Bullet lift, does it exist?

[Brown Dog]

Well,

I'm don't understand that the original question was answered unambiguosly, although I am sure that CatShooter feels HE has answered the question.

Fair point, a question of self-perception .

Just a mumbled apology for the ad hominem stuff then

...I realise I'm giving the misleading impression that it actually is of any import to me...I'm just highlighting how out of kilter with the spirit of this site I find his literalist, ill-tempered and rude posts (particularly when they're wrong!)

Does that shape and/or that associated spin cause the bullet to rise?

I tried to answer that in fair detail a few posts back ?! ...there's lift in various guises depending on how the bullet is presenting itself to the airflow.

Properly spun, does it have lift? A miniscule amount due to magnus /yaw of repose.

Properly spun, does it rise? Emphatically, unequivocally: No.

 

[bwaites]

Fair point, a question of self-perception .

Just a mumbled apology for the ad hominem stuff then

...I realise I'm giving the misleading impression that it actually is of any import to me...I'm just highlighting how out of kilter with the spirit of this site I find his literalist, ill-tempered and rude posts (particularly when they're wrong!)



I tried to answer that in fair detail a few posts back ?! ...there's lift in various guises depending on how the bullet is presenting itself to the airflow.

Properly spun, does it have lift? A miniscule amount due to magnus /yaw of repose.

Properly spun, does it rise? Emphatically, unequivocally: No.

I guess I was the ambiguous one! I understand the sideflow issues, and maybe the actual definition of "lift" was the issue.

So, to put it in my terms:

If the bullet passes through the air with appropriate spin, it does not rise in relation to the direct line of sight between the muzzle and the target, correct?

There may be a "lift" in relation to the surrounding air flow, so that the bullet does turn, secondary to the magnus effect of the side airflow affected by the spin of the bullet, correct?

Bill

 

[AJ Peacock]

A bullet doesn't maintain the same spin to forward velocity ratio during its entire trip though, does it? My understanding is that the velocity degrades at a greater rate than the spin, therefore the 'appropriateness of the spin' changes during the flight and can cause the attitude the bullet has to its trajectory to change somewhat, causing the Yaw of repose to not be consistent and therefore the bullet may appear to have lift.

It is my understanding, that this is why different bullets have different absolute amount of drop over time (the subject of the original question).

As always, I await being corrected ;-)

AJ

 

[Brown Dog]

If the bullet passes through the air with appropriate spin, it does not rise in relation to the direct line of sight between the muzzle and the target, correct?

In terms of lift, if LOS and angle of departure are coincident, correct.

Obviously if LOS is taken as being the axis of the bore and jump causes the angle of departure to be higher than that axis; or if direct fire sights deliberately 'tilt' the angle of departure upwards through the LOS there will be apparent rise in relation to LOS...but clearly not due to lift! ...and I'm certain that's not what you mean!

There may be a "lift" in relation to the surrounding air flow, so that the bullet does turn, secondary to the magnus effect of the side airflow affected by the spin of the bullet, correct?
Bill

...you'll have to dip your finger in your beer and sketch that question out on the table...I'm not sure what you're asking

 

[bwaites]

In terms of lift, if LOS and angle of departure are coincident, correct.

Obviously if LOS is taken as being the axis of the bore and jump causes the angle of departure to be higher than that axis; or if direct fire sights deliberately 'tilt' the angle of departure upwards through the LOS there will be apparent rise in relation to LOS...but clearly not due to lift! ...and I'm certain that's not what you mean!



...you'll have to dip your finger in your beer and sketch that question out on the table...I'm not sure what you're asking

Yes, in re-reading that, it doesn't make much sense, does it?

I'm just going to leave it at that.

My basic question was answered, thank you.

Bill

 

[Brown Dog]

A bullet doesn't maintain the same spin to forward velocity ratio during its entire trip though, does it? My understanding is that the velocity degrades at a greater rate than the spin, therefore the 'appropriateness of the spin' changes during the flight and can cause the attitude the bullet has to its trajectory to change somewhat, causing the Yaw of repose to not be consistent and therefore the bullet may appear to have lift.

Wow, no correction! that's an excellent point.

..'appropriateness' will depend on how you're expecting the projectile to perform way down range. If the spin is such that it overstabilises at it slows, that may or may not matter...depending on what you're trying to do!

But I totally accept your point.

From my perspective (slave to my own experience!) I would see that as increased drag leading to decreased range...but I can see how others might frame that differently!

 

[cheif wsm]

is there any "rule of thumb" so to speak or actual formula around than can calculate, lift and drop of projectiles at extreme ranges?

 

[GeorgeS]

Any bullet that pases through the air in a manor that the axis of the bullet is NOT co-axial with axis of the direction of travel, is displaced from the axis of travel, in the direction of the axis of the bullet. This also applies to other objects (like cats ).

