Physics of shooting a rifle

[tangolima]

Given bullet's BC and target distance, there exists optimum MV for minimum wind deflection.

I didn't realize that till I read an article on airgun pellet. The article gave brief qualitative explanations, but no analytical equations. I tried it on ballistic calculator. It is true. Wind deflection drops with increased MV till it reaches a minimum, and then it goes the other way. Speed doesn't always improve "wind bucking", BC always does.

That's interesting. I will post a link to the article later. Will dig out the Robert McCoy book too to try to derive an analytical solution.

-TL

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[44 AMP]

Wind deflection drops with increased MV till it reaches a minimum, and then it goes the other way.

I can see it going down to a minimum value but don't get how wind deflection could increase again after that. Could you explain that in small words for those of us who aren't very technical minded??

Area for wind to push against, constant, right?
Time wind has to push against the bullet is variable with the velocity. Right??

Faster means less time of flight, so less time for wind to push, so less deflection, Right??

SO, once you reach the minimum amount of deflection, how would further reducing the time of flight by increasing velocity, increase the amount of wind deflection??

I don't get how that could happen.

[stagpanther]

Borrowed this from the Pilots of America forum--italics and color are added by me.

Concerning a recent thread, “Tailwind question”, I read it too quickly and misunderstood what was being asked. My bad.

But in my response, I mentioned that the phase “<wind> blowing from behind” alerted me to a possible misunderstanding of the effect of wind on an airplane in flight, one I’ve seen before. But my answer prompted a disagreement with jimhorner over whether an airplane in flight is “pushed” by a steady tailwind

Oldtimers are certainly sick of it by now, but a while back I posted a compilation of what I called “Stick and Rudder Moments”. https://www.pilotsofamerica.com/comm...s-redux.79699/ I mention that now for the benefit of newbies, and to point out why I think the mental image of wind “pushing” a plane in flight can lead to misunderstandings.

Most relevant was the thought that engine cooling can be compromised when flying with a tailwind:

"5) I’ve heard it said cowl flaps are especially useful when flying downwind, when cooling would otherwise be compromised by the tailwind. More recently, a forum poster here thought winds affected cooling in a Seneca, possibly due to cowling shape. Then he doubled down with: "On my 206 I've notice a 5-10 degree change in CHT based on a strong wind. I am not a fluid dynamics expert, so I have no idea exactly why. Perhaps a slight pressure change in the cowl as I mentioned, or slight turbulence in the relative wind, IDK.""

Another was a pilot’s assertion that his Flight Design CTLSi seemed to run out of nose down trim more quickly when flying into a headwind.

Let me first stipulate the obvious - of course a tailwind results in a higher groundspeed. The question is whether the word “push” is the best descriptor for how the "wind" makes that happens. So, basically a semantic debate.

My assertion remains that once in the air, barring gusts and shear, a plane “feels” no wind. The affects of wind - in this case the additive effect of the tailwind on groundspeed - is simply because the air mass in which the plane is moving is itself moving.

So, in my mental construct there is no “pushing” going on. There is no wind “pushing against” the plane. To a pilot, a plane doing 50kts with a 50kt tailwind will have identical flight instrument readings as the same plane doing 50kts into a 50kt headwind. Under the hood, he or she will be unable to tell which is the headwind and which is the tailwind situation - barring instruments utilizing ground or satellite references, of course. Of course, an experiment could be set up using pressure transducers on the nose, tail and sides of a plane and again, the movement of the airmass the plane is in - the “wind” - will have zero effect on those readings.

I want to thank jimhorner for his thoughtful response - #29 in the “Tailwind question” thread. He clearly outlines that the disagreement may be based on frames of reference: “Its <the wind’s> force most definitely IS pushing your plane, just not relative to the air. It is, however, providing a force which pushes you relative to the ground, and that force is real.”

(italics mine)

There are many frames of reference we could discuss. But point is that as long as we’re airmen discussing airplanes, what we should concern ourselves with is flight through the air and language that best describes said flight. I still feel references to wind “pushing” can and does lead to flawed or inappropriate mental models that can lead to the “Stick and Rudder Moments” that many pilots still fall victim to.

[44 AMP]

Tailwind and headwind effects on aircraft doesn't seem to be well related to wind deflection (wind from the side /at an angle drifting a bullet).

