Attention all ballisticians

[Big Gunner]

Greg

The bullet does not leave the barrel already precessed into the wind. It leaves on a launch angle dead on the to the other balloon.

Yes, the bullet is moving sideways due to the momentum of being fired from a moving platform, but this momentum bleeds off rapidly.

Once the bullet clears the barrel it must align itself to the wind. Both the headwind created by firing, and to the crosswind component it is flying through. Once it does this, the bullet slows down opposite of drag..

Just my thinking anyway.

Lisa

 

[Greg Culpepper]

Lisa,

What wind? There is no wind at a floating, drifting balloon. If a rifle is fired from a drifting balloon there is no bullet precession into the wind because the is no wind (relative to the balloons and the balloon borne rifle). The bullet doesn't drift off the angle of departure because the is no cross component to turn it. The air at the balloon is still (relative to the balloon) because the balloon is traveling coincidentally with the air as it moves over the earth.

What input to the bullet would cause "this (sideways) momentum to bleed rapidly"? There can not be any crosswind component, no wind between the balloons, if the balloons are free to drift in the air. Whatever lateral speed the bullet exhibited over the ground due to the movement of the platform it was fired from (the balloon) would persist even after it, the bullet struck the target on the other drifting balloon. Now the bullet would track an arc over the ground described by its lateral velocity as induced by the balloon's progress over the ground and its (the bullet's) continuously slowing velocity. But it will remain perfectly horizontally aligned with the rifle bore and not turn in relation to the rifle as it would if the rifle were earth bound in the same air.

Greg

 

[alinwa]

IMO balloon-to-balloon is dead air, same as shooting on a planet.

al

 

[Greg Culpepper]

Al

'Zaktly.

Greg

 

[Vibe]

Lisa,

What wind? There is no wind at a floating, drifting balloon.
Greg

Actually there is, or can be. IF the balloons are in a 50mph airstream relative to the ground - the bullet will try and travel a straight line, more or less relative to the ground (since the ground is moving less rapidly). The results will be that the bullet will drift less in the wind than the target balloon and will appear to "drift" upwind or behind the target balloon. Moving points of reference in an other wise stationary inertial situation often produce non-intuitive results. Ever try to play catch across a carousel or merry-go-round?

 

[Greg Culpepper]

Wind between floating balloons? I don't think so Vibe. In any case the bullet has no reference to the ground. It's just in the air.

Unlike a ball thrown across a carousel where to opposite sides are moving ......opposite directions, balloons adrift are stationary in relation to each other, just moving in relation to the earth.

Greg

 

[Vibe]

Wind between floating balloons? I don't think so Vibe. In any case the bullet has no reference to the ground. It's just in the air.

Unlike a ball thrown across a carousel where to opposite sides are moving ......opposite directions, balloons adrift are stationary in relation to each other, just moving in relation to the earth.

Greg

The more I think about it...the more I think you are correct in this. I made the same mistake most people do. The bullet will maintain the 50 mph sideways ground speed because there is no relative force to change that direction. A body in motion will remain in motion...and all that.
But it does serve as a good example in another way.
To an observer on the ground the bullet would appear to be flying with a definite sideways slant from it's true direction of travel, nosed into the wind, in much the same way that a plane would crab in a crosswind.

 

[Gene Beggs]

Interesting Conversation!

The unusual scenarios such as firing a bullet from one balloon to another and playing catch across a carousel really make you stop and think; don't they? Makes my head spin just thinking about it.

 

[Big Gunner]

Greg and Al,

Firstly, if you believe that "wind drift" is a function of time of flight in which the airmass moves the bullet in the direction of travel equal to the airmass' velocity, you're not going to buy this explaination at all.

If, however, you believe that "wind drift" is caused by drag acting on the bullet in a direction different from the direction it was fired due to bullet precession this may make sense to you.

Al, I'm surprised at you. You are the biggest fan of the wind not pushing the bullet equal to its velocity. It's your dropped bullet, fired bullet argument.

Let me try to explain what I think happens:

You fire a bullet from a platform moving in an airmass of 50mp at a target moving in the same airmass.

As the bullet leaves the barrel it is travelling at a forward speed of (say) 3000fps and is moving laterally to the target at 50mph (roughly 725 fps) due to the inertia of being onboard a platform moving at 50 mph when it was fired.

Since I believe that the airmass moving between the balloons will NOT continue to push the bullet at a constant rate of 50mph all the way between the two moving platforms. I think that the bullet will begin to slow down from its lateral travel.

