TargetTalk

I am searching for pictures and/or digital video pictures that show bullet precession and nutation - to show to my shooters.
Can anyone direct me to such pictures/info?

I knew this was out there but had a heck of a time finding it again. The web has filled with a lot of garbage over the years. The main site is

http://www.nennstiel-ruprecht.de/bullfl ... tm#Figures

Pictures, diagrams, and calculations. The yaw of a 38 wadcutter is very interesting. I imagine that the 32 has a similar corkscrew flight. Distance from the target does make a difference.

http://www.nennstiel-ruprecht.de/bullfly/fig26.htm

Larry

Larry:

We run into this confusion between direction and magnitude of yaw angle vs flight path over the years in various discussion forums. Particularly on the rifle side, you will find people who swear that they can actually see the bullet follow a corkscrew path (when humidity etc. is just right).

While it is true that a bullet will precess and nutate along its flight path, the oscillation in alignment does not translate into a corkscrew line of flight. The aerodynamic center of the bullet will revolve around the center of mass, and both will orbit around the line of flight.

Picture it this way: The line of flight describes an arc, and the bullet will track along this arc while the nose may point off the arc. the bullet itself never actually leaves the arc itself however. It may wobble around it (within the radius of its caliber), but never actaully "corkscrews."

Or I could be wrong- but all of the data I have seen about this "corkscrew" phenomenon refers to the direction teh nose is pointed, not where the bullet itself actually goes. A bullet jsut doesn't generate enough aerodynamic forces to overcome the ballistic forces.

Steve "Wrong Before, Could Be Wrong Again" Swartz

Steve -

Good to hear from you. I hope all is still improving with your shooting.

I have no particular attachment to the information pointed to by the links. It looks like it could be at least a partial explanation for why the 32 isn't very good at 50 yards.

The author of the paper seems to have had it reviewed by someone in academia and uses data from the German military and others. The references are impressive, but I don't have the time or interest to check them out.

Larry

Larry:

Thanks- yes I 've been struggling lately (and life sometimes intrudes!) but still hanging in there.

All of the information on the site is accurate (great site by the way- thanks for makign us all aware of it! Great stuff for the "engineering inclined" in the forum).

I don't have a quibble with either the site or what you posted . . . I was just piggybacking on what you said to make a related point about the general misinterpretation of those diagrams.

And yes, we have all seen the 38 and 32 "wobble" imprinted on the paper at even 25 meters! The little "half moon" smudge around the hole shows us how the base is tilted off-axis when it hits the paper. The most accurate 32 load i have consistently prints a very clear clear hal-moon (oddly enough) at 5:00 on the target- whatever that means!

But back t my related-but-not-exactly-on-topic last post. Some people see the "corkscrew" described by the nose of the bullet and think it kmeans the flight path of the bullet is actually corkscrewing, when it isn't. And yes, I have seen the corkscrew vapor trail effect in the early morning on the 1,000 yard line at Camp Perry- very impressive "fishtail" produced by some bullets!

So it's easy to see how the folklore of the corkscrew flight path gets generated and sustained. The center of mass of the bullet pretty much tracks along the "straight arc" [sic] of the flightpath1 , while the nose and tail wobble around it . . . if that description makes sense . . . anyhow, it's just another one of those prevalent myths of shooting that end up being more trouble to explain than they are worth!

Steve

1actually the centroid of the force couple of the c.o.m. vs. aerodynamic center; but both are well inside the bullet body itself

(ps to read the corkscrew graph you just need to realize the axis perpendicular to the flight path is not measured in distance- the units are degrees and direction of the axis of the bullet vs. the flight path . . . so it is showing the "wobble" not "position" of the bullet.)

Thanks everyone for the comments.
The way I see it, the centerpoint of mass can only change position when force from the outside is applied. There are two options:
1 the cp of mass folloes the arc and the tip of the bullet wobbles (a combination of precession and nutation)
2 Like a gyro spinning on a surface the cp of mass follows a circle because of the gyroscopic effect and the friction between gyro and surface. But to my knoledge there is not enough friction between a bullet and air to cause the bullet to spin in a corckscrew around the path of the bullet. Therefor the bullet will wobble around its cp of mass.
But, just for the sake of dicussion, the cp of mass would be positioned at the outer edge of the bullet (the circomfgerence) then the bullet would rotate around this point and it would be seen as a ccorckscrew motion with the outer edge of the bullet as its centerpoint.
Am I correct?

Albert B
The Netherlands

Yes- research (somewhat scientific) done in the late 1800s kind of started this mess- shooting "minnie ball" type rifle ammo with paper screens at v arious distances.

The intent of the study was to measure the disruption to the flight path of various boards/planks placed very close to the flight path of the bullet; an early "aerodynamic turbulence" kind of investigation.

Serendipitously, the cast projectiles were prone to "voids" and had centers of mass significantty offset from the center of rotation of the bullet.

Of course, the author of the study wasn't aware of this initially. What he *saw* was that the flight path of the .50" diameter bullet would "sway left to right" about +-0.2" along it's path- and he was finally able to see the "sway" in all directions.

He then deduced that the bullets were indeed "wobbling" by about half the bullet diameter (in the most extreme cases) around the average flight path.

The author of the study then "spin tested" the projectiles in a smooth bowl and began using only "rotationally balanced" projectiles. The wobble-around-axis went away, and he then noticed that the holes in the paper were oblong, with the axis of the oblong pointed in various directions along the flight path.

Without elaboration, he ascribed this to a "front-back upset" (imbalance) of the projectile and left it at that.

Oh by the way- no, shooting next to boards/planks/structures did not have any noticeable effect on the bullets!

I will dig up the cite of this study at some point if you are interested. The study I refer to is the study most commonly cited when someone insists that projectiles follow a corkscrew path through the air. I finally found a copy of the study (interlibrary loan thorugh Michigan State University) back in 1996 and it was a very interesting book to read.

[Even more interesting to me, a student of shooting folklore/mythology, was how many times very experienced people referred to this study as "proof" of something the study did not find! As with my day job (college professor) it is fascinating how a study will be referenced over and over again- apparently withoutr anyone going back and reading the actual study- until the meaning is changed significantly! Re: the "Hawthorne Study" from org behavior. We are all taught it found "X" when the actual findings are quite different!]

We argued all of this out in the "rec.guns" newsgroup around 1996-1997 and the threads may even still be retrievable.

Thankfully, most people just don't give a rip about stuff like this . . .

Steve

Mann, F.W. (1909) The Bullet's Flight- The Ballistics of Small Arms. Pub by Author, 1909; later reprint 1942 Huntington, West Virginia: Standard Printing and Publishing.

Correction and addendum: They weren't Minnie Balls, they were hollow-base wadcutters. Also, forgot about the other main factor manipulated in the experiment (beside placement of the wooden planks along the line of flight); the author radically deformed the skirts of the hollow bases in order to "juice up" the interaction between planks and bullet (to no great effect).

The whole thing about the bullet wobble was pretty much a pesky side effect. And that turned into the "main [misunderstood] finding!"

Steve