Inflation of the Metalclad Airship, ZMC-2

8/16/2019 Inflation of the Metalclad Airship, ZMC-2

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227arch, 1930 I N D U S T R I A 4 LAND ENGI,VEERISG CHEM ISTRY

Inflation of the Metalclad Airship, ZMC-2Chemical Control during Inflation

A. R . Carr2 and A. C . Good

COLI.EGE O F THE CITY P D E T R O I T KD -4IRCRAFT DEVE1,OPMENT CORPORATION, DE TR OI T, I I C i i

HE first succesful all-m e t a l a i r s h i p , th eT M C - 2 , was designed

and built at the Grosse Ileairport by the Aircraft De-v e l o pin e n t Corporation, adirision of the De troit Air-c r a f t C o r p o r a t i o n , for th eUnited States Xavy. It wasCompleted after five years ofen g in ee r in g re search andstud y. The ship is a single

structural unit in which themetal plating is not only thegas container but also carriesa consideraljle portion of thedirect stresses. It s size of200,000 cubic feet, w hile large

The inflation of th e metalclad airship, the Z M C - 2 ,was carried out in two stages. In the first stage theair in the hull was displaced by carbon dioxide. In thesecond the carbon dioxide was displaced by helium.Carbon dioxide was separated from the helium byscrubbing the gas in a caustic tower and the purifiedhelium returned to the ship. In this manner a purityof over 92 per cent helium was obtained in the gas-filled hul l.

Although the helium has diffused outward at about1 cubic feet per 24-hour day, no appreciable quantityof air has come through the metal wall of th e hul l,

which would cause a decrease in t he lift of the ship.Although th e capacity of t he sh ip, 200,000 cubic fee t,makes it too small for commercial purposes, its per-formance and characteristics prove the feasibility ofsimilar construction for larger, commercial units.This is a distinct contribution to lighter-than-air con-

enough to meet all expeFi-mental requirements, is toosmall to meet the demands of a commercial airship. However,its performance and characteristics compare very favorablywith the non-rigid fab ric blimps used as trainin g ships by theGo vern me nt. Moreover, it proves th at airships can be built,possessing the adv antag es of all-metal construction wit houtadding materially to the weight as compared with fabricships and that these metal ships can be built large enough

for commercial purposes.Description of Sh ip

The X C - 2ma de its first flight on August 19, 1929 (Fig-ure I ) , and passed its filial tests a t Lakehurst about a m onthlater. Its general characteristics and performance d at a aregiven in Tables I and I1 1).

struction.

Ta b l e I -- G e ne r aI C h a r a c t e r i s t i c s of t h e Z M C - ILength of hull. 149 f t . 5 in.Diame ter of hull (max .). . . . . . . . . . . . . . . . . . . . . . . . .Fineness rat io . . . . . . . . . . . . . . . . 2 . 8 3Displacement of hull 202,200 cu. f t .

, , , 50,600 cu. f t .Front bal lonet displacement, , . . . . . . . . . . . . . . . . . . . . . . . . . 22,600 cu. f tRear bal lonet displacement, , . . . . . . . . . . . . . . . . . . . . . . . . . . 23,000 cu. ftRatio of bal lonet volume to hull vo lume. . . . . . . . . . . . . . . . . . 25 per centThickness of ski n. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 .0095 in.Length of ca r. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 f t .Width of car. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 f t . 6 in.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Number of gas valve s. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Number of f i n s . . . . . . . . . . . . . . . . . . . . . . . . . . 8Tota l fin area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 440 sq. f t .Total elevator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190 sq. f tTotal rudder area. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 sq. f t .To ta l au tomat i c rudder a rea . . . . . . . . . . . . . . . . . . . . 95 sq. f t .Su mb er of engines (Wright Whirlwind J-5) , , , , , , , 2Power at 1800 I. p. m . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 440 hp .

9 ft. 2 in.

Th e hull is 149 feet 5 inches long an d 52 feet8 nches in maxi-mu m diame ter. Th e me tal covering consists of Alclad alloysheets 0.0095 inch in thickness. These sheets are sewedtogether by means of a special riveting machine invented bythe Aircraft Developme nt Corporation. Since this coveringcontains the lifting gas, helium, the seams and riveting m ustbe gasproof. This is accomplished by trea ting the lap seamwith a special bit uminous ma terial after riveting. Th e skin

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Propeller diameter (all-metal). . . . . . . . . . . . . . . . . . . . . . . . . .

