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Story-based background (not essential for question)

The Helawi live at the top of a small plateau with steep cliffs all around. There is a carved stairway from top to bottom that allows only one person to pass at a time. This is ideal for defence but dreadful for attack.

The Fikawi are camped at the bottom of the stairway and have been ambushing members of the Helawi when they emerge.

The Helawi have discovered this and want to attack but if they send warriors in single file, they will be killed by the Fikawi.

The cliffs contain large sheets of high quality mica. The Helawi who see buzzards flying from and to the cliffs, decide to make fixed wings of mica. They will glide down at night and attack the Fikawi from behind.

Notes

There is an overhang that prevents a simple bombardment from above. They can't use ropes because this will disturb the birds roosting on the cliffs and the clamour will spoil the element of surprise.


Non-story-based Question

In theory, could a stone age tribe make an air-force of fixed wing hang-gliders and safely glide from top to bottom of a cliff if the air currents were favourable?

In particular, to keep the stone-age feel, could the wings be made of mica or other mineral-based material?

Assumptions

They have access to leather, wood, animal bones and sinews, flint knives, and large sheets of mica.

Sadly, there is no bamboo in this area - just softwoods such as pine.

Mica

"... micas except glauconite exhibit easily observable perfect cleavage into flexible sheets ..."

"... Micas occuring as large crystals are often called books; these may measure up to several metres across..."

https://www.britannica.com/science/mica

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    $\begingroup$ Mica's Specific strength would be terrible for flying. They would have the best luck with leather wings (which still going to be terrible). In this particular case, Helawi should just rope down from their cliffs in a location where Fikawi don't expect them. $\endgroup$ – Alexander Feb 14 '19 at 21:03
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    $\begingroup$ "I particularly want to find out about gliders" Sure, but you're asking how to make usable man-carrying gliders out of stone, mica is both far too brittle & far too heavy for what you want to use it for, you might as well ask for suggestions on how to build a hang-glider with wings made from slate or glass. $\endgroup$ – Pelinore Feb 14 '19 at 21:51
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    $\begingroup$ How big is this tribe? If it's only a few family units, the deaths of several able-bodied young men during the development of such a glider could be devastating. They aren't going to get this right on the first try. $\endgroup$ – Nuclear Wang Feb 14 '19 at 22:02
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    $\begingroup$ @chaslyfromUK Maybe but that becomes a structural nightmare. It would be hard to build that with a good airfoil shape. The bottom of the wing needs to be as smooth as possible. Might be better to invent plywood and build the whole thing out of wood. $\endgroup$ – ShadoCat Feb 14 '19 at 22:46
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    $\begingroup$ how bout Unaided Aerial Vehicle(UAV)? I can only think of stone wraps with banana leaves and launch via tree bunch acting like a level system... you complaint about accuracy and precision but mind u this is stone age ;D $\endgroup$ – user6760 Feb 15 '19 at 1:00
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As @Alexander pointed out in his comment, mica would be terrible. To get it thick enough to be strong enough, it would be too heavy.

Wood and canvas work. Just look at the WWII gliders (wiki). Those started out being towed by propeller driven aircraft to get to altitude but you don't need that here.

Here is a list of gliders used by different countries to give you an idea of different design types.

You could also go with simple parachutes.

However, I am concerned that whatever is preventing a dropping rock from hitting the Fikawi on their heads may prevent the gliders from reaching them as well.

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  • $\begingroup$ Unfortunately they haven't discovered weaving, especially the fine weaving and varnish needed for a glider. $\endgroup$ – chasly from UK Feb 14 '19 at 21:59
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    $\begingroup$ @chaslyfromUK, the gliders don't have to be good, they just have to get people down. I'd be tempted to build a high ridge that goes off to the side and make lots of rope. You send a bunch of people repelling down the final length. $\endgroup$ – ShadoCat Feb 14 '19 at 22:16
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    $\begingroup$ It would not have to be canvas thinly stretch animal skins, or possibly stomach lining or intestines used today to make haggis and sausage casings could be sown together using sinew and bone needles to make the surface of a glider. Similar to the kayaks made in the Northern polar regions. $\endgroup$ – Sarriesfan Feb 15 '19 at 0:47
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In theory, could a stone age tribe make an air-force of fixed wing hang-gliders and safely glide from top to bottom of a cliff if the air currents were favourable?

Well, maybe, but not your stone-age tribe.

The problem is, not neccessarily using mica. If you like, you can wave your hands a lot and postulate that the mica available is found in improbably large perfect sheets. There have been very large mica crystals found in places like Canada and Russia with the desired size, but they are riddled with imperfections, and this will make any large-scale construction extremely difficult. With some practice, the natives might learn to peel off very large, thin perfect sheets and use them for skins. This has problems, since sheets thick enough to be self-supporting will be waaaay too heavy to use, and wrapping thin skins around a wooden wing framework which just happens to have a useful airfoil shape is pretty outrageous. What sort of mushrooms are your people smoking in order to get revelations of the needed geometry?

