Do bullets ever develop aerodynamic lift?

[barnbwt]

Imperator, the vorticity theory as well as pressure theory (velocity theory) all describe the same thing, just different ways of framing the question, and sometimes easier to solve mathematically. Simple wings can be approximated as spinning rods, mathematically.

Grumulkin, a moving surface does drag airflow along with it through boundary layer mechanics, but manifests itself as basically changing the shape that surrounding air must flow over; this is why surface perforations under suction fool an airstream into thinking an angle of attack over the rear of a wing is shallower than it is, delaying the point at which flow separates & goes turbulent. For something as small as a bullet, the effect is tiny, is there, and contributes to spin drift.

 

[Grumulkin]

Spin drift is different than lift. A bullet is not a wing.

 

[Carl N. Brown]

I have heard of allowing for spin drift in long range shooting.
I have not heard of allowing for aerodynamic lift in long range shooting.
Unless convinced otherwise, I continue to believe the effect of lift is negligible.

 

[45_auto]

Spin drift is different than lift. A bullet is not a wing.

Neither is a baseball. But the same aerodynamics that allow pitchers to throw curves, sliders, and sinkers are equally applicable to wings and bullets and sails on boats. Bernoulli's Equation (basically conservation of energy for flow) applies to all, it does not discriminate against objects based on their shape or purpose.

I suppose that you also don't believe that these spinning round rods on this Flettnor Rotor ship are sails?

 

[lysanderxiii]

Very entertaining reading.



Let's pretend we have an airplane wing covered with fabric. We'll also pretend that for no good reason the designer made is so the fabric could move around the long axis of the wing. You could spin said fabric as much as you liked and the wing would still produce lift even if the top was there for a fraction of a second.



Bullets aren't spun to counteract the effects of lift. The location of the center of pressure under and slightly to the rear of the leading edge of a symetrical airfoil (like a helicopter blade) is what produces lift. Also it's the center of lift and not the center of gravity that would determine in which direction an "overturning moment" would be.



So drag will keep a bullet in the air longer? LOL. And, pray tell, in which direction is the lift relative to the ground? Up? Down? Sideways?

Questions for you:

1) What causes aerodynamic lift?

2) What causes a bullet to be unstable?

3) Does lift always result in gaining or maintaining altitude?

 

[X-Rap]

If there were any appreciable lift of a bullet in flight you can bet anyone involved in the manufacturing of ammo or components would be exploiting it. If there is any effect at all it is to the detriment of accuracy.


Sent from my XT1254 using Tapatalk

 

[Grumulkin]

Questions for you:

1) What causes aerodynamic lift?

2) What causes a bullet to be unstable?

3) Does lift always result in gaining or maintaining altitude?

The answers:

1. A difference in pressure.

2. It could be from a variety of causes such as a bullet being out of balance, geing upset on exiting the bore, etc.

3. Of course not.

Neither is a baseball. But the same aerodynamics that allow pitchers to throw curves, sliders, and sinkers are equally applicable to wings and bullets and sails on boats. Bernoulli's Equation (basically conservation of energy for flow) applies to all, it does not discriminate against objects based on their shape or purpose.

I suppose that you also don't believe that these spinning round rods on this Flettnor Rotor ship are sails?

If a baseball were thrown directly forward with all of its rotation perpendicular to the relative wind and if it was smooth with no stitching, it wouldn't do any of those things. A baseball is not a bullet.

That's an interesting ship. I don't know anything about Flettnor Rotors but I could believe they're sails. Once again, they're not bullets.

Sheesh. Some of you guys would believe it when someone told you their Gorilla Magnum was so good the bullets just flattened out with no drop at 1,000 yards. Read ANY credible book on ballistics and you will find nothing that says a bullet develops lift.

 

[lysanderxiii]

The answers:

1. A difference in pressure.

2. It could be from a variety of causes such as a bullet being out of balance, geing upset on exiting the bore, etc.

3. Of course not.



If a baseball were thrown directly forward with all of its rotation perpendicular to the relative wind and if it was smooth with no stitching, it wouldn't do any of those things. A baseball is not a bullet.

That's an interesting ship. I don't know anything about Flettnor Rotors but I could believe they're sails. Once again, they're not bullets.

Sheesh. Some of you guys would believe it when someone told you their Gorilla Magnum was so good the bullets just flattened out with no drop at 1,000 yards. Read ANY credible book on ballistics and you will find nothing that says a bullet develops lift.

What causes the difference in pressure? Your answer is like answering the question of Q: "Why does a car go fast?" A: "Because the wheels turn."