Received December 3 0 , 1929.Chemical engineer in ch arge of control tests during t he inflation.

ib then firmly riyeted tosupporting structu re consist-ing of equally spaced longi-tudinal frames running frombow to stern and a numberof transv erse frames. Insidethe bottom part of the hulla r e t w o ba l lo ne t s , one of22 ,6 00 cu b ic feet and theother of 28 ,000 cubic fee tcapaci ty, the former beingplaced toward the bow and

the latter toward the sternofthe ship. These ballonets arem a d e of rubberized fabriclaced to t he hull an d are filledwith air. The ir purpose iq t oco mD en sat e for change inpressire inside the 1 1 ~ 1 1 ~

The car is suspended fromthe hull by suitable attachm ents. The power plant con+tsof two Wr igh t Whirlwind J-5-A engines carried on tubularoutriggers, one on each side of the car. Eigh t fins, a uniquefeatu re, are equally spaced around the hull. Th e carcon-tains fuel tan ks, control instrum ents, radio, blower, and char ttable, and has places for two pilots, one mechanic, and aboutfour student pilots.

The hull was constructed in two sections erected vertically.When completed, the two sections were placed in a horizontalposition and riveted together. (Figure 2)

Tab le 11 -Pe rfo rmance Da ta of t h e Z M C - IGross lift (100 per cent inflation with 92 per cent pure

helium at 60 F. and 29.92 in. H g ) . . . . . . . . . . . . . . . . . . .Weight emptyUseful load. .

12,242 Ibs.9,113 lbs.3 127 Ills

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Crew (three) . . . . . . . . . . . . . . . . . . . . . . . . 600 Ih?Fuel 200 ga l . ) . . . . . . . . . . . . . . . . . . . . .Oil 2.5 ga l . ) . . . . . . . . . . . . . . . . . . . . . . . .Ballast (50 ga l . ) . . . . . . . . . . . . . . . . . . . .Passengers and cargo. . . . . . . . . . . . . . . .

1 i O O l b i .200 Ibs.420 Ibs.7 0 7 Ibs.

Range with 230 gal. fuelMaxim um possible rangeMaximum speed at 440 hCruising speed at 220 hp.

680 miles

Stat ic cei l ing. , . . . . . . . . . .

Apparatus Used for InflationBecause it is very difficult o separate helium from air, the

infla tion of t heZ M C - 2 was carried out in two stages. I n thefirst stage carbon dioxide displaced the air in the ship. Thecarbon dioxide was introduced at the bottom and the airforced out a t t he topof the hull. I n the second stage the proc-ess was reversed. Helium was led in a t the top while carbondioxide was forced out the bottomof the hull.

When ready for inflation, the hull was suspended in thehangar with the car, fins, valves, and connections in place.The carbon dioxide cylinders were stored between the hulland the wall of the hangar. These cylinders were atta che dby means of two octopus manifolds to a 6-inch pipe line.These manifolds were mad e of pressure tub ing , allowing six-teen cylinders of carbon dioxide to be discharged into thepipe line a t the same time. Th e 6-inch pipe line ran the lengt,hof the ship. This line was tapped a t three points by lines


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March, 1930 I S D U ST RI AL AAYD ESGIlYEERIA-G CHE MIST RY 229

ballonet n-as deflated first, helium being used to replace its and the exhaust line. Eac h sta tion except the exhaust linevolume. Th en the front ballonet was deflated while air was controlled three sampling tubes from the top, e quato r, andbeing admitted to the rear ballonet. d heck analysis indi- bottom of the hull, respectively. Th e arrangement of thecat ed tha t a small amo unt of carbon dioxide was trapp ed in sampling tubes is shown in Figure3. Samples were analyzedthe ballonet folds. Th e inflation was completedon the morn- a t th e four stations simultaneously. These samples wereing of iiu gus t 3. drawn through small rubber tubes inserted into the hull, the

tubes remaining open to insure a fair sample from the ship atall times. Th e gas was sampled a t 10-minute intervalssoth at the time between two successive analyses a t any levelwas one-half hour . Th e results of these analyses are shownin Figure 4. During the carbon dioxide inflation the per-centag e of carbo n dioxide indicate d directly the progress of

the inflation. During the helium inflation the per-centage of carbon dioxide subtra cted from 100 in-dicated approxim ately the per cent of helium inthe ship. White s m odification of t he Hem pelbu ret and a pipet filled with caustic solutionwere used for absorbing the carbon dioxide.

