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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
Bullet lift, does it exist?
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<blockquote data-quote="bwaites" data-source="post: 176829" data-attributes="member: 8733"><p>Another thread posed an interesting question, but that thread was not the appropriate place to discuss it and I apologize for cluttering the thread!</p><p> </p><p>Does bullet lift exist? Evidently Exbal and some of the the other ballistics programs seem to demonstrate that drop of some bullets is faster than similarly shaped other bullets.</p><p> </p><p>My admittedly decades old physics work didn't have a place for that, so I challenged the premise but several members pointed out that I was wrong. </p><p> </p><p>I've done some preliminary google searches and found three different sources who discount bullet lift: The Department of Energy being the most recognized and the first sited. Even Wikipedia has something to say on the subject, although I recognize Wikipedia may not be the most authoritative site.</p><p> </p><p>So here are the quotes, the first from the DOE's "Ask a Scientist" site.</p><p> </p><p><span style="font-family: 'Times New Roman'">Question - Our students claim that when a bullet emerges from a horizontal rifle, it can </span><span style="font-family: 'Times New Roman'">take a rising path due to the spinning of the bullet. Is there a way for a bullet to </span><span style="font-family: 'Times New Roman'">rise? We think that Bernoulli effect is minimal to null in this situation.</span><span style="font-family: 'Times New Roman'">-----------------</span><span style="font-family: 'Times New Roman'">Nathan</span></p><p> </p><p><span style="font-family: 'Times New Roman'">-</span><span style="font-family: 'Times New Roman'">A real bullet shaped projectile.... no. But a round projectile... maybe.</span><span style="font-family: 'Times New Roman'">In aerodynamics there is an effect known as the Magnus Effect. It would likely not apply </span><span style="font-family: 'Times New Roman'">to a bullet because of its shape. The bullet would likely continue to fly with the nose</span><span style="font-family: 'Times New Roman'"> forward and the rifling would turn the bullet around its longitudinal (front to rear) </span><span style="font-family: 'Times New Roman'">axis. However, the Magnus Effect could effect a round projectile as fired from a musket </span><span style="font-family: 'Times New Roman'">or an old fashioned cannon ball. This could cause the ball to rise if were rotated </span><span style="font-family: 'Times New Roman'">properly around its lateral (left to right) axis while in flight. (Think of hitting </span><span style="font-family: 'Times New Roman'">a cue ball with back spin on a pool table. That is the kind of spin it would need.)</span><span style="font-family: 'Times New Roman'">I would expect that this effect would be most apparent with a slow-moving, </span><span style="font-family: 'Times New Roman'">light projectile of larger size. Perhaps it could be seen with slow motion </span><span style="font-family: 'Times New Roman'">photography if a Polystyrene foam ball were pitched with a great deal of back spin. </span><span style="font-family: 'Times New Roman'">I really do not know if it would be significant enough to measure, but it might be </span><span style="font-family: 'Times New Roman'">fun to try.</span><span style="font-family: 'Times New Roman'">Larry Krengel</span><span style="font-family: 'Times New Roman'">=====================================================</span></p><p> </p><p><span style="font-family: 'Times New Roman'">No way.</span><span style="font-family: 'Times New Roman'">The fact that the bullet is spinning means that, on average, it's</span><span style="font-family: 'Times New Roman'">symmetrical about its line of flight, so why would any force exerted</span><span style="font-family: 'Times New Roman'">by air cause it to rise, rather than, say, go to the left? The only</span><span style="font-family: 'Times New Roman'">thing I can think of that would break the symmetry, and result in a</span><span style="font-family: 'Times New Roman'">preferred direction, would be *very* slightly higher air density below</span><span style="font-family: 'Times New Roman'">the bullet than above it, but this difference seems utterly negligible.</span><span style="font-family: 'Times New Roman'">I don't believe a bullet fired horizontally actually rises, or even</span><span style="font-family: 'Times New Roman'">falls more slowly than it would if simply dropped. However, I do</span><span style="font-family: 'Times New Roman'">believe a rifle's recoil could cause it to rotate about its center of</span><span style="font-family: 'Times New Roman'">mass so that the barrel tips upward slightly while the bullet is still</span><span style="font-family: 'Times New Roman'">within the barrel.