Aerodynamic Drag and Gyroscopic Stability

[Michael Courtney]

I thought the readership here might appreciate a recent article in Target Shooter Online:

http://www.targetshooter.co.uk/april-13-issue.html#ballistics

Abstract: This paper describes the effects on aerodynamic drag of rifle bullets as the gyroscopic stability is lowered from 1.3 to 1.0. It is well known that a bullet can tumble for stability less than 1.0. The Sierra Loading Manuals (4th and 5th Editions) have previously reported that ballistic coefficient decreases significantly as gyroscopic stability, Sg, is lowered below 1.3. These observations are further confirmed by the experiments reported here. Measured ballistic coefficients were compared with gyroscopic stabilities computed using the Miller Twist Rule for nearly solid metal bullets with uniform density and computed using the Courtney-Miller formula for plastic-tipped bullets. The experiments reported here also demonstrate a decrease in aerodynamic drag near Sg = 1.23 ± 0.02. It is hypothesized that this decrease in drag over a narrow band of Sg values is due to a rapid damping of coning motions (precession and nutation). Observation of this drag decrease at a consistent value of Sg demonstrates the relative accuracy of the twist formulas used to compute Sg. The relationship between Sg and drag may be used to test the applicability of existing twist formulas to given bullet designs and to evaluate the accuracy of alternate formulas in cases where the existing twist formulas are not as accurate.

Keywords: bullet stability, Miller Twist Rule, plastic-tipped bullets, ballistic coefficient

 

[CatShooter]

While I don't disagree with any observed changes in BC (it's hard to disagree with what been observed, assuming that the observations are correct)...

.. I would disagree with the assumptions of the cause.

Though sometimes blamed for unrelated observations, Nutation does not occur with spinning projectiles such as bullets....

... and precession, once started, continues until the bullet is traveling sideways.

The physics of spinning projectiles is a very complex class of physics, and projectiles, once off their original path, never go back to their original path or orientation.

So called "conning motions" are nothing but classical precession, and as such, never dampen (it would be against the laws of physics)... once started, the conning (precession) continues as long as the bullet travels in an atmosphere, and eventually lead to the bullet tumbling.

Once a bullet points off axis, nothing can bring it back on axis.

 

[normmatzen]

FANTASTIC!
There are some folks out there who can make scientific judgments of our sport's favorite "old wives tales!"

I would like to see what you folks think about such items as barrel tuning for long range, what are first order of importance in reloading accuracy and other "givens" such as the need for a precisely machined action for accuracy? Maybe ideas on what bullet parameters are important such as base to ogive, weight etc.

I have my own theories and have not been proven wrong by my shooting.