naca tm 1019 strength tests on hulls and floats

7/27/2019 NACA TM 1019 Strength Tests on Hulls and Floats

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,.. .. . .. ., ... ,- -, TECHNICAL M~liORANDUMS.,.,


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,,. ..--! .-, %-The same pr oqedure eiapl,o~e:d-n.;w::@l&.qt.zuctures is

followed on floats and hqlls . that iS t the parts are firsttested separately and then tliestiength of the assemblyis tested as a who,le. .-As a..result;: @ree different testproblems are gene-rally intiolved~

1. Testing of floatframesInvestigation of float +ody ,,.. :. , .,,., ..

3; Ikvestigation of flotation gear or, of the, wholefloat system . ,,The tests described hereinafter relate to float sys-

tems for groups I and II. Group I involves aircraft ~r5n-cipally destined for take-off and landing in calm waters ,group IT f or takeoff and landing on waves.AS ultimate laad for the fleat system the 1*55 times

safe load-is specified in all cases.

11. 13xperiments with.1lost .3?ramesFor the proportions of the frames the bottop press,u~eis decisive. In the leading of the frames allowance can

be made for the fact that a part of the bottom.pressureis directly transfiitted (from the skin) to keel and chinestrip, thus giv~rig a pressure distribution as indicatedin figure 2a. Experimentally a uniformly distributed bott-om pressure equal to the assutied highest bot.toa-pressureis applied, thus affording a mar~in of safety (fig. 2b).,..The frames are tested AS in the rib fa:il,ure:testsen t-he:est table (fig. .3)? The frame is fitted with acove-r strip of from 100 to 150 millimeters width. (Undercerthin circumstances the tip-form:ing strip is strength-ened at the inside border %y a rfv.eted seetionin orderto prevent premature kuckling, ~,ince.the coa$it.ions inthe test are much more severe thaninactual service. )The bottom pressure is applied %Y oil pre,ssure, cylindersof 5 square centimeters area. .!?oinsureunifo,rrn distri-but.ionof the pressure a 10 mil:linete$s thic.kfklt strip- ,..is placed on top of the fratie and su>.p.&teilhy taperedwood blocks against which the plungeks .ofthe ,oil-pres-sure cylinder press- (fig. 4)* Thebbttom pressure istransmitted from the frame tij


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NACA Technical Memorandum No. 1019 3

force is divided conformably to the shear distributioninto several concentrated forces {f$g. 5) w~ich are ap-plied tangentially at the frame-c-irctimfer-ence-..heseforces form the reaction forces ,to the bottom pressure atthe frame. lheyare applied as tensile forces by meansof tension straps or wire ropes , the forces (at eitherside of the frame) applied .on the side walls of the frameat small angles being suitably combined t.oform a concen-trated. force by a system of levers. The other forces(applied closer to the top) forming larger angles , areapplied individually by small oil-pressure cylinders: Theforces applied along the frame bottom are linked by straps ,which are flexibly joined at the frame %ottom (figs. 6 andi) and applied as a concentrated force aleng the line ofsymmetry of the frame. For this purpose the arrangementof figure 8, consisting of oil-pressure cylinder and leversystem, is employed.

Obviously the lines of action of all forces must belocated in the plane of the frame web. Unavoidable eccentricities must be equalized by guides. A much simplerset-up is shown in figure 9, but the strength data obtainedwith it are a little too high, and considerable error mayoccur on closed frames. Hence it is only rarely used.

III. Comparison of Strength and Weight of Different.Frame Forms

lhedata from various tests with different frame de-signs have been collected in figure 10. ~he figuresthemselves are indicative of the minor differences inquality ex~sting between the different designs. Particu-lar advantages accruing fro~ the shape of the bottomgirder are therefore hardly to be expected. Obviously,errors, such as unfavorable lecation of the cut-outs andadverse diffusion of load should be avoided. On div:dingthe absorbed ~ottom load by the weight of thefframe, it isfound that a load of from 1.7 to 1.9 t per kilogram offrame weight is taken up, In one inqkance drily is theload capacity >ess, 1.+4, in tNo Case$ higher, 2.2 and 2~3 t.~igures 11 and 12 illustrate some of the examined framedesigns.,..

IV. Breaking. Tests with Floats..The aim liereis>to prove the strength.for all PO$-sible load cases. withalimited number of tests. This is.. -..


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1 1 11 1 Ilml In nllmll II I ,,, ,, ,, I I ., -, 8 8 8,8,8- 8, 8- -. ,-- , .,- . . .

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4 NACA. ?echnical Memorandum No. 1019,.. . :.accomplished by first plotting the moment curves and thecross stress curves for all these loading conditions.Then the curved surface line to the two set$ of curvesis drawn and a load distribution is provided which takesboth the moment and the cress stress enveloping curveinto account. In this manner the number of lead tests canbe reduced to the three,,cases: nose impact , stern impact ,and step impact.

