I am reading Applied Ballistics for LR shooting, aerodynamic jump makes sense but I have one question. It explains a bullet emerges from the bore pointing right and down in a wind from the left, this causes the gyrations that will eventually cause the vertical deflection. What causes the bullet to emerge right and down?/ Left and up for right wind? Pg 86/87 in the book
Aerodynamic jump
- Thread starter Camoman1
- Start date
As the bullet emerges from the barrel it is pointing in the direction of the barrel, but the airflow seen by the bullet, which consists of the combined cross wind and the airflow due to the bullets forward speed, is at a small angle. Because the bullet is gyroscopically stable it will try to turn to face the combined airflow. The gyroscopic response to the bullet turning through a small angle left or right is to make the bullet also turn in a very small angle up or down which is what produces the vertical effect called aerodynamic jump. The size and direction of the vertical angle will vary depending on the spin speed and direction, the direction of the wind and the aerodynamic and mechanical properties of the projectile. It is not a simple relationship.
It has nothing to do with Magnus which is what you often see claimed.
It has nothing to do with Magnus which is what you often see claimed.
dcali
Bullet Maker
Aerodynamic jump can be in any direction. When the bullet is angularly displaced upon leaving the muzzle (wind or bullet imperfection being the main culprits - but you can also induce it with an asymmetric crown), the bullet will veer off in a direction 90 degrees to that displacement (because of gyroscopic precession). Side wind = up/down angle of departure. An up or down wind would cause sideways jump.
People usually discuss AJ in the context of vertical wind deflection or shooting out of helicopters. But its more commonly seen impact is that it's a major source of bullet dispersion in general. Bullet imperfection causes jump in a random direction (increased group size). It's a large part of why match bullets shoot better than ball ammo.
People usually discuss AJ in the context of vertical wind deflection or shooting out of helicopters. But its more commonly seen impact is that it's a major source of bullet dispersion in general. Bullet imperfection causes jump in a random direction (increased group size). It's a large part of why match bullets shoot better than ball ammo.
The vast majority of rifle projectiles are aerodynamically unstable and the majority of barrels have right hand twist so nearly all will tend to react in the same way with the different winds. Air gun pellets, which tend to be aerodynamically stable, react in the opposite directions.
As a pilot, I am looking at this like the bullet is an airplane. When it leaves the muzzle in a right crosswind, the point should turn to the right. (An airplane will rotate into the wind.) If that is also the case for a bullet, the precession will move the tail up and the nose down. This will result in the bullet going lower in a right crosswind. However, that is not what actually happens. If the crosswind is from the right, the bullet actually impacts higher than predicted. Now...if the force acting on the bullet is actually based on airflow over the surface of the bullet, a higher windspeed from the right will add to the usual rightward rotation of the bullet to create greater speed over the top of the bullet, which leads to lower pressure above the bullet, which in turn causes the bullet to impact the target higher than predicted (higher than when there is no wind). I am totally contrary to all the ballisticians in this regard, because I think they are wrong. Keep in mind, there is absolutely no demonstrated capability to measure the exact direction of the tip of the bullet relative to the predicted no-wind direction of the tip of the bullet, so it's just an argument without a definitive answer. Consider this: if the "bullet deflection" proponents are correct, what explains the greater tendency for this effect in lighter (shorter) bullets? Are their tips turned more by the wind? That's contrary to intuition (doesn't mean it's wrong), because the bullet is shorter, so there should be less angular deflection in a shorter bullet. OTOH, if my assumption of "bullet jump" being due to Bernoulli's principle (lower pressure on the side where the airflow is faster), then that would explain greater bullet jump by a lighter bullet, since it is going faster, and therefore rotating faster in the same barrel, compared to a heavier bullet. That would increase the airspeed over the top of the bullet in the right-crosswind scenario, thus creating greater pressure on the top surface of the bullet, and more "lift", especially when acting upon a lighter projectile. I cannot claim that there is no bullet deflection as the bullet exists the barrel in a crosswind, but I simply think that the Bernoulli principle is the major actor in this situation. Again, absolutely no way to measure this, that I am aware of.
I have an idea. I will try at some point to test a left-twist barrel and a right-twist barrel of the same length and same caliber, shooting the same bullet at about the same speed, and see if I can demonstrate the bullet impacting lower in a right crosswind, when fired from the left-twist barrel. If my reasoning is correct, that is what will happen. Also, the slant of the bullet jump angle decreases with distance, which is logical, if it is based on rotational speed, since the bullet is slowing down, producing less lift, yielding a more shallow angle to the bullet jump. Don't kill me through the internet until I've had a chance to do my left-twist/right-twist experiment. I know I'm the only person on the planet who has the opinion I just expressed. That does not make me wrong, nor will yelling at me with erudite pedantry change the way I see this. Let's see what the test shows.
