# How could a sail powered airship work?

I've been wondering if it's possible for an airship to fly by passive means - specifically, no engines (steam or otherwise).

I'm thinking in terms of a world where lighter-than-air gas (and it's containment) is fairly readily available, but that hasn't discovered mechanised propulsion.

The problem is - if you're sailing in water, the water itself provides resistance, and allows you to move in a different direction to the wind. (Whilst you can't sail directly into the wind, you can sail pretty close in a modern yacht).

But I think you're effectively adrift if you're trying to do the same with an airship - and are carried wherever the wind blows, much like a hot air balloon - you've no steerage, and the only way you get to go somewhere different is if the winds at different altitudes are blowing a different direction.

Is there any way this is possible, or am I going to have to think in terms of perhaps solar (convection?) propulsion or magic?

• I'm not a physicist or a... meteorologist? (or whomever is responsible for studying the atmosphere), so I won't answer officially. But my first instinct is that you probably can't sail anywhere you want to go, but using rudders you would be able to veer left or right. Making your ship able to control whether it's more or less aerodynamic would let you resist winds that are heading in the wrong direction until more favorable winds come along... Jan 9, 2015 at 17:58
• I would say yes : en.wikipedia.org/wiki/Gliding Jan 9, 2015 at 18:11
• @Vincent The method of flight, which is what I was referencing, is relevant -- lighter-than-air craft do not glide. Jan 9, 2015 at 19:34
• @EFrog it doesn't work. You are stationary relative to the air you are in so have no steerage way. Boats work by using the difference in speeds between them, the air, and the water. Jan 9, 2015 at 21:51
• The Iain M Banks sci-fi novel "The Algebraist" contains a 'sailing' race within the atmosphere of a gas-giant planet, using the magnetic field of the planet as well as the winds in the atmosphere.
– A E
Jan 11, 2015 at 21:14

I've thought about this a bit in the past. It's possible, but only if the vessel moves vertically between regions of different wind speed.

On a regular ship, lift forces can be generated on the sails based on wind moving faster than the sail. The water provides drag on the ship to keep it going slower than the wind. Essentially, the boat can 'push' against the water to hold it in place and allow it to use its sails to push on the moving air.

In a homogenous mass of air, on the other hand, there is nothing like water to push against. The craft will drift at the same speed of the wind and generate no lift forces on its wings, unless it moves them or does something to get it moving at a different speed as the wind. This is why gliders need to slowly drop. They maintain speed in excess of the ambient air speed by shedding gravitational potential energy.

Some birds, however, have pioneered a method of gliding without power. What they do is to glide back and forth across a shear layer in the atmosphere. The difference in wind speed above and below the shear line allows them to generate lift by moving at a different speed as the wind. This is called dynamic soaring. Your airship could do the same, using wind shears to generate lift using wings/sails. Note that when I say 'lift' I don't specifically mean force pushing the aircraft upwards, but more generally pressure-based aerodynamic force generated by wind moving across an airfoil. The motive force that a sailboat harnesses to move upwind, for example, is often referred to as a form of 'lift'.

It's also worth noting that the different shear layers in the atmosphere behave differently at different times of the day, and under different weather conditions. Your air boat captains would need to be expert weather men, as well.

• Exactly this. It may seem counterintuitive, but on a sailing yacht with retractable keel (e.g. centerboard) pulling it up will actually make the boat heel less (and drift more unless you are running downwind). An airship will drift too if there's nothing to create lift. Jan 9, 2015 at 22:59
• Could magnetism provide the resistance that we need? Jan 10, 2015 at 15:25
• One way of taking advantage of the wind flow at altitudes above or below the airship might be to deploy kites to catch those flows. It might even be possible to use the different directions of flow for resistance in order to perform tacking maneuvers (acting like a boat's keel or centerboard). However I am concerned about low wind conditions that would cause a kite to fail and fall from its intended altitude. Jan 10, 2015 at 21:04
• I had also pondered the kite approach, and was wondering if some sort of drop-glider approach might work too. But then realised that dropping a glider and hauling it in was basically the same as an engine anyway. Jan 11, 2015 at 10:54
• @Sobrique The way I saw it instead of flying a kite above, just let the rigging hang below the gasbag. If the gasbag and sails are far enough apart.. No problem. This is actually a more stable a simpler configuration than having sails on top of something that floats on water. Probably should edit my answer so that this is properly explained. Jan 11, 2015 at 13:40

If the airship travels a set route between a group of cities, they could set up towers at regular intervals with cables connecting their tops. Then as the airship passed over one of these suspended cables, a hook could be lowered to catch it. Once tethered by a hook from its keel to the cable, the airship would only be able to travel along the vector of the cable. The wind would pull the hook taught along the direction that it is blowing and then it would drag the hook along the cable in whatever direction provided less resistance to the vector of the wind.

