Long Range Hunting Online Magazine


Go Back   Long Range Hunting Online Magazine > Rifles, Reloading, Optics, Equipment > Rifles, Bullets, Barrels and Ballistics


Reply

Bullet lift, does it exist?

 
LinkBack Thread Tools Display Modes
  #1  
Old 12-07-2007, 11:33 AM
Silver Member
 
Join Date: Jun 2007
Location: Central Washington
Posts: 491
Bullet lift, does it exist?

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.
__________________

The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.

Thomas Jefferson

http://waitesandbalances.blogspot.com/
Reply With Quote
  #2  
Old 12-07-2007, 11:42 AM
Platinum Member
 
Join Date: Oct 2005
Location: Michigan
Posts: 2,232
My SWAG (simple wild ass guess)

I would 'guess' that actual 'lift' does exist for bullets whose axis of rotation is slightly canted upward from their line of flight. I suspect that different bullet shapes fly at different 'tilts' and this is what causes some of the difference we are all talking about.

The above is simply a guess from a guy that hasn't had enough sleep this week and has had entirely too much caffeine; so take it for what its worth.

Later,
AJ
__________________
If some is good and more is better, then too much is just right.


My mind is a raging torrent, flooded with rivulets of thought, cascading into a waterfall of creative alternatives
Reply With Quote
  #3  
Old 12-07-2007, 11:49 AM
Platinum Member
 
Join Date: Jun 2007
Location: PUEBLO, CO, USA
Posts: 1,096
I dont know about all that, but I do highly suspect the bullet is cast right or left after it exits the barrel such that the parabolic bullet path leans right or left depending on twist direction of the barrel. Often a bore sighted rifle will be 2" left or right at 25yrds depending on barrel twist directionl. I call this Sven's Leaning Parabolic Effect. This would be highly exaggerated at long range if you hadnt benched your rifle to zero the scope. Or it may be just the spin of the earth that causes it.
__________________
svenska50@hotmail.com

NRA Life Member
Reply With Quote
  #4  
Old 12-07-2007, 11:53 AM
Official LRH Sponsor
 
Join Date: Jun 2004
Location: Fort Shaw, Montana
Posts: 6,807
THis is an interesting and very complicated topic for sure. Well over my head ballistically.

However, I have read some extensive studies on 50 cal projectiles tested down on the Yuma test range using radar ballistic equipment at exteme range. They were also able to recover bullets from these tests and found some very interesting things out about bullet flight.

The main thing that effected bullet flight was twist rate from their testing and results. Rifles that had the standard 1-15 twist would have bullets that would travel, meplat on line with trajectory. Meaning that as the bullet traveled its trajectory, the point of the bullet nose would stay on point with the path of the bullet. This was determined by recovering the bullets from the sand fields they were fired into and studing the abrasion on the bullets from impact. The majority of bullet damage was at the front of the bullet.

When they tested some fast twist barrels in the 50 BMG, the results were much different. These were 1-9.5 twist barrels. The results were similiar for all bullet designs, conventional lead core to solids. When the bullets were fired, the bullets nose would stay relatively on track until the bullet began to drop noticably. Instead of the nose following the trajectory path as the bullets from the 1-15 twist barrels die, these bullets remained flat or actually nose up in flight. This was determined by the fact that all recovered bullets had abrasion on the sides and rear of the bullets with no serious damage to the nose of the bullets.

How did this effect maximum range potential, not dramatically but at extreme range, the stadnard twist barrels did drop significantly faster then the fast twist barreled bullets.

They did not come right out and say this was due to "lift". What they did say however was that this was more likely a result of higher BC at extended range because the bullet remained "point into the wind" if you will compared to the standard twist bullets that had "point on trajectory" behavior. Basically saying that the higher RPM bullets had higher BC at extreme range. When I say extreme range I am talking in the +2500 yard range.

Interestly enough, they had lower BC at close range(sub 1000 yards).

Not sure if this adds anything meaningful but it was an interesting read.

Kirby Allen(50)
__________________
Kirby Allen(50)

Allen Precision Shooting
Home of the Allen Magnum, Allen Xpress and Allen Tactical Wildcats and the Painkiller Muzzle brakes.

Farther, Faster and Flatter then ever before.