I have an idea. I will try at some point to test a left-twist barrel and a right-twist barrel of the same length and same caliber, shooting the same bullet at about the same speed, and see if I can demonstrate the bullet impacting lower in a right crosswind, when fired from the left-twist barrel. If my reasoning is correct, that is what will happen. Also, the slant of the bullet jump angle decreases with distance, which is logical, if it is based on rotational speed, since the bullet is slowing down, producing less lift, yielding a more shallow angle to the bullet jump. Don't kill me through the internet until I've had a chance to do my left-twist/right-twist experiment. I know I'm the only person on the planet who has the opinion I just expressed. That does not make me wrong, nor will yelling at me with erudite pedantry change the way I see this. Let's see what the test shows.
Testing with different barrels could work but another option—and probably better—could be to test with the same barrel and just shoot the opposite direction to “turn the wind”. If anyone has access to a big patch of land it should be easy enough to wait until a good strong windy day, and record your shot and target locations with a GPS. The wind doesn’t have to be the same speed for all the shots either, just measure it for all the shots and make sure you know the wind direction reasonably accurately. Because wind can change a lot over the length of a longer shot and can be much more uncertain with a very high max ordinate, it may actually be better to shoot only 300-500 yds, but the stronger the wind, the better. If you only want to resolve the jim_k vs. mankind “is it up or down?” question, you probably don’t need to be super accurate. A shift either up or down of 2-4 inches should be easy to see. If you want a little more rigor, use multiple wind recorders along the path and shoot and record a much higher number of shots.
dcali
Bullet Maker
It's got nothing to do with Magnus - differentials in air speed over the top and bottom of the bullet. What is happening is an initial rotation and/or initial yaw is imparted to the bullet by some means as it leaves the muzzle. Wind, muzzle blast, poor bullet balance, whatever. The divergent angle is going to be at 90 degrees to that initial tipping force because of the spinning bullet (which 90 depends on which way it's spinning).
Think about it from the bullet's point of view. Your velocity is perceived as a straight headwind. Any side wind is added to that, so a left to right side wind means you'll experience a net headwind, but at a slight angle from left to right (slight, because velocity is so much greater than wind speed). That angled combined airflow creates a lifting/overturning force, trying to tip/push the bullet to the right. But because of the (right hand) spin, the bullet goes down rather than right. (And by lifting, I mean lifting sideways, not up. It's confusing, but that's how engineers think).
Reverse the wind direction, and now the bullet is being tipped/pushed left, and the right hand spin will cause it to hit high.
And the longer the bullet, the more impacted by jump it is. That's one reason benchrest shooters use short stubby bullets. They're less sensitive to jump, no matter the source.
Think about it from the bullet's point of view. Your velocity is perceived as a straight headwind. Any side wind is added to that, so a left to right side wind means you'll experience a net headwind, but at a slight angle from left to right (slight, because velocity is so much greater than wind speed). That angled combined airflow creates a lifting/overturning force, trying to tip/push the bullet to the right. But because of the (right hand) spin, the bullet goes down rather than right. (And by lifting, I mean lifting sideways, not up. It's confusing, but that's how engineers think).
Reverse the wind direction, and now the bullet is being tipped/pushed left, and the right hand spin will cause it to hit high.
And the longer the bullet, the more impacted by jump it is. That's one reason benchrest shooters use short stubby bullets. They're less sensitive to jump, no matter the source.
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Well, I'm just going to be different. When a bullet exits the barrel, the air in the barrel, which was in front of the bullet, is compressed, and jets out of the barrel in front of the bullet. I think this will prevent the crosswind from impacting the tip of the bullet first (thus producing the initial motion which makes the bullet yaw). I understand that there is no definitive way to prove, or more appropriately, to demonstrate that the yaw is from crosswind pushing the bullet tip, or to demonstrate that the yaw is "weathervaning". Has the yaw been demonstrated at all? Or has the yaw been proved to be from the push of a crosswind vs. from weathervaning? If the yaw is from the push of the crosswind, the bullet should weathervane, since the tip of the bullet has less cross-section for the wind to act on than the tail of the bullet, ergo the tail should be pushed relatively more, causing weathervaning. That would induce a precession-related bullet downward tipping with a right wind, and that would not make the bullet impact higher (which is what the bullet does). I know you disagree, but I'm just stating my reasoning. I hope you're right. I don't like change. I'd rather be wrong, and not cause any consternation. I'm off this subject until after I do my test, and I'll happily post my failure afterwards. Then everybody can focus on the thing and not the thought process, since very few people believe precession anyway; however, I will just mention that if you have someone on the back of your motorcycle, and they lean left when you try to turn right by leaning right (the passenger may do this to remain upright, I suppose), the motorcycle will forge straight ahead, and their counterleaning can only be overcome by precession. Simply stated, if you absolutely, positively, have to make the motorcycle turn right, exert forward pressure on the right handlebar. That knowledge is more important than my opinions about bullet flight. I have no training in ballistics, only in aerodynamics.
