Aerodynamic Stability of Pellets

[cavedweller]

Unfortunately, there are a lot of myths in shooting, some of them often repeated by people you would hope would know better. Perhaps the most common are that pellets are drag stabilized and that a cross wind blows on the side of a pellet. If you hear anyone say either of them, or read it anywhere, don't listen to or read anything else by them.

People also like to apply school level physics to projectile flight, precession is a favourite thing to talk about, I don't know what nutation has done to be ignored! They also put precession down to the fact that the pellet is spinning, but there are a lot of other things involved as well, such as yaw and yaw rates as the pellet leaves the barrel. Not many mention one of the factors which can be claimed to be the most important in how a pellet behaves, which is aerodynamic overturning moment, which can affect almost everything to do with group sizes. At this time, I am trying to see if there is a connection between gyroscopic stability factor and group size at different ranges and speeds, which can all be lumped together to give close to optimum twist rates and speeds for each range. It is slow work and may not be possible, but at least it keeps the brain cells working.

That’s friicking awesome but do you have time to shoot?

 

[HeyU]

That is all tied in with gyroscopic stability and how spinning bodies react to changes in the airflow direction. I did have a post which tried to explain the mechanism giving the vertical effect of a cross wind, I could look it up if it is of interest.

Thx Bb, I can't remember where here on AGN I read a post some years ago. And I thought the post was put forth well and made sense well enough to stick a picture (in mind), the flight of the pellet reacting to this phenomena. And watch as that plays out and into the impacts on a 5''x5'' paper target.

 

[Ballisticboy]

That’s friicking awesome but do you have time to shoot?

Not with the 70mph winds and rain we have at the moment.

 

[Revoman]

Ballisticboy --=--
I thoroughly enjoy all of your posts. They are highly educational and well thought out. I take verbatim, your information as it all makes sense to me (plus, you are an expert in this field).
While I do have to reread some of your posts to get it all to sink in, they are extremely interesting.
Thank you for your contributions to make my shooting experience better and understanding of what my particular ammunition might be doing on its short flight.
Mike

 

[nomojo65]

Does the speed of rotation affect the reaction of these forces on the pellets and if so how? and also eccentricities of the pellet? ( maybe eccentricity isn’t the proper definition of what I’m asking? )

 

[Troy Bakel]

The Internet is a vast intellectual waste land of beliefs polluted here and there by actual knowledge.
Knowledge is based on validated facts, the exercise of knowledge is wisdom.
Belief is based on whatever your vanity has convinced your ego is true regardless of facts, the exercise of belief is faith, faith that your vanity has a greater than 0% probability of being true.

@Ballisticboy , thank you for your knowledge.

 

[Ballisticboy]

Does the speed of rotation affect the reaction of these forces on the pellets and if so how? and also eccentricities of the pellet? ( maybe eccentricity isn’t the proper definition of what I’m asking? )

The speed of rotation relative to the forward velocity affects the way the pellet reacts, as it changes the balance between the inertial and aerodynamic moments about the centre of gravity. The changes in the relative moments affect the pellet yaw wave lengths, which can give problems at long ranges. The only force the rotation speed can effect is the spin damping force, which will change the spin damping moment and the rate at which the spin is slowing down.

High spin rates in general reduce the effects of any pellet asymmetries once the pellet has left the barrel, in that any forces created by the asymmetry are constantly changing direction, thus creating more of a wobble rather than steering the pellet in one direction. As the pellet leaves the barrel however, high spin rates can make the effects of some asymmetries worse by giving high yaw rates.

 

[Star7]

Thank you much for this illumination. My background in fluids is for visco-elastic systems and this is all new to me. Can you comment on why some pellets are built with longitudinal striations (e.g. superdomes)? Is that for manufacturing or is there a drag related reason?

 

[Leo C]

感谢分享

 

[Ballisticboy]

Thank you much for this illumination. My background in fluids is for visco-elastic systems and this is all new to me. Can you comment on why some pellets are built with longitudinal striations (e.g. superdomes)? Is that for manufacturing or is there a drag related reason?

Unless I am missing something obvious, I cannot see any real aerodynamic advantage to the ribs on some pellet flares. There may be structural advantages in reducing flare mass and keeping the CG forward, there may even be a small increase in spin damping but not enough to make a significant difference to the pellet behaviour. Much of the flare lies in the wake of the pellet head, so there won't be much change in drag with or without the ribs.

 

[dgeesaman]

I always assumed those driving band features were to better engage and center the pellet within the rifling.

We might also consider that design is decades old and may have served a purpose that's no longer significant.

I'm often tempted to believe that pellet designs are all designed for maximum accuracy based on extensive design cycles but the reality is many are sold based on looks and the accuracy difference (loss) is not noticed.

 

[nervoustrig]

The feature Star7 and Miles are discussing are ribs running lengthwise as opposed to ribs around the circumference (e.g. driving bands).

 

[Star7]

Those lengthwise bands may be like the design of fishing lures. Intended to catch the wallet not the fish.

 

[Caliber 22]

High spin rates in general reduce the effects of any pellet asymmetries once the pellet has left the barrel, in that any forces created by the asymmetry are constantly changing direction, thus creating more of a wobble rather than steering the pellet in one direction. As the pellet leaves the barrel however, high spin rates can make the effects of some asymmetries worse by giving high yaw rates.

These two highlighted sentences seems contradictory to each other.
What do I miss here?

 

[Ballisticboy]

These two highlighted sentences seems contradictory to each other.
What do I miss here?

As it says, one is after the pellet has left the barrel, i.e. the pellet is flying in free air and not touching the barrel anywhere. The other is as the pellet leaves the barrel, i.e. the back of the pellet is still in the barrel, but the front of the pellet has left the barrel.

 

[Caliber 22]

So if the 2 bearing surfaces of a pellet is 4mm apart and it leave the muzzle at 900fps, it takes 0.000015 seconds from the head to skirt contact surface. In that short time it can have an influence on stability. WOW. And if the crown is not perfect the stability is even worse.

 

[Caliber 22]

My rifle's barrel has a 1:18 twist rate. That means in 457.2mm the pellet rotates once. Do some calculations and find that in the 4mm from head bearing to skirt bearing it rotates 0.00875 times. With the 5.52mm diameter and a perimeter of 17.341mm of the pellet it means the rotational distance of a point on the perimeter is 0.152mm. That is less than the thickness of 2 x printer papers together.

 

[Ballisticboy]

It is the yawing rate that is produced in the pellet which is the problem, not how much yaw angle is produced.

 

[Ignoto]

Unfortunately, there are a lot of myths in shooting, some of them often repeated by people you would hope would know better. Perhaps the most common are (...) that a cross wind blows on the side of a pellet.

Talking about lag time? Care to elaborate more to the curious ones starving for more info?

 

[Ballisticboy]

Talking about lag time? Care to elaborate more to the curious ones starving for more info?

The lag time was explained in this original post https://www.airgunnation.com/threads/how-wind-causes-pellets-to-drift.1278969/ where its relationship to down wind drift was also explained. Why the wind does not blow on the side of a pellet was also shown in the diagrams.