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My universe is focused on a steampunk-esque atmosphere although with more modern technologies up to the early 2000s (yes I am aware of dieselpunk, atompunk etc, but in here the steampunk aesthetic is merged with newer technologies, which are designed in resemblance to Victorian futuristim for cultural reasons).

In this scenario, one of the characters is creating a flying robot who uses active flapping and mechanical wings on its back. The wings are structured in many ways like a bat's, and are powerful enough to sustain the robot, but my problem lies in the material for said wings (assume the robot is perfectly capable of generating the forces necessary to fly).

The wing membrane needs to endure the forces necessary to keep a 150 kg Robotic humanoid airborne for preferably long periods (ideally a few days), and I'd I'd prefer if the material had the following traits:

  • Being durable enough not to require the membrane to be replaced often (by not needing to be replaced often, I mean that, ideally, the membrane could withstand a reasonable amount of damage before using it for flight becomes highly risky).

  • Hard to catch on fire.

  • As waterproof as possible.

  • Preferably was available/known to humans in our world up to the year 1920 (preferred, but not obligatory, especially in cases of a durable alternative which has all other 3 properties).

Note: The robot is around 2.7 meters tall, with a wing loading of around 93.75 N/m^2 and a wingspan of 16 meters.

Given these traits, what would be the best /most efficient material to use as a membrane for these wings? So far, according to what I've found, a variation of canvas seemed to be a good option at first since it can check out 3 out of the 4 desired traits depending on how it's made and treated but I'm not completely sure.

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    $\begingroup$ You may want to research what exotic materials were used for making aircraft wings before WW1... (Fabric or even paper doped with cellulose acetate.) (And the question lacks the crucial piece of information: what is the desired wing loading?) $\endgroup$ – AlexP Nov 16 '20 at 23:09
  • $\begingroup$ @AlexP I have added the wing loading and wingspan. I'm not really sure about using paper, since the wings follow the bat model and thus require a good level of flexibility both in the skeleton and in the membrane. $\endgroup$ – ProjectApex Nov 17 '20 at 14:20
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    $\begingroup$ Silk, the whole Silk and nothing but the Silk. It is very strong and durable. Not at all waterproof, but it does not mind being wet, at all. Not fireproof, but significantly fire resistant. It does degrade in direct sunlight from UltraViolet, but this process takes many days or few week to even be measureable, and months before it is problematic. $\endgroup$ – user79911 Nov 17 '20 at 14:50
  • $\begingroup$ What about Andean condor bones and leather? Those bird only weight 15kg, but I guess those bones need some badass strength to flap wings. $\endgroup$ – Madlozoz Nov 24 '20 at 16:06
  • $\begingroup$ Harvest bat wings $\endgroup$ – JamesFaix Nov 24 '20 at 16:31
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IMO, Your only hope is silk

However, What does it mean to not be replaced "too often?" What's "too often?" What damage are you imagining that would reduce the lifespan of the cloth? I'm ignoring that requirement since you're presumably looking for suspension-of-disbelief and not a factual solution (the robot puts a bit of a damper on the factual solution). Nevertheless...

All fabrics will burn, but some are more combustible than others. Untreated natural fibers such as cotton, linen and silk burn more readily than wool, which is more difficult to ignite and burns with a low flame velocity. (Source)

But, from that same source we read...

Fabrics with a tight weave - wool, modacrylic, 100 percent polyester and those that are flame-retardant treated are good choices. Heavy, tight weave fabrics will burn more slowly than loose weave, light fabrics of the same material. The surface texture of the fabric also affects flammability. Fabrics with long, loose, fluffy pile or "brushed" nap will ignite more readily than fabrics with a hard, tight surface, and in some cases will result in flames flashing across the fabric surface.

So, a very tightly woven silk with no frills should do the trick. Wool would be a lower fire risk, but it's much heavier and much more difficult to weave into a smooth surface (good for flight).

Silk is also great as it's naturally water repellent.

"Silk, although a natural fiber, tends to repel water" rather than absorbing it.... (Source)

And if you want to waterproof it further, rub it with wax before flight (preferably a wax that remains pliant at flight temperatures).

