Help Me Design An Airship!

[Soterion]

So I'm trying to design an airship that doesn't require an engine for forward motion, that is, it uses the wind to move. Now, obviously you could just have the wind blow it around, but I was thinking some kind of sail configuration would allow it to move forward under control. I'm thinking four: one on top and one on the bottom for forward propulsion, and then one on each side to allow it to move up and down. But I'm not sure how you would attach these sails to the gasbag; masts would probably be too heavy. Your thoughts?

[Jack Squat]

rear elevators would probably work best for elevation, rather than sails. For steering, look at how larger ships (schooners, galleys, clippers, etc.) are designed.

[SDF]

A fantasy one? I'm pretty sure all airships run on magic.
Or a real one? It's a hot air balloon.

[Pyrian]

Water-borne sailing vessels exploit the difference between the current and the wind for their power. Generally speaking, this is all but impossible for an airship. There's no wind to speak of, and nothing to contrast it with. Hot air ballons "steer" by going up and down into different wind zones, and can only travel in directions where they can find wind going. In theory you could exploit air layers, but... In practice I don't think it would work.

[Soterion]

Water-borne sailing vessels exploit the difference between the current and the wind for their power.

No, they don't. They don't work that way at all. Sails provide forward motion via the conservation of momentum in combination with Bernoulli's principle. A sail moving into or across the wind acts as a vertical wing, transforming the impulse of the wind into "lift", which causes the ship to move in a direction perpendicular to the wind. However, the presence of the keel causes the ship to avoid both roll and sideways motion. However, this can be mimicked by any particularly well-devised countermeasure, which is why land-sailing works, despite having no keel on the land yacht.

[Jack Squat]

Water-borne sailing vessels exploit the difference between the current and the wind for their power. Generally speaking, this is all but impossible for an airship. There's no wind to speak of, and nothing to contrast it with. Hot air ballons "steer" by going up and down into different wind zones, and can only travel in directions where they can find wind going. In theory you could exploit air layers, but... In practice I don't think it would work.

You're thinking of steering with the sails as you would a ship, I was just thinking of using them to propel forward. Steering would work the same way as with a hot-air balloon (theoretically with the elevators being used to navigate between air layers, but I'm not an aerospace engineer and I didn't stay at a Holiday Inn last night).

I just pointed towards the designs of ships because the look cooler than the GoodYear Blimp with a mast sticking out of both the top and bottom ends.

[Pyrian]

No, they don't.Yes, they do.


They don't work that way at all.It is impossible for them to work any other way.


A sail moving into or across the wind acts as a vertical wing, transforming the impulse of the wind into "lift", which causes the ship to move in a direction perpendicular to the wind.And that's all you can do if your only input is the wind.


However, the presence of the keel causes the ship to avoid both roll and sideways motion.And that's exploiting the difference between the wind and the current. When there is no such difference, a sailing vessel cannot travel in relation to the wind/current; it is becalmed. This is just as true if their is wind/current as if the air and water are still with respect to land (indeed, land is simply not a factor).


However, this can be mimicked by any particularly well-devised countermeasure, which is why land-sailing works, despite having no keel on the land yacht.I find it interesting that you cite counter-measures while contradicting my assertion that such a thing is required. Land sailing and even flying a kite, insofar as they do not simply go with the wind, requires contact with the ground (which has a velocity distinct from the wind) to function. If the ground does not have a velocity distinct from the wind (normally called "windless"), you cannot land-sail without a powered mechanism (or by exploiting gravity as in a glider, not really applicable to an airship).


You're thinking of steering with the sails as you would a ship, I was just thinking of using them to propel forward.And I'm telling you that without a contrasting velocity or a powered mechanism (or by exploiting gravity as in a glider, not really applicable to an airship), it's impossible. You can only get blown around.

EDIT: One more thing. Once you're airborne, you have your airspeed and you have air currents. If you're not generating airspeed, then you're just going to coast with the air currents. There's not much of a "wind" per se like there is on the ground (or sea) because there's no ground or sea to contrast them with. Once you're traveling at the same speed as the air current you're in, you're becalmed and require a powered mechanism (or another medium/surface going at a different velocity) to exploit to gain momentum against the air.

[RS14]

No, they don't. They don't work that way at all. Sails provide forward motion via the conservation of momentum in combination with Bernoulli's principle. A sail moving into or across the wind acts as a vertical wing, transforming the impulse of the wind into "lift", which causes the ship to move in a direction perpendicular to the wind. However, the presence of the keel causes the ship to avoid both roll and sideways motion. However, this can be mimicked by any particularly well-devised countermeasure, which is why land-sailing works, despite having no keel on the land yacht.

