stagpanther
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Would have been nice if he had referred to "velocity of plane" as indicated airspeed (adjusted for pressure/density) and relative wind as "the wind speed the aircraft experiences."
Physics of shooting a rifle
- Thread starter tangolima
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Oh, yes. Terminology and conditions is the death blow to these kinds of discussions. It makes it very difficult over the internet.
The last few seconds of the video are the most important. You can see the vector of the wind across the ground is exactly matched by an object in the air at steady state flight with equilibrium of forces.
With our bullet, we never achieve that equilibrium. It's not self powered and leaves the barrel with a finite amount of energy therefore it is never in steady state flight. It is constantly changing including its orientation over the flight path. If took snapshots of the bullet and could measure the forces then at each snapshot for those conditions, the bullet would be adopting the "Velocity of the Bullet Relative to the Wind" that moves it as part of that air mass under those conditions.
The physics of aerodynamics does not change, it's just buried in secondary frame of reference as we only really care about the bullet in reference to the ground because our target is on the ground, not the air. It is buried in constantly changing conditions of flight due to the fact our bullet is never in steady state flight.
Our ballistics math is actually pretty crude in the civilian sector. That does not mean it does not predict the flight of a bullet with good agreement, it just not as precise in its mechanics or explaination. I don't have to know many joules in a mole of naphthalenes to figure out the Distance (D=R*T) on a car traveling 55 mph for 60 minutes.
That being said, we have the ability to actually measure this stuff and Department of Defense uses an aerodynamics based approach that seems to work very well.
It's a little tricky to use a plane as an analog to a bullet since a plane is powered and guided while a bullet is an unguided projectile and is neither creating its own velocity via some sort of propulsion mechanism nor steering itself via some sort of guidance system.
If one could make a non-powered, non-guided projectile that stayed in the air for a very long time, it would eventually "assimilate" the sideways velocity of the air mass.
I would be interested to see where the Sierra Engineers claim that a bullet will acquire the full sideways velocity of the air mass it moves through.
Correct. That's what I said.
" In practice, that's not going to happen with bullets because the time they are in the air is not sufficient for them to acquire the same sideways velocity as the air mass. "
??? It doesn't seem like you are disagreeing with what I said. Or rather, I guess it does SEEM like you are but the comment about never achieving equilibrium agrees perfectly with my comment.
That is correct. It is gradually acquiring the sideways velocity (relative to the target) but so gradually that it will never actually get there before it hits the target (or the ground). In fact, it will never get anywhere close to it.
stagpanther
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Except that an aircraft can adjust airspeed and use control surfaces to compensate for drift in the airmass to arrive at ground-based stationary target, no?
Control surfaces and Power application do not change the physics or allow an airplane to alter the laws of physical science.
It makes it harder to see some of the individual effects for sure but it does not change them. It also allows us to see the aircraft performance by comparing steady state flight to maximum forces available. All aircraft performance problems are based upon that relationship.
It is no different than our bullet which never reaches steady state flight. Our bullet just starts out with a finite amount of energy and moves to zero.
Even more important is the fact we are changing frame of reference in our bullet flight calculations. Ballistic calculations are all about ground frame reference not an aerial frame of reference.
That does not mean the physical laws that govern aerial objects get violated, LOL.
stagpanther
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I didn't say or imply that they did. I think I made it fairly clear that the use of control surfaces and airspeed adjustment allows an aircraft to compensate for the drift in the airmass in order to arrive "on target" for a ground-base reference target. I don't think that conflicts with anything you have argued as far as I can tell.
My fault, I missed the ground base reference and you are exactly right. All of us have to be careful as conditions make all the difference.
Our bullet does the same when it picks an angle of repose. To the air, it is moving as an aerial vehicle with the air mass. From the Ground Reference, it has picked an angle that will cause it to arrive on the target as much as it can compensate for given its diminishing energy and other forces acting on the bullet which is why is not an exact match for wind velocity in the ground reference frame.
We are good...it was my bad and I apologize.