What I like most about this solution is that, from the perspective of today's reader, silk was still an uncommon commodity in the early 1900s. It would be perceived as an exotic fabric. Further, even today, we tend to think of silk with a romantic eye, which means it can be used for almost anything and the reader will believe it.

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  • $\begingroup$ Pure natural silk even now is uncommon commodity (mostly due to lots of much cheaper alternatives of comparable quality) $\endgroup$ – ksbes Nov 17 '20 at 10:02
  • $\begingroup$ "Silk was still an uncommon commodity in the early 1900s": Maybe in America? In Europe silk was a pretty common (albeit quite expensive) article since the 11th-12th century. (It was produced locally; first in Italy, then in Spain and, since the 1400s or so, extensively in France.) $\endgroup$ – AlexP Nov 17 '20 at 14:24
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    $\begingroup$ -1 for silk being the only hope because in real life, wings were made out of cotton and linen and those worked just fine. $\endgroup$ – DKNguyen Nov 24 '20 at 1:25
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    $\begingroup$ @DKNguyen It crossed my mind that you might not have understood my previous comment, which I deleted. Compared to silk, cotton and linen burn like matches soaked in gasoline (violating the OP's expectation of inflammability), are as waterproof as window screens (violating the OP's expectation of being waterproof), and are as durable as soggy toilet paper (violating the OP's expectation of durability). If you seriously disagree with any of that, please post an answer explaining why. That would be beneficial to the OP because in real life, wings weren't carrying robots. $\endgroup$ – JBH Nov 24 '20 at 9:37
  • $\begingroup$ @JBH I do not see how those aren't also concerns in real life selection of these materials. These criteria are not unique to OP's world. Neither is the robot because a 130kg robot is the same weight as a heavy human and since it is also the engine, OP's aircraft should actually be lighter than in real life. Depending on how the robot works, it is likely the OP's aircraft, does not even have to deal with oil spray from the engine like real planes do. The only unique thing is flapping. $\endgroup$ – DKNguyen Nov 24 '20 at 14:46
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Asbestos cloth.

asbestos cloth http://alexinsulation.com/dusted-free-asbestos-cloth/

Pretty much any organic fiber will burn merrily. If you want your flying robot to be going thru fires you will need to make its wings out of asbestos. People have been making cloth out of asbestos since antiquity. Charlemagne was said to have an asbestos cloth tablecloth that was cleaned by putting in the fire. Asbestos is made of light minerals and as a fiber is not that dense - lighter than metals or plastic though I think not as light as silk or cotton.

For your robot, incorporated in the asbestos cloth are shining brass threads, to augment its strength. When the wing gets really hot the brass threads might glow for a time after.

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    $\begingroup$ One problem: it's also carcinogenic. $\endgroup$ – The Daleks Nov 24 '20 at 14:45
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    $\begingroup$ Not for robots :) $\endgroup$ – JamesFaix Nov 24 '20 at 16:32
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    $\begingroup$ @TheDaleks Only if they know about it! ;) We made all sorts of stuff, even clothing out of asbestos, before it's evil nature was revealed. Take a peek at asbestos.com/products/textile-cloths-garments $\endgroup$ – PcMan Nov 27 '20 at 20:53
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    $\begingroup$ Thanks for the bounty, @ProjectApex! As re your question I suspect that techniques for making asbestos garments might be trade secrets - I did not turn up any either. I did find an ad from the 1930s that claimed asbestos fiber was "very like flax" so you could have your wings look like gray linen. $\endgroup$ – Willk Nov 29 '20 at 21:51
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    $\begingroup$ @ProjectApex It is made by weaving naturally occuring mineral fibers. asbestos.com/asbestos/types. One thing to keep in mind though is that asbestos cloth on its own is about 16-20 times as weaker than silk for its weight which makes it an awful choice for making wings out of. Individual asbestos fibers are actually much stronger than silk fibers, but they unravel very easily. This means you will need to seal the fibers against unraveling... which was often done by rubberizing it by the way... $\endgroup$ – Nosajimiki Dec 4 '20 at 15:46
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Rubberized cloth

enter image description here

Preferably was available/known to humans in our world up to the year 1920 (preferred, but not obligatory, especially in cases of a durable alternative which has all other 3 properties).

