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ANSWERED on Mon 12 Dec 2016 - 3:32 am UTC by David Sarokin

Question: Arrows

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montecristo 

Customer

 8 Dec 2016 17:46 UTCThu 8 Dec 2016 - 5:46 pm UTC 

I need a graphically supported explanation of why arrows/darts don't fly well without feathers. Clear enough that I can explain to a seven year old what is happening in flight.

 

David Sarokin 

Researcher

 8 Dec 2016 19:52 UTCThu 8 Dec 2016 - 7:52 pm UTC 

montecristo...as is often the case, your deceptively simple question is anything but! There just don't seem to be any straightforward, plain-English, visual explanations of the purpose of feathers on arrows.

The best, albeit illustrationless, example thus far is this one:


http://www.livescience.com/22021-summer-olympics-sports-physics.html

Archery

Action and reaction play a significant role in archery as well. To shoot an arrow straight and true at a target, archers must first impart a forward force on it. To do this, an archer will pull back on the bow string, thus storing potential energy in the string. When the string is released, it imparts this potential energy to the arrow in the form of kinetic energy, propelling the arrow forward.

To keep an arrow on its intended target once it is released, its shaft is tipped at its end with fletching in the form of bird feathers or a plastic substitute — traditionally, three per arrow. Fletching offers aerodynamic stability through air resistance. If some force, such as air turbulence, tries to push the arrow off its straight course, the fletching produces a drag against that change in motion, hindering the movement off course. Sometimes fletching can induce a spin on the arrow, which can further improve its stability and accuracy by equalizing forces from air turbulence.



Hopefully, someone will be able to provide a well-illustrated version of the above!


David

 

montecristo 

Customer

 8 Dec 2016 20:05 UTCThu 8 Dec 2016 - 8:05 pm UTC 

While bowing to the author and her degree in physics, since this is not a private question, I am not convinced by that answer. There is no ‘if’ about it - without feathers, the arrow will invariably be significantly less steady. And I am not convinced turbulence is the main reason - the effect is clear in a still room. Granted, there is always turbulence, but not much. It seems more likely that the feathers push against the fluid and thus achieve stability.

I need to understand this properly because seven year olds ask unpredictable questions.

 

David Sarokin 

Researcher

 8 Dec 2016 20:22 UTCThu 8 Dec 2016 - 8:22 pm UTC 

Turbulence doesn't require the air in the room to be moving. The motion of the arrow through the air is all that's needed to create turbulence, and send an unfletched arrow tumbling. When you stick your hand out the car window, you can create your own turbulence, even if there's no external wind to speak of.

As for your 7-yr old, there's always the "because I said so" approach  ;-)

 

montecristo 

Customer

 8 Dec 2016 21:30 UTCThu 8 Dec 2016 - 9:30 pm UTC 

You are right, and if the arrow is flexing all the time (which slow motion videos on YouTube suggest may be true, although it is less true for darts, which also need feathers), at the back of the arrow it could get relatively turbulent. In fact, if you are even more right, the arrow will always fly in turbulence (and I don't need to be in a car, there is turbulence around my hands as I type). That makes the "if" in the livescience post more questionable. Hopefully I am not being overly pedantic, but the distinction between needing feathers for turbulence and needing it at all seems important (even if it is academic because turbulence will always be present).

 

anonsi 

User

 9 Dec 2016 19:05 UTCFri 9 Dec 2016 - 7:05 pm UTC 

This is a pretty good explanation (the video isn't worth your time, though):
http://io9.gizmodo.com/5949513/why-do-arrows-have-feathers

"If the tip of a non-fletched arrow wobbles out of line with the arrow's flight path, the entire arrow can go off course, and may even start spinning like a cheerleader's baton."

 

David Sarokin 

Researcher

 9 Dec 2016 20:42 UTCFri 9 Dec 2016 - 8:42 pm UTC 

I came across this the other day:

https://www.youtube.com/watch?v=0Y2ILk6PY8E

but didn't post it because (a) the video quality is bad and (b) it's not at 7-yr old level. But we seem to have moved beyond (b), and the video -- poor as it is -- is the best visual explanation I've come across.

Take a look at 1:40, especially....

 

montecristo 

Customer

 10 Dec 2016 23:13 UTCSat 10 Dec 2016 - 11:13 pm UTC 

Does that explain why the fletching? My takeaway was that the fletching creates an additional vacuum, and that is a problem i.e. the fletching isn't added in order to create that vacuum. Then he mentions archer's paradox, but that seems to be something else, which involves arrows flexing. Flexing can't be relevant because darts need fletching too and they don't really flex, being steel, tungsten or other such materials.