Final tests made a t the end of th e inflation periodbu t before the gas was passed through the driergaye the following average results: carbon dioxide,

2.3; oxygen, 0.1; and carbon monoxide, 0.0 percent. After the gas had been circulated throughthe drier, the carbon dioxide had been reduced toan average value of 0.67 per cent. This coupledwith the removal of water vapor brought the

scrubber was dismantled and rebuilt as a drier. It, was fitted purity to above 92 per cent helium, which com pareswith three trays over which the gas from the hull might pass. very favo rably with the results obtained in inflating o therIt was placed in a horizontal position with 950 poundsof ships. Th e helium in the cylinders had a pur ity of 96.6caustic on the trays and the bottom . To prevent the caustic per cent. These final analyses were made in an Orsatap-from being carried over into the hull, two cloth blan kets were para tus, absorbing the carbon dioxide in caustic, the oxygenfastened over the offtake from the drier. This drier offtake in alkaline pyrogallate, and the carbon monoxide in cuprousdischarged into an expansion chamber as a furth er means of chloride solution.preventing the carrying over of caustic. Cau stic indicators Th e condensate drained from the bottom of the ship showedwere placed in the line to detect any caustic which might tra ce s of sodium hydro xide. Analyses of this cond ensatebe carried over. indicated the presence of not more th an0.008 per cent sodium

A blower of 150 cubic feet per minute capacity forced the hydroxide, so it was appa rent that there could be no harmfulgas into the bottom of the drier, caused it to trave l four times effects from this source.

Drying the GasSince n o drier was used, the gas in the hull contained con-

sidera ble water vapo r. Some of this vap or, about250 cc. inall, condensed and was drained from t he hull. To rid the shipof the excess weight due to this water vapor, the tower of the

Figure 3-Position and Number of Sampling Tu b e s on Hull

the length of th e appa ratus, an d discharged it from the top.Drying was accomplished in the intervals between trial

Discussion of Results

flights: After bein gus ed for25 hours the drier was opened.It was observed that some of the caustic had melted andformed icicles which hungfrom the screen whichcomposed the trays. Thedrier was then recharged 90and run for 30 hours more.At this time the gas was Bofound to contain 0.67 percent carbon dioxide. Th ecaustic in the drier had O

served a double purpose.It had not only dried the 60gas but also removed alarge proportio n of the re- 5omaining carbon dioxide.

Chemical Control o\o oTests-Gas Analysis

The purpose of the 3ochemical control te sts wasto show a t all times theexact progressof the infla- 20tion and the purity of thegas in th e ship after infla-tion. To accomplish thisthe gas in the hull wassampled a t five stations-

Since the hull of the ship was constructed as a single com-par tm ent filled with air, th e problem was one of displacing the

e 1 29 8 1 2 9 8 Z-29

the bow, stern, each side, Figure 4-Variation of Carbon Dioxide Content during Inflation


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230 I S D C S T R I A L A S D E SG IA -E ER IX G C H E M I S T RY T’ol. 22 , K O . 3

air wit h helium. Because carbon dioxide is cheap, availablein large quantities, and easily separated from helium, it wasdecided to displace the air with this gas. Thi s method ofinflation had previously been used ona tes t section of th e hullwith good results . An attem pt was made to carry out theinflation in such a manner as to allow the carbon dioxide todisplace the air with as little mixingof the gases as was pos-sible. Since the re are two chief causes of th e mixin g of thesetwo gases-i. e., diffusion and turb ulen t flow of gas from th econtainers-it was necessary to determine the conditions ofoperation w hich would minimize th e effect of these phe-nomena. As turbulence is mainly caused by rapid movementof th e gas, an a ttem pt was made to keep the velocityof theincomin g carbon dio xide well with in th e region of qu iet flow.Diffusion, on th e other han d, depends upon a num ber of fac-tors , among which ar e time, difference in den sity of t he gasec,and the diffusing area. Taking all these factors into account,a rate of input of carbon dioxideof about 10 000 cubic feetper hour way tried, found satisfactory, and maintainedthroughout the inflation.