No, because you can stick a bullet in a wind tunnel and and measure exactly how unstable it will be. Further, take this measured instability, predict performance, and watch bullets in flight to see that theory matches practice. According to your answer instability is unpredictable, and 150 grain Sierra MatchKings, being much more uniform and less upset by muzzle blast should be more easily stabilized (require less twist) than non-uniform cast bullet of the same shape, but we know this not to be the case.

You obviously did not read the posted article, or did not choose to try and understand it.

As to the baseball, why is a rifled Kentucky rifle shooting a spinning round ball more accurate than a tightly patched unrifled round ball out of a smooth bore?

 

[barnbwt]

If there is any effect at all it is to the detriment of accuracy.

I like the above phrase; it seems very useful for these types of conversations. Two types of ballistic effects; useful ones that can be accounted for, and *noise* that simply keeps things interesting when the trigger is pulled

Spin-stability vs. hitting a snowflake on the way to the target

TCB

 

[barnbwt]

What causes the difference in pressure?<Flettnor ship>

Lateral crosswinds. There's a reason the tubes are laid on their sides & pointed in to the wind, though. But Magnus Effect drift is clearly a cause for wind drift, lateral air motion definitely causing a lift effect normal to its flow in addition to simply pushing the round along through drag forces. Lift is intrinsic to drag, after all, since they are the same thing, just pointed in perpendicular directions.

Think about how tiny an effect wind drift has, though, compared to the distances where it becomes a factor. Inches over hundreds of yards. Any lift effect is going to be even smaller than that, and the direction of the effect will be opposite if you are shooting with a left or right cross wind (unless you care a spare reverse-twist barrel in case of a change in the weather ). Coriolis from firing North/South is probably a bigger factor, and it ain't much of one (it's about as 'into the weeds' as I've ever heard of benchrest guys)

TCB

 

[ClemY]

Where I saw bullet lift was shooting a right hand twist Anschutz in a right-to-left cross wind. The bullet is a small cylinder. The spinning cylinder, spinning to the right, in a right to left cross wind will not only be pushed to the left, it will "rise" just a bit. It is really not falling quite as much. What I saw was with 10 to 15 kt. cross wind, bullet drift on the order of .5" to the left and lift, from the zero wind sight point of about .125". Not much, but noticeable.

 

[lysanderxiii]

The answer, my friend, is blowing in the wind....

Aerodynamic lift is caused by asymmetric airflow over a body*. A brick with asymmetric airflow over it will produce lift. So will a bullet. Asymmetric airflow causes the air to increase velocity on one side of the body, and this increase in velocity will cause a drop in pressure. That force is what we call aerodynamic lift. Lift can be to left or right, it can be up or down. That is how fighter jets can pull negative Gs.

A bullet, or any other symmetrical body moving through a fluid at 0 yaw (or angle of attack), will not produce lift. However, the instant its nose moves up or down, side to side, the airflow will become asymmetric, and a force will be generated pushing the bullet toward the direction of its nose. This force will act as a point force through a point known as the center of pressure (CP). For a standard NACA symmetric airfoil the CP is at 1/4 cord. For a bullet the shape of the ogive determines the exact point along the length where the CP is located, but it is generally located in the forward half of the body.

However, according to Newton there is the law of inertia, things like to stay the way there are.

Our bullet has developed a slight yaw in its flight, and now has a force acting upon it, and because of inertia the bullet wishes not move. However, the center of gravity (CG) of the bullet is behind the CP, so a moment has now formed. This is called the overturning moment, the actual value of the force is dependent on the angle of yaw. You can calculate the overturning moment for any bullet and any yaw angle in a wind tunnel.

If unchecked the overturning moment will... well... overturn the bullet, and it will begin to tumble.

To keep the overturning moment in check you need to stabilize the bullet. You could put fins on it, as they do for tank projectiles, or, you can do it through gyroscopic means by spinning the bullet. The gyroscopic effect will push the nose of the bullet back toward the zero yaw position.

If the gyroscopic force is sufficient to over come the overturning moment, the bullet is said to be statically stable. This is (relatively) easily calculated by many on line 'stability' calculators, that assume many things, namely the actual value of overturning moment, and the actual locations of the CG and CP. Which is why they always caveat their static stability factor at 1.5 or 1.4 for stability. In reality, if the static stability factor is above 1.00, the bullet is stable, but since they are assuming so much, they give themselves 40% to 50% 'wiggle room'.