Charles Goodyear invented a process for mass producing rubberized cloth in 1844. This material involved impregnating natural fiber cloth under tension with vulcanized rubber resulting in a material that was highly flexible but not too stretchy, strong, fairly light weight, resistant to both very hot and cold weather, impermeable to air, and very waterproof. All good properties for bat like wings.

Being durable enough not to require the membrane to be replaced often (by not needing to be replaced often, I mean that, ideally, the membrane could withstand a reasonable amount of damage before using it for flight becomes highly risky).

This is where woven natural fibers alone like cotton, silk, and wool really fail. Their edges need to be properly finished or the whole material comes unwoven under very little stress; so, while something like silk might be able to check most of the boxes, a small hole or burn is all it will take to make the whole wing fall apart under the stress of flight. More textured fabrics unravel less easily, but they are also more combustible. Either way, they can not check all of your boxes.

Rubberized cloth overcomes this problem. Like concrete reinforced with rebar, the materials lend what each does well to the other making for something much stronger than either material on its own. The cloth becomes the basic structure of the fabric giving it it's strength while the rubber binds the fibers in a way that prevents them from unwinding or unraveling. So, even if rubberized cloth does get a hole in it, the hole will not just split like purely woven textiles do. This means even very old and somewhat damaged wings will still maintain most of their structure without you having to worry about a cascading failure in the material.

If you need to make the wings even stronger, you can add reinforcing wire to the weave. This is basically how modern tires are made and it's all based on this same process invented by Charles Goodyear.

Hard to catch on fire.

Depending on what exact properties you want the wings to have will determine what fabric you wish to use. If you were to use asbestos as your base material, it will be a bit heavier and weaker than some fabrics, but VERY heat resistant since you are basically looking at what fireman's coats are made out of. Keep in mind that such wings could be used to fly or go through flames unharmed but not at the same time. Asbestos unravels very easily compared to other fibers; so, if you heat soften the rubber too much, the asbestos will not be able to hold the wing together under its own tensile strength when flying. But it can survive the heat well enough that the rubber can melt staying in place, then re-cure after they cool back down.

That said, even without asbestos, vulcanized rubber does not burn very easily at all. Direct exposure to an open flame might melt a hole in your wing, but they will not combust. This is because the rubber will pool at the edges of the hole smothering any flames that might try to burn the inner cloth and making a reinforced edge so that the hole does not easily become a weak point in the structure. So, unless you plan on running into burning buildings with these wing, I would just go with cotton, linen, or silk as the inner material since these will be stronger, lighter, and more flexible than asbestos. Silk would probably be best, but also the most cost prohibitive.

As waterproof as possible.

I think it goes without saying that you won't find an answer that beats rubberized cloth on the water resistance issue. The rubber fills any gaps in the cloth's weave preventing the wings from holding onto or being penetrated by water at all. So, your robot would never need to wait a moment for the wings to dry: one good flap would cast off any droplets that might be clinging to the outside and you are good to go. I'd even say, the wings themselves would make for an ideal raincoat for your robot if the body itself maybe does not do so well with water.

But how strong was it really?

Finding spec sheets that date back to the 1800s is kinda hard, but we know that they would have probably used vulcanized latex over industrial manufactured cotton or linen. So, the specs of that should be very similar this: https://therubbercompany.com/rubber-sheeting/commercial-rubber-sheeting/economy-natural-rubber-sheeting. This material has a tensile strength of 30 kg/cm2. So, even at only 1 millimeter thick, it would still be much stronger than your peak stress expectation.

This spec sheet also tells us that it can operate to spec at temperature ranges of -20°C to 70°C which will cover you for anywhere ranging from well below freezing to much hotter than any desert. That said, many natural fibers can reach temperature ranges in the -150°C to 100°C range without significantly weakening; so, even if the latex weakens, the underlying structure should still hold at much more extreme temperatures as long as the wing is not so damaged that you need to worry about unraveling.

