Tag Archives: sheet metal

THE BIRTH OF THE ATLANTIC AIRLINER

C69LHCONNIEg

Well, today I finally finished the last part of my reflections on the history of the “Atlantic Airliners”, that is, the story how designers and other craftsmen learned to make metal birds that carry passengers safely across the great ocean.

So quick, go to the top edge of this page and click on Early Atlantic Airliners and download the pdf files for free!
(By the way, visit also the beautiful website where I found the above picture:
https://www.scribd.com/doc/280054952/Lockheed-Constellation-From-Excalibur-to-Starliner )

Now the last blog I have to write is to finish this Davis wing story. It’s coming – give me a bit of time.  Lemesee, what was it about again??

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1934: SCHATZKI REVISITED

In the 1930’s the above traffic situation occurred frequently in the canals of Amsterdam: the Fokker Aircraft factory was located in Amsterdam North, a residential and industrial section of town without an airfield, while Schiphol Airport was to the south of the city, on the bottom of a reclaimed lake. For flight tests newly built Fokkers had to be transported through the wondrous and winding canals of the old city. Sometimes, after heavy rainfall, Schiphol was too marshy and flight tests had to be relocated to Welschap, near Eindhoven, a distance of 70 miles or so to the south.

You can read about the tests at Welschap in the revised ‘album’ of Erich Schatzki’s life, when you click here:  alifeofflight2.pdf

Erich Schatzki loved the new country that he had settled in for a short interval of time. Relentlessly moved on by the unfolding of history, events took him eventually to the United States and then to Israel and back again to the USA – like a pendulum, going multiple times back and forth.

For the new data which I was able to add to the earlier description of Schatzki’s life, I am indebted to my old friend Wim Snieder, the writer of the only comprehensive bibliography of Dutch aviation history: “In Vogelvlucht” / Geannoteerde bibliografie over de Nederlandse luchtvaart, vanaf 1784. Uitgever; Canaletto/Repro Holland; 486 pages, ISBN 9789064697340

1939: Erich Schatzki Album

Fokker_g1

As a little boy in Holland I was enraptured by the original shape of the Fokker G-1 fighting plane of 1939. A few years later I became a (small) close witness to the atrocities of the war, as some of my schoolmates and good neighbors in the street where I lived in Amsterdam were deported because they were Jewish. (They never returned.)
Dr.Erich Schatzki

While living now in California and enjoying my own ‘Indian Summer’, I found on the web the designer of the G-1, Erich Schatzki. I also learned that he was a Jewish exile and that he and his family had been on the run for the Nazi’s since 1934. I set out to find more facts about his life.
I have collected these in an ‘Album’, a collection of factual items and a description of some of the fascinating people that he met in his long, adventurous life.

I have added my findings in a new file on my website today. If anybody can tell me more, or if somebody wants to correct an error, please leave a note!

 

1930’s: LIGHT METAL WINGS II

1890: the Wing Otto Lilienthal used
ca 1890: the Organic Wing Otto Lilienthal used

When, in 1953, in my capacity of apprentice in the KLM Maintenance Service at Schiphol, I started one morning to help take off half the wing of a Douglas DC-3, I was most astonished to find that the wing of this rather famous and historic airliner had no sturdy spar in its innards, but that the metal wing cover had a seam from front to back at a position close to the engine, where it was simply bolted to the center section of the airplane.
Only recently I read that this particular construction was called ‘multi-spar’ and invented by Jack Northrop around 1930. In document 3-22(a-b)  3-22-b: Engineering Department, Douglas Aircraft Co. “Development of the Douglas Transport”, Technical Data Report SW-157A, ca. 1933-34, Folder AD-761184-05, Aircraft Technical Files, National Air and Space Museum, Washington, D.C., one can find:

“In the Douglas and Northrop types of multi-cellular wing construction, there are a multiplicity of full length span-wise stiffeners, and the fact that they have no abrupt changes or ‘breaks’ [in their extended shape] results in no concentration of stresses. With the centroids of the stiffeners located at the maximum distances from the neutral axis of the [wing] section, a most efficient structure for absorbing the bending load is obtained.”

