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Airships: Part 4 - In the air

AndyC

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And just to show what METARs and TAFs look like (and why I didn't go into them too far above; the detailed unpacking would take at least an article in themselves and I really don't think authors need to know this level of detail), this morning's METAR and TAF published by Brize Norton (the one I tend to look at most as it's closest to me):

Updated 38 minutes ago:

METAR EGVN 150750Z 20011KT 9999 -RA FEW018 BKN030 15/09 Q1011 TEMPO SCT014=

TAF EGVN 150740Z 1509/1609 18011KT 9999 SCT025 TEMPO 1509/1515 BKN020 PROB40 TEMPO 1509/1515 6000 RA BKN012 PROB30 TEMPO 1515/1519 5000 SHRA SCT020 TEMPO 1601/1609 SCT010=

Once you know what they say, you can read them quite well.

These are valid for Brize Norton and the surrounding area for today, winds light-to-firm from the south to south-west, scattered clouds at higher level with broken clouds lower down, rainy turning into showery as the day goes on, visibility coming in a bit, possibly clearing by about 7 pm.
 

Tabac Iberez

Impetious
Published by SLP
What's really fun is when you consider the twin demons of roll and fluid lift. Normally the former isn't an issue, but when you realize the airship is generally a mild lifting body and the lift that generates will screw with your estimates it gets fun. It then gets more fun when you go into a turn, and the pressure increase on your outbound side causes you to rather alarmingly skid into the turn and roll slightly, which moves thrust axii and then you know the whole balancing act is getting quite frightening!
 

AndyC

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Oh, yes - the relationship between yaw and roll with any lift is possibly disconcerting.

Not many people realise that a yaw (from rudder action, moving the nose left or right) in any winged aircraft automatically induces a roll.
As the aircraft rotates around the axis, one wing (the "outer" wing) moves faster and the other (the "inner" wing) moves slower. The outer wing therefore gets more lift and the inner one less lift, so the aircraft rotates around the ongitudinal axis as well - rolling. (NB - do not ever induce a yaw at a slow speed near the stall speed - the decrease in airspeed of the inner wing can cause that wing to stall. And enter a spin)

Of course, pitch plays a part as well - as an aircraft rolls, the lift vector is no longer vertical, it has a horizontal component pulling the aircraft towards the centre of the turn. The vertical component is less than 100% of what it was previously and must be increased to hold level flight, and the inwards component increases, pulling the aircraft around the turn. On a steeply banked turn, you need a significant pitch increase (and, as it happens, a power increase - a turn with 60 degrees bank ends up pulling two gee and the power has to come from somewhere)

The rudder gives you roll as well as yaw, and, as the aircraft tilts, gives a pitch component as well.

It's all quite fascinating (the interplay of control forces) and does mean that doing is essential - experience in "feel" of controls is crucial for pretty much all aircraft.

As an aside, the skid in a turn can be monitored by putting a streamer outside the cockpit in line of sight. When it is straight in line, you're in a balanced turn; if it's to one side in a turn, you're skidding one way or the other.
 

David Flin

A home of love and laughter.
As a first approximation, and taking account of the different numbers of dimensions, how close is airship movement to sailing boat? It's just that I'm familiar with the latter, and not the former.

Large surface area, affected by wind, variable currents, slow response times to changes, inertia effects.
 

AndyC

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As a first approximation, and taking account of the different numbers of dimensions, how close is airship movement to sailing boat? It's just that I'm familiar with the latter, and not the former.

Large surface area, affected by wind, variable currents, slow response times to changes, inertia effects.
From descriptions, it's a blend of sailing boat and light aircraft, with its own unique changes.

The wind, currents, some inertia are sailing boat-like; the 3-D performance is more light-aircraft like (including on speeds). The response time can be unintuitive - some inputs can cause a surprisingly prompt change (the ballonets), or so I understand.
The large surface area means that different areas of the vehicle are subject to different forces (both horizontal and vertical wind shear).
 

Tabac Iberez

Impetious
Published by SLP
As a first approximation, and taking account of the different numbers of dimensions, how close is airship movement to sailing boat? It's just that I'm familiar with the latter, and not the former.

Large surface area, affected by wind, variable currents, slow response times to changes, inertia effects.
Going by my small boats and unpowered lighter than air flight time (all one hours of it) it's a totally different beast. How you negotiate with the wind is a completely different methodology, since you need to stay entirely in one wind layer or crosswinds ok the envelope will buffet you badly. Likewise, how you handle manuvere will be different too, because as your envelope size increases your aerodynamic witchcraft goes up in regularity. I describe flying a Zeppelin as dancing fairly often, since it's a negotiation between managing your aero profile, working your course versus the environment, and managing your ship.

Heck, I'll even argue against slow response times, because the ship responds to the aero change quickly, but it doesn't start jumping and jiving because, relative to the ship's size, that would take several tens or hundreds times more power than the ship can put out.
 

OwenM

The patronising flippancy of youth
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Reading this reminded me of Jasper Fforde originally had a chapter with his characters escaping in an airship, only for his beta readers to say it got too technical, so he had them flee to Wales instead.
 

Skinny87

Editor, American Breast Enthusiast
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Reading this reminded me of Jasper Fforde originally had a chapter with his characters escaping in an airship, only for his beta readers to say it got too technical, so he had them flee to Wales instead.
Thursday Next does escape in one though, I'm sure - at the end of one of the books
 
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