In reply, a bullet is not like an aeroplane as a plane is not aerodynamically unstable (hopefully) neither is it spinning at around 3000 revs/sec. Yes they both turn into the wind as you say but the aircraft does it because of aerodynamic stability and the bullet does it because of gyroscopic stability. Unfortunately for the bullet there is a gyroscopic reaction to the aerodynamic moments which also produces a vertical change in angle which produces the aerodynamic jump. It is a standard gyroscopic reaction to a moment about the centre of gravity (gyroscopes react to moments, not forces) in that the gyroscope moves at 90 degrees to the direction of the initial moment. Gyroscopic stability relies on this reaction to produce the feedback system which is the basis for gyroscopic stability.
The yawing behaviour can be and has been measured, most easily on large calibre projectiles using the yaw sonde, optical and multi receptor Doppler radar systems, and on bullets using high speed optical systems following the bullet. A friend of mine once built me a rig with an electric motor driving a gyroscope which could turn in any direction. He made it look like a bullet using baked bean cans and yogurt pots! It very successfully demonstrates just how the feedback system used for gyroscopic stability works and clearly shows the vertical angle change from a crosswind induced side yaw angle.
The yawing behaviour can be and has been measured, most easily on large calibre projectiles using the yaw sonde, optical and multi receptor Doppler radar systems, and on bullets using high speed optical systems following the bullet. A friend of mine once built me a rig with an electric motor driving a gyroscope which could turn in any direction. He made it look like a bullet using baked bean cans and yogurt pots! It very successfully demonstrates just how the feedback system used for gyroscopic stability works and clearly shows the vertical angle change from a crosswind induced side yaw angle.
dcali
Bullet Maker
Maybe this will help. The yaw exists the moment that the bullet leaves the muzzle because the muzzle is facing exactly down range, and the airflow relative to the bullet is not because of the wind. That *initial* yaw is what causes the jump. (It would be exacerbated if there were also an initial yaw *rate*, but we'll ignore that and assume the bullet leaves the muzzle straight and relatively undisturbed. There is a good reason, both theoretical and experimental, to believe that's a good assumption).
In the context of a dynamically imbalanced bullet, the instant it leaves the constraint of the bore, the bullet wants to rotate about it's true axis rather than the axis of symmetry. That causes an immediate yaw even in a no-wind situation, but it's in a random direction because we don't know exactly how the bullet is imbalanced ahead of time. The common theme here is that there is an initial yaw relative to the airflow at the moment the bullet is free of the bore. The result is an angular divergence from the ideal trajectory (perfect bullet, no wind), 90 degrees from the direction of the yaw, the sign of which is determined by the twist direction.
things that would increase it are pretty intuitive once you picture what's happening. faster twist, no matter the bullet, will always increase jump. Poor quality bullets would tend to exhibit a greater initial yaw due to greater imbalance (due primarily to jacket eccentricity tolerance), and therefore show greater jump. The shape and mass properties of the bullet also matter, as that determines the magnitude and resistance to the force of the air flow. This is hard to predict, but very generally, longer bullets have greater sensitivity to jump than short bullets.
In the context of a dynamically imbalanced bullet, the instant it leaves the constraint of the bore, the bullet wants to rotate about it's true axis rather than the axis of symmetry. That causes an immediate yaw even in a no-wind situation, but it's in a random direction because we don't know exactly how the bullet is imbalanced ahead of time. The common theme here is that there is an initial yaw relative to the airflow at the moment the bullet is free of the bore. The result is an angular divergence from the ideal trajectory (perfect bullet, no wind), 90 degrees from the direction of the yaw, the sign of which is determined by the twist direction.
things that would increase it are pretty intuitive once you picture what's happening. faster twist, no matter the bullet, will always increase jump. Poor quality bullets would tend to exhibit a greater initial yaw due to greater imbalance (due primarily to jacket eccentricity tolerance), and therefore show greater jump. The shape and mass properties of the bullet also matter, as that determines the magnitude and resistance to the force of the air flow. This is hard to predict, but very generally, longer bullets have greater sensitivity to jump than short bullets.
As a pilot also I'll throw some gas onto this topic. IMO aircraft & bullets have "very" little in common aerodynamically other than they both are somewhat pointed and fly thru the air. We must not confuse rotational "yaw" with "weather-vaning" IMO. Someone make a "tail-less" jet and see how much it "weathervanes" into the wind from crosswind forces. There is how-ever gyroscopic-precession from a rotating propellor. Totally bored and my 2 cents from a retired guy.
