We all know that, as soon as it leaves a rifle's barrel, a bullet begins slowing down.



My question: Does the bullet's rotation or spin, imparted by the lands of the rifle barrel, begin slowing down too, just as a spinning top begins slowing down as soon as the force that imparted the spin is removed from the top?

Finn Aargaard did a test in which a bullet in a rifle with a 1-10 twist barrel at a velocity of 3100 fps had a rotational velocity of 223,200 rpm.
At 500 yards the bullet was travelling at 1700fps but had a rotational velocity of 214,270 rpm.

To calculate the RPM of a bullet - RPM = 720 X Muzzle Velocity / Twist Rate.

Considering that total bullet fligh is measured in seconds, the bleed rate for RPM is very low.

Simple physics, my friend!

Unless some bullet manufacturer has secretly invented a perpetual motion device and found a way to install it into a rifle bullet it is logical to assume that both the velocity and the rotational spin will start decreasing the moment the bullet loses the influenece of the foward force of the expanding gas and the rotational effect of the rifling. Obviously, these two effects cease to work on the projectile the moment it leaves the barrel and the constant force of gravity takes over.

Rick

So...does a spinning object in a in a vacuum never stop spinning? If not...what stops it?

Yeah, and I vote we install zero gravity to make it easier to drag 'em out.

H. C.

Sounds like a good idea but remember.........gravity is more than just a good idea....ITS THE LAW!! Careful you don't get caught breaking it by the Gravity Police.

JV,

actually, it IS gravity that slows rotation, as it takes energy to "lift" the rotating side, and the nose has created a supersonic wave... that wave's freq would be greater than the length of the bullet.



yes, the bullet slows in all positive directions... the overarching name is entropy



fantastic picture of hypersonic shockwave









jeffe

Have you ever read Robert A. Rinker's book: "Understanding Firearm Ballistics?" It's published by Mulberry House Publishing and it is great read. Really blows allot of the old theories out of the water with simple real world examples as well as the complex mathematical formulas to prove it on paper.

It makes for great conversation and debate even though the vast majority of it is totally irrelevant in practical terms.

Rick

I've been a stunt man for over 30 years and the "Gravity Police" have busted me (literally) numerous times.



Rick

JV...

lifting side... should be pretty clear... at any given time the bullet, in crosssection, is turning, right?

1/2 of it is turning "towards" the gravity, and 1/2 is rotating away from the gravity...

since there is no such thing as perpetual motion, this equation is unbalanaced, right? unless you are going to tell me the spin imparts lift.

or, are you telling me that a high bc bullet with a higher twist rifle would slow less over time? I would be dumbfounded if a supersonic bullet can even TELL that there is airbehind the shockwave.. or have i been looking at those reloading books pictures too long that i am missing where the supersonic air actually comes back and hits the bullet?

find the pic that shows a high speed photo of a bullet in flight, i think that was a speer book, and look... the air behind the sonic wave has a different fluid funciton that that of subsonic air.

jeffe
jeffe

Jeffeosso,

gravity has nothing to do with the rotation of a bullet, it does not affect it at all. And there is such thing as perpetual motion, if you shoot a bullet into space, it will keep moving and spinning until it strikes another object.

The torques exerted by gravity on both "halves" of the bullet exactly balance each other, there is only a net force downward, which has no effect on the rotation.
You can trust me on that, I have studied physics for several years. Sorry if my english is too bad to explain things properly.

wally, a spinning bullet is not an airfoil.. there is no bernulli (sp) effect on a spinning opject.

jv,
yep, if you fired a bullet into space, along a path with ZERO gravity, it would go along forever... if you fired it on a planet without an atmoshere, it would fall to ground (gravity) and slow down,as there is no perpetual motion.

perhaps a quick search on entropy would help out..

armstrong's golf ball fell to moon.

a spining bullet has no lift from the bullet.. and BC only inpacts when there's "air",,, yet the bullet would still fall into the gravity well....

okay.. .here;s a good one, since equations always work backwards.. if air slows spin down, then dropping a bullet with NO SPIN would delevope spin from the air..