Arrow fletching can be slanted off parallel with the shaft to make it spin, but all the arrow needs to steer its point into the wind is for its center of pressure to be behind its center of mass. This is called static stability. Since things tumble around their center of mass (CM, aka, the center of gravity), if an air stream pushes harder on one side of the center of mass than on the other, it will push that side away in the direction of airflow, leaving the other side pointing upstream into the airflow. The fletching provides the area needed to ensure the airstream over the shooting arrow applies more pressure on its side of the CM than it does on the arrowhead side, thus keeping the fletched end turned downwind. If you don't believe it, take the head off an arrow, noch the tip end of the shaft, and shoot it backward. No practical amount of spin you can put into its flight will prevent it from hooking around.
I'm sorry I haven't finished illustrating. I am trying to get better at animation to show coning and epicylic swerve and also what it looks like when the bullet path's frame of reference is the air mass or the ground or the bullet. It turned out to be a somewhat bigger undertaking than I first realized.
TL is correct in that precession is the principal actor in spin stabilization. Precession is a phenomenon in which pressure applied to turn the spin axis of a spinning object causes it to react by turning perpendicular to the direction of the applied force. It isn't intuitively obvious to most why this happens, and while I have a good way of visualizing it, I don't want to engage in a lengthy digression when there are so many YouTube video explanations you can find with a single search.
I'll try to get my animations done and lay out a more complete picture.
I'm sorry I haven't finished illustrating. I am trying to get better at animation to show coning and epicylic swerve and also what it looks like when the bullet path's frame of reference is the air mass or the ground or the bullet. It turned out to be a somewhat bigger undertaking than I first realized.
TL is correct in that precession is the principal actor in spin stabilization. Precession is a phenomenon in which pressure applied to turn the spin axis of a spinning object causes it to react by turning perpendicular to the direction of the applied force. It isn't intuitively obvious to most why this happens, and while I have a good way of visualizing it, I don't want to engage in a lengthy digression when there are so many YouTube video explanations you can find with a single search.
I'll try to get my animations done and lay out a more complete picture.
stagpanther
New member
Just from the purely intuitive sense (from years of archery experience and with bullets) I suspect you can't make an apples to apples application of physics of bullet flight to that of arrows--but I look forward to your explanation.
As to the precession thing, I'm not very skilled with physics and readily admit ignorance, but I do know a little about aircraft design. I looked at some of the diagrams and explanations for the forces of precession that affect a bullet's flight and immediately encountered the "given" that there is a pressure/lift differential that causes a pitch up or down of the nose of the bullet. I take this to mean that a bullet design can actually climb--or dive--as a result of a change in its angle of attack to the relative wind. Classic wing design theory holds that this pressure differential that results in lift is a result of what is often equated to the Bernoulli effect; because of the camber in the leading edge of a wing and the distance the relative wind travels over the chord of the wing to the trailing edge, it has further to go over the top surface than the bottom surface and that is that primary mechanism for creating lift (there are some aerodynamic physicists who disagree with this, imagine that ). Bullets obviously do not have a variable camber to create pressure differential and hence climb or dive. Where I do see the similarities between a bullet and an aircraft is the effect of parasitic drag which affects the flow of the relative wind over the surfaces that affect the stability in flight. You can easily see that in videos of vortices streaming off of aircraft's wings. People often think that stall of an aircraft is a function of the airspeed--it is not, it is a function of the angle of attack for which there is an airspeed at which that stall happens.
Fire away!
As to the precession thing, I'm not very skilled with physics and readily admit ignorance, but I do know a little about aircraft design. I looked at some of the diagrams and explanations for the forces of precession that affect a bullet's flight and immediately encountered the "given" that there is a pressure/lift differential that causes a pitch up or down of the nose of the bullet. I take this to mean that a bullet design can actually climb--or dive--as a result of a change in its angle of attack to the relative wind. Classic wing design theory holds that this pressure differential that results in lift is a result of what is often equated to the Bernoulli effect; because of the camber in the leading edge of a wing and the distance the relative wind travels over the chord of the wing to the trailing edge, it has further to go over the top surface than the bottom surface and that is that primary mechanism for creating lift (there are some aerodynamic physicists who disagree with this, imagine that
). Bullets obviously do not have a variable camber to create pressure differential and hence climb or dive. Where I do see the similarities between a bullet and an aircraft is the effect of parasitic drag which affects the flow of the relative wind over the surfaces that affect the stability in flight. You can easily see that in videos of vortices streaming off of aircraft's wings. People often think that stall of an aircraft is a function of the airspeed--it is not, it is a function of the angle of attack for which there is an airspeed at which that stall happens.Fire away!