Eric in DL
Eric in DL
USMCDOC
Silver $$ Contributor
If you shoot 22lr BR, you would see this on a regular basis..
dcali
Bullet Maker
For the curious, this paper shows some methods of testing (inducing) and measuring initial projectile yaw for both artillery and small arms.
https://pdfs.semanticscholar.org/626e/5ee493222ec91dc2e3547e7942c1844f3df0.pdf
https://pdfs.semanticscholar.org/626e/5ee493222ec91dc2e3547e7942c1844f3df0.pdf
Look, I hate to rain on your parade, but aerodynamic jump is settled science and it has been for a long time. My most thorough involvement with the concept of aerodynamic jump was during my time at the U.S. Navy's Fighter Weapons School (known by the civilian public as "Top Gun") back in 1975 flying the F-14 fleet defense aircraft. This aircraft had a 20mm M-61 Gatling cannon and a gun-sight which was significantly more sophisticated than earlier gun-sights. That is because the F-14 had huge computing power compared with previous aircraft as well as a very good radar system which could reliably provide accurate ranging information to the fire control computer.
But aerodynamic jump wasn't invented with the F-14; far from it. It has been a factor in maneuvering gun sight systems well before my time in the military service.
Often, when firing a forward firing gun of the type carried by fighter aircraft, the angle of attack on the aircraft is large. In other words, the aircraft (and its gun) is pointing in a direction quite different from the relative wind. That produces an exaggerated version of the relatively mild crosswinds we target shooters experience and it makes aerodynamic jump an important factor to consider.
The fire control system solution required to hit a maneuvering aircraft under conditions where the range, angle-off, airspeed, altitude, air density, g-loading, angle of attack, are all constantly changing is a difficult one. If the scientists who developed this particular fire control system failed to have a good understanding of aerodynamic jump, they would not have been able to build a gun-sight which would work properly. The fact that they did it, along with my several interviews with the Grumman project manager for the F-14 gun sight convinced me that they did indeed know what they were talking about.
The proof of the pudding is, of course, in the eating. The air-to-air gunnery missions I flew against both a towed banner and a maneuvering "Dart" target (kind of like a huge lawn dart on the end of a long wire attached to a maneuvering tow aircraft) were not only educational, they were fun especially when scoring hits which I did with regularity. Had the gun sight and fire control computer designers been ignorant of what aerodynamic jump is and how it works, I would never had scored any hits.
Previously I had flown many air-to-air gunnery training flights in aircraft such as the F-11, F-8, and the F-4E, aircraft equipped with more primitive fire control systems than the F-14. Nevertheless, their relatively rudimentary gun sight computers took into account aerodynamic jump too and hits on the desired targets were achieved with regularity although in a much more narrow window. Why? Because one factor taken into account was aerodynamic jump. It's old news. People who need to understand it do indeed understand it and they have done so for a long long time.
My posts are not to enlist followers. They are to deal dispassionately and impersonally with a particular topic in a theoretical and experimental sense. It seems there are a parade of people piling on, and I'm proud that they include fighter pilots. I said I am a pilot. I started flying when I was 10 (no, my dad was not a pilot). I am a fixed-wing single and multiengine flight instructor, only straight-wing experienced, although I have the required years of air taxi (Part 135) and flight instructor (Part 141) work in the capacities of Chief Air Taxi pilot or Assistant Chief Flight Instructor, which are required to qualify me as Chief Flight Instructor for a Flag Carrier. I have flown with a Chief Flight Instructor for an Atlanta-based Flag Carrier (training base), as well as learning inverted flat spins from Frank Price in his Bücker Jungmeister two-holer biplane (the one he flew as the German ace in "The Great Waldo Pepper"). I was gifted the E6B whiz-wheel from the man who did the spin testing on the first supersonic USAF aircraft (F-100). I have demonstrated a totally blacked-out landing to a full stop from a plain ILS without PAR guidance (later done out of necessity by one of my co-instructors during the Gulf War - long story). And to my knowledge, I am the only person to successfully recover from a death spiral (another long story). Yes, Virginia, there is such a thing as a death spiral. I will not elaborate, even for my closest confrères, due to the risk from such shenanigans. There is no way I could have been a fighter pilot, though, because getting shot at scares me, and I lose my brain from fear. Sorry, and thanks for your service. Please see my latest post prior to this one, post #15, in which I responded to post #14. Thank you all for your concern over the possibility that I might have made a mistake. I will nevertheless continue as before, even including being the only person to stand up for something or someone I think is right. Fortunately, no harm done.
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