Northrop N9MB as seen at the Air Museum Planes of Fame in Chino Ca, where it is flown regularly.
Northrop N9MB Flying Wing as seen at the Air Museum Planes of Fame in Chino Ca, where it is flown regularly.

In my interpretation this means that the outside skin of the wing (well reinforced with span-wise stiffeners) will absorb all the bending stresses and that one can dispense with heavy spars directly connected to the fuselage. The remainder of the text is too interesting to be omitted, as we, modern airline customers, only too well know how scary modern airliners sometimes flex their wings:

“In a highly stressed airplane, torsional rigidity of the wing is of paramount importance in the prevention of wing flutter at high speeds and torsional deflection of the structure must therefore be kept to an absolute minimum. When under load, there will always be some vertical deflection but this must not be excessive since a wing with large vertical deflections might cause jamming of aileron controls and by no means inspires confidence in the passengers or pilots.”

Also, vibrations can generate most annoying noise (I remember flying in the Vickers Vanguard in 1962):

“If unsupported flat metal surfaces are even moderately large, there is always a tendency for the middle of the surface to vibrate in flight, even when there is no stress. This is termed ‘oil canning’and will, in time, cause fatigue in the sheet metal and in the rivets and cause rivet heads to work and to pop off. These unsupported flat surfaces continually drum and cause a noise that cannot be completely eliminated in a cabin, because part is carried as vibration thru the structure. This is different from ‘wrinkling’of the skin. Wrinkling will be present in every metal wing with a flat metal covering taking stress. These wrinkles are deflections of the skin under load and ordinarily do not have any tendency to vibrate.”

Northrop wing with skin removed showing longitudinal stringers [from: WoodToMetal.pdf  SI 94-7718]
Northrop wing with skin removed showing longitudinal stringers [from: WoodToMetal.pdf SI 94-7718]
The report continues with more on the subject of the wing design for the early Douglas airliners:

“In determining the wing construction of the early Douglas machines single, two, three and multi spar designs were considered as well as shell type and multi-cellular designs. After a thorough investigation of all types the Northrop multi-cellular wing construction was finally decided upon. This type of structure consists of a flat skin reinforced by numerous longitudinals and ribs. The bending is taken by the combination of flat skin and full length [longitudinal] stringers. Three main flat [vertical] sheets or ‘webs’ carry the shear loads. Torsion and indirect stress are carried by the skin with frequent ribs preserving the contour and dividing the structure up into a number of small rigid boxes or cells. Since the major loads are carried in the outer surface of the wing as well as in the in the internal structure, an inspection of the exterior gives a ready indication of the structural condition. The unit stresses in the material are low and therefore the deflections are at a minimum giving a maximum in rigidity. This construction has proven to be a happy medium of those considered since it combines practically all of the advantages of each; namely, very small unsupported areas, extreme lightness for its strength and rigidity; also ease of construction, inspection, maintenance and repair.“

Douglas DC-3
Douglas DC-3

For the early Douglas airliners:

“The Northrop wing being comparatively small, it is economical to have many of the stringers run from the top to the bottom of the wing as shear webs or spars. However, when the principle is carried out on a larger scale, as in the DC-1 with its deeper wing, it is more efficient to have only three shear webs or spars. Thus it was not necessary to evolve a new type of structure but merely to adapt a time proven type to the dimensions of the DC-1.” [end of quote]

Detachable wing of DC-3
Detachable wing of DC-3

The exterior wing was fastened to the center section with a great numbers of bolts. It was my task to receive each bolt, nut and washer that became undone and secure them in a numbered hole in a plywood board. In the end there were 20 boards with a total of 652 bolt sets. My mentors /colleagues worked according to strict KLM protocol  [see the following drawing which I owe to Mr. Wim Snieder, The Hague, Holland]  and had the use of an overhead crane.