If you launch a long (stable) stick at "0" degrees (on your vector range table) from a catapult, but the stick is point at 350 degrees, then the stick will start traveling at "0" degrees, but immediately start changing direction and continue to change direction, until it is traveling at 350 degrees, at which point, it will then travel straight at 350 degrees unless other external forces are also applied.

This is NOT rocket science. I did this stuff in the 8th or 9th grade.

If we rotate the vector range table so it is vertical instead of horizontal, so now we have a launching catapult that is based on a vertical angle...

... we launch the stick at +30, but the stick is pointing at +20 degrees. The stick will drop at a faster rate, because there is a positive pressure on the upper surface, and a negative pressure on the lower surface, so the pressure differential vector is added to the vector of gravitational direction, and the stick drops faster, because the total "down" force is greater than gravity alone.

If we change that...

... we launch the stick at +20, but the stick is pointing at +30 degrees. The stick will drop at a SLOWER rate, because there is a positive pressure on the BOTTOM surface, and a negative pressure on the upper surface, so the pressure differential is SUBTRACTED from the gravity directional vector, and the stick drops SLOWER, because the total "down" force is LESS than gravity alone.

This is NOT rocket science. I did this in the 8th or 9th grade.


Now... if we replace the stick with a bullet, NOTHING CHANGES. The fired bullet starts dropping at that famous "16 feet per second, per second" rate, and as soon as it starts to drop (very slowly at first) the axis of the bullet is greater (a larger up angle) than the axis of travel.

Two things happen at this point. The pressure under the bullet is higher than the pressure on top of the bullet, and the bullet is (in colloquial terms) "LIGHTER" than it's mass would suggest, so it falls slower that the law of "16 feet per second, per second"... but in physics, we don't do "colloquial" explanations.

In physics, there are two separate things going on, and we measure each, and combine the vectors.

The bullet is falling at "16 feet per second, per second", because that is a law of physics, and (unlike speeding laws) cannot be broken... so we calculate the drop based on "16 feet per second, per second".

But we have a second, real physical thingie going on, and that is the pressure differential, which (contrary to that world respected source of information, Wikipedia), causes the bullet to rise. This rate of rise is easily calculated - it is the acceleration of the upward force, times the mass of the bullet... just like calculating the acceleration of a car with a gasoline engine.

To the physicist, the solution is easy, and complies with all laws - the bullet is falling X units, and the areodynamics is lifting the bullet Y units - X is bigger than Y, so the formula is... Drop = X" - Y"... and Drop will be less than predicted by the "16 feet per second, per second" law.

bwaites... if you wanna debate this, you are a fool of the first order. This is basic high school science.


.

I guess that your 8th or 9th grade physics teacher didn't do you any favors, because the acceleration due to gravity is not "16 feet per second per second" but rather 32 feet per second per second. That IS rocket science, as well as simple physics, since rockets have to contend with the reality of gravitational constants.

Moreover, while the trajectory of a stick launched at X degrees but with a non-coaxial attitude is deflected in response to air pressure in a relatively predictable way, that is decidedly not the case for a bullet spinning rapidly owing to the response of an object experiencing gyroscopic force in addition to wind pressure and gravity, as can be readily ascertained - from an 8th or 9th grade physics experiment - and an additional force is imposed at right angles to the externally-imposed force.

You may choose to look this up. As an example, if a bullet is spinning clockwise along its axis of travel, and is subject to a crosswind from right to left (wherein the center of pressure of the bullet is ahead of the center of mass, which is almost always the case), the force tending to deflect the bullet path and particularly the bullet yaw, to the left, will also, owing to the bullet's spin angular momentum, cause the nose of the bullet to point upward. The bullet will thus rise, and strike the target both to the left of the aim point and above the point at which it would have hit had there been no wind.

Given the flaws in your argument, I would not inveigh against others simply attempting to ask a question as if they were fools of the first order, when it is clear that your abusive and incorrect answer clearly identifies the fool in this debate.

 

[GeorgeS]

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.

Question - Our students claim that when a bullet emerges from a horizontal rifle, it can take a rising path due to the spinning of the bullet. Is there a way for a bullet to rise? We think that Bernoulli effect is minimal to null in this situation.-----------------Nathan

-A real bullet shaped projectile.... no. But a round projectile... maybe.In aerodynamics there is an effect known as the Magnus Effect. It would likely not apply to a bullet because of its shape. The bullet would likely continue to fly with the nose forward and the rifling would turn the bullet around its longitudinal (front to rear) axis. However, the Magnus Effect could effect a round projectile as fired from a musket or an old fashioned cannon ball. This could cause the ball to rise if were rotated properly around its lateral (left to right) axis while in flight. (Think of hitting a cue ball with back spin on a pool table. That is the kind of spin it would need.)I would expect that this effect would be most apparent with a slow-moving, light projectile of larger size. Perhaps it could be seen with slow motion photography if a Polystyrene foam ball were pitched with a great deal of back spin. I really do not know if it would be significant enough to measure, but it might be fun to try.Larry Krengel=====================================================

No way.The fact that the bullet is spinning means that, on average, it'ssymmetrical about its line of flight, so why would any force exertedby air cause it to rise, rather than, say, go to the left? The onlything I can think of that would break the symmetry, and result in apreferred direction, would be *very* slightly higher air density belowthe bullet than above it, but this difference seems utterly negligible.I don't believe a bullet fired horizontally actually rises, or evenfalls more slowly than it would if simply dropped. However, I dobelieve a rifle's recoil could cause it to rotate about its center ofmass so that the barrel tips upward slightly while the bullet is stillwithin the barrel.Tim Mooney

The second from a blogger who is interested in guns:

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).

