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The reason for ridges on a pellet skirt.

Printed From: Kiwi Airgunners
Category: General
Forum Name: Flub's General Discussion
Forum Description: General discussion goes in here.
URL: www.kiwiairgunners.co.nz/forum_posts.asp?TID=343
Printed Date: 27 Nov 2024 at 11:01am
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Topic: The reason for ridges on a pellet skirt.
Posted By: Novagun
Subject: The reason for ridges on a pellet skirt.
Date Posted: 15 May 2017 at 8:54pm
The reason for striations on pellet skirts. (Ridges on the side of the skirt)
 

The following is my contribution to another forum. I reproduce it here to see what happens.

I have no authoritative quotes to substantiate this information apart from a discussion among some knowledgeable shooters at the local airgun club. It is subject to someone who has better information.

The reason for the striations on some pellets is the Magnus effect. In brief this means that the spinning of the pellet as it travels produces a low pressure area to the side of the pellet into which it is sucked. That gives a pellet deviation to the side of the direct line from muzzle to target. Left to a clockwise rotating pellet. 

At short ranges this deviation is insignificant with airguns. In powder burning high powered rifles it is also insignificant even at ranges of say 600 yards because the bullet exits the muzzle very fast, up to 3000 fps and does not loose significant velocity during its flight although it does loose some. The important point to note is that the spin rate imparted by the rifling is very fast, up to 70,000 rpm and for the duration of the bullet flight remains constant. Therefore the magnus effect is constant for the whole of the bullet trajectory. Because it is travelling so fast it makes no difference to the deviation from the direct path from muzzle to target. That is why among other reasons that rifle bullets do not have striations.
On the other hand an air rifle pellet exits the muzzle at say 850 fps and because it has little energy it drops velocity quite quickly. However the spin rate of the pellet remains, for practical purposes, constant. Over the course of its travel the pellet spin remains the same so the magnus effect that is drawing the pellet off course remains constant. However because the pellet is bleeding velocity quite quickly the magnus effect has a greater offset effect per unit of distance traveled at lower pellet velocity.
The way to offset the magnus effect is to slow the rotation of the pellet as it travels so that at greater ranges with the pellet travelling slower the offset per unit distance is less. The striations are to slow down the pellet spin and consequently reduce the Magnus effect. 

I think this deviation from the straight path is also called spin drift.


Replies:
Posted By: Novagun
Date Posted: 15 May 2017 at 8:57pm
Yes that is right the pellet does decelerate quickly but according to my information the spin rate does not. The striations are to slow the spinrate down by way of drag as the velocity of the pellet decreases so that the the magnus effect is less as the pellet velocity decreases. The further out from the muzzle the longer time makes the drag slow the pellet more. That is what the striations achieve. 
Sure it must be that as the pellet leaves the muzzle the spin rate is high and the striations must increases the circular motion of the air and have a high magnus effect early in the pellets' travel.
I suppose the pellet designers expect trade offs until harmony is achieved. ( Never is though)

I think the smooth skirted JSB and H&N pellets are the best. Better than RWS Superdomes.

Posted By: Pauly5
Date Posted: 15 May 2017 at 9:00pm
That's interesting. I would say that most pellets used by the throngs have smooth skirts. Your example of the 850 fps pellet - any idea on how far off it would travel over say, 50 m?
Does caliber matter?


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Posted By: Novagun
Date Posted: 15 May 2017 at 9:10pm
I don't think calibre matters nor does velocity at least at small speeds.
The magnus effect is used markedly on cricket balls, golf balls, basket balls which are all slow moving.
With rifle bullets I think high power overcomes many occurences because bullets go really fast.

Posted By: Novagun
Date Posted: 15 May 2017 at 9:15pm
How far off does it move the pellet. Who knows, who cares but it is definitely there. If you put the pellet in the pigeons  head that is good.
In my case all my five shot one hole groups are just as good.

Posted By: Pauly5
Date Posted: 16 May 2017 at 8:00am
But what if you're OCD? Is that one hole group exactly where you aimed?
I would say the cricket ball moves because of drag on one side and shiny the other.

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Posted By: Novagun
Date Posted: 16 May 2017 at 10:07am

Yes but it also has seams which must shift air around its surface. And tennis balls, they both get lobed with spin.

Posted By: nunga
Date Posted: 16 May 2017 at 12:14pm
Paul is right about the cricket ball, thats why they shine only one side of the seam to get the ball to swing through the air when bowling. the seam of the cricket ball normally stays straight up and down when bowling


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2nd place HP springer - WFTF World Championships 2014

Posted By: pete
Date Posted: 16 May 2017 at 7:30pm
yer need to google "flettener rotor"
Mr Flettener built a ship that used the magnus effect to power it..

.


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career Infinity .22 PCP
Crosman optimus 1.77
Hatsan Mod 75 .22
Swiss Arms brake barrel pistol 1.77
Crosman 1322.

Posted By: Pauly5
Date Posted: 16 May 2017 at 7:43pm
I did. Very interesting. So a pellet would either go up or down if there is a side wind. It would need a vertical wind to move sideways. Have I got that right?
So in theory in a certain side wind, your pellet may not drop as much as you would expect. As nova said probably nothing noticeable but all the same interesting.

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Posted By: xyon
Date Posted: 18 May 2017 at 12:08pm
I'm pretty sure this statement is wrong.
"In brief this means that the spinning of the pellet as it travels produces a low pressure area to the side of the pellet into which it is sucked. That gives a pellet deviation to the side of the direct line from muzzle to target. Left to a clockwise rotating pellet. "

There is no difference between the left or right side (or top and bottom for that matter) when the pellet is rotating.
The magnus effect will change the flight path when the axis of rotation not in the same direction as the air flow over the projectile (eg a crosswind).  See http://en.wikipedia.org/wiki/External_ballistics#Magnus_effect" rel="nofollow - http://en.wikipedia.org/wiki/External_ballistics#Magnus_effect



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My favourites
springer - Diana 52
PCP - FX Wildcat III

Posted By: Pauly5
Date Posted: 18 May 2017 at 12:22pm
If I understand you right, there will be no effect unless there is a wind acting from the side. So really no impact until there is wind.

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Posted By: xyon
Date Posted: 18 May 2017 at 3:59pm
I hadn't read your last post before I wrote mine. Yep spot on, a side wind will cause an elevation change, how much I have no idea.

I would have guessed ridges on a pellet would make the effect more pronounced, as that is why golf balls are dimpled and not smooth ??

I have wondered why some pellets have ridges, but since most of the good ones don't seem to I haven't bothered trying to find out.



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My favourites
springer - Diana 52
PCP - FX Wildcat III


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