But then again, inertia rears its head again and once the nose is put in motion it wants to stay in motion, and the gyroscopic force tends to cause an overshoot of the zero yaw position and the bullet's nose will yaw again in the opposite direction, and the whole gyroscopic thing starts over again.

At this point, three things can happen, 1) each time the nose yaws the gyroscopic force pushes it back and the magnitude of the yaw decreases with each cycle (dynamically stable), 2) the magnitude of the yaw remains the same through each cycle (dynamically neutral), or 3) the magnitude of the yaw increases with each cycle (dynamically unstable).

Calculating dynamic stability is much more complicated (and easier done by watching the bullet in question during its flight) than static stability. Also because there are so many more variables involved, dynamic stability can change over the flight of a bullet. Some bullet designs leave the muzzle in a condition of dynamic instability and calm down to be dynamically neutral or stable down range. (The M193 bullet is dynamically neutral through all of its flight, see the above trace of the nose, note that the maximum yaw angle does not decrease.)

So, the answer to the OP's question is, yes aerodyamic lift is created, HOWEVER, as noted above, lift does not always mean "toward the sky", so no, they do not gain altitude, and because the gyroscopic forces on the bullet are always pushing the bullet nose back and forth, the lift vector is also moving back and forth (actually it pushes in in a circle, but that's for later), so the net result is the yaw angle lift forces over time tend to cancel out and the bullet travels pretty much exactly along a point mass trajectory (if there are no cross winds, of course).

The Magnus effect has been well covered, so we'll leave it at that.

_________________________
* With the Magnus effect, if you look at the flow lines, the airflow is highly asymmetric, this is caused by the spin of the object and its relationship with the relative wind.

 

[.308 Norma]

When you fire a bullet from a firearm gravity will cause it to start dropping as soon as it leaves the muzzle. However, do any bullets ever develop any aerodynamic lift that will keep it up longer than if it were to just free fall?

Thanks

JJ

Nuh-uh. A bullet does not "start dropping as soon as it leaves the muzzle" unless the firearm's barrel is either level or actually pointed down. In most cases, the muzzle is elevated, which causes the bullet to climb until it reaches its mid-range trajectory. Many ballistic charts list "mid-range trajectories" showing the bullets in flight are actually above the lines of sight while they're on their ways to the targets.
I don't know anything about Magnus Effect, curve balls or sailboats, but I know if you want to hit a target 250 yards away, you elevate the muzzle of your firearm by setting your scope/sights so that your firearm's muzzle is elevated when you are aiming at the target. Either that or you "hold high."

 

[JellyJar]

Well .308 Norma, you are correct. What I really meant is that gravity acts on the bullet as soon as it leaves the muzzle pulling it toward the center of the Earth. So as it goes up due to its trajectory unless it is going faster than the escape velocity of the Earth's gravity well it will eventually come back down.

And I thought this was going to be a simple yes or no question? :banghead:

 

[Grumulkin]

As to the baseball, why is a rifled Kentucky rifle shooting a spinning round ball more accurate than a tightly patched unrifled round ball out of a smooth bore?

I thought we were discussing aerodynamic lift in a shot bullet. Are you now saying that a spinning round ball develops aerodynamic lift?

Nuh-uh. A bullet does not "start dropping as soon as it leaves the muzzle" unless the firearm's barrel is either level or actually pointed down. In most cases, the muzzle is elevated, which causes the bullet to climb until it reaches its mid-range trajectory. Many ballistic charts list "mid-range trajectories" showing the bullets in flight are actually above the lines of sight while they're on their ways to the targets.

It would be more accurate to say that gravity starts affecting bullet trajectory as soon as it leaves the barrel.

 

[Sav .250]

That question/answer will have no affect on my daily life.

 

[45_auto]

Are you now saying that a spinning round ball develops aerodynamic lift?

That was pretty much the premise of my introductory aerodynamics class about 40 years ago. Haven't seen anything to invalidate it since then. The information is freely out there if you're willing to make the minimum investment to even slightly educate yourself. If you're not so inclined, feel free to continue to deliver your rather amusing perspective on aerodynamics!

I would imagine that the Kutta-Joukowski theorem (early 1900's) would be the most applicable to lift developed by a spinning bullet if you're interested in trying to quantify it.

http://hyperphysics.phy-astr.gsu.edu/hbase/fluids/kutta.html#c1

If you're really interested in the aerodynamics involving a spinning ball, NASA's high school level (grades 9-12) explanation on the spinning ball is here:

https://www.grc.nasa.gov/www/k-12/airplane/bball.html

All that is necessary to create lift is to turn a flow of air. The airfoil of a wing turns a flow, but so does a spinning ball. The exact details are fairly complex and are given on a separate slide. Summarizing the results, the amount of force generated by a spinning ball depends on the amount of spin, the velocity of the ball, the size of the ball, and the density of the fluid.