Part of the KLM protocol for dis-assembling wing of Douglas DC-3
Part of the KLM protocol for dis-assembling wing of Douglas DC-3


We finished unbolting the [half] wing by 3 pm and went for tea, delivering on our way the boards with fasteners at Testing for examination on hair cracks and corrosion.

http://www.airnieuws.nl/phregister/476/luchtvaartnieuws.html
http://www.airnieuws.nl/phregister/476/luchtvaartnieuws.html

1930’s: LIGHT METAL WINGS I

main spar box Boeing
picture source: navyaviation.tpub.com

“…The ability of sheet metal to carry an increasing load after it had begun to buckle – which in conventional structures was regarded as failure – was crucial to the development of metal airplanes. It had first been discovered in 1925 by Dr.Herbert Wagner, who was then working for the Rohrbach Metall-Flugzeugbau in Berlin, Germany, but his findings were not published until 1928 in English by NACA. Northrop’s work was done independantly. Wagner went further than Northrop in his analysis of the way in which a thin sheet of metal behaves when supported at the edges, as it is in airplane structures, and he evolved the theory of the diagonal-tension field beam to explain it. This theory, and elaborations of it, formed the basis for the development of a/c structures from the mid 1930’s onward. But it was not applied to the early Northrop airplanes or the Douglas DC-1-2-3. Northrop’s construction gave a good enough ratio of strength to weight for these airplanes, and the use of Wagner’s theory would have added to the complication and cost of design…”

The above quotation is from: Ronald E.Miller; David Sawers, “The Technical Development of Modern Aviation” (Routledge & Kegan Paul: London; 1968) p.65
In August 1933 Paul Kuhn  wrote an explanation of Wagner’s theory as NACA Technical Note No. 469 “A Summary of Design Formulas for Beams Having Thin Webs in Diagonal Tension”,  Langley Memorial Aernautical Laboratory. Washington,. A copy of this paper may be downloaded from the Herbert Wagner page on this website.

1890: THE TIN AIRSHIP

SCHWARZ, David
Born: Zalaegerszeg, Hungary, 20 December 1850
Died: Vienna, Austria, 13 January 1897
Timber merchant; airship designer
SCHWARZ, Melanie: his wife and business partner
SCHWARZ, Vera, their daughter; opera singer

SchwarzAirship FotoHungary

A tin airship was brought to flight by David and Melanie Schwarz from Agram, Dalmatia (now Zagreb, Croatia). At the end of the nineteenth century, it was they who built the first metal airship in the world. The story is well documented by now (see links below) but remains remarkable because they were the predecessors of Graf von Zeppelin, who is generally assumed to have been the first to build the historic metal steerable, lighter-than-air vehicles that now carry his name.
It is also remarkable because here a woman played a decisive role in the construction of a flying machine. (In the first half century of aviation history there have been a good number of courageous and successful female pilots, I know, however, of no other example of a woman who was involved in the business of building a flying machine, nor have I ever heard of major contributions in this field by ladies such as Mme Blériot, Mrs. Boeing or Frau Heinkel…)
scaled-209x400-David-Schwarz-und-Frauy

The husband, David, was a man of some importance, a timber merchant who every year spent long months in the forest, overseeing logging operations. His desire was for a magical, mighty machine that would be able to lift the cumbersome trunks of trees straight up and out of the hilly terrain. His thoughts materialized into the design of a rather large metal cylinder, filled with hydrogen gas. The pressure inside the vessel would equal the outside air, so as to avoid extreme forces on the shell. In order to be able to levitate, the total construction, plus its load, would have to be lighter than the air it displaced, according to the Law of Archimedes.
Being an avid reader of technical books, he had learned of the miraculous metal aluminium (or aluminum in English speaking countries), known since 1825 as silver-from-clay. As Wikipedia states: “Aluminium is the third most abundant element (after oxygen and silicon), and the most abundant metal in the Earth’s crust.” Yet it proved extremely hard to extract from its host, the ore bauxite. Indeed, the first small quantities produced were so costly that they were used only for art objects and expensive cutlery at the court of Napoleon III.

