Thesis (B.S.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1934.MIT Institute Archives copy bound with: Thompson, Ransom S. and Sylvester, Edmund Q. Current trend in automotive engine design (1934). MICROFICHE COPY AVAILABLE.by Lorimer C. West.B.S

West, Lorimer C. (Lorimer Clifton)

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Design of an Apparatus for TestingAeroplane Fabric at Low TemperaturebyLorimer C. WestMassachusetts Institute of TechnologyMay 1934Cambridge, Mass.,May 22, 1934.Mr. A. L. Merrill,Secretary of the Faculty,Massachusetts Institute of Technology,Cambridge, Mass.Dear Sir:I herewith submit for your approval my thesisentitled "Design of an Apparatus for Testing AeroplaneFabric at Low Temperature."Sincerely yours,ACKNOWLEDGMENTI wish to express my appreciation to ProfessorSchwarz for supervising this thesis and for his suggestionsas to the best way to test the fabric. Thanks are alsodue to Professor Berry and Professor Wklk@- for theirsuggestions on the construction of the apparatus.Table of ContentsIntroduction---- VDesign of Refrigerating Apparatus 9Calculations of Refrigerating Apparatus - - - - - 11Design of Mechanical Apparatus 14Assembling for a Test. - - - 16Appendix. 18Fig. 1. 002 ContainerFig. 2. Inside CoverFig. 3. Outside CoverFig. 4. Outside BoxFig. 5. Inside BoxFig. 6. Frame for Fabric.Fig. 7. EccentricEccentric StrapEccentric RodRib and- Stiffener.Fig. 8. Assembled Refrigerating Unit.P - 1. Apparatus Set up.P - 2. Inside of Apparatus.P - 3. Inside of Apparatus.YI N T 'R 0 D U C T I 0 NCountries that have such severe winters that railwayand waterway transportation is stopped find the aeroplane anideal means for connecting sections that would be otherwiseisolated.' In these sections temperatures as low as - 600F.are commonly encountered and under these conditions metalsare very brittle and break easily under a shock load. Theeffect has been studied quite thoroughly on metals, but littleor no- thought has been given t o the possibility that thefabric covering of the wings also becomes brittle. In the caseof metals, those whose impact values are low at low tempera-ture have been avoided in design and if the fabric too shouldprove to be brittle at low temperatures it is certainly ad-visable to use a covering of greater strength.In testing materials the most desirable test isthat in which the conditions are exactly similar to actualconditions, but this is very often impossible. However, itis better to have some results showing approximately thebehaviour of the material in question than none at all. Beforedeciding upon the most suitable method of testing aeroplanefabric.,. the method of attaching it to the wing and its actionon the wing while in flight should be known.The ordinary fabric covered aeroplane wing is com-posed of three main parts, a front and a rear spar, ribs, andfabric. The spars are usnally long wooden members at rightVIangles to the direction of flight. On these are spaced theribs at distances increasing from the fuselage to the tip ofthe wing. The ribs are about 1/2" wide and are the forms forthe shape of the wing. The fabric is placed over thisskeleton frame work in the untreated condition and is thengiven a coat of dope and allowed to dry. The fabric is thensewed to the wing and the ribs and several coats of dope areapplied till the fabric is as tight as a drum. As a resultof this tightening process, the ribs are well defined and thefabric is pulled over their comparatively sharp edges.In flight, an aeroplane is supported by lift on thetop of the wing and by an upward d'eflecting force on the bottcmof the wings. Approximately two-thirds of the supportingforce is derived from the lift on the top surface of the wingsso the fabric has considerable tension in it. There is notonly this tension, but also the fabric is undergoing a con-tinuous flexing action due to the vibration of the motor.The frequency is the same as the revolutions per minute ofthe motor and the amplitude is approximately one-eighth of aninch. It is this last action that is believed will cause anyweakening in the fabric that may occur at low temperature, andit is very likely that the weakening will take place where thefabric passes over the sharp edges of the ribs. From the fore-going it is evident that the fabric should be subjected to aflexing test and the afparatus should be so designed that con-ditions at the ribs may be studied.VIIThe next