NOT

drop an airfoil, situated that it falls straight down, and the designed upper surface would cause the airfoil to "lift" and cause the airfoil to arc and then stall...

if you dont believe me, make a paper airplane and drop it from waist high.. you can't make it fall STRAIGHT but you can a bb or a bullet..


sorry fellas, gravity makes things fall.. BC and velocity define the distance.... and interesting experiment.. you can look it up in any ballastics table... a bullet fired from and exactly horizontal barrel will hit the ground (assumed flat) at exactly the same TIME for any velocity from 1500 to at least 4000 fps.. this is gravity...

in fact, if a bullet was dropped from the exact same height and time, it would hit the ground at exactly the same time...

the only thing that changes is distance...

a fella named galaleo(sp) figured that bit out... followed by another chap named issac

jeffe

Sorry, jeffeosso, but you appear not to have a solid grasp on the relevant physics.

The Bernoulli Effect does come into play in the case of a rotating cylinder or bullet. More precisely, the Kutta-Joukowski Theorem comes into play. Rotating cylinders do generate lift, and consequently there is aerodynamic drag that counters that rotation.

Gravity does not directly slow the rotation of a bullet. If a bullet were fired in a perfect vacuum, it would continue to rotate forever regardless of the effects of any gravity field -- provided, of course, that some other force does not stop its rotation. That's not completely true, since tidal forces (which are the result of gravity) affect masses that are not perfectly rigid. Those forces cause a transfer of angular momentum between, e.g., the earth and the moon. That is why the moon's rotation slowed until it was tide locked with the earth. But such tidal forces are absolutely negligible when considering a bullet's rotation. Similarly, they are almost completely negligible when considering the rotation of earth orbiting artificial satellites, which are often spin stabilized, just like a bullet is. Atmospheric drag on the rotation of such a sattelite is of far greater magnitude and concern than is tidal drag caused by gravity.

Quote:

"That's not completely true, since tidal forces (which are the result of gravity) affect masses that are not perfectly rigid. "

Now the question is...if we were to dump a barge full of Viagra into the ocean would there still be waves at the beach?

Rick

"a barge full of Viagra into the ocean would there still be waves at the beach?"

Don't know ... but the fish would have some fun!

info sponge...
ah, yes... those spinning satellites you mention have a limted life time, for a couple reasons
1: the amount of fuel they must consume to maintain both spin and position (altitude)
2: micro gravity constantly pulling them into the gravity well.

skylab and mir for examples of falling into the well.

those that "think" a mass shot into space will keep going are buying the over simplification, as that statement assumes no gravity and no mass. satellites fall back into the earth due to micro gravity and lack of fuel to maintain orbit... comets, for the astronomical example, are kept in orbit DUE TO GRAVITY, and yet are extra solar.

i am floored that someone who could explain why halley's comet returns would even pretend to believe that gravity has no effect on the spin of bullets.


Now, please go do the paper airplane and bullet experiment...

or, explain how a non spinning bullet picks up spin by dropping it due to air or gravity?

can't be done...


jeffe

infosponge..



go read your link again...



the spinning cylinder in those experiments is presented as longitudinal in reference to air flow... not inline with the airflow. this is called a rotating sail.. not a bullet.













your idea is 90degrees out of phase.



or, to put a fine point on it, your example has zero bearing on bullet flight, as the long side of a bullet does not fly into the wind... futher, if relevant, you would be stating that a bullet shot into a crosswind would RISE rather than drift, except when the crosswind is exactly inline with the opposite side of spin,





btw, the japanese have been using cylindrical metal sails for decades on transport ships,,, presented vertically and only allowed to spin in a CROSS WIND...



or, you could read "texas on the rocks" for a laughable utilization of this type of SAIL



jeffe

last in this series..


and, to argue that a bullet's spin could cause lift, infosponge, you are actually stating that given enough spin, a bullet would RISE from the muzzle in a horizonal barrel, and wouldn't ever fall, given your stipulation that gravity has no or minimal effect on spin and that this Kutta-Joukowski Lift Theorem has anything to do, what so ever, with a bullet...

are you really saying a faster spinning bullet with rise or fall slower than a slower spinning bullet, to the extent that it's creating enough lift to delay impact/gravity?

newton is "spinning" in his grave

jeffe

Lloyd,

I believe that the answer to your original question is YES, spinning bullets do eventually experience a slow down of that spinning...and Gremlins make it happen!

Rick

William Davis, writing in the American Rifleman magazine in December 1982 states that a 150 grain .30-'06 bullet fired horizontally at 2,900 fps from a 5 foot height would hit the ground 1,330 ft down range after a flight time of .600 seconds.

The same bullet dropped from a five foot height strikes the ground in .557 seconds...or .043 seconds sooner than the one fired from the rifle.

For "practical" arguments sake I guess it could be said that the two strike the ground at the same time...but technically they do not.