404px-Eros-piccadilly-circusX
Aluminium Eros at Piccadilly Circus London, ca. 1895

Production on an indutrial scale had to wait until in 1886, when Charles Martin Hall in Ohio, USA, and Paul Héroult in France invented the electrolytic process of refining aluminium at practically the same time, using an electro-oven.
       This development was only possible after the perfection by
George Westinghouse (and predecessors) of the electrical transformer, a device that could deliver to the oven extremely large currents at low voltage. The oven required a vast amount of electric power, which had become available on an industrial scale after Man had learned how to build hydro-electric power stations (for instance at Niagara Falls, 1895 and Neuhausen Switzerland, 1888). The production of relatively cheap aluminum became from then on feasible and Schwarz’s dream came into the realm of reality.
It goes without saying that in order to become truly dirigible, his tin cylinder would also need a motor with propeller and rudder (although for lifting tree trunks out of the woods a cable balloon might have served the purpose). The practical, portable combustion engine was put on the market around 1885 by Gottlieb Daimler and Carl Benz.
Summing up, we may say that the light-weight metal steerable airship could not have been built before 1890 and that Schwarz’s invention represented the cutting edge of technology.
Schwarz first approached the Austria-Hungary War Ministry, but received little interest in his ideas. He found more resonance in Russia and a first attempt by him to build a metal airship was made in St.Petersburg. When these attempts failed Schwarz returned to Zagreb.

Zagreb Trg1880

In 1894 he got involved with the German entrepreneur Carl Berg from Ludenscheid, Westfalen. Carl’s firm specialized in the production of aluminum flat sheet and rolled shapes with various profiles. The factory obtained raw metal from the first European aluminium smelter in Neuhausen, Switzerland (1888), later known as Alusuisse. Berg saw great potential in Schwarz’s project and decided to help him transform ideas into hard reality. In fact, it was Berg’s engineers who made the definitive calculations and ultimate design for the airship. On paper it looked sort of like a giant spray can lying on its side: a cylinder with a flat bottom and a conical point. An open gondola hanging from the cylinder would hold the pilot, the Daimler engine (16 hp) and the steering controls. Via belts the engine drove no less than four propellers, one of them a horizontal one to aid levitation. According to one account the ship measured 38 meters (125 ft) from tip to tail; its diameter was 12 meters (40 ft). The aluminium skin was 0.2 mm thick (equal to four sheets of kitchen aluminium foil) and riveted air tight on a skeleton of thirteen aluminium hoops and longerons of angle profile. Important: at the highest point of the cylinder was a hydrogen release valve that could be opened from the gondola.
Berg and Schwarz came to the agreement that Berg would assume all further costs. He would produce the parts that were to be assembled under the supervision of Schwarz at Tempelhof Airport in Berlin.  It took till the summer of 1896 to get the metal airship ready. Then it was discovered, during the last preparations for the first flight, that the so-called hydrogen gas supplied by a German chemical factory was not of sufficient purity; its specific weight was not low enough in comparison with the air it was displacing and so the loaded airship would not float upwards. Further tests had to be postponed.

Ehrengraeberjuedx
Central Cemetery of Vienna – Israelite part

To the great dismay of his family and business associates, David Schwarz was hit by a fatal stroke while in Vienna on the 13th of January, 1897. The Jewish community of the city of Vienna gave him a funeral with all due honor and a monument at the Zentralfriedhof of the city.
Carl Berg feared he was now stuck with a bizarre and rather fantastic-looking aluminium cylinder whose inventor and promoter had taken his leave forever. However, high-quality hydrogen gas was delivered in Berlin at that same day and Melanie Schwarz came to the rescue. Contemporary sources state that she was a “delicate yet unbelievably energetic lady”. Apart from caring for her family she had always assisted David in his endeavors. Everybody was surprised when she took charge of the project. The preparations for the first flight were resumed resolutely. She engaged a Mr. Jagels, a military man who had, as he said, some experience in ballooning and who was prepared to wager his life for a modest compensation .