An airship will not be able to tack into the wind. The net force will be at least partially in the direction of the wind, and at best you can move sideways while doing so. Thus eventually (quickly) your velocity in the direction of the wind will equal the wind, and you will not have any airflow over the sail.

[Pyrian]

Listen to the smart guy with an airplane for his avatar! :smallcool:

[Mando Knight]

An airship will not be able to tack into the wind.
Yes, it will. Airplanes are designed to be able to do so against their engine output.

The net force will be at least partially in the direction of the wind, and at best you can move sideways while doing so. Thus eventually (quickly) your velocity in the direction of the wind will equal the wind, and you will not have any airflow over the sail.
Using control surfaces, you can induce a "lift" over the surfaces, causing moments, so that you can (within reason) control the relative velocity of the wind and the moment balance.

The problem comes when you want a fully sail-powered ship: you can't have the wind push you when the wind is your relative velocity vector. Wind creates lift and drag, not thrust.

[Jack Squat]

And I'm telling you that without a contrasting velocity or a powered mechanism (or by exploiting gravity as in a glider, not really applicable to an airship), it's impossible. You can only get blown around.

Right. That's all I'm saying the sails would do. They just have a greater area of contact with the air current than the vessel itself, thus propelling it more efficiently than the vessel without sails.


Yes, it will. Airplanes are designed to be able to do so against their engine output.

But airplanes work because they're forcing themselves through a body of air, not riding along with it. An airship that's moved by the air currents can't go against the only thing moving it.

[Pyrian]

Mando Knight: That works for a glider, which can exploit gravity, trading altitude (or relative altitude if there's an updraft) for forward momentum. An airship at neutral bouyancy without a powered mechanism cannot exploit a lack of wind, which is exactly what it will inevitably experience. EDIT: Okay, I see you updated for the situation at hand. :smallbiggrin:

EDIT:
Jack_Squat: Okay, but what's the point? A hot-air balloon gets "blown around" with near-total efficiency already; the time between entering a new air current and equalizing with it is all but negligible, to the point where you feel little or no wind while in one (unattached to the ground, of course).

[Jack Squat]

Jack_Squat: Okay, but what's the point? A hot-air balloon gets "blown around" with near-total efficiency already; the time between entering a new air current and equalizing with it is all but negligible, to the point where you feel little or no wind while in one (unattached to the ground, of course).

Looks cooler, that's really about it. I figured this was for a book or a game or similar, not that he was actually building it, in which case he should probably consult a team of engineers rather than a DnD forum.

[Mando Knight]

An airship at neutral bouyancy without a powered mechanism cannot exploit a lack of wind, which is exactly what it will inevitably experience.
This is true, but it can still remain stable without relying on a power system. Sails won't work as a means of propulsion, but it can be used as a control surface: spoilers, ailerons, elevators, stabilizers, rudders, "wings" to provide a touch of extra lift when needed, etc.

[Pyrian]

Y'know, I think we might just not be thinking EPIC enough, here. What about an airship on the scale of the largest flying fortresses of Girl Genius? Or even larger? Theoretically, I think such a beast might be able to have a gigantic keel-like sail in a lower part of the atmosphere and sails in the jetstream (or whatever) and function like a sailing vessel in that manner.

[Soterion]

What about changes in buoyancy? If Pyrian is correct, then the airship drops ballast, causing the airship to rise and causing a flow of air over our baffles (or whatever) giving the ship the ability to turn against the wind. Maybe that would work.

[Zephykinns]

Easy, make it like a F-16 powered by magic! it would be awesome!

[RS14]

What about changes in buoyancy? If Pyrian is correct, then the airship drops ballast, causing the airship to rise and causing a flow of air over our baffles (or whatever) giving the ship the ability to turn against the wind. Maybe that would work.

That could work in theory. You'd need the ability to drop an immense amount of ballast, and an immense amount of low-density gas to be able to sustain this. Better would be a bellows system allowing you to essentially inflate or deflate it at will (which requires an engine of some form to power), and sails to drive it.

[SDF]

It's like driving a kite!

[Force]

Step 1: Get a 5th level wizard.
Step 2: Have him cast Fly.
Step 3: ???
Step 4: Profit!

[Mando Knight]

What about changes in buoyancy? If Pyrian is correct, then the airship drops ballast, causing the airship to rise and causing a flow of air over our baffles (or whatever) giving the ship the ability to turn against the wind. Maybe that would work.

...That's silly. Why would I do that when I could just use a proper engine and do things more efficiently?

[Tricksy Hobbits]

I'm not sure if the sails would work but attaching themn wouldn't be too difficult as only hot air ballons have just a gasbag. most airships have lots of smaller gasbags in them and the ovoid shape from canvas stretched over a steel frame, so the masts could be attached to the frame.