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Airships: Part 5 - Navigation and Weather

David Flin

An evil Socialist, apparently.
Many thanks for that. Love the line about the ground having the right of way.

Mind you, I'm having difficulty working out why an airship might want to hug the ground at any time other than take off and landing. Anti-submarine work in the middle of the Atlantic, maybe. But, as far as I can see (pun unintended), observation is a big role for an airship, and height gives you a more distant horizon, all else being equal.
 

AndyC

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Stealthy transport (eg through a valley), transit through the area is essential but weather up top is dreadful, need to get a close look at something for whatever reason, low-level bombing run (not the ideal craft for it, but sometimes needs must)...

Of those, the first two are most likely (the second can happen quite suddenly if the weather comes in).
 

Tabac Iberez

Impetious
Published by SLP
Mind you, I'm having difficulty working out why an airship might want to hug the ground at any time other than take off and landing.
Crosswinds. Remember, you're in a giant balloon that doesn't have independent propulsion as it has negotiating rights with the sky. If you're headed north and the only north wind is at 3000 ASL and the ground is 2700 ASL, then you options are horsing at three hundred feet off the ground or engaging in Dynamic Lift Exercises higher up and hoping you don't hit a southbound wind.
 

AndyC

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Actually, with me reading into more depth in the Imperial Airship Scheme and the specifics, practicality, and economics behind long-distance airship transport (as well as reading some more primary documentation about the R101 (and R100 and intended R102 and R103) for the PoDs article(s)), it turns out that low-level flight (for our modern definition of "low-level" being below about 3000 feet) is actually the standard default for airship transport. As in - cruising at about 1500 feet.

A transport airship is constructed to be more resilient and long-lived than the military airships that would operate at up to 20,000 feet (which themselves could be fragile when handled too abruptly in the thicker air of low level - it's one reason behind those lists of airship accidents that say "xxx broke apart in calm weather mid-flight"), and thus heavier. The (negative) lifting weight minus the vehicle weight provided the payload weight (fuel, ballast, passengers, crew, and cargo). The lift reduced as the vehicle climbed (as the air outside became less dense, the difference between the hydrogen or helium inside the gasbag and the air outside diminished), so lift was greater at low level than at altitude. In addition, you'll get dynamic lift from the shape of the airship moving through the air, which is itself also greater at low level due to denser air.

The passenger airships of the Thirties (R100, R101, Hindenberg, Graf Zeppelin) were designed for a low altitude "pressure height" - when the pressure outside equals the pressure of the fully inflated gasbags, you don't want to go any higher for risk of a rupture. You can vent lifting gas out of valves if you do, but then you're a bit low on gas when you descend.

Ballonets (as described in my earlier article) help here; effectively giving you a dynamic adjustment of your pressure height, but when the ballonets have been completely emptied of air (and are thus flat inside the envelope), you're at the design pressure height. For a military airship, this will be a lot higher than for a passenger airship (the loss of efficiency is acceptable to give you that wide operational envelope).

The R100, R101, Hindenburg, etc, therefore conventionally cruised at around a mere 1500 feet. This was actually seen as a feature rather than a bug - the view for passengers was fantastic. It did mean, though, that all I've discussed about low-level weather and operations at low level were very much the standard operating procedure for passenger airships.
 

Alex Richards

She needs an artificial Mountain, not AV
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Hmm. 1500ft cruising altitude.

The Empire state building at 1,250ft was in part designed as a mooring mast- essentially that's a building tall enough to project into the standard cruising altitudes (or very nearly) of most airships.

However the buildings around it top out at about 700ft (or certainly did at the time) with the exception of the Chrysler Building at 1,000ft

So lets assume 700ft buildings are manageable, 1,000ft buildings are just about doable and 1200 foot buildings need to be avoided.

...

Good god airships would have problems in modern cities.
 

Tabac Iberez

Impetious
Published by SLP
Good god airships would have problems in modern cities
Everything has problems in modern cities. Most major metropolitan centers in the US have a altitude "floor" of five to eight thousand feet, below which you are technically endangering the city by flying at reckless altitude.
 
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