Berlin_Tempelhofer_Feld_1907 (1)
Berlin: Tempelhof Field

Filled with almost pure hydrogen gas, the tin cylinder finally elevated itself from German soil in the presence of a vast crowd on the 3rd of November, 1897. A hard and cold wind blew from the east. Jagels had practiced ballooning under simple circumstances; now it was demanded of him to observe a multitude of variables such as wind, altitude, obstacles, engine revs and desired course, while at the same time handling the engine, the drive belts and the rudders.
The ship did lift off, but thereafter things went wrong. The drive belts jumped from their wheels, the propulsion failed and the little ship was carried off, out of control, by relentless and swirling winds to a height of more than four hundred meters. Caught in a basket and at the mercy of the elements, this height is frightening to even the most obliging person and it is understandable that Jagels did the only thing that he could possibly think of: he yanked hard at the cord that opened the safety valve. Unfortunately, just like Blanchard ninety years earlier, he let too much of the good gas escape. The ship, suddenly having lost its buoyant force, dove down and, zigzagging like a punctured child’s balloon, struck the earth at an oblique angle. Fortunately, the skipper was able to jump just before the metal cylinder flattened the gondola against the ground and so saved his dear life.
Melanie showed a remarkably modern talent for public relations. She dispatched the following telex to Carl Berg:
“HYDROGEN FILLING AND LIFT OFF  FULLY SUCCESSFUL” STOP
“SHIP ATTAINED 1000 FT MADE 2 TURNS” STOP
“DRIVE BELT PROBLEM CAUSED PREMATURE LANDING” STOP
“SHIP DAMAGED” STOP
“JAGELS UNHURT“ STOP
“MELANIE SCHWARZ“  STOP

idem1897

Unfortunately, this diplomatic account of affairs could not withhold Berg from withdrawing from the project. He had the remnants of the ship melted. (One of the curious properties of the new metal was that it could be completely recycled.)
Melanie appeared one more time on the stage of history when a certain Count von Zeppelin approached Carl Berg to embark with him on a new project for a metal airship. “This ship will be completely different. It will have an exo-skeleton of aluminum girders that will be covered by  watertight fabric. The gas will be held inside in a row of conventional balloon bags.” Berg was highly interested but felt himself tied by contract to the Schwarz estate. To nullify the obligations, the following proposal was made to the heirs: during a three-year period the Schwarzes would be paid the equivalent of 15,000 Reichsmark, with a royalty of 10,000 Reichsmark for every airship delivered, with a maximum of thirty airships. To their surprise, the generous offer was turned down by the guardian of the Schwarz children, Herr Czillac from Fiume. This male meddling infuriated Melanie and she personally made an appearance at Berg’s headquarters. She was willing to tear up the contract for an immediate payment of 15,000 Reichsmark. “Cash in hand,” she must have reasoned, “that silly old fool Zeppelin won’t ever amount to anything!”
In hindsight, this is a shame, of course, because the thirty airships mentioned in the proposal would already be built by Graf von Zeppelin before 1915, and the previously mentioned three hundred thousand Reichsmark would have been just the sort of money that a mother-alone-with-children could have put to great use.

Epilogue

800px-Wien_-_Staatsoper_(2)
Vienna Staatsopera

All together we may safely state that Melanie did well in the end. Her daughter Vera Schwarz (188?-1964) became with dedicated maternal care a famous soprano, appearing in all the major opera houses of Europe and the United States, often together with Richard Tauber. From 1938 to 1948 she lived in exile in the U.S. Upon her return to Vienna she became a sought-after teacher, giving well attended master classes.
In 2011 a street in Vienna’s 23rd district was named Vera-Schwarz-Gasse in her honour.