</span><span style="font-family: 'Times New Roman'">Tim Mooney</span></p><p> </p><p><span style="font-family: 'Times New Roman'"><strong>The second from a blogger who is interested in guns:</strong></span></p><p> </p><p><span style="color: #333333"><span style="font-family: 'Times New Roman'">In external ballistics a number of forces act on bullet flight, making for a royally nasty set of differential equations to solve. The basics, such as gravity and aerodynamic drag (including crosswinds), are conceptually pretty straightforward. One that I find interesting for some strange reason is the Magnus effect. The magnus effect is what makes curve balls curve. Topspin on a ball makes it fall faster than it otherwise would (provides "lift" downward), backspin makes it fall more slowly than it otherwise would (provides "lift" upward).</span></span></p><p> </p><p><span style="font-family: 'Times New Roman'"><span style="color: #333333">Things get interesting when dealing with bullets. Since they spin around the axis of travel, the aerodynamic forces caused by the spin cancel each other out. However, add in a crosswind, and things get interesting.</span></span></p><p> </p><p><span style="font-family: 'Times New Roman'"><span style="color: #333333">Take your standard bullet fired from a right-hand twist barrel. (Imagine a barrel, take your right thumb, point it along the direction of your imaginary barrel, and curl your fingers toward your palm. The bullet, which is traveling in the direction of your thumb, spins in the direction your fingers curl.) Looking from behind the bullet, the spin appears clockwise. (The right side is moving down, the left side is moving up.)</span></span></p><p> </p><p><span style="font-family: 'Times New Roman'"><span style="color: #333333">Now, add in a cross wind coming from the right.</span></span></p><p> </p><p><span style="font-family: 'Times New Roman'"><span style="color: #333333">The wind hitting the bullet from the right sees topspin. The right side of the bullet is spinning down, creating "lift" to push the bullet downwards. (like a curveball) This causes the bullet to fall faster than it otherwise would, causing the bullet to hit the target lower than it otherwise would. opposite effect. The spin creates lift, and the bullet falls more slowly than it otherwise would, causing the bullet to hit the target higher.</span></span></p><p> </p><p><span style="font-family: 'Times New Roman'"><span style="color: #333333">The magnus effect on a normal bullet is small, small enough that it is almost always disregarded in small arms ballistic calculations, (Field artillery is another mater.) but, for some horribly geeky reason, I find it rather interesting.</span></span></p><p> </p><p></p><p><span style="color: #333333"><span style="font-family: 'Times New Roman'"><strong>And finally, Wikipedia:</strong></span></span></p><p> </p><p><span style="font-family: 'Times New Roman'"><span style="color: black">The Magnus effect in external ballistics, also known as 'spin drift'</span></span></p><p> </p><p><span style="color: black"><span style="font-family: 'Times New Roman'">Another context where the Magnus effect can be found is advanced </span><a href="http://en.wikipedia.org/wiki/External_ballistics" target="_blank"><span style="font-family: 'Times New Roman'"><span style="color: #002bb8">external ballistics</span></span></a><span style="font-family: 'Times New Roman'">. A spinning bullet in flight is often subject to a sideways wind. In the simple case of horizontal wind, depending on the direction of rotation, the Magnus effect causes an upward or downward force to act on the projectile, affecting its point of impact. Even in a complete calm, with no sideways air movement at all, a real bullet will experience a small sideways wind component. This is because real bullets have a </span><a href="http://en.wikipedia.org/wiki/Flight_dynamics" target="_blank"><span style="font-family: 'Times New Roman'"><span style="color: #002bb8">yaw</span></span></a><span style="font-family: 'Times New Roman'"> motion that causes the nose of the bullet to point in a slightly different direction from the direction in which the bullet is actually traveling. This means that the bullet is "skidding" sideways at any given moment, and thus experiences a small sideways wind component.