Aerodynamic Drag and Gyroscopic Stability

[Michael Courtney]

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

April 13 Issue

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

 

[elkaholic]

VERY interesting Michael! Even though you feel there would be a trade off in finding a sweet spot "at long range" it seems to me that there might be some merit for tuning at a given range competition. What do you think? Also, what would be the practical application "yardage wise" for say a .7 b.c. bullet with a mv of 3000' recognizing that bullet composition varies.......Rich

 

[Michael Courtney]

VERY interesting Michael! Even though you feel there would be a trade off in finding a sweet spot "at long range" it seems to me that there might be some merit for tuning at a given range competition. What do you think? Also, what would be the practical application "yardage wise" for say a .7 b.c. bullet with a mv of 3000' recognizing that bullet composition varies.......Rich

You would be "tuning" for a given air density, not just a given range. Changes in humidity, temperature, and air pressure could shift the gyroscopic stability. Also note the increase in the error bars for the BC near the "sweet spot." Trading increased shot to shot variations in BC (+/- 2 to 5%) to gain a slight improvement is a losing proposition, in my opinion.

The main advantage of higher BC is that it delivers greater energy downrange and higher retained velocity to ensure expansion/energy transfer to the target and that higher BC gives the shooter greater forgiveness on errors reading the wind or shifting wind. Thus the benefit from higher BC depends on the wind and the shooter's ability to read and compensate for it.

Another factor to consider is shot to shot variations in BC. I'd prefer a bullet with a BC of 0.600 with a +/-1% shot to shot vatiation (0.594 to 0.606) to a bullet with a BC of 0.700 with a +/-2% shot to shot variation (0.686 to 0.714). Few shooters consider the need for a consistent BC and its relative importance to a high BC.

 

[elkaholic]

Very good and overlooked point concerning consistent b.c. I suspect most of us are seeking that in our group size, but don't fully understand how we achieved it!.........thanks/Rich

 

[royinidaho]

A very great article.

I'll be months digesting the info.

Very impressive work.