Once you get past that, I suspect sufficiently thin mica would actually work, since animal glue ought to adhere satisfactorily, and very thin sheets ought to bend well.

So, if you want to stretch credulity to the breaking point, you might, maybe, if the reader has been doing 'ludes or maybe X, and is feeling very mellow, get away with it.

No, the problem is that of first flight. The tribe lives on top of a small plateau. The only way to learn how to control a glider is to practice, and this is usually done on low slopes with just enough slope that the novice doesn't get too high and get badly injured in the inevitable crash. In your case, the only way to learn is to strap a glider on and run over the edge. Y'know, earn while you learn.

And this is not going to end well unless you invoke authorial privilege in ways which will cause most readers to throw the book across the room.

EDIT - there was actually a related incident (almost) during WWII. Problem Allied officers were held in Colditz Castle, and, as much out of boredom as anything else, worked on the Colditz Cock, which was not finished before the end of the war.

This was a two-seat, overwing glider which was built in an attic and intended to be hauled up on the roof of the castle for launch. It might, just possibly, have worked, but that is mainly because it was designed and built by officers who had some experience in operational aircraft. I certainly would not have wanted to be part of the maiden flight.

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    $\begingroup$ "this is not going to end well unless you invoke authorial privilege in ways which will cause most readers to throw the book across the room" a [+] for that line alone, honestly, if I found the OP's idea in a book I was reading it'd be enough to get an author on my "don't buy anything he's written ever again" list $\endgroup$ – Pelinore Feb 14 '19 at 23:31
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All the ideas about "let's port an idea to an earlier age" ride on the wave of thought inertia. "How could they not have thought about it earlier?!" Such things include steampunk (Victorian era information technology), Ancient World steam engines, your Stone Age flying, etc.

The problem with this approach is less that people did not come up with proper ideas. While this is true to a some degree, a diligent study shows that most ideas were suggested multiple times. If an approach or a device does not take off (sometimes, literally), the reason is some times the lack of demand (people don't know what to do), but in the most cases the lack of materials to implement the idea properly or the missing economic sustainability (it's too expensive to operate). Or everything together.

To give some examples: The lack of civilian supersonic travel is a prime example of missing sustainability. All those almost full-titanium prototype bombers that never went into production scratch at the line of missing or too expensive materials.

Materials are the key

Gliders were mentioned.

Yes, you probably could build a somewhat working glider from your local wood and local fabric. It would be not good enough in most cases.

The balsa wood hat its fame in the early aviation days not without a reason. The duramold and delta-wood (quite complicated, mind you, that's finest mid-20th century tech!) were invented not without a reason. (Funnily enough, some secrets on fabricating these are now lost, because we have better materials now.) And don't get me started on lightweight and durable fabrics, nylon, aviation varnishes and resins.

Technology is hard. Material science is hard. The reason no one basically built a proper glider before 20th century has something to do with missing knowledge of aerodynamics, yes. But also with lacking materials.

A classic old wives tale is about a local idiot, who builds wings 'cause he wants to fly as a bird, climbs the local bell tower and crashes down. The problem is that the wings would be in most cases designed as wearable items (thus having too little wingspan) and that the materials available to the local madman around 17th century are not really balsa and nylon and silk.

Frankly, I fail to see how your Stone Age tribe has access to such materials in such a variety. You can put them where balsa grows, but they'd be like half an Earth away from where the silk worm lives.

And don't even get me started on engines. Basically, anything less than an internal combustion engine has too little power/weight ratio to move an aircraft. A heaver that air aircraft that is.

Zeppelins? No.

But proper dirigibles are another, quite tearful story. You'd need proper varnish and material for the insulation. You'd want a hard hull. You need a carcass from a quite good steel or aluminum that is. You'd still need engines. You need a source of a lighter-than-air gas. Hydrogen is highly flammable, hello Hindenburg! You'd still need some natural source of it (or of helium, even harder) or an industrial way to produce it.

Basically, except hot-air balloons for reconnaissance (used in wars in 19th century, if I recall correctly), anything lighter-than-air is too high-tech for Stone Age. And I doubt hot-air balloons too.

What can we do?

All my text above describes why do materials matter, why logistic chains are important, and why we did not have Stone Age aircraft in our history on our Earth.

You could change exactly this. In your alternate reality, balsa trees and silk-producing bugs are neighbours. Or your Stoge Age tribes domesticates damn huge spiders and learns to milk them for web.