. a) Nose Impact.I?igure 13 illustrates the experimental set.-up. Thefloat is joined at the nose to the test column. The mo-ment is applied hy a pair of oil-pressure cylinders.

3) Stern ImpactThe set-up is practically the same as in the preceding case , except that the other end of the float is clampedto the test column.

c) Step ImpactThe experimental arrangement is shown in figure 14.The float is held by anchor bolts at the two points of

attachment. The load is applied at the individual framesby means of cradles. A system of levers combines theindividual forces into one force. The load is again ap-plied by a pair of oilpressure cylinders mounted at eitherside of the float.d) Results of Tests. .

The described expey.~ ment.s invol~ed a float withespecially low unit weigh~.. On such small dimensionalparts it is essential that the areas of pronounced localsagging, which might induce premature failure , he continu-ously o%served during the test. If such places are no,ticed, the test should he interrupted and. the areasstrengthened, as ly joining the stringer sections to theframe %y means of riveted angles, for instance. Suchcomparatively rapid alterations make it possible to avoidunnecessary labor at the point where no previous clarityexists a%out the necessity ~of junction and stiffenerplates, and assure a particularly light construction ofadequate strength. Subsequenlito the corresponding smallreinforcements the required 1.55 times safe lead isappliedand hence the strength of the float proved.

~mportant for the construction are the stresses ap-


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NACA,TeChnkcal, Memorqg.dum.~o. 1019 5

plied in the test. .,The maximum loq;g~tudinal stress inthe .case of stern .impact~oc.curs at. $he rear .j,mintt Here...the compressive stiess in the~-.coye,ri-ng-mo~rit1.* 1900.: kilograms per square, centimet,er .,,,.I:nthe ,.ca.s!eqf.hose-impact the maximum Iongitudinal stress iS lecated at thefront junction., The ultimate stres.sof the skin amounted, to 1200 ki~ogra,ms..per. square centimeter pressure. Themaxi-mum shearing stiess in the case of, step,:i-m.pact-ccursat frame ,1.1.A -shearing stress of 560 kilograms. persquare cent imeter was, r.cache.dwith-eat inducing fa,lure.The loading was .no,~~on-ttg~~d, t,~ failure , since the f~~atwas intended f or other. .t,ssts.in connection with the air-frame. . , , ,, , 0 .,

V. !i?estswiih Float SystemsThis test involves the float with the flotation gear.Since fuselage and wings of the, airplane had beendestroyed in previous load tests the float supports weremounted nn a specially designed framework. Figures 15and 16 illustrate the exgeritierktal arrangenent. The flotat ion gear was inverted and attached to a beam B pivotedin A. The lateral brac,ing consisted of a cross beam Csolidly connected with B. The dissimilar rigidity ofbeam C from the wing of the airplane conditioned severalchanges in the cable braces from these prescribed forthe completely mounted machine. These modifications,however , are not important.The loading of the f~ot-ation gear for centricallyapplied forces is ef.fecte.db-y means of oil pressure. oilcylinders F, attached by means of special cages E 4 torail D anchored to the floor, exert a dc+wnward pressure ,the cylinders acting over 1 set. of levers G each nn beamsH placed over the float frames. For the torque provided

in some of the loadings the end.frames, of tke fleat carryriveted %eams J at the ends of which moments are appliedby means of weights.. The loadapplied h~ -the-oil pressureis checked bya dynarn,ome~,e~~uspe~de.d at :peint K of beamB pivoted in A.. !Phis checkwas possible in the symmetri-cal load cases ;hly, in;~unsymmetrical.l o~d cases the beamsB and C were.fixed toward the f~~~fii order te preventoverturning of whole unit. . .... ,The tests themselves fell into three groups: .,.A)~nvestigation of the .extknt towhich the assump-tion serving as basis of the,~d~cti~atl~i holds true in

that the fl~at bo&ies Can be regarded as being rigid,


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6 NA CA T.echn ica.1$le@@Sa.a:@vm.o..1019

that is , their .forrn change sr.emaim.::o...small that they be-come ne.gligible.n the solution wf.::,the.ndeterminate systern. To this end the form chamges.tthe- attaehment pointsare recorded under- toti:s.on.al and [email protected],tresses .... . ,, :$

B) Yor the man; load cases .t.obe verified the stres$distribution was measured- on: cables ,and struts. The ten-sile stresses in the cables and the c-oppressive.and bendingstresses as ..-wellas the shape of the deflection.curve ofthe strut s:were recorded .with tens bmeters. ,The loadingwas raised considerably beyond the sare load to the vicin-ity of the breaking pointi. This, of course, occasionedform changes which somewhat detract from the behavior inthe subsequent tests. - In this manner the stresses incables and struts under theoretical breaking load areestablished.

C) 3reaking tests afforded the breaking load of thestruts under the beau-column stress which occurs. Sincesome of the cables were dimerisioned as high as the struts ,they were replaced by the next heavier size before thebreaking test as cable,failure was, of course, to beavoided. The ultimate load of the endangered cables wasestablished separately by te~sile test.a) Measurements

a) Check on load by dyaamoaentera%) Twisting on float body . .

