For you ballisticians.

[Vibe]

,,,,,with our discussion.

Let's assume the straight-line over-the-ground course from the muzzle to the target at 1000 yards is due north or better referred to as heading '0' degrees. Our bullet starts out on a heading of '0' degrees, its ground track is also '0' degrees as there is no crosswind in the first 500 yards. If the bullet is launched at 3000 fps it feels a relative wind right on the nose of 3000 fps. Of course, it is constantly decellerating after leaving the muzzle.

Reaching 500 yards, our bullet enters a direct crosswind from left to right at 10 mph. The crosswind results in a bit of drag on the left side of the bullet and it turns half a degree left, into the crosswind, to streamline itself with the new relative wind.

The new relative wind has a vector component of the negative of the remaining forward velocity, plus the 10mph at right angles to it. Now the bullet is being accelerated in both of those directions - negative to the direction of travel. If 0° is X then the crosswind is along the Y axis.

The bullet is now flying perfectly streamlined thru the airmass on a heading of 359.5 but its nose is pointing half a degree left of course relative to the ground. If not for the fact that it is constantly slowing down, the bullet would not drift at all!

OK. But only because it will not drift in a vacuum.
But neither would it change heading or pointing direction.

As velocity continues to decrease, on a heading of 359.5, the bullet begins to drift downwind over the ground. Heading does not change but ground track does, and that's what we are concerned with; ground track.

"Heading" actually will change slightly as the direction of travel and the direction of the crosswind become less and less perpendicular...but that's a bit of nit picking in this example. It really only becomes important in Als example of the upward "wind" seen during falling. This is because, while under the influence of the crosswind, the ground track path will be curved, not straight - and the longer it is in the crosswind, the more "off course" the bullet will seem to be.

Perhaps the reason so many have trouble accepting my explanation is because they think the bullet turns much farther than it actually does. Of course, I realize we often exagerate things to make a point, but Alinwa's drawing is a bit misleading in that it depicts the wind correction angle as approximately 30 degrees! No one would believe a bullet turns 30 degrees back to its original heading upon exiting the crosswind at 601 yards and of course, it doesn't. It was only crabbed (relative to the ground) half a degree (heading 359.5) to begin with so when it exits the crosswind at 601 yards, the bullet turns half a degree to the right, streamlines itself with the new relative wind and is now on its original heading of '0' degrees. From 601 yards to the target winds are calm, there is no crosswind component so there can be no drift.

the problem is that while the bullet WAS in the crosswind - it was accelerated in the direction the wind was going, and therefore still retains all of the velocity transferred to it in the Y axis direction. Once the crosswind is no longer accelerating it, this becomes a constant Y direction velocity in the direction of the wind...until an opposite crosswind cancels it out...Of course this Y axis component is also now degrading and a rate exactly proportional to the rate that the X axis velocity is degrading.

 

[Vibe]

Getting fixated on the bullet's attitude is a mistake. It's attitude is not WHY it drifts.

al

It's attitude is only why it drifts as little as it does - it reduces drift (by presenting the smallest frontal area toward the source of drag)...but it does not eliminate it.

 

[Gene Beggs]

"The bullet is now flying perfectly streamlined thru the airmass on a heading of 359.5 but its nose is pointing half a degree left of course relative to the ground. If not for the fact that it is constantly slowing down, the bullet would not drift at all!"

Gene,
I was doing okay until the above statement. In my pea-brained mind the airmass is moving right, the bullet is within the airmass, and therefore the bullet will drift right even if there is no slowing down due to drag. Please explain your reasoning behind "If not for the fact that it is constantly slowing down, the bullet would not drift at all!" If I am standing at the firing line and observing the bullet (superman eyes) relative to the target, I will observe a right drift. I agree it will drift much less if there is little lag, but it will still be seen as drifting,
Thanks,
Richard

Richard

I'm sorry you're confused about this. Here, you will find an in depth explanation of what's happening.

http://benchrest.com/forums/showthread.php?t=43279


Gene Beggs

 

[Gene Beggs]

Thank you sir!

It's attitude is only why it drifts as little as it does - it reduces drift (by presenting the smallest frontal area toward the source of drag)...but it does not eliminate it.

Well stated.

Gene Beggs

 

[model14]

Richard

I'm sorry you're confused about this. Here, you will find an in depth explanation of what's happening.

http://benchrest.com/forums/showthread.php?t=43279


Gene Beggs

And I am sorry you will not explain your "no lag, no drift" statement to me. I guess if the bullet is rocket propelled and has no lag that it will not drift. Bull; I don't believe it! Please lower yourself to my level for a moment and explain this to me. I have read the lonnnggg thread on this subject and it does not answer my question. I started this thread, remember? I was doing fine until your last post.
Thanks,
model14

 

[alinwa]

It's attitude is only why it drifts as little as it does - it reduces drift (by presenting the smallest frontal area toward the source of drag)...but it does not eliminate it.