(Damn huge spiders need a yet another change of the reality, say, it's damn hot in the Stone Age and damn huge is not really very large. Either that or damn huge spiders have lungs. This, however, needs a further explanation, why do we not have a civilisation of damn huge spiders long before human Stone Age, but I digress.)

Or your Stone Age guys found some alien technology. And could reproduce some of it. Or at least they learned aerodynamics. Throw also in thermodynamics and quantum theory, because why not? (You'd have then Iulius Caesar killed with a laser, like in one very short Sci-Fi story, but that's another point and I digress again.)

Or the same alien ship would have transported tons of natural silk for whatever intergalactic fashion reason. Or, even better, liquid synthesised components of spider web in industrial quantity. That'd be a non-renewable source, of course, but your tribal shamans could (miraculously) learn to produce spider web and spider silk cloth from it. And we already agreed that balsa trees just grow around where the tribe lives. Or they were great hunters making composite bows routinely and hence they have some technology resembling delta-wood using carefully collected wood, antlers, bee wax, goat intestines and what not.

Oh, and as already mentioned, being locked out on a plateau does not help.

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    $\begingroup$ I'd object to your glider analysis. Light weight is only important if you need large glide ratios or soaring. WWII troop gliders like the Waco used standard plywood and metal fasteners for the frames and wings. The gliders posited in the OP only need a high-enough glide ratio to land without seriously injuring the pilot. $\endgroup$ – WhatRoughBeast Feb 15 '19 at 0:21
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    $\begingroup$ If you look at what Octave Chanute and Otto Lilienthal were playing with, you could build gliders like that clear back to the time of ancient Egypt or before -- if you can somehow get away from the whole "flapping = flight" mindset. I don't think leather wings are light enough to work, but the Egyptians were certainly known for high-quality, lightweight textiles. $\endgroup$ – Mark Feb 15 '19 at 2:38
  • $\begingroup$ WhatRoughBeast: At WW2 the humanity was pretty good at aerodynamics. If you have to discover these laws first (and especially if your flight is down from a plateau and it's essential to have enough lift), you'd want better materials. Mark: Yeah, I wrote "basically" because of Lilienthal and Co. I'd say it can be surely said that aviation before 20th century was in its infancy. $\endgroup$ – Oleg Lobachev Feb 15 '19 at 22:27
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In theory, could a stone age tribe make an air-force of fixed wing hang-gliders and safely glide from top to bottom of a cliff if the air currents were favourable?

Yes, it can be done with mica in light thin sheets, but only if your Helawi take a lesson from the insects.

enter image description here

  • Dragonfly's wings are corrugated on a fine scale such that they are superbley strong, and flexible in ways that enable superb manouverability - they are bendy when acted on by forces from above but rigid when bearing the thrust of a downstroke or a tight turn. In the 2010 study the thickness of the wing in some places proved too small for the scanning electron microscope to resolve. The wings are arranged into veins and nodes connecting them - all serving to distribute strain for maximum utility.

  • A thin and flexible pine frame reinforcing thin sheets of mica with plant fibre veins, all fixed together with a refined animal protein glue (collagen in meat boiled down to gelatin) - all you need is a stone with a concave surface and some meat, water and fire. (I wouldn't try to sew mica - it doesn't possess the tensile strength to cope with being threaded).

None of this could be invented in twenty five minutes, over time expertise would need to be developed by trial and error and occasional inspiration of the Lithogeniuses of the tribe, they would need time to develope tactical defence.

As to Tactics.

I recommend that you send an assault force down silently, as you said, to wait behind the Fikawi, then a second wave slowly and very noisily glide down with flaming torches and whissles/cries, maybe throwing rocks/flaming balls of fat to attract the attention of and disorient the enemy, the first wave taking this as a signal to attack from below. They don't like it up 'em.

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    $\begingroup$ If you're going to downvote, please leave a comment saying why. $\endgroup$ – BLT-Bub Feb 14 '19 at 23:25
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I think we can come up with a scenario where your stone-age glider would work better by looking at the lift equation, $F_\mathrm{lift} = \frac{1}{2}CA \rho v^2$, where $F_\mathrm{lift}$ is the lift force provided by the wing, $C$ is the lift coefficient (usually of order 1), $A$ is the wing area, $\rho$ is the density of the fluid, and $v$ is the speed of the wing relative to the fluid. For flight, the lift force must counteract the gravitational force, $F = -mg$, where $m$ is the mass of the wing and pilot and $g$ is the local gravitation acceleration. We have a few options:

Increase $v$: Your plateau might have very high winds. Doubling the wind speed will lead to four times the lift! Unfortunately, steering the glider would be difficult. Hopefully the Fikawi are camped downwind.

