For other reasons, your third link is of great interest to me too! Unfortunately I can't read German at all well. I can copy and paste text from at least some PDFs but it seems the Fraktur font gets garbled when I do, so Google Translate is no help.
Glancing through it I can see that indeed there is talk of airship travel here. I recognized a reference to Dr. Durr, who was Zeppelin's chief engineer/designer.
This being a 1909 publication, it is a bit premature, though a decade hence an airship line from Berlin to Baghdad would be quite feasible, given the political right of way and suitable base infrastructure.
I always thought "Berlin to Baghdad" referred to a railway scheme however. That, once fully built, would be far more valuable. (Don't know how it would cross the Dardanelles Strait though. Building a bridge would probably be somewhat beyond early 20th century capability I'd think, and it would interfere with shipping too. A tunnel would be even harder to build. A ferry would probably be the way to go). Trains can stop at little stations on the way and not lose too much time, and in an era where passenger trains had right of way a modern fast train could be very fast indeed. Berlin to Baghdad is valuable not so much to connect one terminus with the other (especially in a era that did not yet realize how much oil can be found under Mesopotamia!) but for all the options for travel between intermediate points.
An airship as I have felt obliged to point out, is not something that can be brought to moor at an intermediate stop and payloads exchanged quickly. My grand notion of huge airships accommodating a series of hook-on airplanes introduces something like the quick-change capabilities of a train to the steady, majestic progress of an airship--assuming that shifting winds do not obligate the airship to divert hundreds of miles off its nominal great-circle course!
To be sure, the relatively small airships Zeppelin could make in 1909 were suited to much shorter ranges than say the famous "Africa ship" of WWI. From 1909 to 1914, DELAG operations flew over 34,000 passengers...but none of them were passengers in the sense that they paid to be taken from one point to another on a scheduled flight. They were in fact joyriders, people who wanted to see the world from above, who wanted to experience Germany's most advanced technology first hand, to be able to boast of having done these things.
Here are statistics for a Zeppelin typical of this period:
LZ 13
hull number LZ 13
Type G
designation Hansa
Length in m 148
Diameter in m 14
Volume in cubic meters 18,700
number of cells 18
Weight in kg 15,400
Payload in kgs 6,330
motor number 3
Engine output in hp 170
Total output in PS 510
1. ride 07/30/1912 01.08.1914
Off-duty 07/29/1914 August 1916
owner DELAG German Army / Navy
commander Mr WE Doerr
Mr. A. Heinen
Shipyard Friedrichshafen
Speed in m / s 22.2
remark - 399 trips for DELAG
- total 44,437 km
(Translated from German, original can be found in link to LZ-13 text above)
I chose Hansa as an example because if we consider that the wartime pressures have somehow greatly accelerated the rate of progress at the Zeppelin works, so that roughly triple the rate of progress happens that did in years of peace OTL, then by the end of this war in 1908, the firm ought to be roughly at a par with OTL 1912--Hansa having first flown in that year OTL.
OTL when the author of the link prompting this post dreamed of global German air service, ships rather less advanced than this were the inspiration ready to hand.
At a maximum airspeed of 22.2 m/sec, in the ballpark of 43 knots, this airship is just barely getting up to minimum speeds that empirically were found to be needed to handle the more severe but not uncommon contrary winds. To operate reliably it needs more like 60 knots, which would require nearly 3 times the power with this sort of drag, and at that airspeed would need to be stronger by nearly a factor of 2! Its payload of over six tons however would put any 1912 airplane in the shade. However it clearly is not up to a transoceanic run, probably not even up to a ferry run across the Baltic to Stockholm--she did visit Copenhagen once however.
A ship like this is simply not up to the task of regular passenger service in the sense of filling a niche that is irreplaceable and therefore reliable. What will it take?
Now look at a ship like this instead:
Here we see something a lot more like what we imagine when we think "Zeppelin;" the resemblance to the final generation of the most advanced rigid airships is much closer. This ship flew OTL just two years after the maiden flight of the Hansa above. We see a number of radical improvements at a glance: a much improved streamline shape; engines mounted on the hull and directly driving pusher props in more or less streamlined engine cars; a cruciform tail instead of the older box kite type. On top, observe 6 bumps--these are the Venturi hoods of an internal ventilation system that gently circulates air through the interior, sweeping up small deposits of hydrogen leaking from the hydrogen gas bags and venting it out of the ship before it can accumulate to dangerous levels. Also there are internal walkways inside the hull giving crew access to the interior, so they can observe the state of the gas cells and other structural status, and make adjustments or repairs in flight.
This however is not a Zeppelin; it is the work of Dr. Johann Schuette, at the Schuette-Lanz works in Mannheim. He sold this ship to the Austrian army and as its advantages proved out hoped to take over the rigid airship business from Zeppelin. Unfortunately for him however, the OTL Great War led to the German government pooling patents, and Zeppelin was free to incorporate all his above innovations without paying royalties. As a result, a typical Zeppelin of the final war year looks much like this, enjoying all the advantages Schuette hoped to be paid for developing--and more.