The bullet once leaving the barrel will immediate start to slow its forward velocity due to drag, as well its "lateral momentum" (or "with the wind" momentum) of 50mph because the bullet is not precessed along this axis of momentum. It's momentum slows from 50mph with the wind to 49.99 mph with the wind, due to loss of enertia (by drag).

Once that happens, the bullet sees an apparant crosswind (granted, its only a 1fps wind, but its crosswind nonetheless) and the bullet begins to precess into this crosswind.

Drag is constantly slowing it down, and it is constantly precessing further and further into the wind it encounters because it keeps bleeding the momentum imparted on it from being fired from the moving platform and the wind does not continue to carry it along at the inital 50mph.

As the bullet slows down in the direction of drag, (or opposite the direction of its nose) and because there is precession into the wind, the bullet is no longer slowing down in the same direction it would if there were no precession.

The "drift" becomes different and the bullet does not strike the target as if there were no wind at all, even though the bullet is fired in a moving airmass.

Does it strike considerably "downrange" because it was fired from a moving platform in a moving airmass and carried with it inertia? Absolutely.

Does it exhibit "wind drift" even though it was fired from a moving platform in a moving airmass? Absolutely.

Does it exhibit the same "wind drift" as if it were fired from a fixed point on the ground at a fixed point on the ground? No way!

That's my story I'm sticking to it. At least for now.

Lisa

 

[Greg Culpepper]

Vibe,

The more I think about it...the more I think you are correct in this. I made the same mistake most people do. The bullet will maintain the 50 mph sideways ground speed because there is no relative force to change that direction. A body in motion will remain in motion...and all that.



But it does serve as a good example in another way.
To an observer on the ground the bullet would appear to be flying with a definite sideways slant from it's true direction of travel, nosed into the wind, in much the same way that a plane would crab in a crosswind.

To an observer on the ground the bullet would be flying with a sideways component and its path curving more and more as it slowed from drag. An observer on one of the balloons would think the bullet was going straight and the ground was spooling out from under him.

Greg

 

[Greg Culpepper]

Lisa,

I'm sure I speak for Al also when I say bullet drift has absolutely nothing to do with time of flight. My very first post on this subject many months ago began with this declaration. You're preaching to the choir, girl. A bullet backs down its own axis proportional to its time lag which is caused by drag. The more drag, the more time lag, the more drift. Time of flight has nothing to do with drift.

We agree. Until you discuss the bullet losing its sideways velocity after its fired from a balloon. It doesn't have any sideways velocity in relation to its balloons, its rifle, its shooter or its air. It flies straight in the air between the balloons. The wind doesn't push it sideways. There is no wind. Its the ground that is moving under the air and the balloons and the rifle and shooter and fired bullet.

The bullet fired from a drifting balloon doesn't see an apparent crosswind, ever.

That's my story and I'm stickin' to it to.

Greg

He said, she said. As a thrice divorced man I hope this doesn't end badly for us. I'd miss Boyd (some) but I'd really miss our dialogin'.

 

[alinwa]

Lisa, I can't agree...

We need to think of "wind" like a physicist thinks of "heat".... there is no such thing, except relatively speaking.

I'm serious with the planet example. I'm setting and typing in a 1000mph "wind" due to the earth's rotation, but to feel it you'd have to be in space and dabble my finger into the airstream. God knows, there's a wind here..... (groan )

The idea that the bullet knows where the "wind" is, or the ground..... is the problem. It doesn't......The bullet just IS, it's in a static environment from balloon to balloon. It has a 50mph sideward component and it keeps it, it HAS to because there's no indication (force) for it not to. There is no crosswind between two balloons. The bullet DOES NOT lose inertia any more than it does when fired N-to-S from ground-to-ground (like we do every day) It DOES NOT "turn to point into the wind" from any perspective.

If I fire from the Northern Hemisphere over the Equator and the bullet lands in the Southern Hemisphere it's the same as balloon to balloon. . . . . But ONLY at the Equator. IN FACT, there's LESS DRIFT or deviation of any sort balloon to balloon than there would be at say WA State where the Coriolus Effect dictates the rifle and the target have different velocities. Two balloons in any speed of consistent wind make a static test environment regardless of position in the airstream.