Just from the purely intuitive sense (from years of archery experience and with bullets) I suspect you can't make an apples to apples application of physics of bullet flight to that of arrows--but I look forward to your explanation.
As to the precession thing, I'm not very skilled with physics and readily admit ignorance, but I do know a little about aircraft design. I looked at some of the diagrams and explanations for the forces of precession that affect a bullet's flight and immediately encountered the "given" that there is a pressure/lift differential that causes a pitch up or down of the nose of the bullet. I take this to mean that a bullet design can actually climb--or dive--as a result of a change in its angle of attack to the relative wind. Classic wing design theory holds that this pressure differential that results in lift is a result of what is often equated to the Bernoulli effect; because of the camber in the leading edge of a wing and the distance the relative wind travels over the chord of the wing to the trailing edge, it has further to go over the top surface than the bottom surface and that is that primary mechanism for creating lift (there are some aerodynamic physicists who disagree with this, imagine that
Fire away!
Arrow definitely flies differently from bullet. That was the point quoting it as example in the first place. One is easy to understand fin stabilized, whereas the other is complicated spin stabilized.
I took ground school long ago and the last entry in my logbook was over 25 years ago, so a lot details about aeroplane have gone rusty. The lift of a plane have different components. Bernoulli effect optimized by the wing's airfoil cross section is the major one, no doubt. There another component is simply the apparent wind striking the underside of the airframe. You feel it when sticking hand out of a moving car. Bullet fly experiences the same I think. Nose-up attitude climbs and nose-down dives.
Stalling in aeroplane is indeed due to angle of attack. But only advanced planes have sensors that detect angle of attack directly (remember Boeing 737-max fiasco?), so airspeed is often used as indicator. It is only valid on the condition that the pilot is trying to maintain altitude by changing plane's attitude.
Thanks Unclenick to give inputs. Looking forward to seeing the illustrations, which are top notch as always.
-TL
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It is more than just spin stabilized although that is one component of the overall stabilization picture.
Unless we are talking Canard Aircraft the normal relationship to achieve stability is what UncleNick related:
Bullets at rest or traveling slow enough do not have this relationship. Most bullets are designed so that the Center of Pressure (CP) is in FRONT of the Center of Gravity (CG) unless they are traveling fast enough. Center of Mass and Center of Gravity are interchangeable as long as the acceleration of gravity for the environment is constant, in other words we aren't talking about a different planet or an extremely tall object.
Due to the effects of normal shock wave formation in compressible aerodynamics the relationship of the CP to CG changes once the bullet is fired. The CP moves rearward with normal shock formation until it is behind the CG and our bullet becomes stable.
That movement is one of the reasons for aerodynamic jump and is part of the process the bullet undertakes in picking the angle of repose it requires to move as a part of the airmass.
You are correct.
Any object traveling thru the air will obey the same physical laws. However, they all do not have the same forces application acting upon them.
A rocket, a piston engine airplane, a balloon, a bullet, and an arrow all adhere to the same physics.
When having a discussion without a fundamental understanding of the basics it makes little sense to jump around to different aerial objects whose behaviors are different only because of the force application. It leads to wrong conclusions about what is going on with our object.
Of all of them, an airplane is steady state flight is the most useful object for understanding the basic principles of an object moving thru an air mass. That is why is used to teach Aerodynamics.
I agree that bouncing around to different aerial objects will not be useful.
stagpanther said:
Yes. It's messy and irregular. When the bullet clears the muzzle, the gas behind it in the barrel, being lighter than the bullet, accelerates and blows past the bullet. So, the bullet actually has a tailwind for a brief period between interior and exterior ballistic effects called transitional or intermediate ballistic period. That tailwind increases the bullet's forward velocity without increasing its spin (because it is no longer constrained by the bore and rifling). The effect on velocity is commonly on the order of three percent or so of the peak bullet velocity. During the transitional period, and especially in the first few calibers of travel out of the barrel (eleven calibers were measured by the military for artillery), if there was any flaw in the muzzle crown's axial symmetry or if the bullet was at all tilted in the bore, the muzzle blast will play unevenly off its base, and this contributes significantly to initial yaw and to lateral drift. But even if a bullet has no initial yaw coming out of the muzzle, as soon as it is clear of the bore, gravity starts pulling it down, and unless it is fired straight up, that starts to pull it into the trajectory arc. The headwind experienced by a bullet is always tangent to the trajectory arc, so as that arc turns down, that headwind starts to angle up, and that puts a vertical overturning drag on the bullet because its gyroscopic stiffness tries to keep it on the original bore line. As a result, the headwind itself becomes the source of an upward overturning force that initiates coning motion.