Correct me if I'm wrong, but don't aircraft have to alter the positions of their control surfaces to counter the effect of wind from the side, below, or above?

A bullet can't do that.

[tangolima]

Originally Posted by 44 AMP

I can see it going down to a minimum value but don't get how wind deflection could increase again after that. Could you explain that in small words for those of us who aren't very technical minded??



Area for wind to push against, constant, right?

Time wind has to push against the bullet is variable with the velocity. Right??



Faster means less time of flight, so less time for wind to push, so less deflection, Right??



SO, once you reach the minimum amount of deflection, how would further reducing the time of flight by increasing velocity, increase the amount of wind deflection??



I don't get how that could happen.

Wind deflection is result of two factors; TOF and the force that pushes the projectile accelerating sideways.

TOF decreases with MV. As stated by unclenick's posts in the thread, the origin of the sideways force is the drag. The drag increases as the MV. Above certain MV, the 2nd factor becomes dominant, so the wind deflection comes back up.

This phenomenon happens with low BC projectiles. Airgun pellet has low BC (&lt;0.05), so it is more noticeable. But I wouldn't be surprised light varmint bullets at super high MV have the issue.

-TL

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[stagpanther]

Tailwind and headwind effects on aircraft doesn't seem to be well related to wind deflection (wind from the side /at an angle drifting a bullet).

Correct me if I'm wrong, but don't aircraft have to alter the positions of their control surfaces to counter the effect of wind from the side, below, or above?

A bullet can't do that.

Exactly--bullets don't have control surfaces (at least the ones used in conventional small arms); and the reference of wind (really the dynamic movement of airmass) "drifting" the bullet as opposed to "pushing" it is also "more" correct. Just like the pilot above observes, the difference between "pushing" and "drifting" may seem like a small question of semantics, but it can lead to a basis of faulty assumptions to further modeling. I'm not a fizzics expert but 25 year's of glider pilot experience ingrained this fundamental concept in real-world situations. Wind--or more precisely the movement of the airmass, will always have an effect on an object flying through the air, regardless of what direction vector the wind is relative to the flight path of the object. Our points of reference as shooters are mostly static ground-bound, so those effects are generally seen only as variations in elevation and windage at bullet impacts (keep in mind I'm not discounting the forces of inherent bullet stability, drag, coriolis, pressure wave etc).

[Unclenick]

Bullets don't get pushed or drifted by wind. "Wind drift" is a misnomer maintained out of historical habit. It is what F. W. Mann tried unsuccessfully to demonstrate, likening the bullet's flight through air to a boat drifting downstream or a leaf tossed on the wind. But the numbers don't add up. Even a boat placed in a running stream or a leaf released by a tree don't instantly start moving at the speed of the flowing fluid they find themselves dropped into. First, the motion of the fluid exerts drag on the item in question, and that force starts overcoming it's static inertia and moving it. As it picks up speed, the difference between its speed and that of the fluid diminishes, so the drag force drops off, so the acceleration drops off, and the object's speed closes in on the fluid speed more and more gradually as it gets closer to it. This continues until any difference in the two speeds is lost in the noise and the two appear to be moving together. In the time of flight of a bullet, it can account only for a small fraction of the actual bullet deflection from its zero wind POI (as I showed by calculation for a 22 LR in the attachment to one of my earlier posts on page 3 or 4, IIRC).

At any rate, the official correct physics term is wind deflection, even though we keep calling it wind drift. The same is true of spin drift, which is actually a deflection caused by the yaw of repose slightly favoring one side in deflecting the bullet's headwind off the ogive of the bullet nose.

[stagpanther]

Bullets don't get pushed or drifted by wind. "Wind drift" is a misnomer maintained out of historical habit. It is what F. W. Mann tried unsuccessfully to demonstrate, likening the bullet's flight through air to a boat drifting downstream or a leaf tossed on the wind. But the numbers don't add up. Even a boat placed in a running stream or a leaf released by a tree don't instantly start moving at the speed of the flowing fluid they find themselves dropped into. First, the motion of the fluid exerts drag on the item in question, and that force starts overcoming it's static inertia and moving it. As it picks up speed, the difference between its speed and that of the fluid diminishes, so the drag force drops off, so the acceleration drops off, and the object's speed closes in on the fluid speed more and more gradually as it gets closer to it. This continues until any difference in the two speeds is lost in the noise and the two appear to be moving together. In the time of flight of a bullet, it can account only for a small fraction of the actual bullet deflection from its zero wind POI (as I showed by calculation for a 22 LR in the attachment to one of my earlier posts on page 3 or 4, IIRC).