Decrease $g$: The Helawi glider would work a lot better if your stone-age tribe lived on a planet with less surface gravity and a similar atmospheric pressure as Earth. At $g = \frac{1}{2} g_\mathrm{Earth} \approx 5~\mathrm{m/s}^2$, the glider would generate double the lift of the same glider on Earth.

Increase $\rho$: One way to increase $\rho$ is to increase the total pressure. As long as you keep the partial pressure of oxygen near its value on Earth, people can survive with a fairly wide range of pressure. Nitrogen narcosis causes symptoms similar to alcohol consumption at pressures greater 4 times Earth's normal atmospheric pressure. That would give the glider 4 times the lift! With some mild evolutionary adaptations, your people might be able to withstand such total pressures with no ill effects or maybe up to 10 times as much pressure. I think a planet with 10 times Earth's atmospheric pressure might be the best way to fly with a stone glider.

Another option for increased $\rho$ would be to keep the pressure of the gas the same, but make the gas heavier. This seems a little far-fetched from an elemental abundance perspective, but maybe your atmosphere is 20% oxygen and 80% xenon by volume. At 1 atm of pressure, the glider would produce (0.2*16+0.8*131)/(0.2*16+0.8*14) = 7.5 times as much lift! Some other heavy non-toxic gas might also work. However, xenon and many other heavy inert gases have anesthetic effects, so some biological adaptation from normal humans would probably be required.

Decrease $m$: As mentioned in the other answers, mica isn't a great material for gliders. But if your locale had native aluminum metal, maybe they could pound it into wings. Despite aluminum's high reactivity for oxygen, native aluminum metal apparently exists in some places, although it seems to be mixed with other, more dense metals. However, aluminum isn't very stone age and you can only get so far by reducing the weight of the glider. The weight of the pilot would eventually be the limiting factor. Could you make the pilot smaller? I could imagine a hummingbird-sized person flying around on a thin sheet of mica.

Increase $A$: This would be difficult without making the mica break. The mica might be reinforced by wood and leather, but the available materials are really going to limit the size that the wing can be without collapsing.

In conclusion, if the story must take place on Earth with a history similar to ours, I suppose that high winds (high $v$) and small, possibly pygmy-sized, people (low $m$) would might be your best bet. Without doing some experiments, it's unclear how well a glider constructed from mica, leather, and wood would perform under current conditions on Earth. As suggested in the other answers, looking at early twentieth century methods of airplane construction would help, although these relied on cotton fabric and doping substances that would probably not be available to stone-age culture. Could the fabrics be replaced with pitch-coated animal skins? I don't know. However, the above points show that if you are flexible with the conditions of the planet in your story, you can get much better gliding performance with stone-age construction.

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  • $\begingroup$ So basically a "not really plausible" then? maybe the answer could benefit from a short summation / conclusion? $\endgroup$ – Pelinore Feb 15 '19 at 1:28
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    $\begingroup$ Aluminum isn't malleable enough to cold-work, unlike copper or gold. $\endgroup$ – Mark Feb 15 '19 at 2:43
  • $\begingroup$ @Mark That's a good point. I'm not sure if a stone-age culture would be able to do anything with relatively pure aluminum metal, even if they had it. How hot would they have to get it to be able to work it? Would it start to burn first? $\endgroup$ – WaterMolecule Feb 15 '19 at 17:23
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Unless you can successfully build and test hot air balloons being airborne is not the answer here.

Make long ropes and scale down the backside of the mountain. When you have enough hiding in the back then you come out and attack.

Throw down,catapult, or etc boiling water, rocks,acid, flaming pitch, or poison (or etc) from above to land on your enemies.

Depending on height archers might be a great way to attack them.

All this without leaving your camp.

Team 1 maintains a constant barrage on the folks below.

Team 2 charges down the mountain as fast as safely possible in great numbers.

If the cliffs are not sheer drop offs you might be able to sled down them, as in straight down the side.

If you planned in advance you could have made a path all the way down free from rocks and debris.

Then you get in steerable wagons, and go shooting down the mountain at great speed. Who knows, you might be able to reach 60mph, if your carts hold together.

Think of it this way.

A perfect single lane road just wide enough for your carts plus a safety margin. Its important to bank the sides as much as possible and as steep as possible so you can roll giant rocks down at enemies without them flying off the side of the mountain prematurely. By the time the reach the bottom there momentum should be crazy. A solid stone, round boulder at 100+mph, it bowling time. Strike!!!

Think Indiana Jones, and the giant round boulder rolling down the narrow corridor behind him. Except this time you have 50 ready to go to smash your enemies at the bottom. Maybe you can even add spikes(flaming even) to them.

You can send carts careening down the road from the top building up great speed, and knock down people coming up the road to attack you.

Think human bowling.

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