Agreed.

To further elaborate, it's attitude is the result of the bullet fighting for stability while negotiating changes in environment.

Serendipitously "most stable" and "least drag" coincide for elongated, spin-stabilized projectiles.

For round balls all spin-stabilization does is get rid of the "curve ball" effect.

al

 

[Vibe]

Agreed.

To further elaborate, it's attitude is the result of the bullet fighting for stability while negotiating changes in environment.

Serendipitously "most stable" and "least drag" coincide for elongated, spin-stabilized projectiles.

al

That depends upon how loosely you interpret the definition of "most stable"
A bullet with a dynamic stability factor around 1.0 to 1.5 would do this...spin one up to a factor of 10.0 and it won't care about changes in the environment...it will gyroscopicaly continue to point the same direction even in a 100mph crosswind. Which is why over-stabilized bullets are not often as accurate.

 

[mks]

Where it's pointing question...
On something like a typical Matchking, where is the center of pressure relative to the center of gravity? When the force of a cross wind is applied from the side, at what point is the force considered to have been applied? If a bullet is turned, by applied force, where is the pivot point?

Another question
When the wind blows on a cylinder, at right angles to its axis, what happens to the air pressure on the windward and lee sides?

The CP is forward of the CG. Pressure is high on the windward side and low on the leeward. The pivot point is the CG. http://www.nennstiel-ruprecht.de/bullfly/fig5.htm

Cheers,
Keith

 

[tobybradshaw]

I guess if the bullet is rocket propelled and has no lag that it will not drift. Bull; I don't believe it!

I'm sure Gene can handle this, but here's a short thought experiment:

You have an airplane that can fly 100mph. You fly from "calm" air (no air mass movement relative to the ground) into a crosswind from 3 o'clock. The crosswind is moving 100mph relative to the ground. How much does the airplane "drift" downwind (3 o'clock to 9 o'clock direction)?

What happens if the crosswind speed is 50mph?

Toby Bradshaw
baywingdb@comcast.net

 

[alinwa]

For a given cross wind, the heavier bullet will not be accelerated across the flight path by the force of the cross wind as much as the lighter bullet (remember Newton?), and will therefor have less wind drift. Also, BC is measured with no wind component and all drag is strictly along the straight line flight path, so any effects of cross wind are not taken into account.

M14

Back to the OP.

model14, your initial assumption is wrong. The bullet IS NOT accelerated across the flight path by the force of the crosswind.

This is the disconnect, this is where the weirdness starts........ Here's the thing; If you SHOOT a bullet and you DROP another bullet they'll "both hit the ground at the same time" right? The two motions are independent of each other.


Likewise if you SHOOT a bullet through a crosswind and DROP a bullet through the same crosswind they should move over the same. Right??


Wrong!


If you SHOOT a bullet and DROP a bullet they WILL NOT both be blown over equally. The sideward acceleration from the wind isn't analogous to the downward acceleration due to gravity. The bullet is not blown sideways by the wind.


Regarding your second statement about BC being measured along the line of flight, "drift" occurs when the drag is NOT acting straight down the flight path. BC is a "measurement" of drag. High BC= less drag and low BC = more drag. Turn the drag offline and drift occurs.

hth

al

 

[alinwa]

That depends upon how loosely you interpret the definition of "most stable"
A bullet with a dynamic stability factor around 1.0 to 1.5 would do this...spin one up to a factor of 10.0 and it won't care about changes in the environment...it will gyroscopicaly continue to point the same direction even in a 100mph crosswind. Which is why over-stabilized bullets are not often as accurate.

Not to quibble, but the generally accepted reason that overspun bullets are less accurate has to do with eccentricity, CG not matching CF and core slippage.

al

 

[Gene Beggs]

Fair enough

And I am sorry you will not explain your "no lag, no drift" statement to me. I guess if the bullet is rocket propelled and has no lag that it will not drift. Bull; I don't believe it! Please lower yourself to my level for a moment and explain this to me. I have read the lonnnggg thread on this subject and it does not answer my question. I started this thread, remember? I was doing fine until your last post.
Thanks,
model14

Richard, you're right. You started this thread and I owe you an explanation of what I meant when I said,

"If not for the fact that it is constantly slowing down, the bullet would not drift at all!"

So you think what I said is "Bull?" I'm ashamed of you! And you,,, a pilot! Your primary flight instructor would hang his head in sorrow!