At any rate, the official correct physics term is wind deflection, even though we keep calling it wind drift. The same is true of spin drift, which is actually a deflection caused by the yaw of repose slightly favoring one side in deflecting the bullet's headwind off the ogive of the bullet nose.

Thank you for the reponse. I know it's irritating to put up with stubborn comments sometimes from people like me. I think wind deflection is still a better term than "pushing" Anyway, I hereby promise to the forum I will never bring it up again.

[Unclenick]

If even F. W. Mann couldn't get past "wind drift", you have nothing to apologize for.

[stagpanther]

If even F. W. Mann couldn't get past "wind drift", you have nothing to apologize for.

Well, in that case, the real point I was trying to make is that...

[davidsog]

Correct me if I'm wrong, but don't aircraft have to alter the positions of their control surfaces to counter the effect of wind from the side, below, or above?

Only in relation to the ground to hold a constant heading or position. Otherwise the aircraft does not feel the wind.

Great explanation in this book:

https://www.kevinkoperski.com/post/stick-rudder/

[mehavey]

Anyone who's had to hold hard rudder & aileron landing into a strong crosswind . . . .
https://www.youtube.com/watch?v=NglxhkfP1ds&t=409s

[stagpanther]

For all airspeeds there is an associated angle of attack to the apparent wind at which an aircraft can be stalled. In the past few months commercial airliners have been able to exceed 700 or more mph indicated groundspeed over the US and when transiting the pond; nobody is breaking the sound barrier. Friend of mine just posted this yesterday doing a regional flight over the US.

[stagpanther]

I'll just drop this right here...

[mehavey]

I think we now have the epitome of
"circular arguments at cross purposes"


.

[stagpanther]

What? You don't like my drawing? I am an artist you know.

[davidsog]

I think we now have the epitome of
"circular arguments at cross purposes"

Bingo.

There are quite a few terms confused and convoluted to obscure a very simple truth.

"the aircraft does not feel the wind". Consequently, a bullet does not feel it either.

However, the airmass is moving and we will perceive the "wind drift" when our frame of reference is two stationary points on the ground that are NOT moving in the airmass.

Convoluting separate concepts of turbulence at airmass shear does not help illustrate that fact of physics.

Even in turbulence, an airfoil maintains a constant air density as it only produces the lift required. We perceive that maintenance of constant air density as movement about the CG and because our frame of reference is seperate in not being attached to the aircraft...we feel the acceleration.

[stagpanther]

airfoil maintains a constant air density

Errr...what??? I'm taking a wild guess and assuming what you mean is that due to camber/variable airfoil the Bernoulli effect (some people have argued that a sheet of flat plywood can fly quite well under the right circumstances) creates a net positive lift pressure under the wing when splitting the airflow over the wing?

I'm not arguing the drift vs push thing--what I'm pointing out is that in general where there's wind, there's turbulence, especially near the ground. Of the three sources of disturbed airflow I show in my crude drawing above (there are others, like downdrafts/catabatic winds) chances are very high you're going to be exposed to at least one of them while shooting from the ground.

[44 AMP]

When sideways (off centerline axis) pressure is applied, aircraft and boats "skid". They get moved off course. SO do bullets. Aircraft and water craft have control systems to compensate for that. Bullets do not.

Can we agree on that?

Car's tires have traction, which "anchors" them to the ground more firmly than boats in the water or airplanes in flight, they don't drift as much, but you can feel your car being rocked by a gust of sideways wind, so its obvious the pressure is there.

A bullet cannot compensate for sideways force the way a steerable vehicle can. So only the shape, surface area and speed are factors in how much drift from a given "push" the bullet undergoes.

It was stated that as the bullet goes faster the effect of "wind drift" goes down, (meaning the amount of off axis movement is reduced) and that makes sense to me.

