on each, in a paralle logram of forces, as

6shown in Fig. 1J.The resultant loads on the rivets are: Rivets A and E 450 lb.,. B F 652 lb.., C 173 lb. D 507 lb.Spacing of Rivets.Keel and keelson butts and laps should be trebleriveted, whilst shell p late edges, floor plates and frameneb laps should be double. In monocoque fuselages,single riveting is usual. When speaking of double andtreble riveting , it is assumed th at " chain rive ting "is meant. "' Reeled " rive ting is confined to th e flange sof angles, channel stiffeners, etc ., where the wid th of

material cramps the pitch ing. The definitions of theabova terms will be clear from Fi g. 12 .In a small fitting or clip the question of spacing therivets will be decided by the design of the detail as awhole. In general, however, it may be said that norivet should be nearer the edge of a plate than twiceits diameter, and the minimum distance between rivetsshuuld be three diameters.In metal spars, fuselages, and seaplane floats andhulls, general rules may be laid down to cover mostcases. In the long lines of rivet s used in these s truc-tures a spacing of 8 diameters is usual, except wherewatertightness is necessary. Fo r hulls and floats &pitch of 4 diameters should be used in all shell seams,attachment of frames and stringers to the shell, andsuch important structural members as keel and keelsonbutts, floor plates, etc.

The above ruling of 2 diameters between centre ofrivet and edge of plate should be observed, and, wheredouble and treble lines of riveta arc used, a spacing of3 diameters should be allowed between centres of lines.Where single straps are used on one side of the jointonly, they are made of material of the same thicknessas the par ts they join, or of one gauge thicker. Doublestraps, one on each side of the joint, are made of thenext size above half the thickness of the parts theyjoin.For large-scale riveting, such as that on flying-boathulls, Table 3, of appropriate diameters for given thick-nesses of material, may be simplified, and is given here,together with lap and strap widths in accordance withthe rules just stated.

TAULE 8.Thick-nessofPlating.

141618-20

Dia-meterofRivet.in .1

Width of Lap.Single.

in.0-750-650-50

Double.in .1-351-100-90

Treble.in .1-901-601-25

Width of Strap.Single.

in.1-501-251 0

Double.in.2-652-201-75

Treble.in .3-753 1 52 '50

Precautions and Workshop Practice.The effect of hammering up a rivet head is felt notonly on the head itself, but also on the plate imme-diately round the rivet. At each point the plate isstretched very slightly. In long rows this str etch ingbecomes quite apprec iable, and sufficient to cause" wind " in a spar or bu ilt-up stru t. On a hull orfuselage it will cockle the plate along the length of thelap. The troub le can be preve nted. A spar should bebolted up with service bolts at every fourth hole alongall its flanges before the first rivet is put in. Therivets should then be inserted in the intermediate holes,starting from the ends and middle simultaneously inall flanges. Similar!}7 in a hull or fuselage, the plateshould be bolted in position at every fourth hole, andthe riv etin g should no t " grow " round t he edge fromone point alone. The more it is spread abo ut with inpractical limits the better. If the lap is arranged neara longitudinal stringer, the stiffness of this will preventundue cockling and the resultant drumming of the plate.A similar precau tion is to swage the p late edges(Fi g. 13). In fuselages and metal-covered wings, where

D I T C H

SINGLE LAP OR SEAM

ST \

DOUBLE L A P O R SEAMCHAIN RIVETED T RE BL E L A P OR SE A MC H A I N R I V E T E D .

SINGLE OTRAP ""f M-7 ^ K ^ t ^

O O U B L E R I V E T E DS T R A P TREBLE R IVETED STRAP

DOUBLE R I V E T E DDOUBLE S T R A PF IG . 12. -o

R E E L E D R I V E T I N G4 7%


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40SUPPLEMENT TOF L I G H T M AY 29, 1931THE AIRCRAFT ENGINEER

the covering is corrugated, this, of course, occurs auto-matical ly.The question of badly-formed rivet heads is frequentlyraised by inspectors, who are countered by " knowing "foremen claiming that , since rivets are only used in

TECHNICAL LITERATURES U M M A R I E S O F A E R O N A U T I C A L R E S EA R CHC O M M I T T E E R E P O R T S

These Reports are published by His Majesty's StationeryOffice, London, and may be purchased directly from H.M.Statio nery Office at th e following addresses : Ad astral House,King swa y, W.C.2 ; 120, George Street, E dinb urg h ; YorkStreet, M anchester ; 1, St. Andrew 's Crescent, Cardiff ; 15,Don egal! Square, W est, B elfas t; or throu gh an y bookseller.

shear, the head shape is imm aterial . W hilst this is tosome ex ten t t ru e, i t does not go the whole way. Ahalf-formed head (Fig. 14) indicates that inefficient orinsufficient hammering has been applied to it, with theresult that the shank of the rivet is not properly ex-pand ed and pressed home into i ts hole. A single rivetin a long row badly formed may not be of importance,but one should suspect the joint i f there are severalt h u s .

FIG 14-Ha nd rive t ing is expensive and often unsa t isfactory .