I'm just pullin' your leg and I'm smiling as I type this.

Richard, you may have missed it in that looooooooong thread refrenced earlier, but there IS a detailed explanation there of why and how bullets drift in crosswinds. I'll go over it once more for your benefit but when you have time, you should carefully review that "loooooooong thread." I really knocked myself out on it.

The easiest and most effective way to explain to YOU how bullets drift in crosswinds is to relate it to flying. You are a pilot and already know how bullets drift but perhaps you have forgotten or maybe someone has convinced you that bullets and airplanes behave differently. To quote you, "Bull!!!!"

I can just hear some of our readers groaning,

"Aw man,,,; here he goes again, flyin' off on a tangent to nowhere. When is this guy gonna' realize that airplanes and bullets are two different things?"

Dear reader,,,if that's what you were thinking, please excuse yourself at this time because I'm not talking to you, I'm talking to Richard and others with open minds. With that said, let's get on with it.

Richard, do you remember back during your pre-solo flight training when your instructor introduced ground reference maneuvers? S-turns over a road, turns about a point, rectangular courses, following roads etc. We will now get in your Cessna 182 and follow a road.

That's right! We are going to fly down a long, straight road that runs as straight as an arrow due south from the airport. We will be flying at fairly low altitude so you can easily see what the aircraft is doing in relation to the ground. Skies are clear, visibility unlimited, winds are strong out of the west at 30 mph. (Typical Midland, TX conditions; huh? )

We are now airborne, at 500 feet above ground level. Your Cessna 182 is showing an indicated airspeed of 150 mph, there is a 90 degree crosswind from right to left at 30 mph. Your assignment; fly directly over the road to the destination 150 miles ahead. With me so far?

As an experienced pilot, you know that with a direct crosswind from right to left at 30 mph, you cannot fly a heading of 180 degrees and wind up directly over the road 150 miles ahead. Of course not,, if you flew a heading of 180 degrees, the same as the course over the ground, you would wind up 30 miles left of course in one hour.

So what do you do? Sure, you apply a wind correction angle to the right, nose into into the crosswind. For the sake of discussion, let's say the wind correction angle is 10 degrees right, which would be a heading of 190 degrees. As long as airspeed and wind remain the same, the aircraft remains directly over the road; no drift!

So here we are tracking merrily down the road, right on course; the aircraft heading is 190 degrees flying streamlined thru the airmass at 150 mph, the airmass is moving from right to left at 30 mph. Everything is in balance and as long as the aircraft doesn't slow down, there is no drift. But,,,,,

,,,,let's start reducing power and gradually slowing the airspeed down as a bullet would do. As airspeed decreases you notice the aircraft begining to drift to the left. The slower the airspeed the greater the drift. Winds have not changed; they are still out of the west at 30 mph, aircraft heading is still the same, 190 degrees, the only thing that has changed is airspeed which is slowly but steadily decreasing.

Now do you understand? Airplanes and bullets have a lot more in common than you thought; huh?

Richard, I must excuse myself and get my gear together. I have a long drive tomorrow. I'm taking the weekend off for myself! Yep, I'm going to Albuqurque, NM to shoot in a match. Can't wait! I'll get back to you next week and see how you're doing. Hang in there!

Later

Gene Beggs

 

[Big Gunner]

This is where I disagree

I've stayed out as long as I could, but my opinion is that airplanes and bullets may be very similar, but airplanes are not flown in the manner that bullets fly.

Sorry, I should have taken Gen's advice and stopped reading his post

I don't agree with Gene's explanation above for one very important reason: A spin stabilized bullet will precess into the wind more and more as the wind increases in percentage of forward velocity.

In the above example, the airplane is ALWAYS at a heading of 190 degrees. A bullet would precess further and further into the wind as its forward velocity slowed. Again in the above example, the original crosswind of 30mph was 1/5th of the forward speed of 150 miles per hour. As the aircraft slowed down to 90 miles an hour the same 30 mph crosswind would be 1/3 of its forward speed but it would still stay on heading of 190 degrees.

A bullet would have turned into the wind more.

Taken long enough (if gravity didn't pull the bullet down before it had time to happen) the bullet would turn competely into the wind if the forward velocity stopped but it was still spinning. It would come to an aerodynamically stable position in the airflow.

The aircraft could do the same thing (essentially hover in the air) it it turned directly into the crosswind, and had no forward motion.

