In the question What would an age of sail flying ship look like?, I wrote an answer which defines the type of flying sailing ships that I am considering for this question.

To clarify matters, keel material - wood in the age of sail, but metal later on - resists any movement as if by friction, but resists movement much less along one axis, and also resists somewhat less when turning. This resistance has nothing to do with air, it simply occurs relative to the largest object nearby, typically the planet.

If one was to push a length of keel, it would resist lateral movement, but would 'slide' forwards and backwards with only a little friction. If a length of keel was placed at an angle to the ground and pushed at one end, it would fall down because it could turn, though it would do so more slowly than expected if it was an object subject only to gravity.

However, if a length of keel was placed at an angle to the ground and held at that angle while being pushed, it would ascend at an angle somewhat less than the angle at which it was being held, depending upon how fast it was moving and its lateral friction coefficient. The higher the lateral friction coefficient, the less leeway a keel will make. This is all completely independent of the atmosphere, if any.

This means that as long as a vessel with keels is moving, it can maintain or increase its altitude. Because of the higher lateral friction, a vessel can be sail powered in an atmosphere - the keel gives the vessel something to push against so that there are differential forces between the wind and keel, allowing movement and steering in directions other than with the wind.

If a vessel with keels in gravity is overloaded or moves too slowly, the keels' angle of attack will be too great, resisting forward movement, and leeway will become greater than headway, resulting in loss of forward motion and a loss of altitude.

An object with multiple keels cannot be "anchored" in a gravity well: even the best keel material does not have infinite lateral friction, nor does any keel material have zero friction to headway. The best that can be expected is a greatly reduced rate of descent. In order to move, a keeled vessel requires an external source of kinetic energy - hence all such vessels require wind power, some other motive power, or gravic potential energy that can be traded for kinetic energy.

When we have magical keel material that allows flight with a smaller volume of material than an airfoil, when propellers can be made by placing angled keel material on the edge of a powered wheel, then only scarcity of that keel material might lead to the development of aerofoils, or perhaps they might be invented even if they are more expensive than keels, if they had some utility that is not served by keels.

For purposes of this question, let us assume that basic keels are a relatively rare form of wood, and are about as expensive as a rare hardwood such as Ebony. More efficient metallic keel material is possible in the age of steam.

Aside from this keel material, we may assume that the environment is much as Earth. Square sails which simply provide drag were invented around 3000 BC, and only allow sailing with the wind. Lateen sails were invented around 900 AD, and allow sailing into the wind, since they act as an aerofoil, but it was not until DaVinci first described man-made wings that their use as an aerofoil began to be recognized, and not until the 19th century that aerofoils began to be used for flight.

Additionally, keels operate by a different principle to aerofoils: aerofoils work by passing through a fluid medium, while keels resist movement in all directions, but offer much less resistance to movement along one axis, relative only to the keel and the rest of the universe.

For purposes of this question, let us assume that keels were discovered around 500 AD.

So, the question is: would aerofoils be invented earlier or later relative to other technologies in such a world, and to what uses might they be put?

  • $\begingroup$ I think the first part of your question is hardly objectively answered. It's your world, you decide the inventions timeline. $\endgroup$ – L.Dutch Jul 10 '18 at 8:25
  • $\begingroup$ @Raditz_35, have a look at my edits. $\endgroup$ – Monty Wild Jul 10 '18 at 10:16
  • $\begingroup$ Apologies for being slow, but would you please confirm my understanding of the original answer. There is a plank of "Keelwood" oriented N/S and angled up at 5 degrees at the N end. If I push this from the south with a really long pole then it will climb into the sky at the same 5 degree angle for as far as I can push it, rather than just bumping along the ground. Have I understood correctly? $\endgroup$ – KerrAvon2055 Jul 10 '18 at 10:36

As far as I've understood, your magic keel is nothing more a device that has negligible drag in one direction and as much drag as possible in another one (please note that a standard keel has high drag in only one direction, and negligible drag in the other two directions, the trick is that flotation covers one direction).