Web Page: www.apsrifles.com

allenmagnum@gmail.com
Reply With Quote
  #5  
Old 12-07-2007, 12:12 PM
Silver Member
 
Join Date: Jun 2007
Location: Central Washington
Posts: 491
I do recognize that BC is at least speed dependant, and spin may also play a part.

Kirby, in that study, did the bullets actually DROP faster or was their speed decreased more by their yaw and thus they dropped in a shorter distance?

Bill
__________________

The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.

Thomas Jefferson

http://waitesandbalances.blogspot.com/
Reply With Quote
  #6  
Old 12-07-2007, 12:24 PM
Gold Member
 
Join Date: May 2001
Posts: 920
Quote:
Originally Posted by AJ Peacock View Post
I would 'guess' that actual 'lift' does exist for bullets whose axis of rotation is slightly canted upward from their line of flight. I suspect that different bullet shapes fly at different 'tilts' and this is what causes some of the difference we are all talking about.

The above is simply a guess from a guy that hasn't had enough sleep this week and has had entirely too much caffeine; so take it for what its worth.

Later,
AJ
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.


.
__________________
Here, kitty kitty kitty.

LRH member #23
Reply With Quote
  #7  
Old 12-07-2007, 12:49 PM
Gold Member
 
Join Date: May 2001
Posts: 920
The problem with the theory of bullet tracking the arc (following the tangent of the path, co-axially) is the law of "Conservation of momentum" which specifically applies to gyroscopic motion.

An object that is spinning (like a bullet) has what is called "Rotational momentum"... this is kinda similar to the momentum of things moving forwards - they don't stop without changing something else.

Well, rotational momentum cases this thing called "The moment of progression", or "yaw"

People say that bullets follow the arc, but if you understand the laws of rotational momentum, you will realize that is impossible, for the following reason.

If you are behind a bullet (or gyroscope) that is rotation clockwise (like a bullet), and you try to raise the back, or lower the front, what happens IMMEDIATELY, is the nose (front) turn LEFT, the exact amount that you raised the rear or lowered the front.

If you do the math on a long range bullet, it might be launched at 3 degrees up, and a mile away, be falling at 15 degrees down...

If the theory of bullets tracking the arc was true, then the bullet would also be pointing 17 degrees to the left (from the firing position)...
... which is obviously NOT the case, because we could NOT hit anything with it if it were true.

It make no difference what the spin rate is - if the bullet attitude (direction of axis) is changes, the there is a 90 degree additional change of the SAME AMOUNT.

And there is that pesky question... Newton's Law says that there can be no change in an body's movement without an external force - where is the force to push a bullet's nose down, when in fact, the aerodynamics are trying to lift the nose, and gravity is pulling the back down cuz the center of gravity is BEHIND the center of aerodynamic drag... so the bullet wants to fall tail first, NOT nose first.

If you doubt this... go to the dime store and get a common gyroscope and play with it for a day - it will WAKE YOU UP FAST to a lot of the BS about bullets flight.

The sand marks are NOT a reliable test of attitude - only down range high speed photos can determine these things.


.
__________________
Here, kitty kitty kitty.

LRH member #23
Reply With Quote
Reply

Bookmarks

Thread Tools
Display Modes


Similar Threads for: Bullet lift, does it exist?
Thread Thread Starter Forum Replies Last Post
Does an economy Borescope exist? 300magman Equipment Discussions 2 07-05-2011 12:26 AM
Sako 22-250 bolt lift silverado Gunsmithing 1 04-04-2011 07:26 PM
Muzzle Brakes & bullet lift?? David Long Long Range Hunting & Shooting 10 08-21-2009 11:30 PM
Hi from Alberta, Canada. Does good 300win brass exist? predhunter Member Introductions 2 08-19-2009 07:10 AM
Do any Obermeyer (SP?) blanks still exist? JustC Rifles, Bullets, Barrels and Ballistics 9 01-09-2003 03:05 PM

Current Poll
Do you archery hunt for elk?
YES - 34.40%
118 Votes
NO - 48.69%
167 Votes
Not yet, but I plan to. - 16.91%
58 Votes
Total Votes: 343
You may not vote on this poll.

All times are GMT -5. The time now is 07:45 PM.


Powered by vBulletin ©2000 - 2014, Jelsoft Enterprises Ltd.
Content Management Powered by vBadvanced CMPS
All content ©2010-2014 Long Range Hunting, LLC