Mesh wings for ultralight, ultra-small flying things? and other exotic ramblings

[Flyingwingbat1]

Recently, I picked some Goat's Beard (aka "giant dandelion poofs") for an experiment. After breaking the seed stem from the fluffy parachute portion, I trimmed the parachute into a rough delta-wing shape and tried slope-soaring it with a piece of cardboard. It worked surprisingly well, considering my doubts beforehand. The flying speed is around .5 to .6 mph, and I'd guess it's mass at 1 mg, with a mesh area of ~ 1 in^2. RE in flight is roughly 400! Call it anti-dynamic soaring, if you will.

It got me wondering whether ultra-light mesh could be used for extremely small flying things, since the RE is so low anyways that going for high L/D becomes an academic exercise in futility. With super-low RE, we get into Stokes Flow, where the air acts like syrup due to the viscous effects prevalent at such a small scale. I'm thinking a lighter mesh wing would behave much like a solid wing at very small scales due to the viscosity.

As far as how light solid and mesh surfaces could get, I did some calculations.

A single-layer graphene sheet would be ~ 660 MICROGRAMS per square METER, based on Avogadro's number (6.02 x 10^23 atomic mass units per gram), carbon's atomic weight, and the average distance b/t carbon atoms in the hexagonal graphene structure. Remembers, this is for a SOLID sheet, if you can call anything "solid" when it's only 1 molecule thick.

Now, imagine the 1 meter square made of carbon nanotubes woven into a mesh with 1 micrometer openings. I forgot the exact diameter used in my earlier calculations (~5 nanometers IIRC); when all was said and done, it came to 13 micrograms per square meter!! Also, those nanotubes can be made smaller, down to ~ 1 nm in diameter, so divide 13 micrograms by 5 to get the absolute lightest, most ethereal, invisible "film" possible.

[JetPlaneFlyer]

Surely the 'poofs' on dandilion seeds cannot actually produce lift as such (ie it cannot use horizontal airspeed to produce verticel lift force).
They just produce drag which slows their descent. If you slope soar with one then it could climb if the air is rising faster than the descent speed of the 'poof' but it really not 'flying' in the conventional sence, it's just falling slowly.
Because the 'poof' cannot produce lift from forward airspeed adding power would be pointless, all it would do is give the poof some horizontal velocity but it would still fall at the same speed.
The same would apply to a mesh wing unless the ratio of solid/hole was high then it would start to work like a conventional wing, elbeit a very inefficient one.

I suspect that if mesh wings worked at very low Re then you would find them in nature on insects.

Steve

[Texas Buzzard]

Surely the 'poofs' on dandilion seeds cannot actually produce lift as such (ie it cannot use horizontal airspeed to produce verticel lift force).
They just produce drag which slows their descent. If you slope soar with one then it could climb if the air is rising faster than the descent speed of the 'poof' but it really not 'flying' in the conventional sence, it's just falling slowly.
Because the 'poof' cannot produce lift from forward airspeed adding power would be pointless, all it would do is give the poof some horizontal velocity but it would still fall at the same speed.
The same would apply to a mesh wing unless the ratio of solid/hole was high then it would start to work like a conventional wing, elbeit a very inefficient one.

I suspect that if mesh wings worked at very low Re then you would find them in nature on insects.

Steve
.................................................. .................
Good thinking Steve. I have nothing to add.

[MarkusN]

Read again. He offset the CG by removing most of the fluff and leving just a sector, creating a delta. Which apparently did go into some forward movement.

It seems to work in nature. Just ask spiders. What do you call these floating spiders on a string in English? Gossamer, I think. Again just passive against falling though.

[nmasters]

I suspect that if mesh wings worked at very low Re then you would find them in nature on insects.

The smallest flying insects have “wings” that look like feather dusters. These creatures operate at Re < 1,000. Some of the larger of the fuzzy wing group are the Plume Moths (http://images.google.com/images?hl=en&client=firefox&rls=FlockInc.:en-US:official&hs=dw9&q=plume%20moth&um=1&ie=UTF-8&sa=N&tab=wi) . High speed photography has shown that the Plume Moths beat their wings horizontally thus demonstrating that there is some dynamic lift. Many of the larger moths have fuzzy trailing edges but that may have as much to do with sensory organs as aerodynamics.

--Norm

[Flyingwingbat1]

Interesting, NMasters! Thanks for the moth info.
FWIW, the L/D of my offset-trimmed Goat's Beard parachutes is something like 1:4! It's atrocious, no doubt, but just enough to slope-soar with. Untrimmed parachutes did not work, nor did maple-seed helicopters, or any other pure falling objects. An object must have SOME non-zero L/D for the slope-soaring to work. Now to wait on the fancy nanotube stuff...