Fired over the poles the bullet would impact weirdly due to the same Coriolus, this is what vibe was trying to say. BTW, Coriolus IS COOL!!!! (irrelevant but cool) If you ever get a chance it's well worth it to take a nerf football to the fair and ride inside the rotating drum known as the Cyclone, The Bamboozler, Gravitron, Rotor, whatever it's called on your fair circuit. Play catch, or try to! Warn others on the ride.......you'll be hitting them, not'cher buddy.

al

 

[Vibe]

The "drifting balloon" scenario got me to thinking some.
I think Greg has got the "drifting balloon" shot down. It's the transition from stationary shooter into a moving airmass that seems to give folks fits. But in the balloon situation - at all times during the flight, the bullets axis remains pointed at the target balloon (side to side), even though it's line of motion relative to the ground is in a different direction ( as it includes the 50 mph lateral velocity of the shooter/target/airmass).

To put a similar analogy to it - in a constant crosswind, from a stationary shooter, to a stationary target, a shot aimed directly at the target along a line of sight direction - would miss.
Being blown downwind. We've all seen this happen at the range when we do not hold off or compensate for the wind. But when we aim upwind in a hold off - the bullet continues to point in the direction we aim, all the way from shooter to target. Even though it's "flying" in a curved path. In a "from right to left" crosswind, we aim to the right of the bull, the bullet is pointed in that direction, leaves the rifle pointed in that direction (from left to right), and ends up entering the target (hopefully in the bull) still pointed in that direction, even though it is now moving from Right to left under the influence of the wind. And the stronger the cross wind the more pronounced the effect. So in a CONSTANT cross wind, much of the "precession" is done upon firing by simply aiming upwind of the target. What seems to be tough is that the bullet will change the direction that it points under the influence of a crosswind that changes, in either direction or magnitude, AFTER it has left the shooters position.

 

[bsl135]

My $0.02

I’ve read thru most of the above. Rather than address each post, I’ll attempt some explanation, in my own words, from the top.

Headwind/Tailwind
A bullet launched into a headwind will experience greater aerodynamic drag than a bullet fired into still air. The greater aerodynamic drag will act to decelerate the bullet at a greater rate, which will cause the bullet to strike the target at a lower velocity, and at a later time than a bullet fired into still air. The increased time of flight gives gravity more time to accelerate the bullet downward, so the bullet strikes low. The reverse happens for a tail wind, except in reverse. Reduced aerodynamic drag, faster, less time, etc.

I read the excerpt from Bill and Ted (Sierra’s Ballisticians, post#19) and found their explanation to be accurate.

Crosswind
Everyone now seems to be in agreement regarding what happens in a crosswind. As the definition of stability requires, the bullet will ‘weathervane’, meaning it will turn to point its nose straight into the oncoming airflow, which is slightly angled to the LOS. Since the drag acts straight back thru the bullets’ axis, and that axis is angled to the LOS, the bullet is ‘dragged’, horizontally, away from the line of sight.
There are differences between ‘fin stability’ and ‘spin stability’ (cp behind cg, or vise versa), but the way a projectile weathervanes is the same for both kinds of stability. A bullet won’t point its nose differently than an arrow just because its cp is in front of its cg. Stability is stability, and it means: “aligning the projectile axis with the oncoming airflow”.

Balloons
The question was asked: If a bullet is fired from one balloon to another, both drifting in a 50 mph crosswind, will the bullet exhibit wind drift?
This is a tricky question, because it requires we define a reference frame. If the reference frame is the balloons, the wind doesn’t have a crosswind component, so the bullet flies from one balloon to the other as if it was fired from a stationary bench to a stationary target thru still air. I don’t believe the bullet would lose any of its (ground referenced) cross-velocity, why would it?
To an observer on the ground, the bullet would travel laterally with the air mass but not because it’s drifting or being deflected, but because it was fired from a platform which gave it a lateral velocity (relative to the ground) and nothing acted against that lateral motion. To the observer on the ground, the bullet moves laterally by an amount equal to the wind speed times the time of flight (the full time of flight).
In neither of the above reference frames does the bullet act as it does when actually fired from a stationary platform into a crosswind. In this ‘real world’ scenario, the bullet drifts by an amount equal to the cross wind speed times the lag time; lag time being the difference between the bullets’ actual time of flight, and the ‘vacuum’ time of flight.

