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All Springer/NP/PCP Air Gun Discussion General > "Bob and Lloyds Workshop"

External Ballistics of Pellets by Ballistician Miles Morris

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rsterne:
By TPL (Timo)


--- Quote from: ballisticboy on February 26, 2020, 09:57:53 AM ---This is complete bunkum based on a total lack of knowledge on the basics of aerodynamic stability. It also fails to explain why a wadcutter pellet is apparently still stable despite the vast majority of its drag being at the front rather than the back of the pellet.

--- End quote ---
I made a pair of pictures based to same diameter same length bullet by Kolbe's calculator which I have tested in practic and found to be spot on by few %. We must call this slug because Kolbe does not include pellet form. But anyway there is something interesting for this particular question.

First picture is sharp nose blunt base bullet and what interest us the most is subsonic speed.



...compared to blunt nose boat tail bullet of same size.



Remarkable difference in BC and drag. I'm sure you as a ballistic expert can explain this to me.

rsterne:
By MJP (Marko)....

Oh man, your static force picture is way out. First of all the normal force is pointed inward from the arc of flight, and so is gravity, you have one force forward and that is the momentum of the pellet, in tangent of the arc of flight.
Then there is air friction slowing the pellet down force vector pointing straight against from the direction of movement. From your picture I would say the pellet has no mass and it would just float to space.

Marko

rsterne:
By superchikn (Ray).

My head hurts!

rsterne:
By ballisticboy (Miles).

Not sure which picture it is you appear to be miss-understanding bit I will try to work through your comments.


--- Quote from: MJP on February 26, 2020, 12:05:09 PM ---Oh man, your static force picture is way out. First of all the normal force is pointed inward from the arc of flight, and so is gravity, you have one force forward and that is the momentum of the pellet, in tangent of the arc of flight.
--- End quote ---

The normal force on a projectile is defined as any force at 90 degrees to the projectile axis so it can be up, down to either side, any direction as long as it is at 90 degrees to the central axis based on the projectile axis based system. Normal forces are usually produced by the projectile being at an angle of yaw to the airflow with the force being in the same direction as the yaw. Momentum is not a force, it is normally taken as mass times velocity. The diagram is of aerodynamic forces. Projectile weight from the acceleration due to gravity is not included as it is not an aerodynamic force and makes no contribution to the stability moments since by definition it acts through the centre of gravity.


--- Quote from: MJP on February 26, 2020, 12:05:09 PM ---Then there is air friction slowing the pellet down force vector pointing straight against from the direction of movement. From your picture I would say the pellet has no mass and it would just float to space.

Marko

--- End quote ---

Air friction is only one small part of the total projectile drag force which consists mainly of form drag, friction drag, base drag (the main contributor in many but not all pellets) and yaw induced drag. Again, because the line of action of the drag force acts through or very close to the centre of gravity, it produces negligible stabilising moments. I am not sure why you want mass in a diagram of aerodynamic forces, particlarly as mass is not a force and is irrelevant to stability having no moment about the centre of gravity.

Perhaps I did not explain things clearly enough in the OP.

rsterne:
By ballisticboy (Miles).


--- Quote from: TPL on February 26, 2020, 11:49:42 AM ---I made a pair of pictures based to same diameter same length bullet by Kolbe's calculator which I have tested in practic and found to be spot on by few %. We must call this slug because Kolbe does not include pellet form. But anyway there is something interesting for this particular question.

First picture is sharp nose blunt base bullet and what interest us the most is subsonic speed.

...compared to blunt nose boat tail bullet of same size.

Remarkable difference in BC and drag. I'm sure you as a ballistic expert can explain this to me.

--- End quote ---

I would suggest that you are attempting to use the Kolbe calculator way outside its limits of applicability. I believe it is based on the Bob McCoy equations and the drag calculation is probably the same. Bob made the assumption that a flat meplat makes no contribution to drag in the McDrag software. This was a perfectly reasonable assumption for the shell and bullet meplats the program was designed for. However, when meplats start to get larger than 10-15% of calibre then the errors start to get large and when it is as big as in your example then it is completely wrong. The program is assuming the flat front face makes no contribution to the drag which is obviously wrong. I know about Bob McCoys software as we exchanged aerodynamic prediction programs when we were both working for our respective government departments, that is how RARDLIFT ended up on the JBM website.

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