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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
enviromental effects on trajectory
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<blockquote data-quote="Ivanhoe" data-source="post: 22558" data-attributes="member: 665"><p>There are 3 atmospheric factors affecting aerodynamics; density, viscosity, and sound speed. Density is literally mass per unit volume, i.e. kilograms per cubic meter (or slugs per cubic foot, slugs being what engineers use for mass in the English system). Viscosity is resistance to shear; even gasses have a little bit of viscosity. Sound speed is just that, the speed of a sound wave thru the gas. I believe density is the primary factor affecting velocity loss, sound speed 2nd, viscosity 3rd.</p><p></p><p>And velocity is what you've got to worry about, as it is the dominant thing that affects trajectory.</p><p></p><p>Density is a function of temperature, pressure, and gas composition. Density mainly reduces with increasing altitude due to lower pressure. Believe it or not, humid air is <strong>less</strong> dense than dry air at the same temperature and pressure. Each water molecule takes up a lot more volume than the nitrogen or oxygen molecule that it replaced. And higher density means higher drag. </p><p></p><p>As a way to keep this stuff straight, how much aerodynamic drag is there in outer space (i.e. a vacuum with zero density)? Zero drag. How much drag is there on bullets traveling into water (much higher density)? Lots. Therefore increasing density means increasing drag.</p><p></p><p>Sound speed is dependent on temperature and gas composition, with increasing temperature causing an increase in sound speed. Sound speed affects the strength of the shock waves created by the bullet (and thus the wave drag on the bullet), can't remember offhand if a higher sound speed means higher or lower wave drag. </p><p></p><p>Dont' forget that higher temperatures generally mean higher bore pressures, which means higher muzzle velocity, which is the dominant effect on downrange trajectory.</p><p></p><p>Darryl's description if humid air not rising is incorrect. If humid air didn't rise, we would never have thunderstorms or tornadoes. There are circumstances in which there isn't much vertical movement of air, the temperature inversion layers over L.A., Phoenix, and Denver being good examples, but this is not a humidity thing. The effect of natural convection (i.e. the warm ground heating the lowest air layer which then rises) is probably minor compared to changes in muzzle velocity and air density.</p></blockquote><p></p>
[QUOTE="Ivanhoe, post: 22558, member: 665"] There are 3 atmospheric factors affecting aerodynamics; density, viscosity, and sound speed. Density is literally mass per unit volume, i.e. kilograms per cubic meter (or slugs per cubic foot, slugs being what engineers use for mass in the English system). Viscosity is resistance to shear; even gasses have a little bit of viscosity. Sound speed is just that, the speed of a sound wave thru the gas. I believe density is the primary factor affecting velocity loss, sound speed 2nd, viscosity 3rd. And velocity is what you've got to worry about, as it is the dominant thing that affects trajectory. Density is a function of temperature, pressure, and gas composition. Density mainly reduces with increasing altitude due to lower pressure. Believe it or not, humid air is [B]less[/B] dense than dry air at the same temperature and pressure. Each water molecule takes up a lot more volume than the nitrogen or oxygen molecule that it replaced. And higher density means higher drag. As a way to keep this stuff straight, how much aerodynamic drag is there in outer space (i.e. a vacuum with zero density)? Zero drag. How much drag is there on bullets traveling into water (much higher density)? Lots. Therefore increasing density means increasing drag. Sound speed is dependent on temperature and gas composition, with increasing temperature causing an increase in sound speed. Sound speed affects the strength of the shock waves created by the bullet (and thus the wave drag on the bullet), can't remember offhand if a higher sound speed means higher or lower wave drag. Dont' forget that higher temperatures generally mean higher bore pressures, which means higher muzzle velocity, which is the dominant effect on downrange trajectory. Darryl's description if humid air not rising is incorrect. If humid air didn't rise, we would never have thunderstorms or tornadoes. There are circumstances in which there isn't much vertical movement of air, the temperature inversion layers over L.A., Phoenix, and Denver being good examples, but this is not a humidity thing. The effect of natural convection (i.e. the warm ground heating the lowest air layer which then rises) is probably minor compared to changes in muzzle velocity and air density. [/QUOTE]
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