If enough towers provided enough cables, a network could be assembled which might provide access to most of the cities for each major wind pattern.

There would be seasons when "you just can't get to EastPort by air".

Occasionally, airships would stall along the way when the wind blew exactly perpendicular to the cable. In such cases, the captain would have to change altitude in search of different winds.

There would also be a little bit of a gamble available for brave captains... "We're going to release this slow cable we're currently riding and float free towards those hills. If the winds are with us, we can snag the EastPort cable outside of Regis and cut three days off our journey. If the winds aren't our friends, well... I always wanted to see what was beyond the forbidden zone."

• At this point, there's no need for airships at all. Just use ziplines or little hand-cranks. Jan 9, 2015 at 18:34
• A zip line would have to be much stronger, to support the weight of the vessel & cargo. The balloonway would just have to be strong enough to provide the equivalent of a keel. Plus there are safety issues: if a zip line breaks, you crash; if the balloonway line breaks, you just let out a bit of gas and make a soft landing. Jan 9, 2015 at 19:23
• Wind perpendicular to the cable is not bad. It allows travel in both directions. Jan 9, 2015 at 21:44
• Indeed - a modern sail can sail almost upwind (I think the best is about 35 degrees) which means it can make progress in almost any direction due to tacking. The only real downside of the guide cable is that if the wind happens to be blowing too close, you're scuppered. And it isn't quite what I had in mind for my setting (which I'll grant I didn't mention, so it's still a good answer) which was something that was long range exploration and staying aloft for extended periods of time. Jan 10, 2015 at 15:30
• @Sobrique: Of course the engineers laying out the balloonway's cable routes would take prevailing winds into account, and choose routes that allowed for tacking, instead of a simple shortest distance between two points. (Much as railroads curve around hills to keep a workable grade.) As I recall, Jack Vance wrote a number of books ("The Anome" is one) in which such a balloonway is featured. Jan 10, 2015 at 19:38

If the airship can adjust its buoyancy during flight, you could make it work. Like @vincent mentioned, think about how a glider works. It somehow gets into the air, then uses the downward gravity vector in combination with its wing surfaces to produce forward motion. It can steer in any direction, though its forward motion will be relative to the prevailing wind.

A neutrally buoyant airship would be at the mercy of the wind. But if it adjusted its buoyancy to become heavier than air, now it has a downward gravity vector. In theory, it could deploy some airfoils and fly just like a glider.

Eventually it'll get too close to the ground, so it'll need to reinflate its gas bag. Now it becomes positively buoyant, and it could use its upward vector to continue steered flight.

By repeatedly diving and climbing, an airship like this would be able to control its direction somewhat even when the wind isn't blowing the right way.

• There are autonomous underwater "gliders" that work on this principle. No fundamental reason why an airship shouldn't do the same. Turns a large excess of buoyancy (which can otherwise be a control problem) into a virtue. Jan 11, 2015 at 13:49
• +1 for using bouyancy as a counter force (acting a bit like the keel of a boat, but in the vertical axis) Jan 18, 2015 at 21:29

Your hunch is correct, a lighter-than-air craft with no mechanical propulsion would be completely at the mercy of the winds.

A sail boat is able to control its direction thanks to its keel, which effectively "slices" a route through the far denser water it is in and allows the boat to redirect the resultant force of the winds striking the sails (changing sail positions also helps with navigation, although that's more for controlling how and how much of the wind strikes it). The rudder allows the boat to change the direction of its keel and effect course changes.

A lighter-than-air craft, on the other hand, while you could put a keel on it it would have no effect except to increase the amount of wind hitting the craft and, thus, increase how quickly it is moved by the wind. You would have no control over your direction, and trying to resist would be completely ineffective because you have nothing against which you can generate a counter-active force (not unless you drop an anchor, that is).

The one aspect you could control (potentially at least) is your altitude: By increasing or decreasing your buoyancy you can increase or decrease your altitude; this could be advantageous if, for example, the winds at a higher altitude are going in the direction you want. The particulars of how effective this would be are beyond my expertise, but you do have things like jet streams at high altitudes that are not affected by the ever-changing winds you or I are used to at ground level. Since these are pretty constant and can be easily mapped, you could have rapid passive air travel via these.

In our modern world in fact we do have such passive lighter-than-air craft; we call them hot air balloons, and while their lift is provided by a gas burner heating up the air inside the balloon, the principle of their navigation -- or rather, their lack thereof -- applies here.