Lisa Spendlove

 

[Tony Shankle]

ok, I'll play

Ok Richard,

Now picture that I have slipped up behind your Cessna (Al's flying the chopper and I am hanging out the door with my duct tape) and strapped a stage of a Saturn V rocket on your tail. Rocket fires for 7 nano seconds V-maxing your Cessna removing the wings, sending you in to a 4,000 cycles per second spin at 3200 fps before the rocket tears away. Now don't worrry about tumbling because I carefully strapped on some weight to keep your center of gravity behind your center of pressure although I have to warn that some weird things will happen with your lunch due to angular momentum.

We probaby have a bit of Initial Yaw caused by the imperfect alignment of my rocket (damn it Al, hold the chopper steady) and made worse by vibration and blast debris from the rocket but don't worry because it will diminish in the next 200 yards as the fuselage goes to sleep. Now while you are spinning up to that 4,000 cycles per second you will feel some other nodding/corkscrew motion at first from the Nutation but it wont last long due to aerodynamic drag. Now you are up to gyroscopic stability and you will gain rigidity in space but you pay some of the price with precession.

At the same time lets not ignore that once the wings and the rocket fell off you have been falling at 32 feet per second per second but don't worry because this flight wont last long. You are on a long curved trajectory a little nose up and to the right and gravity is pulling you in a gradually increasing and accelerating downward path but you will probably take on a constant trajectory attitude even though that nose is still moving in a small swinging arc that will move more and more down and in the direction of rotation. All of this spinning is causing you to spin drift about 9" to the right of the landing strip but Gene is a real bud and moves it over. The earth has also moved a bit due to your South heading so our tireless friend Gene comes out of the wind tunnel and moves the landing stip over another 2.5 inches (Coriolis) (thank God for Gene ).

Now you are halfway home and all lined up (with some minor wobble) and WHAM...Vibe turned on the exhaust fan in Gene's tunnel which hits you with a 30mph 3 oclock cross wind that last less than a tenth of your 1.2 second flight. You quickly find that wobbling nose turned to the right (into the wind) and while it is hard to notice, your time of flight has been extended due to drag . You gain lateral momentum which will stay with you until the end unless you get an equal and opposite force. Just as fast as you turned into that wind you turn back out when it stops but you are now pointed to the left of that strip and it keeps getting worse (although not as bad as it would if the wind continued). Something else happend, a strange downward force (Magnus) came along with that wind but instead of effect on the CG it seems to like the CP and you have a new Yaw to worry about now.

With all that moving and jerking and yawing and spinning you say to yourself "if I ever get out of this just let someone say the word Stable to me" but all is quickly forgotten as Toby uses his advanced math skills to tell Gene where to move the landing strip. Al and I buy a round of stiff drinks as a way of saying we're sorry for our antics and we give you one of Boyd's rifles as a present.

Thank you for flying WhatIF Airlines!

 

[Boyd Allen]

Al,
Can you post Mann's experimental procedure and results relating to the difference in response to wind of dropped and fired bullets? It has been several decades since I read the book. That is your main source on this point isn't it?

 

[alinwa]

Hey...... I don't even KNOW this guy!!! He just flagged me over and asked "can you catch that plane?"

And I said yup

Next thang I know he's hangin' off a strut with an oversized JATO in one hand and duck tape in the other yelling "can you invert this thing? I can't reach!!"

So's I'm jackin' the collective and slappin' my tail trying to gain more roll (don't NEED a seatbelt since't I'm clenched like a limpet....) and this dude sez "HEY, can you draft in his propwash to null this thing outabit??? You're shakin me all up!!"

null out?? c'MON man! I'm yawed up like a funky monkin' a football, leadin' with my heels.......I mean all's I can see is ground....... I'm following that liddle skylane by keeping the hairs on the back of my neck centered up.....both feet on the panel.....

jeepers


So's I reaches out and snags that ducktape and deftly WRAPS it around his head......STOPPED HIS MUZZLE!


then that booster fired

!!

And that's all I remember.

"rigidity in space" my achin' @$%^!! I still walk like a rooster..... but at least now I know the guys name and program...!

WhaTiF Airlines all right.... at least WARN a guy! I got cawffee all over the keyboard!!


steenkin' shankle


sheeesh


al

 

[Gene Beggs]

Finally!!!

I've stayed out as long as I could, but my opinion is that airplanes and bullets may be very similar, but airplanes are not flown in the manner that bullets fly.

Sorry, I should have taken Gen's advice and stopped reading his post

I don't agree with Gene's explanation above for one very important reason: A spin stabilized bullet will precess into the wind more and more as the wind increases in percentage of forward velocity.

In the above example, the airplane is ALWAYS at a heading of 190 degrees. A bullet would precess further and further into the wind as its forward velocity slowed. Again in the above example, the original crosswind of 30mph was 1/5th of the forward speed of 150 miles per hour. As the aircraft slowed down to 90 miles an hour the same 30 mph crosswind would be 1/3 of its forward speed but it would still stay on heading of 190 degrees.