Schwarz

http://austria-forum.org/af/Wissenssammlungen/Biographien/Schwarz,%20Davidhttp://www.zeit.de/wissen/geschichte/indexhttp://www.zeit.de/2013/34/zeppelin-erfindung (david schwarz)http://www.britannica.com/EBchecked/topic/1350805/history-of-technology/10452/Petroleumhttp://de.wikipedia.org/wiki/David_Schwarz:

http://en.wikipedia.org/wiki/David_Schwarz_(aviation_inventor)
http://en.wikipedia.org/wiki/Vera_Schwarz

ARENSE, J.L. : Zeppelins en de Luchtscheepvaart”
De Bataafsche Leeuw: Amsterdam
1990   ISBN 90-6707-250-8

“The Metal Airship” article in: Aviation Enthusiast, July 1992

FOR BOOKS ON AIRSHIPS:
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http://www.lindbergh-aviation.de/

1936: INSIDE THE ZEPPELIN

Luftschiff Hindenburg (LZ-129), Speisesaal

With some disappointment Carla looked through the downward slanting large windows: there was still nothing more to see than grey fog, a pea soup that seemed to touch the windows and that moved past with considerable speed. Of course she had it wrong, she realized, the ship was moving forward at one hundred twentyfive kilometer per hour and it was the fog that was stationary.

They had departed from Frankfurt yesterday and that had been an exciting experience. The ship had been moored to the landing tower at a height of about thirty meters and hundreds of people had swarmed below it. Commands and shrill whistle signals could be heard, hawsers were cast off, even a ship’s bell was sounded. She had heard a steady discharge of water – that was ballast, she was told. Then the ship had moved slowly away from the mast, going backward and upward in a stately manner, while the crowd below was cheering and waving. She had felt a shudder going through the mighty structure surrounding her, followed by a steady vibration when the engines took hold of the airship and started to propel it forward. The “Hindenburg” had freed itself from the airport departure site and had at first flown low over the city until it had begun to rise gradually into the clouds.

800px-Zeppelin_Postkarte_1936_a
THE HINDENBURG ON A TRIAL FLIGHT IN 1936

Obviously that’s where they still were at present. Carla had slept very well and after having been awakened by a light knock on the door, she had dressed quickly in order to hasten to the dining room with almost childlike impatience. Coming from the ladies’ rest room on the lower deck, it struck her that the corridor was sloping slightly downward toward the rear and that the ship was rolling and pitching lightly. Much gentler than an ocean liner, but, as she moved towards the tail, the movement became more noticeable. The engines hummed far away, not loudly and not bothersome, but rather emanating a feeling of security.

The breakfast room was empty except for two well-dressed gentlemen, most likely business men on their way to Brazil. As Carla walked past them to the large picture window at port, she smiled and exchanged a friendly “grüß Gott”. She had been hoping for a cheerful sunny morning, but outside there was only grayness. All the same, she peered out intently for some minutes as if to grasp the true meaning of the passing cloud patches.

She could not help overhearing the conversation of the two gentlemen. It also concerned the clouds.
“I think we’ll probably stay in the clouds today as long as possible,” said the larger of the two, a military looking man. “In that way the Captain saves his hydrogen gas. See, we are heavily loaded – the fuel tanks are almost completely full – we are near maximum weight. If we get out over the clouds into the sun, the gas cells will heat, the gas expands to maximum and it could well be that the safety valves will bleed it off. But, see, we can’t afford that – we need all the gas to stay afloat, especially during the night. That’s why we are staying in the clouds now. Do you want some more coffee?”
“Yes, please.”
“There is one more thing that shows that we are heavily loaded: we’re cruising with the nose slightly up. That means the forward speed gives the ship some lift on its hull, just like the wing of an airplane. If we were to fly horizontally, with static buoyancy, we might loose height gradually.”
“We could jettison ballast?”
“Yes, we could, that’s true, but we are at the beginning of the journey and the Capitan must make best use of his resources – we may get into a situation later where he needs the ballast badly. Don’t forget that the ship gets slowly lighter all the time because it uses its fuel. In a while we’ll get to a warm country with a much lighter ship. You wouldn’t want to be in a situation where you couldn’t descend because you were too light, would you now? Or where you kept going higher and higher? Haha!”