There is another detail of the interior of the SL-2 that should be mentioned. All Zeppelins, from the first failed attempt to the final product, used some form of aluminum for their rigid members. SL-2 did not--instead Schuette had developed a form of plywood. All of his company's constructions used this wood product. Why use wood when aluminum was already available? The catch is, the early Zeppelins used pure aluminum, which is light, but not very strong. Compared to pure aluminum, Schuette's plywood members had a superior strength to weight ratio!
Again, bad timing for Schuette--by 1909 a German firm had developed, patented and published details on a new alloy of aluminum, copper, maganese and magnesium they called Duralumin. Somewhat denser than pure Al, it was however much stronger, and once Zeppelin adopted it, the weight of the rigid frames came down considerably. Meanwhile, in actual flight trials, the SL plywoods proved vulnerable to deterioration caused by humidity--the wood would soften, warp, and worst of all the glue would fail. The wood was superior to pure Al but not Duralumin, and would fail completely in conditions aluminum took well. (Actually Duralumin would corrode, but a coating of pure aluminum generally would prevent this from becoming a problem).
Had Schuette switched over to Duralumin immediately instead of sticking with wood, perhaps his products would have stolen the show from Zeppelin. But I suspect the Count had too much momentum by 1914 OTL; had the Great War been averted somehow so the firm could not simply steal the patents legally, I doubt the courts would uphold attempts to enforce them regarding the aerodynamic shape or even the cantilevered symmetrical control surfaces. There could be other approaches to achieving suitable internal ventilation too.
ITTL on the other hand, Dr. Schuette presumably exists, but he did not begin his efforts to surpass Zeppelin's design flaws until witnessing a failure in 1908.
Conceivably he could instead set out to make his improvements upon observing the flaws evident in ships like Hansa, but it would take him many years to realize them if OTL is any guide. Also besides the plywood, his first design (which did incorporate plywood as well) used a very different structural approach, arranging the rigid members in a diamond form. It was quite beautiful actually:
But unfortunately it turned out to be more draggy than he figured, presumably because the air flow across the diagonal members generated a lot more turbulence than he figured it would. See also how the two engine cars are suspended below with no rigid attachment; he hoped that this would help protect the lightly built hull from sudden shocks, as a car impacting the ground would simply cause the suspension member lines to go slack thus relieving the hull of their weight. A pretty good plan I think, but it would prevent the evolution of airships incorporating elements into the hull volume thus lowering drag.
To be sure, as long as airships must be lifted with hydrogen instead of helium, some elements such as engines must be kept out of the hull anyway. OTL LZ-14, Hansa's immediate successor, sold to the German Navy, sought to lower drag by fairing over the gap between suspended elements and the hull, with the result that during a storm with rapidly changing pressures, sparks from the engine got taken up and wafted into the hull where hydrogen venting due to sudden rises in altitude from the cells' emergency valves was set off, resulting in a fire that swept the length of the ship and causing the loss of all hands in the crash--this was the first incident in which lives were lost related to Zeppelins OTL.
So, although the war and the Kaiser's enthusiastic personal support combined with newly evolved conventional wisdom that sees Zeppelins maintaining a lead over other aircraft all combine to have accelerated the Zeppelin design I am guessing by some 4 years, the way forward is not so straight and clear. OTL the Zeppelin firm under the Count and Dr Ludwig Durr running design was rather conservative. Here with the push for wartime improvement and the enthusiastic Kaiser looking over their shoulders we might hope their thinking is a bit more bold and flexible, but realistically it is going to be some years before they have airships fast enough, strong enough, well designed enough and big enough to inaugurate scheduled revenue passenger services.
I do think though they can and will do this. OTL once Zeppelin got the support of military procurement funding, they immediately sought to develop civil applications in parallel. Hansa-like ships are certainly good enough for the revenue joyriding DELAG profited from OTL immediately. Meanwhile, possible early routes once they get some ships going faster include ferry routes--an airship is faster than a surface vessel, so flights to London from Amsterdam or Bremen, flights to Stockholm from Berlin, politics permitting flights from Trieste to Rome might all be in the cards within just a few years, on ships maybe twice the mass of Hansa and in some ways more resembling SL-2. Also, although airships are poor competition for a well developed rail network, Germany and Austria-Hungary have just incorporated vast new territories of client states formerly under Russian rule; these lands are much poorer in rail kilometers per square km, and what rails they have are Russian gauge. The prospects for overland point to point flights might then be temporarily improved until new construction can catch the region up. (Also there are cargoes that can benefit from bypassing surface transport; if advanced airships capable of keeping station amid shifting winds can be developed a permanent niche might open up).
By 1914 or maybe earlier, something should exist that can cross the Atlantic carrying revenue passengers.
I don't know about Berlin to Baghdad; that is mostly overland and railroads or if we delay enough, automobile/truck highways should take care of it.
OTOH links to German East Africa are definitely something that ought to happen; it would be most sensible I think to go from Trieste or some other Dalmatian port over the Med to the Suez canal thence coastwise along eastern Africa. However, if air links to the Black Sea are somehow favored, going south across it, across Anatolia and to Baghdad on the way to the Persian Gulf and on to East Africa that way might be possible, assuming a friendly regime in Turkey and Mesopotamia.