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.

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.)

Now, add in a cross wind coming from the right.

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.

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.


And finally, Wikipedia:

The Magnus effect in external ballistics, also known as 'spin drift'

Another context where the Magnus effect can be found is advanced external ballistics. 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 yaw 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.(yaw of repose) 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 does not act upon the bullet's center of gravity, but the center of pressure. This means that there is a Magnus force that affects the yaw of the bullet. The Magnus effect will act as a destabilizing force on any bullet with a center of pressure located ahead of the center of gravity, while conversely acting as a stabilizing force on any bullet with the center of pressure located behind 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.

Interesting question, and the answer is interesting as well. I addressed it in a post above in response to "catshooter," but the simple answer is: yes, bullet lift does exist.

Interestingly, while the Magnus Effect, described by others here, does operate in a crosswind to lift or depress the axis of bullet travel, it does so not only to a lesser extent, but also in the opposite direction of, the more noticeable effect attributable to gyroscopic forces.

Assuming a clockwise spin from a view at the back of the bullet (right twist - the most common), and assuming a crosswind from left to right, the Magnus effect will slightly ****** the fall of the bullet from what would otherwise be the case of the bullet falling in still air - as well as cause a left to right displacement of the bullet, from the lateral wind force. In the case of a bullet that has a center of pressure forward of the center of mass, however, the bullet will not only be displaced from left to right, but also deflected angularly from the axis of flight. That is, the nose of the bullet will experience a moment tending to rotate the bullet horizontally around its center of mass, which would normally - in a projectile moving through the air without spin - introduce a yaw, deflecting it yet further to the right.

A spinning projectile, however, will respond to that force by moving at a right angle to the force, owing to the gyroscopic spin angular momentum. In this case, the bullet nose will be pushed downward, counteracting the lift of the Magnus Effect, and causing the bullet to drop owing to increased pressure on the upper surface of the bullet.

Conversely, if the crosswind is from right to left, the nose of the bullet will be deflected upward, even as the bullet drifts left. The attitude change of the bullet will produce lift, retarding the fall of the bullet. The bullet will thus hit to the left of, and above, the point of aim, just as the bullet would hit to the right of, and below the point of aim in a crosswind from left to right. This effect is larger than that of the Magnus Effect, and so will only be moderated slightly by the Magnus Effect.

So: a long-winded answer that sums up to: for a right hand spin, in a right to left crosswind, spin angular momentum will pitch a typical bullet's nose upward, resulting in lift, and a rise of the bullet above the flight path it would take in still air.

There are other issues, of course, such as the Coriolis Effect causing lateral drift when shooting in a northerly or southerly direction, planetary spin causing an apparent rise or fall when shooting in an Easterly or Westerly direction, and vertical and horizontal/pitch and yaw moments imparted to the base of the bullet as it leaves the vibrating and twisting barrel, from lack of orthogonality of the bullet base with the axis of the barrel, and force asymmetries from gas flow out of the barrel owing to bullet base irregularities, barrel crown imperfections, etc. - all of which contribute to the fascinating subject of ballistics research.

 

[cape cove]

Do you realize this is an 8 year old thread.

 

[Barrelnut]

Do you realize this is an 8 year old thread.

Obviously not. But think George is right on with the physics.
All I really know is the Applied Ballistics App takes wind into account when figuring the bullets drop for a target. So I really don't have to think about it.

 

[GeorgeS]

Do you realize this is an 8 year old thread.

Do you realize how old the dictionary is - yet people still refer to it as a reference? If a thread starts "Help - my refrigerator isn't holding its temperature," or "Windows 98 keeps crashing!" I don't respond. If it starts with a question about physics and fundamentals that hasn't been answered - dude, that's worth recording an answer to, because it's an eternal question. Particularly if you spot an error that will mislead someone, like catshooter's mysterious gravitational constant of 16 f/s/s.

 

[cape cove]

It was a simple question that you still haven't answered. Imagine all these people being led astray for the last 6-8 yrs. Some may have even went to their grave thinking about the 16 ft/sec constant.

 

[GeorgeS]

It was a simple question that you still haven't answered. Imagine all these people being led astray for the last 6-8 yrs. Some may have even went to their grave thinking about the 16 ft/sec constant.

Ah - well, then. We should probably burn all the books, because having accurate information for people to read in the future really is unimportant. I'll delete my post - you'll delete all yours, too, I imagine? By the way - what brought you to an 8 year old post?