Carla felt a bit of a shiver – there was much more involved in this type of travel than she had ever suspected.
“But in that case he would valve off the gas, wouldn’t he?”
“Of course, I was only joking. But the essence is to maintain control of the airship in an optimal way by preserving as much as possible of both gas and ballast. Remember, we can use the resources – fuel, gas and ballast – only once. When they’re gone, they’re gone for ever. That’s to say, until we can land.”
“Same with water?”
“Less so. We can collect rainwater and condensation on the hull and replenish our stores. But that takes time.”

LZ_129_Hindenburg_interior

The weather outside cleared somewhat. The pea soup became less dense and wispier. Occasionally there were even clear spaces between the shreds of fog. The men behind her continued with their breakfast and the expert on zeppelin travel volunteered more information:
“On the other hand, of course it is advantageous to fly as high as possible.”
“Because there the air is less dense?”
“Yes, the air is thinner; the ship has less drag. That means less effort to propel it. So we can make the same speed with less fuel, or we can go faster with the same fuel – so we arrive earlier in Rio.”
“So, we should fly as high as possible?”
“Yes, but there are limits. First of all we get trouble breathing. Especially the older passengers may not find it very comfortable at two thousand meter. The engines also will have their problems; they deliver less power per liter of fuel. Then there is the cold, mind you. At two thousand meter it may start to freeze. So the ship has to be heated, for the passengers first of all, and that takes fuel again…”
“Yes, but if there are no clouds at two thousand meters, the sun will heat us. You said just now that the gas would get warm …”
“Indeed, by radiation, but the air temperature is low and the air will have to be heated!”
“What fuel do we use?”
‘Diesel, we have four twelve hundred horse power diesel engines.”

Carla no longer followed the conversation. Typical boy’s talk. Men were awful… They could go on and on about something technical. She was more interested in the clouds. They were unmistakably becoming less and less. Occasionally she could even see a small patch of ocean surface. Ripples in a grey sheet: waves seen from a great height. It was only for a moment that she could see the sea and the thought flashed through her that somebody on a ship could have seen the Hindenburg. She imagined being on a ship underneath an endless cover of clouds and then suddenly in an opening, an airship appears and is gone again immediately. A strange thought. Would there have been somebody, there at sea? Or were they the only ones present in this space: a surreal dining room at five hundred meters, surrounded by clouds? Was she dreaming? Did the sea down there really exist? And had the zeppelin been visible from the sea even if nobody had been there to see it? Was there a reality outside the things that you saw yourself?

Fortunately, at that moment more passengers entered the room and a friendly elderly couple invited her to join them. They talked about their destinations and where they came from and soon the disturbing thoughts were gone.

————————————–
ADDENDUM

Luftschiff Zeppelin LZ-129 ‘Hindenburg’ was the largest operational passenger airship between the wars in Germany. It had a length of 800 ft and a diameter of 135 ft.
Starting March 1936, it flew a regular airservice between Germany and South (later also North) America, until it was destroyed by fire
in May 1937 at Lakehurst, New Jersey, USA. In 1936, 17 round trips were made across the Atlantic, carrying an average of 65 passengers with a crew of 56. In that season the ship flew 192,000 miles, carrying more than 2000 passengers and 160 tons of freight and mail. A westward trip
took an averige of 65 hours depending on weather conditions; eastward it took ten hours less. The one way fare to the USA amounted to $400.–.

for more reading, see this most interesting Wiki site:
http://en.wikipedia.org/wiki/Lz-129

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