They go into a more detailed explanation using Kutta-Joukowski in their next slide here:

https://www.grc.nasa.gov/www/k-12/airplane/beach.html

The details of how a spinning ball creates lift are fairly complex. Next to any surface, the molecules of the air stick to the surface, as discussed in the properties of air slide. This thin layer of molecules entrains or pulls the surrounding flow of air. For a spinning ball the external flow is pulled in the direction of the spin. If the ball is not translating, we have a spinning, vortex-like flow set up around the spinning ball, neglecting three-dimensional and viscous effects in the outer flow. If the ball is translating through the air at some velocity, then on one side of the ball the entrained flow opposes the free stream flow, while on the other side of the ball, the entrained and free stream flows are in the same direction. Adding the components of velocity for the entrained flow to the free stream flow, on one side of the ball the net velocity is less than free stream; while on the other, the net velocity is greater than free stream. The flow is then turned by the spinning ball, and a force is generated. Because of the change to the velocity field, the pressure field is also altered around the ball. The magnitude of the force can be computed by integrating the surface pressure times the area around the ball. The direction of the force is perpendicular (at a right angle) to the flow direction and perpendicular to the axis of rotation of the ball.

Wikipedia has some basic info:

https://en.wikipedia.org/wiki/Magnus_effect

The Magnus effect is the commonly observed effect in which a spinning ball (or cylinder) curves away from its principal flight path. It is important in many ball sports. It affects spinning missiles, and has some engineering uses, for instance in the design of rotor ships and Flettner aeroplanes.

 

[Carl N. Brown]

Cast lead bullets, try as hard as one may, are not perfectly balanced.

Fired from a smoothbore a patched round ball will start to drift toward its off-center of gravity.

The effect of spin from rifling on a round ball is to even out the effect of its off-center of gravity. That's nothing to do with aerodynamic lift.

 

[Carl N. Brown]

"The Magnus effect is named after Gustav Magnus, the German physicist who investigated it. It describes the force generated by fluid flow over a rotating body, at right angles to both the direction of flow and the axis of rotation."

The axis of rotation of a bullet passing through the air is at 0° to the flow of air over the bullet and the direction of air flow is at 0° to the the axis of rotation. The conditions of the Magnus effect do not apply to a bullet flying through the air: the bullet woulld have to be moving at 90° to the airflow and moving at 90° to its axis of rotation (ie, flying stably sideways through the air spinning and not tumbling).

 

[Grumulkin]

Oh my... I give up. Peace and good cheer unto all who wish to believe a bullet develops aerodynamic lift.

 

[climbnjump]

The axis of rotation of a bullet passing through the air is at 0° to the flow of air over the bullet and the direction of air flow is at 0° to the the axis of rotation. The conditions of the Magnus effect do not apply to a bullet flying through the air:

No, the axis of rotation of a bullet passing through the air is not at 0 degrees to the flow of air over the bullet. There is a very small angle - on the order of 0.1 degree - called "yaw of repose".

http://www.exteriorballistics.com/ebexplained/5th/42.cfm

http://www.nennstiel-ruprecht.de/bullfly/yawrepf.htm

http://www.nennstiel-ruprecht.de/bullfly/fig24.htm

https://thearmsguide.com/5346/long-range-shooting-external-ballistics-spin-drift-13-theory-section/

 

[azrocks]

The answer, my friend, is blowing in the wind....

Aerodynamic lift is caused by asymmetric airflow over a body*. A brick with asymmetric airflow over it will produce lift. So will a bullet. Asymmetric airflow causes the air to increase velocity on one side of the body, and this increase in velocity will cause a drop in pressure. That force is what we call aerodynamic lift. Lift can be to left or right, it can be up or down. That is how fighter jets can pull negative Gs.

A bullet, or any other symmetrical body moving through a fluid at 0 yaw (or angle of attack), will not produce lift. However, the instant its nose moves up or down, side to side, the airflow will become asymmetric, and a force will be generated pushing the bullet toward the direction of its nose. This force will act as a point force through a point known as the center of pressure (CP). For a standard NACA symmetric airfoil the CP is at 1/4 cord. For a bullet the shape of the ogive determines the exact point along the length where the CP is located, but it is generally located in the forward half of the body.

However, according to Newton there is the law of inertia, things like to stay the way there are.