Rick

Rick,
the falling is gravity ... a constant.. .. and that i would imagine he fired a LOS not LOB

jeffe

Sorry, Jeffe...but you are mistaken on this point.

Refer to pages 164-165 of the book I mentioned earlier on: Understanding Firearms Ballistics by Robert A. Rinker. He deals with this theory in detail and explains why the dropped bullet will beat the fired one to the ground by a slight margin.

Mr. Rinker was formally educated and trained as an industrial engineer and also has a degree in Aviation science. He has vast experience with firearms up to and including large shipboard Naval guns.

Your theory only works in a vacuum...which, after all this debate, I am starting to abhor as much as nature does!

Rick

rick..
i think *I* am starting to draw vaccuum... between my ears!!!

i've got a couple emails out to some ballistics folks for a firm answer

jeffe

Correct, there must be a crosswind component in order for the spinning cylinder to generate lift. However, whether there is crosswind or not, the boundary layer of the spinning cylinder rotates with the cylinder, the surrounding air does not, and there is thus parasitic drag induced by the momentum transfer in the transition from the boundary layer to the surrounding air mass -- that transition is not discrete, discontinuous, or frictionless. That drag (as well as mechanical loses in the bearings and such of the rotating cylinder) must be overcome for rotating cylinder "sails" or "wings" (these have been done as well) to show an increase in overall efficiency over non-rotating sails or wings. Regardless, boundary layer drag does slow the rotation of a bullet. It doesn't do so by much over the course of flight of the bullet, but what slowing of rotation does occur is far more attributable to aerodynamic effects than to vanishingly small tidal forces due to gravity.



And for what it is worth, spin stabilized sattelites do not generally use their limited fuel supply to adjust their spin rate, which varies hardly at all over their useful lifetimes without any need for such adjustment. The effects of gravity on rigid body rotations are negligible over anything but very long timespans (typically millions of years -- as in the earth/moon tidal system, unless you are talking about an extraordianrily intense gravity field -- like that near a blackhole.) The time of flight of a bullet is many orders of magnitude less.

2: micro gravity constantly pulling them into the gravity well.

skylab and mir for examples of falling into the well.

No, sir. It is not gravity ("micro" or otherwise ) that caused the orbits of these (or other) sattelites to decay. Orbital decay of sattelites is caused by atmospheric drag, since sattelites (particularly those in low earth orbit) are not operating in a perfect vacuum, but rather in the wisps of the earth's atmosphere remaining at their respective altitudes. Without the effects of atmospheric drag, gravity is perfectly content to maintain masses in a stable orbit. The moon has remained in such an orbit with respect to the earth for billions of years without "falling into the well." In fact, it is constantly falling into the well, but it is also moving at a high enough tangential velocity that it keeps (and, fortunately, will continue to keep) missing the earth. A stable orbit is a perpetual free fall.

Furthermore, when tidal forces due to gravity are small compared to the conserved angular momentum of a nearly rigid body, and when that rigid body is rotating in a vacuum and otherwise free from forces opposing its rotation, then the body will continue to rotate at a nearly constant rate regardless of the presence of the gravity field. For proof, look at the near constant rates of rotation (without any need for continually adding additional angular momentum) of the various outer planets in their orbits around the sun.

The experiments with bullets travelling in a spiral downrange has been done many times and the main reason for this is that bullets are not perfectly balanced. The imbalance causes the bullet to travel in spiral, the more the imbalance the larger the spiral.

Also a couple of physical laws come into play. Primarily the gyroscopic effect, (precession) where force applied to a spinning object results in movement 90 degrees to the direction of the force applied. Gravity applies a constant force (down) which causes a bullet to push right (from a lefthand twist barrel). Another property inherent to gyroscopes is that a spinning object resists (and counters) all changes in direction. As the bullet tries to push right, the gyroscope tries to push it left again. The higher the spin of an object the more resistant to changes in direction.

Gentlemen, please help explain this phenomemn(and excuse the spellling)

If you talk to long range hi-power shooters about sight adjustments for target shooting at extended distances, they will tell you that for an unexplained reason(at least not well known) it becomes apparent that your ammo does not only travel in an arc up and down but also in a spiral movement.

IE: from a 100 yd zero the group shifts right at 300, high at 600(compared to ballistic chart) and left at 1000. Causing the shooter to adjust first left at 300, lower than thought at 600 and back to center and finally right at 1000. All rounds fired in very calm conditions. Of course example above is dependant on right or left hand twist.