[nmasters]

the L/D of my offset-trimmed Goat's Beard parachutes is something like 1:4! It's atrocious, no doubt
no no! That's about average, for butterflies :D

[Flyingwingbat1]

Actually, Butterfly L/Ds aren't all that bad; a friend of mine from Boston gets the dead butterflies from an insect garden, dries them out, and sets them up as walkalong gliders! He gave me one, and it flies pretty nicely, if a bit fast. The thin wings allow for an L/D of 3:1 or so , versus the 1:4 L/D of my Goats-beard "gliders"

[MarkusN]

The first Rogallo wing gliders were also at 4:1.

[nmasters]

Actually, Butterfly L/Ds aren't all that bad; a friend of mine from Boston gets the dead butterflies from an insect garden, dries them out, and sets them up as walkalong gliders! He gave me one, and it flies pretty nicely, if a bit fast. The thin wings allow for an L/D of 3:1 or so , versus the 1:4 L/D of my Goats-beard "gliders"
Oops, I read your 1:4 backwards. Butterflies have L/D ~ 4 to 1 or about a 20 degree glide slope. I'd assume that mother nature has optimized their wing loading so that the weight lose from dehydration would probably decrease the glide a bit, but that's just a guess. The same weight lose would improve the sink rate and that's not a guess

[Flyingwingbat1]

I think the L/D of a dried butterfly depends a lot on how the person prepared it. My friend forms the lower aft wings to have a lot of reflex, while the upper wings are left flat. He uses nail polish to help strengthen the body (hmm, for once, "fuselage" is an inappropriate term!) The wing position matters, too, for balance reasons.

[BMatthews]

Have you thought about trying to make your own balsawood "butterflies"?

Indoor wood of around 1/64 thick or sand down some lighter 1/32 wood to make it into 1/64 or even less if you're brave. Overall the weight shouldn't be any worse than the dried butterfly and may even be better. Also you could form an S curve into the wing's camber.

Back when I was doing a lot of indoor modeling and was a starving student I couldn't afford indoor wood so I made my own by sanding down 1/32 sheet. Use fresh sandpaper, not a lot of pressure, lift on the return stroke and check thickness often. You'd be surprised at how quickly you'll have a stock of .015 to .010 wood.

[Flyingwingbat1]

So, when you're sanding those sheets down so thin, do you sand from both sides (by flipping the wood over now and then) to prevent it from curling? What grit(s) work best?

[MCarlton]

IIRC the tiniest midges beat their wings approx 1000 times per second, using a sort of elastic muscle reflex doodad. Studies show this to be a vibration more than a flap, so at this extremely low Re, are we not looking at something more akin to one of those wind up bath toys with flipping legs?

Does anyone know the relative Re of a human being moving through water and a midge moving through air?

[reinking]

A quick search of Google Scholar using the search terms "dandelion seed lift" produced about 1,750 pages. Here are a few articles found by that method-

"Long-distance dispersal of tree seeds by wind"
"Various flying modes of wind-dispersal seeds"
"Predicting Plant Migration Rates in a Changing World: The Role of Long-Distance Dispersal"

You might be able to find a better combination of search terms and find some interesting stuff.

Bob Reinking

[Flyingwingbat1]

The fluffy seeds all appear to use the same drag-producing method of numerous fine fibers to increase their Cd, due to the huge exposed surface area. IIRC, wikipedia listed a swimmer's RE at __x 10^6, quite high, but that's due to water's high density and the characteristic reference length (the person's height). Midges in air have REs around 50-100 at the most. Stokes flow occurs at REs less than 1; in other words, the viscous forces dominate the inertial forces in the fluid.

[nmasters]

like this?
http://urbana.mie.uc.edu/yliu/Images/Stokes_Flow_Around_Many_Cylinders.bmp
Amazingly this site (http://urbana.mie.uc.edu/yliu/Software/) was at the top of the list that Google image returned for “Stokes flow”

[MCarlton]

Are these computations based on clean air? It occurs to me that at a microscopic scale, the flow model above must be subject to disruption from airbourne particles, dusts, pollens and the like.

Does, therefore, microscopic aerial debris affect the viscosity of air? So, for example, is it easier for a midge to fly through air which is dirty (smoke/dust etc) because the air is more dense?

Obviously for larger objects, the suspended particles are pushed aside, but if the mass differential between the suspended particle and the body moving through the air is much less, I assume the particles will not move so easily?