The re is more possibi li ty of damag e to the surro un dingpla te , and the heads a re frequent ly poor . Pne um at icrive t ing is sat isfactory when careful ly carried ou t . Thetool is of the kind used in shipyards for l ight caulking.But the best workmanship is obtained from a singlepressure machine, such as was i l lustrated in F L I G H T(THE AIBCHAFT ENGINEBH) , October 25, 1928, page 936.This implies bench work, and is sui table for spars,fuselage fozmers and hull frames. I t csannot be used ina t taching skin pla t ing to inte rna l s t ruc ture , a l thoughthere are cases where large panels of plat ing may beput together before erect ion.Aluminium-alloy rivets should not be used in theannealed cond it ion. No t only will they never developtheir ful l strength, but they are l iable to induce corro-sion in the surrou ndin g plate. The usual procedure isto anodically coat them first and to follow this by fullheat t roa m ent to 480 C. + 10, and quench ing. Ifused within an hour, they are soft and pl iable, and wil lage-harden to the i r ful l s t rength la te r .It is argued that in hull and seaplane work the rivetheads are the corrosion danger points, part icularly asthe hammered-up ends cannot have an anodic surface.A method of overcom ing this f aul t , which is employedby Messrs. Saunders Roe, Ltd. , is to hammer up on theinside of the hull , at least up to the waterl in e. This isonly possible on hulls with an extremely accessibleinte rna l s t ruc ture , and there a re some points where i tcan no t be done. The principle may be applied in rivet-ing together panels of shel l plat ing before erect ion.Whilst the obstruct ion to the air flow caused by therivets may be comparat ively small on large machines,it becomes of importance on the fuselages and mainplanes of small fast cra ft. In such aero plan es the skinis usually too thin to al low of countersinking, and asat isfactory solut ion has yet to be found. W here theskin is supported by a robust member, this may becountersunk and the skin forced down into the hollow,causing in effect a countersink on the outer surface.Bu t i t is only a pa rt ial solut ion. Elsewhere the headsmay be made flat instead of domed, a process difficultto carry out without cracking the edge of the rivet head.The aircraft industry awaits a real ly neat method ofachieving a smooth surface to a thin riveted shel l .

FUB THE R EXPE BIMEN TS ON THE BEHAVIOUR OF SLNGLBCRYSTALS OP ZIN C SUBJE CTE D TO ALTE RNAT ING TOESIONALSTRESSES. By H . J . Gough, M.B .E., D.S c, and H . L. Cox,B.A. W ork performed for the D epa rtm ent of Scientificand Ind ustria l Research. R. & M. No . 1322 (M. 68). (20pages and 9 diagrams.) Augu st, 1929. Price Is. 6d. net.

These experiments orm p art of a lengthy investigation in to the theoryof fatigue and relate mainly to experiments on single crystals of variousmetals. From the results of a previous ex per im ent ' on a single crystal ofline, it was concluded that the formation of twins in zinc occurred on planesof the 1012 typ e and th at the pa rticular o perativ e twinning pjane (of thelx available) was determined chiefly by the direction of slip on the originaltbasal plane and possibly, to some extent, by the relative magnitudes of thenormal stresses on the possible twinning planes. In this previous experimentthe orientation of the crystal was such that slip on the original basal planeoccurred in one d irection o nly and one pair of com plementary twins onlywas observed.From the resu lts it was predicted t ha t if a test w ere made on a crystal ofBUitable relative orie ntation of the crystallograp hic a nd straining axes suchtha t all three slip dirertions became operative then the opeivtiri' twinningplanes should change with the slip direction . The present experiment wasplanned in order to test this prediction. Again, in the previous experiment,of the two possible pairs of complementary twinning planes associatedwith any one slip direction, it appeared probable that the choice of theoperative pair w as influenced by considerations of normal stress on thetwinnin g plane. The present experimen t would, it was hoped, throw furtherlight on this aspect of twinning.The slip plane of zinc is the basal plane (00 01); the slip direction is the mosthighly stressed prim itive direction ; deformation by slip is controlled by thecriterion of maximum resolved shear stress. The twinnin g planes of zincare the six planes of the general type 1012.With the specimens employed in the present tests, due to the relativeorientation of crystallographic and straining axes, and to the type of appliedstressing, three slip directions become operative in tur n. The results showdefinitely: (a ) One pair only of complementary twinning planes appearin the a rea associated with each operative slip direction, thus reducing thetotal of six possible twinnin g planes to three pairs ; (6) a change in slip direc-tion is accom panied b y a chang e in the id enti ty of th e pair of operative twin-ning planes; (c ) in an y o perative slip direction, the twinning planes con-taining that slip direction do not appear: (d ) norm al stress alone does not

determin e the choice of operative twinning plane. If conditions (a), (Mi(c) and (d ) are fulfilled, only on e sequence of op era tiv e twin ning planes canresult, offering tw o altern ative phases of the sequ ence. Both phases havebeen observed on different specimens.The im port ant conclusion is thus reached t ha t th e occurrence of twins, uswell as slip bands, is controlled by the simple criterion of maximum resoiveushear stress on the slip plane. " Koy. Soc. Pr oc ," A. vol. 123, pp. 143-167 (1929) and li. * "No. 1183.t Associated with the initial structu re of the unstressed crystal.AIRSCREWS FOR HIG H SPEED AEBOPLA NES. i>y n-Glau ert, M.A. Com mu nicated by the Directo r of ScientificResea rch, A ir M inistry. R. & M. No . 1342 (Ae. 474)-(18 pages and 7 diagrams.) Jun e, 1930. Price Is. net.Since little is known of the characteristics of an airscrew at the his'a r* '^of adva nce which occu r with mo dern racing aerop lanes , it 19 desiv.Miexamine theoretically the most suitable type of airscrew lor mU l: - i i -future high speed aeroplanes, to determine the efficiency ol these au *\1*"and to consider the possibility of improving the low static thrust which IMI'- " vd lidit f i era ting near th e sta te of maxim um ci.jVJ ^f f i c y 01

pand solidity of an airscrew operatin g near the stat e of m jJ ^Another appro xim ate formula has been derived for the efficiency 01 (escrew, and these formula; have been used to determine the mosi typ e of airscrew for a high-speed aeroplane. A simple formula nas

The Aircraft Engineer May 29, 1931

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Transcript of The Aircraft Engineer May 29, 1931