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  • $\begingroup$ My main concern with this option would be how much the rubberizing process compromises the flexibility of the membrane, since I'd need it to fold up in potentially awkward angles when not in use. $\endgroup$ – ProjectApex Dec 4 '20 at 21:57
  • $\begingroup$ @ProjectApex Vulcanized natural latex is the same stuff party balloons are made out of. Since your wings don't need to be more than a millimetre thick, flexibility should be more than enough $\endgroup$ – Nosajimiki Dec 6 '20 at 8:18
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I believe people in the past already faced and solved this problem. Consider any material used to make sails. They were designed to be durable and generally not affected by the water. The only problem is it would be rather heavy so it would need to be modified to fit your design. However sailcloth gives you a number of choices such as flax, hemp or cotton.

Another important factor I think you should take into account is combination of fibers and weaving styles.

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Thin enough Aluminum, like thick aluminum foil

Silk is obviously the answer normally because its' density is lower than Aluminum and its' tensile strength is higher.

However, you want fire/waterproof?

Then I think Aluminum is better.

Being durable enough

I'd say both Silk and Aluminum (Foil) are about the same at durability (Not thinking about water/fire, only at the force being applied to the material)

Aluminum has an ultimate tensile strength of about 310MPa, while Silk has 130~1410MPa (source), but the average is about 500MPa?(source) Not sure what silk would have been available back then

Both would be ok in terms of durability

Fireproof

Aluminum being a metal, it won't burn like Silk, it'll melt (at 660 degrees celcius, which is still higher than Silk's flash point of about 450 degrees celcius)

Waterproof

Aluminum oxide is stronger than aluminum, and it doesn't peel off like rust, so it protects the metal underneath from further oxidation (or you could just coat it with anti-oxidation stuff)

Preferably was available to humans in our world up to the year 1920

This is the most fitting part, as I see it

Aluminum production was available since 1852, but it was super expensive at the time (1.5 times the price of gold)

However, Electrolytic Production was discovered around 1883, and thanks to it, the price went down to $0.5 per pound in 1894, which fits nicely with that early 1900s theme, since aluminum was probably considered a new industrial material, with its' properties of being light and its' new Production method making the price suddenly low. (something like what graphene feels like to us?)


So with that said, since it's not a fabric, can the wings catch air?

I'm not sure, but I think appropriate design of the wings could make it work.

I believe that a slightly cupped wing design would work, trapping the air beneath the foil.

Aluminum is 1.9 times denser than Silk (silk density) , so for the same amount of materials, you would get a 2 times heavier wing with slightly lower tensile strength.

But if the wing is powerful enough... maybe?

And I personally think Aluminum is more fitting than Silk in that "steampunk-esque atmosphere" in terms of aesthetics too

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In the book Cove, the main characters assemble a mechanical dragon using the instructions of a genius mechanic. The material they used for the wing membrane was a very fine form of chainmail. The actual metal is fictional but the concept remains the same.

So my solution is to use titanium chainmail fine enough to prevent air from flowing through. Why titanium? It's lighter than steel, twice as strong, and follows that steampunk aesthetic

As for durability, as I said, it's titanium, super strong, and if you want you could even make it an alloy to increase its durability.

For waterproofing, it is made of metal, which doesn't absorb water, and since it's tight enough to prevent air from going through, it should stop water as well.

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  • $\begingroup$ This could in theory meet the 3 material properties the OP wants, but such chainmail could not be made with early 1900s tech. Perhaps some manner of lamellar might work though. $\endgroup$ – Nosajimiki Nov 24 '20 at 23:16
  • $\begingroup$ so a less fine form of chainmail but with a form of rubberized coating $\endgroup$ – Firestryke Nov 24 '20 at 23:18
  • $\begingroup$ Actually, in the book, they have pretty primitive tech. It would take a long time to make, but still possible $\endgroup$ – Firestryke Nov 27 '20 at 23:57

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