</span><a href="http://www.nennstiel-ruprecht.de/bullfly/longr.htm#header_longranges" target="_blank"><span style="font-family: 'Times New Roman'"><span style="color: #002bb8">(yaw of repose)</span></span></a><span style="font-family: 'Times New Roman'"> All in all, the effect of the Magnus force on a bullet is not significant when compared to other forces like drag. However, the Magnus effect has a significant role in bullet stability because the Magnus force <em>does not</em> act upon the bullet's center of gravity, but the center of pressure. This means that there is a Magnus force that affects the <em>yaw</em> of the bullet. The Magnus effect will act as a <em>destabilizing</em> force on any bullet with a center of pressure located <em>ahead</em> of the center of gravity, while conversely acting as a <em>stabilizing</em> force on any bullet with the center of pressure located <em>behind</em> the center of gravity. The location of the center of pressure depends on the flowfield structure, in other words, depending on whether the bullet is in super-sonic or sub-sonic flight. What this means in practice depends on the shape and other attributes of the bullet, in any case the Magnus force greatly affects stability because it tries to "twist" the bullet along its flight.</span></span></p></blockquote><p></p>
[QUOTE="bwaites, post: 176829, member: 8733"] Another thread posed an interesting question, but that thread was not the appropriate place to discuss it and I apologize for cluttering the thread! Does bullet lift exist? Evidently Exbal and some of the the other ballistics programs seem to demonstrate that drop of some bullets is faster than similarly shaped other bullets. My admittedly decades old physics work didn't have a place for that, so I challenged the premise but several members pointed out that I was wrong. I've done some preliminary google searches and found three different sources who discount bullet lift: The Department of Energy being the most recognized and the first sited. Even Wikipedia has something to say on the subject, although I recognize Wikipedia may not be the most authoritative site. So here are the quotes, the first from the DOE's "Ask a Scientist" site. [FONT=Times New Roman]Question - Our students claim that when a bullet emerges from a horizontal rifle, it can [/FONT][FONT=Times New Roman]take a rising path due to the spinning of the bullet. Is there a way for a bullet to [/FONT][FONT=Times New Roman]rise? We think that Bernoulli effect is minimal to null in this situation.[/FONT][FONT=Times New Roman]-----------------[/FONT][FONT=Times New Roman]Nathan[/FONT] [FONT=Times New Roman]-[/FONT][FONT=Times New Roman]A real bullet shaped projectile.... no. But a round projectile... maybe.[/FONT][FONT=Times New Roman]In aerodynamics there is an effect known as the Magnus Effect. It would likely not apply [/FONT][FONT=Times New Roman]to a bullet because of its shape. The bullet would likely continue to fly with the nose[/FONT][FONT=Times New Roman] forward and the rifling would turn the bullet around its longitudinal (front to rear) [/FONT][FONT=Times New Roman]axis. However, the Magnus Effect could effect a round projectile as fired from a musket [/FONT][FONT=Times New Roman]or an old fashioned cannon ball. This could cause the ball to rise if were rotated [/FONT][FONT=Times New Roman]properly around its lateral (left to right) axis while in flight. (Think of hitting [/FONT][FONT=Times New Roman]a cue ball with back spin on a pool table. That is the kind of spin it would need.)[/FONT][FONT=Times New Roman]I would expect that this effect would be most apparent with a slow-moving, [/FONT][FONT=Times New Roman]light projectile of larger size. Perhaps it could be seen with slow motion [/FONT][FONT=Times New Roman]photography if a Polystyrene foam ball were pitched with a great deal of back spin. [/FONT][FONT=Times New Roman]I really do not know if it would be significant enough to measure, but it might be [/FONT][FONT=Times New Roman]fun to try.[/FONT][FONT=Times New Roman]Larry Krengel[/FONT][FONT=Times New Roman]=====================================================[/FONT] [FONT=Times New Roman]No way.[/FONT][FONT=Times New Roman]The fact that the bullet is spinning means that, on average, it's[/FONT][FONT=Times New Roman]symmetrical about its line of flight, so why would any force exerted[/FONT][FONT=Times New Roman]by air cause it to rise, rather than, say, go to the left? The only[/FONT][FONT=Times New Roman]thing I can think of that would break the symmetry, and result in a[/FONT][FONT=Times New Roman]preferred direction, would be *very* slightly higher air density below[/FONT][FONT=Times New Roman]the bullet than above it, but this difference seems utterly negligible.[/FONT][FONT=Times New Roman]I don't believe a bullet fired horizontally actually rises, or even[/FONT][FONT=Times New Roman]falls more slowly than it would if simply dropped. However, I do[/FONT][FONT=Times New Roman]believe a rifle's recoil could cause it to rotate about its center of[/FONT][FONT=Times New Roman]mass so that the barrel tips upward slightly while the bullet is still[/FONT][FONT=Times New Roman]within the barrel.