With this design (without flotation) your sail flying ship is not really different than a plane since it's missing the main concept: buoyancy. What if the most feared weather for the sailors occurs? Which, by the way, is dead calm and not, as commonly mistaken, tempest. Moreover, keels work only if there's some relative velocity with respect to the fluid in which they are placed, the more speed the more "lift" they can generate. Without airfoil technology, sails can work only by pure drag and can't really go upwind, and they are efficient as long as there's speed respect to the fluid. This means that if you place your sailship in a flow you either have some efficiency on the keels OR on the sail, not both. If you anchor your vehicle to the ground it can act as a kite because of the keels. If you let it follow the flow the sail will make it match the flow speed, allowing it to move, but in this case, the keels won't work since they won't have a significant relative speed respect to the fluid.

Unfortunately, this is a logical design issue: despite dimension and building technology you can't use sails as propulsion and keels as lift. Unless, you know, the keels are "magical" as you said but then in a magical world is only up to you to decide what should happen because of variables that are true only in your world.

In "our" world you need some sort of "mechanical "propulsion, in this case you can use a "propeller" made by inclined keels on a turning wheel or you can just use some oars (you don't even need to move them up and down, you only have to turn them by 90°, changing the keel direction with respect to their movement). You can't use a paddlewheel since you don't have different fluids' density near the flying ship.

Stated that you need a mechanical propulsion, you don't have flotation and all your keels works as a drag maximizer you don't have a vessel but a strange (inefficient) airplane instead. With enough speed -technically- also a brick can actually fly, the issue is the efficiency, and that's why we developed airfoils.

With these premises I would confidently bet that airfoil would have been developed earlier since it would have been a HUGE advantage for both "lifting" keels and propellers (propellers are designed as airfoils). Also, magic keels, at least the ones that are supposed to generate lift, could improve their efficiency by using an airfoil profile (not really sure since they are magical, but seems plausible). As soon as someone noticed anything similar to an airfoil effect the economic pressure for this technology would have been more intense than the one for nuclear weapons during the cold war, therefore flying vessels would have been designed with airfoil way before we did.

We successfully used vessel propeller long before the built of the first successful airplane not because the physical principle was unknown but because of the economic pressure (no one believed in planes).

PS: you asked about the airfoil "invention", I replied about the airfoil successful usage because it's quite difficult to discuss when some scientist should have the right intuition. We knew the airfoil principle since Bernoulli, yet we were able to use it to make something fly only a century ago.


Sail flying ship, at least at the beginning of their history, will move thanks to wind.

Since they already have a means of stay lifted in air, we have no direct use for aerofoils here, as we have no need for lifting force.

However we would soon face the same struggle which was found by the early sailors: the wind not always blows in the direction you want to travel. Therefore the concept of aerofoils can be used to sail into the wind, allowing more versatility into navigations until suitable motors are invented for the ships.

Regarding the first part of your question, I think you can freely decide the invention timeline of your story.


Your question implies a causal link between the discovery and exploitation of the friction properties of your special wood to allow flying vehicles to exist .. and the development of a method for harnessing the power of the wind to move said vehicle.

Of course there is no causal link .. the discovery of the Bernoulli effect had no impact on kite design until modern days when the intersection of material science and extreme sports/military applications brought about the parasail. I think that development of sails has a long history without any sign of airfoils being involved, so two threads of technology will only be intertwined if you write it that way

If in your world the development of the sail follows ours then we start with someone holding a blanket up with a stick on a raft. That leads to square sails that still only catch a following wind, then lateen rigs that catch the wind coming from the side of the ship, and finally sails laced on a vertical mast that take on an airfoil shape arrive.

My answer is that the discovery of keel materials that allow a ship to fly would spur the development of every form of propulsion except for sails .. steam powered turbines driving propellers, petroleum based combustion, solar panels making steam or converting photons into electrons, anything would be better able to exploit the power of flight more than sails. The only reason for having a keel is to mitigate the sideways force exerted by the sail when sailing into the wind. The necessity of the keel wastes the majority of the force exerted on the sail so nearly anything else would be an improvement.


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