My question
I’ve always wondered: WHY is it that crosswind deflection is precisely proportional to lag time? It would be intuitive if wind deflection were proportional to total time of flight, that would be like the ‘feather in the wind’ visualization. Even if wind deflection were not neatly proportional to any ‘easy’ quantity, I could accept that as a non-intuitive complexity of ballistics. BUT, since it’s mathematically provable that wind deflection is directly proportional to lag time, I’ve always wanted to know why, and what exactly that means.
I’m almost satisfied with my understanding of the concept when thinking about the ‘no deflection for thrust = drag; zero lag time’ case, and the ‘upwind deflection for thrust > drag; negative lag time’ case. But there’s still something mysterious about the fundamental nature of lag time that I can’t quite put my finger on.

So, why’s it so simple?

 

[alinwa]

But there’s still something mysterious about the fundamental nature of lag time that I can’t quite put my finger on.

So, why’s it so simple?

Here we go again OOhhhhh I'm gonna; get allover in trouble A'gain......


Here's why I think it's different (and so COOL! I TRIED hard to get this across in TMOAWDT before I backed out)

The mysterious part is because the actual wind has little to do with the thang..... (whereas with a feather or an airplane it has ALL to do with it)

The driving force is the bullet "backing up" from it's frame of reference. The driving force IS NOT the wind. The wind only gives it direction.

This is essential.... the bullet's passage CREATES the imbalance which translates to "wind drift"....

The correlation between "lag time" and this "backing up" is direct, they're the same thing, therefore the relationship is nearly linear.

jeepers I hope this makes more sense than last time!

al

 

[bsl135]

Al,
Wind does more than define the direction of the deflection,
it also defines the magnitude. Faster wind = greater deflection.

Unless you mean; all the wind does is orient the bullet...
then a greater cross wind speed means a greater angle between the LOS and the bullets axis (drag direction).

I'm with ya (sort of)

Can you point me to your prior attempts at explaining your ideas so I can catch up?

 

[alinwa]

Al,
Wind does more than define the direction of the deflection,
it also defines the magnitude. Faster wind = greater deflection.

Unless you mean; all the wind does is orient the bullet...
then a greater cross wind speed means a greater angle between the LOS and the bullets axis (drag direction).

I'm with ya (sort of)

Can you point me to your prior attempts at explaining your ideas so I can catch up?

EXACTIMALLY!!!! Whether I'm right or wrong, THIS is what I've been trying to say since TMOAWDT. (The Mother Of All Wind Drift Threads)

In the last thread on the subject "For You Ballisticians" http://www.benchrest.com/forums/showthread.php?t=64966 I tried to illustrate it from the Reference Frame of the bullet..... it backing up like a car.

hth

al

 

[alinwa]

Unless you mean; all the wind does is orient the bullet...

We also need to lose this reference, as Boyd Allen keeps pointing out IT ALSO APPLIES TO A ROUND BALL, no "pointing" nor orienting of the bullet...... Because everyone from Vaughn to Rinker To McCoy refer to "drag acting along the long axis if the bullet" we tend to make this a part of the thang, IT AIN'T!!

The reorientation of the bullet is the result of it searching for stability in the airflow, it in and of itself has nothing to do with drag.

al

 

[Boyd Allen]

Al,
All of the energy that it takes to start, and keep a bullet moving laterally comes from somewhere. Where might that be? Let me give you a hint. Lacking a tail wind componet, nothing propells a bullet forward, beyond the muzzle, except its momentum. Are you saying that it comes from that ? If so, where is the corresponding loss in velocity? Nothing is for free...yes? Also, since drag increases with velocity, how can drift increase as the bullet slows, if drag is decreasing?

 

[alinwa]

Al,
All of the energy that it takes to start, and keep a bullet moving laterally comes from somewhere. Where might that be? Let me give you a hint. Lacking a tail wind componet, nothing propells a bullet forward, beyond the muzzle, except its momentum. Are you saying that it comes from that ? If so, where is the corresponding loss in velocity? Nothing is for free...yes? Also, since drag increases with velocity, how can drift increase as the bullet slows, if drag is decreasing?

OK, I dunno if this will work but:


The energy to start the bullet laterally "comes from" deceleration, the wind blowing on the nose of the bullet, but crooked to the flight path. MOST of the "wind" is from the speed of the bullet itself, NOT from the wind that's blowing....

The reason that it keeps increasing is simple, in the same way that a constant acceleration of 1G can bring a spaceship up to lightspeed over time, ALL value is added ......there's nothing at all to decrease it's sideward component since deceleration is more than enough to override any gain from momentum. Plus the momentum moment itself is so closely aligned with the flight path that it offers little in the way of centrifugal (centripetal?) resistance.

al