I am beginning to think this is like why planes fly, no one knows.

 

David Sarokin 

Researcher

 11 Dec 2016 00:13 UTCSun 11 Dec 2016 - 12:13 am UTC 

You're right...he's suggestive but not explicit. I think I just filled in the blanks in my head.

These two links (for rockets...but it's the same with arrows) get to point rather simply:

https://van.physics.illinois.edu/qa/listing.php?id=2140
"...The fins counteract sideways motion of the rocket. Air flows smoothly past them if the rocket is traveling along its axis. If there is any sideways motion, then the air striking the fins pushes the rocket back towards straight motion. It's not perfect -- the rocket may still go around in circles, but the idea is to make the path straighter than it otherwise would be."


and some more detail (with a discussion of the center of gravity and the center of pressure):

http://forum.kerbalspaceprogram.com/index.php?/topic/107327-a-simple-guide-to-rocket-aerodynamics/
"...Fins are protrusions outside the rocket's airframe. Their goal in life is to lower the center of pressure on the rocket. Therefore, the most ideal fins are rather small and as far towards the bottom of the rocket as possible.

Fins keep the rocket stable without much drag in a very clever way. When the rocket is travelling straight up, fins have very low drag. However, if the rocket begins to tip slightly to one side, the fins' drag increases, making the rocket briefly more stable. They also exert a force on the base of the rocket ("Lift") that causes the rocket to remain pointed up. Lift plays a far larger role in keeping your rocket pointed in the right direction than drag does, but both are quite capable of helping...."



Hope that helps some.

David

 

montecristo 

Customer

 11 Dec 2016 10:48 UTCSun 11 Dec 2016 - 10:48 am UTC 

I'll take that. It's not easy for me to visualize but that Kerbal link is pretty clear.

 

David Sarokin 

Answer

 12 Dec 2016 03:32 UTCMon 12 Dec 2016 - 3:32 am UTC 

You must have one inquisitive 7-year old on your hands!

One more link worht visiting (still...no good images, though):

http://www.nar.org/nar-products/rocket-stability/

Search the page for 'fins'. As the text explains, the fins do an important aerodynamic job of restoring stability when the rocket (or arrow!) wobbles. The wobble exposes more fin surface to the air rushing past which creates lift that pushes the rocket in the direction opposite the off-axis wobble.

The weight of the fins is also a crucial factor, as it helps adjust the relative positions of the center of gravity and the cent of pressure. I can't quite explain this...but it's important.

Have a lovely holiday.

David

 

montecristo 

Customer

 12 Dec 2016 09:04 UTCMon 12 Dec 2016 - 9:04 am UTC 

 

Roger Browne 

Researcher

 12 Dec 2016 09:33 UTCMon 12 Dec 2016 - 9:33 am UTC 

At the age of seven, experimentation may work better than a theoretical explanation.

Stand on a lawn and throw a bamboo skewer up into the air. It almost always lands at some awkward angle, and won't stick into the grass. Find a feather and tape it to one end of the skewer. Now you can throw the skewer into the air and it will come down point-first. A seven-year-old can intuitively understand this as being caused by the feather "catching the air" and dragging the tail of the skewer back a bit, leaving the point to touch the ground first.

Now teach the seven-year-old how to throw a dart so that it sticks into a dartboard. Cut the fins off the dart, and it will become hard to make the dart stick into the dartboard. Unless we can hold the dart at the exact center of gravity when we throw it, it won't stick in. But with the addition of fins at the back (to keep the rear end of the dart pointing exactly rearwards), the point at the front will hit the dartboard at the correct angle to stick in.

Angled or twisted feathers also impart rotational motion which stabilises the arrow or dart through gyroscopic effect, but I don't think a seven-year-old needs to know about this yet. After they have played with a toy gyroscope, perhaps.

 

montecristo 

Customer

 12 Dec 2016 16:18 UTCMon 12 Dec 2016 - 4:18 pm UTC 

Roger, I will try those experiments with them.

Darts is how this came about - frustrated that standing right next to the board doesn't make the darts fly better, they took off the fletching, in the hope that the reduced weight would help.

It's the same pair of children that this was about, half their lifetimes ago: https://uclue.com/?xq=5865

 

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