question is the proper size of specimen touse. The average rib spacing near the fndselage is approximate-17 12"1 and this distance increases gradually to about 20" at thetip. Naturally for the same amplitude the largest angle offlexure will occur in the shortest span so a 12" span shouldgive results for the worst case. In sewing the fabric to theribs, the stitches are about three inches apart and since itmay be desirable to give a tension test to the specimen afterits flexing test, holes should be avoided. From a sample 10"wide it should be possible to obtain three specimens threeinches wide and free from holes.To test the fabric for use at - 600F, tests shouldbe made at lower temperature,- down to-80OF for example. Thesimplest means for producing those low temperatures is in aninsulated box with solid carbon dioxide as the coolant.The design of such a box cannot be done by the use of exactmathematics, but must be based upon previous experience andan approximate calculation may be made to obtain a rough ideaof the heat flow into the box and the rate at which the coolantmay be expected to sublime. The size of specimen chosen was12" x 10" and since other apparatus was to be placed in thebox, the container f or the specimen was made 16" x 16"1 x 12".A three-inch wall of solid carbon dioxide was to surround theapparatus on four sides and three inches of insulating materialwas to surround the whole. This original plan was shown toVIIIProfess or Wilkev'. He was of the opinion that the apparatus wassuitable for the temperature desired and calculations as suggested.by Professor Berry upheld his statement- Professor Walkersuggested that the carbon dioxide be immersed in a bath ofalcohol in order to give better conduction. The carbon dioxidesublimes and the resulting gas would hinder the heat transfer.Professor Berry also made a valuable suggestion" After thecarbon dioxide sublimes it is still at a low temperature and ifpermitted to escape directly into the air, it would represent aloss. He suggested that the apparatus be so designed that thiscold gas surrounds the whole apparatus and thereby cuts down theheat losses. Despite Professor Walker's opinion that threeinches of insulation was sufficient, it was felt advisable tomake it four inches in case the workmanship was such that cracksexisted in the outer box and raised the heat losses.9Design of Refrigerating Apparatus.To fulfill the above suggestions the apparatus wasdesigned as follows. The illustrations in the appendix willmake the explanation clearer. The box cohtaining the specimen(Fig. 1) was necessarily made of metal to hasten the heat trans-fer and to hold the liquid alcohol. Copper would have beendesirable but it was too expensive so galvanized iron was used.A box 22" x 22" x 12" was made of this metal and then a wallof the s ame material was put in the box t o f orm a space 3" widearound the inside. To hold the four inches of insulatingmaterial around the metal box, a wooden box (Fig. 5) 30" x 30" x 16"are made. In the final set-up the tops of these boxes (Fig.8)Cwas in the same plane and a low box (Fig. 2.) 30" x 30" x 5b"covers them. This latter box contains four inches of mineralwool which was used as an insulator throughout. It is compara-tively cheap and it has the same coefficient of conductivity asthat of cork* 'loles 1}" in diameter were drilled in the coveringbox (Fig. 2.) directly above the carbon dioxide. The purpose ofthese will be apparent shortly. As stated before Professor Berrysuggested that the still cold carbon dioxide gases be utilizedto reduce the heat losses. To do this another wooden box (Fig.4)was made of such dimensions that when the foregoing apparatus wasset up in it an air space one-quarter of an inch separated them.The action is as follows. (See Fig. 8.) Heat entersthe apparatus and causes the coolant to sublime. The resultant10gas would fall into the specimen container or into the insulat-ing maperial because of its density if it were not for the factthat the edges of all the boxes are covered with fe.t which en-ables the covers to fit tightly. Pressure builds up in the carbondioxide container and forces the gas out into the air space throughthe holes previously mentioned* The ccmbination of pressure amddensity causes the gas to cover the top of the box and then falldown the dides and then cover the bottom where it exits throughsix holes drilled -near- tte center of the outer box. Holes