Stratification of th e carbon dioxide an d air was qui te

complete. This was partially visible to the naked eye. Dur-ing the inflation the interior of the sh ip was illuminated.Through the peep holes one could observe the moirture con-densing out of th e air and lyingas a blanket of fog on the layerof cold carbon dioxide in the lower part of th e shi p. -1s thecarbon dioxide rose higher an d higher in th e hull, the layer offog preceded it. Results of th e gas analysis also verified th ecompleteness of stratificationas shown by Figure 4, whichgives the p ercentage of carbon dioxide a t various levels inthe hull plotted against time during the inflation. Th e lowersampling tubes almost immediately indicated100 per centcarbon dioxide. Th e rise in percentage of carbon dioxidewasrapid, once appreciable quan titiesof the gas appeared in anytube. About 33,000 cubic feet of carbo n diox ide were lobtin purging, which is extremely low.

During the helium inflation stratification was not nearlyso complete. This was due chiefly to th e great difference indensity betv-een helium and carbon dioxide. The inp ut could

not be kept within the region of qu ietflow without consider-able diffusion. While the volumewas small and the area ex-posed to diffusionwas not great, stratificationwas fairly com-plete, as shown by Figure 4. When appreciable quantitiesof helium h ad d isplaced the carbon dioxide, trouble began.

Some of the causesof the rap id riseof helium in th e exhaust

before the scrubbingwas started w ere as follows: Th e dif-fusing area, which was abo ut6000 square feet, had been at itsmaximum for some time; the gas came into contac t with thetops of the inflated ballonets, causinga surging motion;the rate of inp ut of helium had been increased. How everit is dou btfu l if diffusion between carbon dioxide and heliumcould have been prevente d.

The insertionof a drier or refrigerator between th e scrubberand the hull would be a decided improvem ent. Spacingthe sampling tubes symmetrically above and below theequ ator of th e ship would givea more accu rate check of theprogress of inflation. The rate of inpu tof helium was abo ut10.000 rubic feet per hour.

Conclusions

The results show that the method employedi b relativelysimple and efficient for inflating thi s typ e of ship. One hundred per cent ballonet could not be employed in a shipof thissize. In a large ship, where 100 per cent ballonet may beused, the usual methodof inflation can be carried out.

Fina l conclusions concerning th e diffusion of helium fromthis ship have not b een reached,as it has been under stu dy fortoo short a time. However, approximately 100 cubic feet ofhelium are added every24 hours to replace leakage froin thehull. Th e purity of the gas in the ship has not decreased dueto inward leakage of air. Th is fact isa decided advantageover the fabric ship, which allows appreciable inward leakagof a ir wit h a correspond ing loss of lift.

Literature Cited

1 ) Fritsche, J le c h. E n g , 61 9 5 1 9 2 0 ) , gives complete description ofdesign, construction, and erection of the Z M C 2 .

A Derivation of Duhring’s Rule’A . McLaren White

G E O R G I ~ C H O O L O F TIXHKOLOGY, T L A N T. 4 , GA.

CRIKG the last few years the necessity for obtaininga simple yet accurate method of relating vapor pres-

Dures and temperatures has led to the rediscovery of

Duhring ’s rule. Thi s relation has given valuable resultswhen app lied to solutions of sa lts in wa ter I ) , to solutions oforganic liquids (4) ,and to pure liquids. Duhring’s rule hasbeen regarded as entirely empirical, though from its wideapplicability and validity it would seem that it should havesome thermod ynam ic basis. It is the object of th is paper topoint ou t how Drihring’s rule niay be reconciled with therm o-dynamics.

Inasmuch as the D uhring relation in rolres vapor pressuresand te mperatu res, it seems logical to believe tha tit must beconnected with the Clausius-Clapeyron equation. Thisequation in i ts approximate form may be stated as

where p is the vapor pressure, T he absolute temperature,

and AH th e heat of vaporization. Th e vap or is here as-

d In p/dT = A H / RT 2 (1)

Received November 29, 1929.

sumed to be a perfect gas, and the volumeof the liquid negli-gible compared w ith th at of th e vapor. If the h eat of vapori-zatio n is assumed to be co nsta nt over a small range of teni-perature, this equation may be integrated, obtaining

This equation proves experimentally to yield a straight lineover small ranges of temperature.

Now for txo different substances,a and b, let the vaporpressures be equal at absolute temperatures,T , and Tb.Substi tuting in Equation2 and equating the result

Documents

Inflation of the Metalclad Airship, ZMC-2