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Re: Vine seeds become 'giant gliders'

Postby ARP » Sun Feb 14, 2016 10:12 am

"Taking the L/D concept a little further down the aerodynamic path:-

If an object is placed in a vertical airstream such as they do for freefall simulation, is it drag or lift that is keeping the skydiver up? The pressure on the under side of the skydiver is higher than that on the upper side. Once this pressure difference matches the weight of the skydiver balance is achieved and he/she no longer falls to the ground. ( Terminal velocity) The vectors are aligned so are drag and lift the same thing with only the resultant vector described by a L/D ratio?"


A quote from a NASA source:-

Definitions of Lift and Drag

Since the fluid is in motion, we can define a flow direction along the motion. The component of the net force perpendicular (or normal) to the flow direction is called the lift; the component of the net force along the flow direction is called the drag. These are definitions. In reality, there is a single, net, integrated force caused by the pressure variations along a body. This aerodynamic force acts through the average location of the pressure variation which is called the center of pressure.
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Re: Vine seeds become 'giant gliders'

Postby Rick Masters » Sun Feb 14, 2016 1:00 pm

February 12. 1915
The evolution of the Etrich "Taube"
https://www.flightglobal.com/pdfarchive/view/1915/1915%20-%200106.html
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Re: Vine seeds become 'giant gliders'

Postby ARP » Sun Feb 14, 2016 1:22 pm

On similar lines Jose' Weiss glider:- http://www.littlehamptonfort.co.uk/wp-c ... -Weiss.pdf
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Re: Vine seeds become 'giant gliders'

Postby Rick Masters » Sun Feb 14, 2016 3:02 pm

Superb document. Thanks.
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Re: Vine seeds become 'giant gliders'

Postby KaiMartin » Sun Feb 14, 2016 11:03 pm

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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Re: Vine seeds become 'giant gliders'

Postby Frank Colver » Sun Feb 14, 2016 11:48 pm

Interesting that Jose Weiss had developed the concept of positive tip twist to stabilize his "flying wing" gliders. That was an early time for that use (now common) which surprised me. He was definitely "ahead of his time". :thumbup:

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Re: Vine seeds become 'giant gliders'

Postby ARP » Mon Feb 15, 2016 6:18 am

Kai:-
"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."

I know your interest is in the physics so here is some "pseudo" physics to wrestle with:-
http://www.discovery.com/tv-shows/mythb ... -in-syrup/

As I said the resultant lift off of the spider to an observer has been described as kiting. The word may not suit your definition of the science behind it but it does show that nature can launch the spider into the air in in way considered in general parlance as kiting.

The science is interesting though and worth further consideration. Has there been much research at this end of the "flight" envelope?
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Re: Vine seeds become 'giant gliders'

Postby ARP » Mon Feb 15, 2016 6:59 am

Frank,

I think others were working on the concept of auto stability at this time and came up with similar solutions as did Dunne:- http://simanaitissays.com/2013/10/15/du ... eroplanes/

Whether by design or accident flexwings provide washout and add to stability of the gliders we fly today.

Tony
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Re: Vine seeds become 'giant gliders'

Postby ARP » Mon Feb 15, 2016 10:48 am

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Re: Vine seeds become 'giant gliders'

Postby KaiMartin » Mon Feb 15, 2016 3:34 pm

ARP wrote:I know your interest is in the physics so here is some "pseudo" physics to wrestle with:-
http://www.discovery.com/tv-shows/mythb ... -in-syrup/

The myth busters swam in a syrup with viscosity of about 0.5 Pa s. This is 500 times the viscosity of water. However, I did deliberately mentioned honey which is more in the range of 10 Pa s. At the speed we can move our limbs this makes a crucial difference. In honey the relevant Reynolds number drops below 10. At this low Reynolds number all flow is laminar. That is, no vortices. Human swimming heavily relies on vortex creation. Without it you cannot produce sufficient lift to keep the head above the surface.

With the 200 times more runny syrup the mythbusters used, the Reynolds number of limb movement through the fluid is 20 times higher in the range of 50 to 100. This happens to be well above the threshold for vortex creation. Consequently, the mythbusters had no problem swimming in the goo.

Here is a solidly fact based, yet very readable lecture on the topic of Low Reynolds number swimming, akka swimming in honey:
http://www.sfu.ca/~eemberly/phys347/lec ... ynolds.pdf

By the way, there is a simple but impressive way to get a feel for the transition from laminar flow to vortex generation: Grab a base tube (the good old round type) like a sword on one end. Jump in a pool. Swing the tube like Jedi lightsabre. Up to a certain speed the lightsabre moves smoothly through the water. But if you want go faster the tube starts to wobble wildly and present much increased counterforce. The theory behind it: Reynolds numbers grow proportional with speed. Depending on the swiftness of the swing the tube works the water at different Reynolds numbers.

The science is interesting though and worth further consideration. Has there been much research at this end of the "flight" envelope?

See the PDF linked above. Bacteria and other single cell organisms including sperm have to cope with single digit Reynolds numbers. Their methods of propulsion have been studied for decades. For manned flight this not even a remote option, though. There is no way to bring down the effective Reynolds number by five orders of magnitude compared to a regular hang glider. Since we are so large and the air is so thin we have no alternative to embrace vortices and turbulent flow.

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