You could use "ground contact" airship travelling.

Over the open sea, this is quite simple: from the airship, lower a "water kite" type of device into the water with a 100 - 300 m long rope. It is permanently in the sea (with seawater as ballast), but can be pulled up when approaching land, when wanting to "sail" fast right into the wind, or in emergency situations (like, umh, pirates). The angle of the kite against the rope could be varied from above, or by a man on that water kite, and this is how the steering is done.

Over land, it is more difficult. I can imagine a "step by step" approach though, using multiple rope anchor points in a sequence. One full sequence is:

1. The airship is shaped like a vertically aligned airfoil, and held at some angle by a rope that is anchored to the ground. Like a kite (but with the whole setup 90° turned), it then assumes a stable angle to the wind, which can be chosen by varying the angle between rope and aircraft.

2. When it reaches its target position, another rope is lowered and anchored to the ground somehow (we'd use automatic drills now, you could use a guy lowered in a basket).

3. The first rope is released (remotely controlled with a smaller control rope) and pulled in by the airship. It can even have a small balloon at the rope's end so that entangling the rope with obstacles due to pulling it over ground is not an issue.

4. The new anchoring rope is gradually extended until reaching a target position, and the process is then repeated.

The anchoring rope could be 1500 - 3000 m long maybe – rough guess :) With this, each step would cover, say, 1 - 2.5 km of way.

• Do you want to merge this with your earlier post? It would be a good combination. Jan 25, 2015 at 23:49

Yes, but it is more complex and heavier than normal sails.

First, you have outrigger sails or vanes to control the rotation of the vessel.

Second, you have wing blades that you can rotate around their vertical axis. These blades have a rigid profile so by directing one edge towards the wind, the curvature of the blade deflects the wind in one direction and the vessel gains thrust in the opposite direction. In theory this would allow moving upwind. In practice compensating for the gas bag drift would be pretty hard, which explains why such contraptions are only seen in stories with magical or super-science levitation. But it should be possible, just not sensible.

EDIT: Note that this rather obviously requires a difference in the wind affecting the gas bag and the sails. This difference exists since the sails and the gas bag are at different altitudes. Also air currents are actually three dimensional, so the sails would need to be controllable on more than one axis to take advantage of updrafts and such.

• I'm not at all convinced that "wing blades" would work. While you might get some net deflection of the force from the wind from them, the body of your craft and in particular the giant bag o' gas suspending it are going to be providing massive surfaces for the wind to push against -- without your "wing blades" being far beyond "unreasonably large to the point that the entire resources of a large nation are required for the construction of one craft", you're just not going to overcome that. Jan 9, 2015 at 18:57
• @Kromey I think I mentioned this issue in my answer? In any case while the blades must be large, they only need to be large in relation to the windward crosssection of the gas bag. Which increases slower than the lift from the gas bag does. Jan 9, 2015 at 19:27
• Do not want to bump with an edit, but I have been thinking about this and I think it would really be practical, but it would have to be specifically designed for sail propulsion and not really look like a normal airship because of the need for the gasbag and sails to have different winds. (-> large vertical distance) Also the ship would be unable to go upwind at all, although in some cases the wind at different layers has different directions. But they used to cross oceans with ships with similar limitations... Jan 26, 2015 at 7:41

Hopefully you're not stuck on the aesthetics of sail rigging, as I will answer the question of how to turn an airship into a dirigible without mechanized propulsion or sails and "fly by passive [analog] means".

Before humanity discovered mechanized propulsion, we used animals (including humans) for 'power'. I see slaves turning screws that power propellers or trained wildlife that can wear a harness and still fly, as options. Dragging yourself along with harpoons would be a lot of work, but it would get you there.

If the average wind speed on the planet is close to zero knots, I don't see a problem. Analog propulsion has been done on Earth in a balloon but I'd bet it was quite a workout and that the weather was very cooperative: (if propellers are out, we'll just go with Blanchard's '85 trip)

In 1784 Jean-Pierre Blanchard fitted a hand-powered propeller to a balloon, the first recorded means of propulsion carried aloft. In 1785 he crossed the English Channel in a balloon equipped with flapping wings for propulsion and a birdlike tail for steering. Airship -Wiki

• Not so much the aesthetics, as looking for something that was perpetually airborne. A sailing ship can just keep going without needing to carry fuel with it. Now, in theory you could desalinate for water, and fish for food as you went, and you wouldn't need an engine. Jan 10, 2015 at 15:18
• @Sobrique I am assuming the terrain is without navigable water and is otherwise inhospitable to travelers. Only that would make these economical. Also, don't forget about scurvy; you'll always need to make port once in a while to restock and to, you know... Jan 10, 2015 at 20:11

Now if your terrain was mostly plains, you could have some sort of hybrid of balloon and "wind wagon" (http://www.kshs.org/kansapedia/wind-wagons/12239 ), with the balloon acting as the sail, and the wagon as the keel.