Our bullet has developed a slight yaw in its flight, and now has a force acting upon it, and because of inertia the bullet wishes not move. However, the center of gravity (CG) of the bullet is behind the CP, so a moment has now formed. This is called the overturning moment, the actual value of the force is dependent on the angle of yaw. You can calculate the overturning moment for any bullet and any yaw angle in a wind tunnel.

If unchecked the overturning moment will... well... overturn the bullet, and it will begin to tumble.

To keep the overturning moment in check you need to stabilize the bullet. You could put fins on it, as they do for tank projectiles, or, you can do it through gyroscopic means by spinning the bullet. The gyroscopic effect will push the nose of the bullet back toward the zero yaw position.

If the gyroscopic force is sufficient to over come the overturning moment, the bullet is said to be statically stable. This is (relatively) easily calculated by many on line 'stability' calculators, that assume many things, namely the actual value of overturning moment, and the actual locations of the CG and CP. Which is why they always caveat their static stability factor at 1.5 or 1.4 for stability. In reality, if the static stability factor is above 1.00, the bullet is stable, but since they are assuming so much, they give themselves 40% to 50% 'wiggle room'.

But then again, inertia rears its head again and once the nose is put in motion it wants to stay in motion, and the gyroscopic force tends to cause an overshoot of the zero yaw position and the bullet's nose will yaw again in the opposite direction, and the whole gyroscopic thing starts over again.

At this point, three things can happen, 1) each time the nose yaws the gyroscopic force pushes it back and the magnitude of the yaw decreases with each cycle (dynamically stable), 2) the magnitude of the yaw remains the same through each cycle (dynamically neutral), or 3) the magnitude of the yaw increases with each cycle (dynamically unstable).

Calculating dynamic stability is much more complicated (and easier done by watching the bullet in question during its flight) than static stability. Also because there are so many more variables involved, dynamic stability can change over the flight of a bullet. Some bullet designs leave the muzzle in a condition of dynamic instability and calm down to be dynamically neutral or stable down range. (The M193 bullet is dynamically neutral through all of its flight, see the above trace of the nose, note that the maximum yaw angle does not decrease.)

So, the answer to the OP's question is, yes aerodyamic lift is created, HOWEVER, as noted above, lift does not always mean "toward the sky", so no, they do not gain altitude, and because the gyroscopic forces on the bullet are always pushing the bullet nose back and forth, the lift vector is also moving back and forth (actually it pushes in in a circle, but that's for later), so the net result is the yaw angle lift forces over time tend to cancel out and the bullet travels pretty much exactly along a point mass trajectory (if there are no cross winds, of course).

The Magnus effect has been well covered, so we'll leave it at that.

THIS is the correct answer, and should be the end of the thread.

It also happens to agree with what I've learned from the excellent coffee table book, "Modern Exterior Ballistics" by Robert McCoy.

Yes, bullets with anything but zero yaw in flight create lift. But with a spin-stabilized bullet the lift vector is rotating, so the net lift is - effectively - zero.

 

[hdwhit]

...do any bullets ever develop any aerodynamic lift...

In short, no.

The "Magnus Effect" which many have posted about can generate inconsequential amounts of lift when an outside force (i.e. a crosswind acting perpendicular to the direction of flight) acts upon the bullet; the bullet's normal flight does not "develop" a Magnus Effect.

 

[azrocks]

In short, no.

The "Magnus Effect" which many have posted about can generate inconsequential amounts of lift when an outside force (i.e. a crosswind acting perpendicular to the direction of flight) acts upon the bullet; the bullet's normal flight does not "develop" a Magnus Effect.

Lift is produced, and it's independent of the the magnus force. Lysader describes it quite well above. It is not lift in the traditional sense because the lift vector is continually rotating due to the spin imparted on the bullet. But at any given point in its rotation, if there's more than 0-degrees yaw, there's going to be lift.

Also, magnus forces are generated by unequal pressures on either side of a spinning body, the unequal pressure being the result of viscous interaction between the fluid (air) and the spinning body. No outside force (like a gust of wind) is required to elicit magnus effects. It's the same phenomenon one sees with a curve-ball in baseball.

 

[benEzra]

The axis of rotation of a bullet passing through the air is at 0° to the flow of air over the bullet and the direction of air flow is at 0° to the the axis of rotation.

Initially, yes. 600 yards out, no; at that distance, the bullet is flying with a slightly positive angle of attack relative to the local airflow, so the Magnus effect comes into play, resulting in an aerodynamic force to the side, which can cause six inches or so of deviation at 1000 yards. Again, look up "spin drift" in the context of exterior ballistics.