ARP wrote:There are far more insects that fly than there are birds, bats, frogs, squirrels and aeroplanes all put together. Because a fly operates in the same medium and the air is more viscous to its wings than say that of a bird, it is still 'flying', that's why we call it a fly.
Note, that the wings of most insects are in the single mm range. While this may seem tiny, it is still three orders of magnitude larger than the diameter of spider thread. Coincidently, our beloved hang glider wings are larger than insect wings by about the same factor. This reflects in the aerodynamic. Insect wings operate not quite like hang gliders or air planes. There is a reason why there are no insects dynamically soaring the wind faced sides of houses or rocks. But insect wings still induce vortices to generate lift. Compared to the vortices our wings induce, the swirls done by insects are pretty short lived.
By contrast, a single strand of spider silk at single digit m/s speed fails to stir vortices at all. And yes, this is a very significant difference.
The term 'kite' was probably derived from the name of the bird that appeared motionless in the air as if held there by a tether to the ground.
BTW, in German the toys are called "Drachen" which was taken from the word we use for dragons. After all, we westerners adopted the technique from Asia in the 16th century where dragon designs are traditional theme. The German term for hang glider is "Drachen", too
That said, I am not so much interested in words as I am in physics.
Whether or not this force is purely aerodynamic or there is some element of electrostatic force involved is not clear but the resultant is the spider gets lifted into the air in much the same observed manner as power kite operator does when jumping from level ground.
There is no kite at the end of the spider thread. The thread itself does not operate on principles similar to a kite, either. End of story.
I use my arms in a birdlike wing flapping motion and can generate reasonable thrust and lift to propel myself completely underwater. Not sure where my configuration sits within the Reynolds number scale but I am able to make good headway (...)
Good observation!
There are actually "water tunnel" facilities which exploit the fact that the at the same size and speed the Reynolds number is about 15 times larger than in air.
Due to the density of water your under water motions are probably in the Reynolds number range of 1e4 to 1e5 -- comparable to a model air planes. Notice, how easily you can stir a vortex.
I have not tried it in honey though, preferring it spread on my toast.
Better don't jump into a deep pool of honey. You'd probably die a sweet but horrible death by suffocation. The only chance would be to move as little as possible and try to float with the mouth at the surface.
p.s. I do not profess to understand the formula for working out Reynolds numbers but one of the main factors in the equation is chord length. The silk projected by the spider is of very small diameter but as its chord is its length, not its width, and that is aligned with the airflow, then the calculation might show that it has a high relative Reynolds number?
Well, I used the Reynolds number argument a bit sloppily. Rigorously, the Reynolds number can only be used to compare similar geometries at different speeds, sizes and mediums. So what I am really saying is: If you were to try and produce aerodynamic lift with a micron thin thread it will fail to induce vortices and thus will not work as expected.
As for the actual geometry: A long, thin upward pointing thread with the wind blowing parallel to the ground does not only not lift. It actually pushes down and tires to align with the stream lines of the wind.
---<)kaimartin(>---
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