An interesting (to me, anyway) idea is that you could make the wagon carry fairly heavy ballast, then if danger threatens on the ground, toss the ballast overboard and escape through the air.

No, a free-ballooning (sail powered) non-magical airship won't work.

For two reasons, and you may be surprised by the reasons.

First, yes the airship can steer...a bit...by varying it's altitude instead of by using 'sails'. Using favorable winds at different altitudes is how free-balloonists have steered for over a century. Sails don't work for steering - there is no other medium for a keel or rudder to push against.

However, there's a big difference between balloon-style steering and actually reaching a destination. Powered dirigibles often needed to make multiple approaches to the landing field due to an unexpected crosswind or navigational error, and a powered fly-around could easily take 30 minutes. An unpowered fly-around might take hours...or be impossible entirely.

Aside: Non-magical airships cannot just drop into a cow pasture in an emergency, either. That's called a 'crash'. Ground crews of 40-50 were needed to walk larger Zeppelins into the mooring mast. Airships may be buoyant, but have a lot of inertia and sail area. Several ground crewmen were swept into the air --and killed by the fall-- by minor errant breezes. You can use heavy tractors today instead of large crews (Goodyear did), but you still need to get the airship and the tractor close enough, and it's still dangerous. Remember that this means the non-magical airship must arrive at destination at least 3-4 hours before sunset to be landed safely...unless your landing field has hectares and hectares of non-intrusive illumination and a ground crew trained for night landings.

The other reason a sail-powered non-magical airship won't work is that they are simply too big and fragile to survive ordinary weather for long. Most non-combat losses of airships were due to simple weather. Example: The USS Shenandoah was literally ripped apart in midair by an otherwise unremarkable summer thunderstorm over Ohio. Powered airships generally cruised outside of a weather front, searching for a weak spot to punch through, The Graf Zeppelin once spent 6 hours over the South Atlantic scouting (successfully) for her safest course of action through a rather mild front. A free-balloon simply lacks that ability, is likelier to be sucked into the worst storm cell nearby and promptly destroyed.

One other major drawback of unpowered non-magical airships, though not a critical fault, is their inability to maintain accurate navigation. Powered airships, before LORAN and GPS, relied upon celestial navigation and detailed ground charts. This meant popping above the cloud deck several times each day while over the ocean to get a sun/star position fix. Powered, this is a quick, accurate operation. Unpowered adds time and positional uncertainty to the journey (how far off were we blown by those five layers of crosswinds?), increasing the likelihood that the airship will miss it's destination and need to (sigh) fly-around.

You could exploit differential velocities of wind at different altitudes. The effect is similar to sailing, with the lower-velocity wind acting as a "drag / anchor medium", and the higher velocity wind acting as a "propulsion medium".

The aircrafts would be made out of two parts, connected by several hundred meters of rope:

1. Above, the lighter-than-air vehicle. It would be kite-shaped, with an airfoil cross-section, and connected like a kite to the drag device below. The mounting mechanism must allow adjusting the angle against the rope in two directions, as this is what allows steering.

2. Below, a drag device, similar to a huge parachute.

There are always wind speed differences between different altitudes, and if only the difference between slow surface wind and faster higher-altitude wind. This however might require to fly the drag device / parachute 10 - 20 m above ground level, which can be risky due to possible entanglement with obstacles.

This idea is a bit like the proposal by Ville Niemi here, but I think, different enough :)

Sails work because of the resistance created by the keel (daggerboard, centreboard) in the water to the wind above. So you would need to create some kind of resistance for your airship sails to work against. But you have no other medium, only the air. Someone once tried something similar; it failed and they died (Salomon August Andrée's Arctic Balloon Expedition of 1897). But it doesn't mean it cannot be done, just that we haven't figured out how to do it yet. :)

It's possible to fly without power. In 2008 there was a talk at the Chaos Computer Congress titled Flying for free - Exploiting the weather with unpowered aircraft. With enough information about the weather they manage to travel 3,000km without any power.

– Mark
Apr 14, 2015 at 1:07

How gung-ho are you on avoiding the "mechanized propulsion" bit? Is this a steampunk kind of society? If not, you could always use targeted lasers ala Breakthrough Starshot. Admittedly, this is very advanced tech and wouldn't gel with a steampunk kind of society.