A bullet would have turned into the wind more.

Taken long enough (if gravity didn't pull the bullet down before it had time to happen) the bullet would turn competely into the wind if the forward velocity stopped but it was still spinning. It would come to an aerodynamically stable position in the airflow.

The aircraft could do the same thing (essentially hover in the air) it it turned directly into the crosswind, and had no forward motion.

Lisa Spendlove

Thank you Lisa! I think you're right. In recent times as I have been mulling this very thing over in my mind, there was always a haunting doubt about whether the bullet would remain on heading as its forward speed decreased. I think you are right. I hope Bryan Litz will join in this discussion.

In the case of the airplane, I can MAKE it stay on heading with the controls but once a bullet leaves the muzzle, it's on its own and would turn increasingly more into the crosswind as its forward speed decreased. YES!!!!! Thank you, thank you, thank you!!!!

Wow! I knew if I just kept putting out the bait, someone smarter than I am would clear this up! Terrific! I'm certainly not the sharpest tool in the shed; there are many things about external ballistics that I do not understand but thanks to you, this piece of the puzzle has been found.

Lisa, I had already signed off for the night but I couldn't sleep. I'm glad I logged back on because your post made my day; no, my year!

Please, please, stay with us in this discussion and help us clear up misconceptions that many of us have. We need people like you. You obviously have a great understanding of the subject, and please, tell us more about yourself. From what you said in your post, I know you're a pilot; what else?

WOW!!! I'm ecstatic! Hope I can sleep tonight. I have a long drive tomorrow.

Wow, even hard headed old Captain Beggs can still learn a thing or two.

Thanks again Lisa!

Later,

Gene Beggs

 

[Gene Beggs]

Al, I haven't,,,,,

Hey...... I don't even KNOW this guy!!! He just flagged me over and asked "can you catch that plane?"

And I said yup

Next thang I know he's hangin' off a strut with an oversized JATO in one hand and duck tape in the other yelling "can you invert this thing? I can't reach!!"

So's I'm jackin' the collective and slappin' my tail trying to gain more roll (don't NEED a seatbelt since't I'm clenched like a limpet....) and this dude sez "HEY, can you draft in his propwash to null this thing outabit??? You're shakin me all up!!"

null out?? c'MON man! I'm yawed up like a funky monkin' a football, leadin' with my heels.......I mean all's I can see is ground....... I'm following that liddle skylane by keeping the hairs on the back of my neck centered up.....both feet on the panel.....

jeepers


So's I reaches out and snags that ducktape and deftly WRAPS it around his head......STOPPED HIS MUZZLE!


then that booster fired

!!

And that's all I remember.

"rigidity in space" my achin' @$%^!! I still walk like a rooster..... but at least now I know the guys name and program...!

WhaTiF Airlines all right.... at least WARN a guy! I got cawffee all over the keyboard!!


steenkin' shankle


sheeesh


al

,,,,laughed so hard in many a day!

We are making terrific progress; aren't we?

With the help of those smarter than we are, we will figure this stuff out yet; huh?

Later

Gene Beggs

 

[alinwa]

Al,
Can you post Mann's experimental procedure and results relating to the difference in response to wind of dropped and fired bullets? It has been several decades since I read the book. That is your main source on this point isn't it?

well, no...... I've dropped bullets myownself too.


Mann's procedure was to build a stand with a plumb bob and a landing target and drop onto the target while recording wind speeds. His apparatus was guyed and precisely build and was very repeatable.

but it showed him little.

Here are his words:

"There is no attempt in this article to make any suggestions or explanations of the cause of the major part of wind deflection which occurs...."

He recognized the discrepancy.

al

 

[model14]

To all that have hung in there with me :

Richard's Hypothesis:
A rocket propelled, spin stabilized, projectile, that has a constant ground speed (on board super accurate, super quick response, GPS coupled to auto-throttle), will always fly in a perfectly straight line relative to the ground, regardless of the wind speed or direction.

Corollary:
If I shutdown the rocket, the projectile will start to drift. The amount of drift before impact will be a function of the velocity at shutdown, cross-wind speed and the BC (sectional density and form factor).

Can we all support my hypothesis? Does it hold true for both subsonic and supersonic flight? This is important because our bullets may go subsonic before reaching the target.

If you can get me past this (i may not fully understand it but I am ready to believe it), I have several other questions that apply to the practical side of this discussion, but I don't want to go there until we reach agreement. Feel free to modify my hypothesis until we all agree.

Richard