My personal pet theory is goes along with the spinning sail effect. And don't forget that the bullet in not exactly smooth, it does have engraved rifling.

leadfoot

Bullet rpm decays due to drag, and does so very slowly. Gravity is not a factor in this. Fired and dropped bullets will strike the surface at the same time in a vacuum, not is the atmosphere, where the fired bullet lags slightly due to aerodynamic lift.

Bullets fired from a right hand twist barrel thru a right-to-left cross wind will impact high, or vice versa, or versa vice, your choice. The subject is aerodynamic jump. Projectile jump is caused by differences between the axis of rotation in the barrel and axis of rotation about the longitudinal GC as it leaves the muzzle. It is random and causes dispersion.

Most bullets have a dual mode of precession as they leave the barrel, FAST and SLOW, which is called an epicyclic precession. It's a cute little thing that kinda looks like a flower bloom, which is why they call it a rosette pattern. The fast cycle usually nulls within a few hundred yards, the slow cyle changes only as a factor of it's GS, or gyroscopic stability factor. And that changes as the bullet goes down range, well, because GS increases as velocity decays. Tricyclic precessions are possible too. It gets REALLY weird when your try to keep track of all this crap on the firing line, so I'd suggest you don't. But if you do, don't forget about the Yaw of Repose which results in the nefarious "spin drift", which is not drift, but rather a deflection.

The really neat thing about this is that I didn't make it up. Rinker discusses some of these issues, so too does Vaughn. My favorite is McCoy. All of you disbelievers, read 'em and weep.

Simply put...
A. Any object moving in the air experiences drag from that air which will cause it to slow down, no matter in what direction it is going.
B. Just as a baseball is curved by a pitcher by spinning it so will a bullet curve in like manner. Not as effectively since it has no threads to catch the air.
Any surface imperfection (treads of a tire, fins on a rocket, paddles on a boat, surface imperfections on a bullet) will grab the medium through which an object spins and cause both drag and curve. The effect magnitude depends upon five things.
1. The mass of the spinning object
2. The speed of the rotation
3. The size of the surface imperfections
4. The medium through which the object is passing
5. The amount of time that all of these factors are in effect

After listening to all of this debate I am certainly going to feel allot better about missing a shot in the future.

Damn...with all that physics stuff and words I can't even say screwing around with my bullet I think I have a great excuse for missing what I was aiming at!

Rick

Yeah, after I read up on all of that stuff I had more excuses than Carter has peanuts! And the boys at the range...didn't know what to think! That comes under the heading of "If you can't dazzle them with brilliance, baffle them with BS".

.44, Vaughn says that the engraving of rifling has little influence on RPM decay because they are moving within the boundry layer of the flowfield, this being one reason for the relatively slow decay.

44,
evertying you said applies to subsonic objects...

look at the pic again, and try to imgain WHERE the air is dragging on the spin...


or, I do find this laughable, that we are projecting that a bullet, shot in gravity, but in vaccuum, will spin forever... that's perpetual motion, folks... and it just don't exist in any form of gravity or atmosphere... people have been trying for that for only 2000years or more...

TANSTAAFL ...

jeffe

Do you really believe that a spinning bullet can trace a helix with the center of the helix outside
the bullet itself ?. What forces would be reqired to force such a motion?. Don't tell me Vaughn
said so, he didn't.
Take Care!

Dan,

If you really want to fry your brain try studying the dynamics of firing weapons from fast moving objects. It's amazing that any plane was ever shot down with guns being fired from another plane. Tracers help allot but even with that it's still a pretty amazing feat.

Rick

The fact that a bullet is moving supersonically doesn't mean that its rotation is free from boundary layer drag. The bullet is rotating in air that is rarefied compared to that compressed by the shockwave. It is not rotating in a vacuum any more than a speedboat hull is not at all wet just because it is forming a bow wave. Similarly, it is not free from the effects of crosswinds or other movements of the surrounding air any more than that speedboat is free from the effects of currents in the surrounding body of water.

You don't understand conservation of angular momentum or the thermodynamic restrictions on perpetual motion, jeffeosso. Angular momentum is conserved; it cannot be destroyed, just as mass/energy cannot be destroyed. It can be transfered. If there is no available mechanism to effect such a transfer of angular momentum, then a rotating mass will continue to rotate forever. That does not violate the Second Law of Thermodynamics. Gravity does not cause a transfer of angular momentum except through tidal forces. The tidal forces on a rotating bullet are extraordinarily small. They are completely negligible in considering the decreased rotation rate of a bullet during its course of flight.

Your intuition misguided by misunderstood physics does not change those facts.