However, it was also stated that when a bullet goes faster than the point where minimum drift happens, the amount of drift goes back up, and that I cannot figure out. Still hoping someone more skilled in this matter can explain it to me in a way I can grasp.
Thanks.

[stagpanther]

So only the shape, surface area and speed are factors in how much drift from a given "push" the bullet undergoes.

I would add time (duration) of flight exposed to the wind to that.

[davidsog]

I'm taking a wild guess and assuming what you mean is that due to camber/variable airfoil the Bernoulli effect

CL = L/qS

p = Density

q = 1/2 * p * V^2

L = Lift = weight in steady state flight = constant

S = Lifting Surface Area = constant

When an aircraft moves thru the airmass it must maintain that constant density to remain in flight. When that density changes due to a change in air mass velocity at the shear.....
The density remains constant from the aircrafts FRAME OF REFERENCE as the aircraft will remain at a constant density. YOUR Frame of Reference is not the aircrafts and because you are not in that same Frame of Reference you will feel it as motion as the aircraft keeps that same density.

All motion is relative to a frame of reference.

https://physics.info/frames/

When sideways (off centerline axis) pressure is applied, aircraft and boats "skid". They get moved off course.

That sideforce is called spindrift. It is a factor in long range precision shots.

[tangolima]

Originally Posted by 44 AMP

It was stated that as the bullet goes faster the effect of "wind drift" goes down, (meaning the amount of off axis movement is reduced) and that makes sense to me.



However, it was also stated that when a bullet goes faster than the point where minimum drift happens, the amount of drift goes back up, and that I cannot figure out. Still hoping someone more skilled in this matter can explain it to me in a way I can grasp.

Thanks.

I did in post #155.

-TL



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[JohnKSa]

I don't think that's an explanation that provides any sort of intuitive insight. I suspect he's looking for answers to these questions that will provide an intuitive grasp of the situation.

Drag always goes up with velocity, so it's not simply a matter of drag increasing. WHY does the increased drag, even with lower TOF result in more POI change due to wind?

Intuitively, with a shorter time for the wind to operate on the bullet, one thinks it should move less. What effect is resulting in more POI change due to wind even though the wind is the same and has less time to operate on the projectile?

You mention that this doesn't happen all the time. What are the specific circumstances under which increasing velocity also increases POI change due to wind and how do those circumstances result in this effect?

[davidsog]

I don't think that's an explanation that provides any sort of intuitive insight.

There is a fundamental misunderstanding in the term "Wind Shear". There seems to be some belief that turbulence is wind shear. It is not. Wind shear is a very isolated phenomenon that occurs between airmass in only four situations. Turbulence occurs in all airmasses. We feel turbulence because we are not physically a part of the aircraft. That is why they tell you to remain seated with your seat belt buckled when experiencing it.

Wind shear is a very different animal. In order for your bullet to experience Wind Shear effects it would have cross gust front of a Thunderstorm and the wind shift line between airmasses. They convergence of those two things MIGHT create a shear. Mostly it will just make turbulence.

The other conditions for Wind Shear are simply not something you would ever experience on a rifle range as they occur at altitude.

[tangolima]

Originally Posted by JohnKSa

I don't think that's an explanation that provides any sort of intuitive insight. I suspect he's looking for answers to these questions that will provide an intuitive grasp of the situation.



Drag always goes up with velocity, so it's not simply a matter of drag increasing. WHY does the increased drag, even with lower TOF result in more POI change due to wind?



Intuitively, with a shorter time for the wind to operate on the bullet, one thinks it should move less. What effect is resulting in more POI change due to wind even though the wind is the same and has less time to operate on the projectile?



You mention that this doesn't happen all the time. What are the specific circumstances under which increasing velocity also increases POI change due to wind and how do those circumstances result in this effect?

Wind deflection and drag, see unclenick's explanation in post #70. Basically the force that makes the bullet move sideways is syphoned off the drag. The magnitude of the cross wind determines how much of the drag force got diverted.

With high speed and low BC, the drag force increase, so does the force that move the projectile sideways. If the reduced TOF can't offset the effect of the increased force, wind deflection comes back up.

I will dig up the McCoy book to get a analytical solution when I have a chance. For now just try this on a ballistics calculator. Keep increasing MV and observing the change in wind deflection.

-TL

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