. c,)Angle of twist at end of forward starboard strutby goniometer and plumb line ;d) Deflection.of float bodies . .. .e) Deflection of struts

,.f) Stresses in struts; .4 t-e.nsoqe$ers each %t star-bo.ardstruts , ,,centep aRd (float) end;.2 each at beginning(bo~y end), Pr altogether10 tensome~ers per stTVt, (The portside.strutsgenerally carry -only 2 tensometers each.)., ,,g) Stresses ic the cable~with1 tensometer each.. . --- .,h) Shock absorption Of flotation gear by distancetape. on floats and.flotaticm..g.e.ar - Measure-ment .b,yleveling g~ge-. .; ...... -..


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8 NAC& Technical Memorandum No. 1019

float and portside cabane fitting and ?IY a twisting. momentat a frame. The moment alout the lateral .a.xisand thecross-wind force were chiefly taken at the f orebody.

There were 16 loading areas in all (exclusive of theweight equalization). The 10 higher loads were applied hYoil pressure, 5 by weights; whereas IImoment was applied%y pulleys and dynamometer. The forces 6600 and 1080 lcilo-grams are the external forces, all others are reactionforces an~ moments. The .Ioad is a~plied in single stages.Since the s?ecified safe load factor of the float is only1.55, the float-support struts were strengthened at thehigher load stages. Then the load was raised to 1.8, andthe strength of the air frame for this loading conditionherewith proved.

Translation by J. Vanier,National Advisory Committeefor Aeronautics.

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IIAGA Technical Hemortium XO.1OI9

Figure la. - Portable oilpressure pumpwith controls and testinstruments for two oilcircuits.

a)

b)

Figure 2.- (a)Mathematicalbottom pressuredistribution on the floatframe. (b)Experimentallyapplied bottom pressuredistribution.

Figs. la,lb,2,3

Figure lb. Controlapparatusfor automatic oilpressure distributionof six oil circuits.

Figure 3.- Dispositionof a frameon the test board.


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NACA Technical Memorandum No.1o19 Figs. 4,5,6,7

,.. .Figure 4.- Distributionof pressureover the frame.

Figure 5.- Shear distri-bution overthe frame and analysisin five concentratedloads .

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Figure 7.- Differentversion ofstrap fitting accordingto fig. 6.

Figure 6.-Applicationof forcesacting along frame bottombyflexibly attached straps.


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NACA Technical Memorandum No.1OI9 Figs. 8,9,10

Figure 9.- Simple testrigfor frames which gener-ally give too highvaiues and is therforerarely used.

mo ]24.0 /0,5 10.34. 11,89

0 120 los Izol1220 308 1,5 1,65 3,55

1220 310 1,2 1,50 2,66

1Z20 .310 1,2 1,60 2,66

1220 310 0,5 1,10 3,11

1280 350 2,0 2,50 4,85

12s0 350 1,8 2,25 4,26

t2a0 350 2,0 2,50 4,26

1280 350 2,0 2,25 4,26

W60 350 1,5 2,20 3,97

Boi+onpr.ssU1-e

F07iL4Iwjlc.m

21,6

39,5

29,6

22,2

22,2

25,9

33,7

>1,6

31,6

31,6

29,4

s00 00.000 Qo.

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NACA Technical Memorandum No. 1019 Figs. 11,12,13,16

Figure 11.- View of.. exploredframe designs.

Figure 12.- View ofexploredframe designs.-

Figure 13. Strength testof float inthe case of bow impact.

Figure 16.Arrangement fortesting the strengthof a flotation gear.


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4(Oil c y fj da ) I I 1Ii L -1- 41

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fid 11 u M II & 1 x u u II 1A /

.Fi~re .14. Experimentalsetun fortesting float un~er:stepimpact.

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:Figure 15.- PDiagrammaticsketch of experimentallayout fortesting IFthe strength of float sunports. G


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Pure torque

Symmetricloadin~ .

Eccentricloading

::m~;:;ic.-..-.............SymmetricloadingEccentricloading

Type of load I Load patternTorque at$ront endframe. W ITorque atrear endframe.

Symmetricalstep impact.

Loaded tolZd=320mkg

Md=320mkg

P = 4500kg

P=3700kg

.P=1200kg

Md=240mkg

P=1500kgMd=.300mkg

P=4700kg

p=4300kg

(L900kg)-

Notes, jTwisting measure-ment at floats ,;

$$.ressmeasurementsfor deferminingloaddistributionoverstruts.Measurementsat sev-eral load stages to1.5 times safe load.

Loading--tofailure.Frontstrut collap-s.e~.Supplemental test,$fie E struts K.adedtoa ure. Rear star-board strut breaks.Loading to failure.of rear nort-sidestrut;-

Outline of 16 individualtest series.

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naca tm 1019 strength tests on hulls and floats

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