arealso drilled in the final cover (Fig.3) to coincide with the holesin the inner cover so that if it is necessary to replenish thecarbon dioxide, it may be done by removing the corks and droppingthe required amount directly into the container.The desired temperature is maintained by adding morecoolant than is necessary and then raising the temperature bymeans of electric bulbs in the specimen box. The bulbs are lowin the box-as a reasonably good temperature gradient may beexpected. The rapid flexing of the sample will also cause theair to be in continuous motion so there is more reason to Dxpecta rather uniform temperature from top to bottan of the box. Ifthe amount of refrigerant in the apparatus should be too greatit may be possible to maintain a constant temperature by puttinga rhoestat in series with the lights and adjusting it until areasonable equilibrium is obtained. A potentiometer and themo-couple provide a flexible means of measuring the temperature.11Calculations of Refrigerating ApparatusAs stated before, an exact calculation is impossiblefor an apparatus of this types The calculations are furthercomplicated by the space containing gaseous carbon dioxide inmovement. There is no data on hand that would give the propercoefficient of conductivity through the film on each wall ofthe space. The safest thing to do is to assume that the spaceis filled with air that is not moving. In that way a higherheat loss will be computed and the results will be on the safeside. The box will be treated as a sphere. The dimensions aretaken from the figures in the Appendix and the following listgives the coefficients of conductivity in B. T. U s per squarefoot per inch per degree Fahrenheit per hour.MATERIAL C OEFFIC IENTWood .26Mineral Wool .29Air film (inside) 1500Air film (outside) 212Q .4-T (T - T2)(1-1)4 (1 -+ 4 (1-1).4(V V2  (V2  3) A 3 Al 4 V4 V5 ) 51 k2 24 l 70 - (-80)1 ) 12(11 T5.29.. I1_ ) 12 4 ..1 ....-( 1 2 - -)12(15 15.75) 1500 (15.75 16.002) (16.00 16.75).26 .261|2 X 1675= 18841.005 . 147518841.1952+ .00000528 + .0129 + .0298= 1575 B.T.U. / hour.It requires 242 B.T.U. per hour to sublime a pound of dryice, so according to the heat loss, six and one-half pounds of theice will be used each hour. This figure is much too high andshould not be used as an indication of the actual consumption.A test was run on the apparatus to determine the tempera-ture that could be obtained in it. The box will hold approximatelyone hundred pounds of the refrigerant but only twenty pounds and agallon of radiator alcohol were put in. The temperature wasregistered by a thermocouple placed at t he top of the box. The188413lowest recorded temperature was-840F and at the end of the testthe thermocouple was pulled to the bottom of the box and-87 0 F wasattained there. The distribution evidently is very good and withthe apparatus running the flexing of the fabric should make thetemperature even more uniform.14Design of Mechanical ApparatusIn an airplane wing, it is the fabric between the ribsthat rises and descends, but to produce those same conditions isa bit difficult. Since the motion is relative, the same respultsshould be obtained by fixing the moving fabric and causing therib to move up and down. The purpose is to cause flexing of thefabric passing over the ribs and it is evident that that isachieved equally well by making the ribs vibrate. A rectangularframe was made as shown in Fig. 6 to accommodate the specimen andthe ribs (Fig.7) is represented by a strip of wood 1/2" x 1/8" x12" glued and nailed to a 3/4" x 3/4n x 12" block for a stiffener.The stresses involved in the mechanical equipment ofthe apparatus are so small that it is useless to figure thecorrect sizes for the various parts. The results would be toosmall for practical purposes. The vibratory motion is obtainedfrom a cast iron eccentric (Fig. 7) fastened to the shaft of asynchronous motor turning over at 1800 R.P.M. A steel strap(Fig.7) was made to fit over the eccentric. The motor and itsaccessories were placed outside the box so it was necessary totransmit the motion from the eccentric to the rib by means of asmall steel rod (Fig.7). A right and a left hand thread werecut on the ends so that the rod could be screwed simultaneouslyinto the eccentric strap and the rib* A steel block (Fig.7)was attached to the rib by screws and a thread cut in it to takeone end of the rod.15All the moving parts require