The other option you could take advantage of is using Bernoulli's equation (https://www.princeton.edu/~asmits/Bicycle_web/Bernoulli.html) which governs the conservation of mass of the flow of liquids which, in short, states that the flow of fluids (which, I believe, certain (if not all) gases are considered a part of) must remain constant at all points, and that variations in volume or pressure will cause systemic shifts to maintain this principle. This is the premise upon which the Venturi Effect is based. https://en.wikipedia.org/wiki/Venturi_effect

In other words, if your society could somehow generate changes in the flow of your atmosphere (giant fans or vacuums perhaps?) at intervals along sailing routes, perhaps something like this could help create lift for your airships? Also worth investigating is the principle of lift as seen on an airfoil. Though this follows the same theory as Bernoulli's principle, it's probably helpful to understand on your way to formulating a way to get your airship to stay, for lack of a better term, in the air!

https://en.wikipedia.org/wiki/Continuity_equation#Fluid_dynamics

Good luck dude!

One of the possibilities I'd considered for something like this was magnetism. The way magnetic levitation works is that moving a superconductor through a magnetic field creates an infinite reverse current, which repels it - it therefore creates a virtual 'tram line' of magnetic potential.

Drop a magnet down a copper pipe, and it'll create inductive drag. I was wondering if this drag effect could be used with the earth (or a.n.other planet's) magnetic field, and provide the 'resistive' force to push against, that the sea would provide a sailing ship.

• Earth's magnetic field is far too weak for this.
– Mark
Jan 11, 2015 at 0:43
• YEah, I figured as much. Maglev works with 'proper' magnets and the distances are centimeters. I also suspect that if the planetary magnetic field was strong enough, there would also be all sorts of implications. Jan 11, 2015 at 10:53

Maybe a gas "bag" is very tall and is shaped like a keel. Then kites like governable parachutes both fore and aft as well as in the middle are flown at an altitude of higher velocity air . Really good co ordination between the fore and aft kite "pilots" should make it possible to orient the ship in the lower velocity air and make it possible to do tacking maneuvers. At least I think that is something readers could buy.

I also like the glider/airship hybrid mentioned earlier. You only need a way to compress and release the gas on demand to make it work. Rowing machines hooked up to propellers for emergencies might also be a good idea.

• The "tall gas bag" idea requires something much taller (thousands of feet tall) than is reasonable.
– Mark
Apr 14, 2015 at 1:11

Um, idk if this really counts as "sail", but heres what I got.

Lets take your airship. Assuming its sails are placed how you would see on like a galleon, remove them. Place them onto the side of the hull. Now them equal in terms of surface area, weight and structure if possible.

Now, you have 1 sail on each side of the ship, and you can pull in the sail or push it out. That part is very important.

Assuming you are facing exactly where the wind is going, both sails(extended the same percentage) will create equal drag on both sides so that the airship is pushed along the wind. Now lets say one of extended farther than the other, say the right sail is 70% exposed/extended while the left sail is only 30%. Now, the level of drag on either side is now unequal, imbalanced(insert thanos reference). Due to this, the airship will then veer to a desired direction, manipulated and controlled by how far extended each sail is.

Now, I think its pretty good, but it has a few limits.

for example,if turne enough, the rear of the ship( or any part of the ship) will block wind from the sail,and will normalize into the wind.I'll call this understeering

Now, if you made sails that the winds can push regardless of the ship orientation(say, having sails bigger than the entire ship not including sails) the difference of drag on each sail will be too big, and the ship will broadside(showing full side profile) the wind, both rendering both sails useless/ineffective at catching the wind, so no getting out of there until the wind rotates your ship into a position/orientation where the sails work, but your airship will probably have overturned, and crashed. I'll call this oversteering.

Now, assuming your smart enough not to under or oversteer, your ship should be able to veer left and right(or any direction, as long as placement of the sails allow for it).

Whether this would actually alter course effectively, idk. But good engineering?

• Sure you can use differential sails to turn, but without an effective keel, be it in water as for boats or the magnetic fields of the planet or some other field to produce a different thrust vector, the ship as a whole will always drift in the direction of the wind. Apr 12, 2021 at 19:54
• Yeah, I didn't actually think about that specific part. With out a keel, altering course would be difficult using this method, but I got a question: I'm not too knowledgeable about air currents and such, but what's stopping you from using a well placed fin as a keel? Not countering, just wondering if that fin would be able to solve the problem. Apr 12, 2021 at 19:57
• Again you might be able to alter the ship's orientation, but ... Apr 12, 2021 at 20:00