[/FONT][FONT=Times New Roman]Tim Mooney[/FONT] [FONT=Times New Roman][B]The second from a blogger who is interested in guns:[/B][/FONT] [COLOR=#333333][FONT=Times New Roman]In external ballistics a number of forces act on bullet flight, making for a royally nasty set of differential equations to solve. The basics, such as gravity and aerodynamic drag (including crosswinds), are conceptually pretty straightforward. One that I find interesting for some strange reason is the Magnus effect. The magnus effect is what makes curve balls curve. Topspin on a ball makes it fall faster than it otherwise would (provides "lift" downward), backspin makes it fall more slowly than it otherwise would (provides "lift" upward).[/FONT][/COLOR] [FONT=Times New Roman][COLOR=#333333]Things get interesting when dealing with bullets. Since they spin around the axis of travel, the aerodynamic forces caused by the spin cancel each other out. However, add in a crosswind, and things get interesting.[/COLOR][/FONT] [FONT=Times New Roman][COLOR=#333333]Take your standard bullet fired from a right-hand twist barrel. (Imagine a barrel, take your right thumb, point it along the direction of your imaginary barrel, and curl your fingers toward your palm. The bullet, which is traveling in the direction of your thumb, spins in the direction your fingers curl.) Looking from behind the bullet, the spin appears clockwise. (The right side is moving down, the left side is moving up.)[/COLOR][/FONT] [FONT=Times New Roman][COLOR=#333333]Now, add in a cross wind coming from the right.[/COLOR][/FONT] [FONT=Times New Roman][COLOR=#333333]The wind hitting the bullet from the right sees topspin. The right side of the bullet is spinning down, creating "lift" to push the bullet downwards. (like a curveball) This causes the bullet to fall faster than it otherwise would, causing the bullet to hit the target lower than it otherwise would. opposite effect. The spin creates lift, and the bullet falls more slowly than it otherwise would, causing the bullet to hit the target higher.[/COLOR][/FONT] [FONT=Times New Roman][COLOR=#333333]The magnus effect on a normal bullet is small, small enough that it is almost always disregarded in small arms ballistic calculations, (Field artillery is another mater.) but, for some horribly geeky reason, I find it rather interesting.[/COLOR][/FONT] [FONT=Times New Roman][/FONT] [COLOR=#333333][FONT=Times New Roman][B]And finally, Wikipedia:[/B][/FONT][/COLOR] [FONT=Times New Roman][COLOR=black]The Magnus effect in external ballistics, also known as 'spin drift'[/COLOR][/FONT] [COLOR=black][FONT=Times New Roman]Another context where the Magnus effect can be found is advanced [/FONT][URL="http://en.wikipedia.org/wiki/External_ballistics"][FONT=Times New Roman][COLOR=#002bb8]external ballistics[/COLOR][/FONT][/URL][FONT=Times New Roman]. A spinning bullet in flight is often subject to a sideways wind. In the simple case of horizontal wind, depending on the direction of rotation, the Magnus effect causes an upward or downward force to act on the projectile, affecting its point of impact. Even in a complete calm, with no sideways air movement at all, a real bullet will experience a small sideways wind component. This is because real bullets have a [/FONT][URL="http://en.wikipedia.org/wiki/Flight_dynamics"][FONT=Times New Roman][COLOR=#002bb8]yaw[/COLOR][/FONT][/URL][FONT=Times New Roman] motion that causes the nose of the bullet to point in a slightly different direction from the direction in which the bullet is actually traveling. This means that the bullet is "skidding" sideways at any given moment, and thus experiences a small sideways wind component.[/FONT][URL="http://www.nennstiel-ruprecht.de/bullfly/longr.htm#header_longranges"][FONT=Times New Roman][COLOR=#002bb8](yaw of repose)[/COLOR][/FONT][/URL][FONT=Times New Roman] All in all, the effect of the Magnus force on a bullet is not significant when compared to other forces like drag. However, the Magnus effect has a significant role in bullet stability because the Magnus force [I]does not[/I] act upon the bullet's center of gravity, but the center of pressure. This means that there is a Magnus force that affects the [I]yaw[/I] of the bullet. The Magnus effect will act as a [I]destabilizing[/I] force on any bullet with a center of pressure located [I]ahead[/I] of the center of gravity, while conversely acting as a [I]stabilizing[/I] force on any bullet with the center of pressure located [I]behind[/I] the center of gravity. The location of the center of pressure depends on the flowfield structure, in other words, depending on whether the bullet is in super-sonic or sub-sonic flight. What this means in practice depends on the shape and other attributes of the bullet, in any case the Magnus force greatly affects stability because it tries to "twist" the bullet along its flight.[/FONT][/COLOR] [/QUOTE]
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