lubrication. Theeccentric was lubricated by a grease cup screwed into the strap.The steel rod has to be lubricated and it also has to be heldvertical otherwise there will be a tendency to tear the fabric.Both of these requirements were satisfied by screwing a bronzeplate (Fig.8) on the bottom of the outer box, and bolting asimilar plate to the specimen container. Under each one of theplates was placed a piece of felt impregnated with oil. Holeswere drilled and reamed in the plates so that they are justlarge enough to prevent the rod from sticking in them. Theboxes were so adjusted that the holes in the bronze plateswere directly in line.As stated before, electric bulbs supply the heatnecessary to raise the temperature. There are four of themconnected in parallel so that the failure of one will not ruina test. A switch on the outside of the box controls the lights.The photographs in the Appendix show the apparatus inthree different positions. P-1 shows the apparatus set up readyfor a test. The eccentric and electric motor were disassembledpreviously so they are not shown. The main purpose of thephotograph is to show the method of supporting the apparatus.P-2 is an interior view showing the arrangement of the boxes,the electrical circuit, and the angles to which the fabricframe is bolted to prevent it from moving.P-3 is similar to P-2and differs from it only in that the bulbs were inserted and theframe with the fabric prepared for a test was put in.16Assembling for a TestTo run a test the following procedure is suggested.One end of a piece of fabric 24" x 104" is tacked to the undersideof the frame and is pulled over the rounded ends of the framewith a tension just sufficient to remove all the wrinkles and thefree end is tacked as before. A coat of dope is applied and whenthis is dry the rib is sewed to the fabric - precautions beingtaken to have it centred and perpendicular to the frame. Anumber of coats are then applied till the desired tautness isobtained. It is assumed that the refrigerating apparatus isassembled as shown in Fig. 8 with the exception that the coversare removed. Screw the rod about one-eighth of an inch into therib and set the frame into the apparatus and bolt it down. Putthe eccentric and strap loosely on the shaft and screw the rodinto the strap until the fabric is horizontal on the frame.Adjust the eccentric so that the line joining its centre and thecentre of the shaft are horizontal and then tighten the set screwin the eccentric hub. If that is not done the movement of thefabric will not be uniform in each test. In one case theoscillations may be entirely above a horizontal and in anotherthey may be entirely below the horizontal, so that the fabricwill not be run under the same conditions each time. Consequent-ly any conclusion that might be made would be erroneous. Insertthe bulbs and put the coolant into the container and place thecovers on. The apparatus is now ready for a test.No work has been done on the low temperature testing17of fabrics before so no definite procedure can be outlined.It may be advisable to make several runs at room temperature,and for various lengths of time to determine how long thefabric remains unchanged and then run the cold tests for thesame time. Part of each specimen should be subjected tomicroscopic (onsUB) test. Such tests may be of no avail, butthe investigator may have a knowledge of textiles and be in aposition to suggest better test methods.18APPENDIX.ApparafScol "c06 126 GaeC#f~in~'r.F VIII II II I1% 1*9 -+-Aft*33- 4-$In$de Coter(loy.#Pe) Ou tside Cover ("P/e)/6NJITNIiSca/o-!| F:., i z 1e.'mu Iinw..n.A V- 3' ;( -,II'IIt IIt IjI IiI~I; ~IILate.am~aa~..sa..aIa a a a a JIg IOots le Rot (4*pin) IN0.4 rfBox (f* Piae)Scq/e .FoN /i e.-I,Ig1114 I INILI/0 I pI'-q~-mm--mi1'2$cca/e - i/q If S/5~.P ~ For Fa /ric~.1-i4 I$c ce rEc<n I /cF C eCe n f 4 icAt iA n SOL;Ec CentIe -- Ca@r ohFcc e n f r/C sfrat-- StRe/Ectenitic Red - S/eelR MC hadsftfcenr- P, Ine.P/cen5211YiF~jy.Ro t e sc -a/e7ItIiI-i- 40&/Minera/ Woo/rLmmui.mhmLNireral/ b!oo rn-N'..XXN ! XX7,$7777,7/IJyCrOSS &cino AsnLelie fr,1 erz fifty V'.tAcQ/e- 1 r F4// J'ize.N NNNNzzzF4OFhI;'I0*I,P-Ip -~P - 22'll .. -4soNtit,P -357,

Design of an apparatus for testing aeroplane fabric at low temperature

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Design of an apparatus for testing aeroplane fabric at low temperature

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