Note that there is also a lift on the bullet angled off the tangent line of the trajectory that moves the whole bullet and doesn't just act to overturn it. As precession causes coning to set in, the direction of that "lift" goes around the clock, causing the bullet's center of mass to travel in a spiral. Each time coning goes around the clock it pushes the bullet back where it came from, so this helix simply circles the trajectory path. I calculated it for a 30-caliber match bullet once long ago, assuming about one moa of yaw, and it worked out to be something like 0.02" in diameter, IIRC. I've forgotten the rate of twist and the bullet weight, but it's a small orbit that shrinks as a properly stabilized bullet goes down range because the coning radius spirals inward on such a bullet. In any event, you have a couple of cyclical motions here, such as the coning and the helical corkscrewing. The term epicyclic means to have multiple cycles at once, as when the moon orbits the earth, but the earth also drags it along an orbit around the sun at the same time, not to mention the whole galaxy turning, so the moon's motion is epicyclic, and so is a bullet's.
When I first started shooting Service Rifle matches back in the '80s, I noticed that when the sky was clear on Viale range, and you stood at the 600-yard line watching guns shooting at the slight upward angle that requires, you could see the mirage of the bullet wake when a rifle fired, and some were pretty straight, but some were a very pronounced helix, indicating bullets with less or more initial yaw, and the same gun might fire one and then the other or anything in between. This causes a small difference in ballistic coefficient from shot-to-shot, of course, but not nearly enough to spoil a good score.
The transitional ballistic period, the moment the bullet exits muzzle, is a critical moment. The stabilization mechanism hasn't been fully established. Slight perturbation during this period would have profound effect on the rest of the flight. Along this thought, do you think a muzzle device of enough length may do good by shielding the bullet from cross wind? 11 calibers for a 6mm bullet is 2.67". I will look for a 3" long flash can to try. I currently have a shorty on my .243 win.
-TL
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stagpanther
New member
The reason I have a problem with some of the definitions is that they sometimes confuse what we on the ground observe as wind blowing through trees, flags etc. with relative wind to an object in flight--it is independent of this wind, it is what the flying object "sees"--hence it is relative to it only.
tangolima said:
Yes. Boattails are sensitive to uneven or asymmetric wind deflecting of the slope of the boattail, tending to impart slow drift away from the intended trajectory in the direction opposite the gas deflection. The final stabilization mechanism in the relative wind that David defined can't begin until the bullet has exited the temporary blast sphere of accelerated gases, as we've all seen in Schlieren shadowgraphs of a firing rifle. Until that moment, the bullet has no slowing drag, and any hint of starting gyroscopic stabilization in that short time period would be a response to the tailwind. Since the twist direction for response to the tailwind is from the front of the bullet, that precession response will be backward from the precession off-axis drag on the nose will induce later. So if it made any significant attempt to stabilize in that short transitional period, it would have to be undone after it cleared the blast sphere.
tangolima said:
That muzzle blast sphere is already a shield for a foot or two. If you install a muzzle brake, though, it will reduce that sphere's size. The sphere is comprised of gasses that have exponential deceleration as they expand, which gives them more and more area of atmosphere to lose kinetic energy against. It's not quite a perfect sphere, but close enough that its force will be close to obeying the inverse square law that the strength gravity and light and magnetism from an isotropic source do. In other words, double in size and have four times the deceleration, so this stuff slows down quickly.
This suggests a gun with a muzzle brake will not only lose the last 3% of acceleration from muzzle blast, but the bullet will experience wind a foot or two sooner.
As far as seriously affecting wind, Harold Vaughn got his gun club to let him install 100 yards of sewer pipe to give them a zero wind firing point. Doing that for longer ranges is limited by the height of the trajectory, as the sewer pipe gets pretty costly when it's ten feet wide. But stopping the wind takes that sort of effort.
Damn! I thought I was on to something that would work. Nothing new under the sun. I'm getting a plain thread protector then.
-TL
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