Muzzle brake

[Greg Culpepper]

Al,

Well done. In the words of the late, great Ed McMann, "Correctomundo, oh great one".

At the conclusion of the last wind drift extravaganza it was my hope and belief that you had gotten your head wrapped around that subject. Now I know you did.

I don't know anyone who works harder at getting it right whether learning or instructing. Thanks for all your help to me along my way. I'm sure that a certain pilot down in Odessa will smile when he reads your post.

Greg

 

[Vibe]

It isn't that the bc varies, we're just using the wrong drag model and have to adjust for that shortcoming.

Greg

True there are various models, and the BC is represented as differing numbers when each is used..But the BC varies because the drag coefficient varies with the velocity. The amount of drag on a bullet is different at 3000fps than it is at 2000fps, and even more different at 1000 fps - regardless of the model used.

 

[Greg Culpepper]

Boyd,

I hope you will do you're own testing, particularly given your preference for using empirical evidence to prove the science. As for me following your (and everyone's) advice on flags, I might just be too lazy to actually get to it. But thanks just the same.

Greg

 

[tobybradshaw]

I still can't wrap my head around the idea that a 3oz superball hits my head with twice the force of a 3oz lead sinker..... Maybe I'll go conduct some experiments

al

Three points.

First, although the superball changes the momentum of your head twice as much as the lead sinker does (assuming both impact with the same velocity, and that the collisions are perfectly elastic or perfectly inelastic), the force experienced by your head depends on the RATE of momentum change, not the AMOUNT of momentum change. GIGO.

Second, the imprecision (and, in your case, length ) of verbiage is exactly why mathematical models are used by (apparently, the few) people who really want to understand what is going on in the universe.

Third, mathematical/physical models can generate counterintuitive (but correct) predictions, verifiable by experiment. Your (or at least my) intuition tells you (me) that a moving bullet has the same mass as a bullet at rest, that your (my) watch runs at the same rate on the ground floor of your (my) house as the second floor, that a moving object will slow down unless force is applied continuously, and that a bullet is blown sideways by a crosswind. All of these intuitions are wrong, and have been shown to be wrong. All the words in the English language won't change that.

Toby Bradshaw
baywingdb@comcast.net

 

[Boyd Allen]

Al,
I have always understood that wind resistance (drag) causes a bullet to loose its velocity at an increasing rate, to decelerate, but in my opinion you should really think about not using the phrase "backing up" to describe what is happening. As commonly used, backing up is used to describe actual position change of a person or object in the direction that is opposite from its front. This is not the same as decelleration, which is a loss of speed. I think that your explanation looses clarity through this non-stantard usage.

 

[Greg Culpepper]

Vibe,

Of course drag varies with velocity. But if a drag model and the actual bullet fit each other, the observed cd will not vary from the predicted bc. In the interest of full disclosure, I confess I am talking WAY above my pay grade on this issue. But we do have honest to gosh rocket scientists on this board. Maybe Bryan will speak up.

Greg

 

[Vibe]

The FASTER we spin them the FASTER they align themselves with the airstream... (well, almost....they don't align perfectly but 98%??? Yaw of Repose???.... that's nothing more than a SWAG) .......

http://www.nennstiel-ruprecht.de/bullfly/fig24.htm

If a bullet flies stable (gyroscopically and dynamically!) and the transient yaw has been damped out, usually after a travelling distance of a few thousands of calibres, the bullet´s axis of symmetry and the tangent to the trajectory deviate by a small angle, which is said to be the yaw of repose.

For bullets fired with right-handed twist, the longitudinal axis points to the right and a little bit upward with respect to the direction of flight, leading to a side drift to the right. The yaw of repose, although normally measuring only fractions of a degree, is the reason for the side deviation of spin-stabilized bullets.

Now...... on to the tipping over to follow the flightpath. IF IT'S TRUE that the bullet "balances" or centers itself on the apparent wind vector then it follows that the bullet WILL tip over in its flight path. Given enough (but not too much) stability.

At this point if anyone questions where the bullet is pointing then we need to understand that this is another discussion. If there are those who DO NOT agree that a properly stabilized elongated bullet points into the apparent wind then this need be approached separately IMO.


So, if we find that a properly stabilized bullet MUST keep its tip very nearly aligned between its CG and the apparent wind direction........ IF WE AGREE that the bullet aligns with the felt wind vector.......... then we can go back to the gravity thing.

We've been ignoring gravity but it hasn't gone away

After that one second the bullet is feeling another "wind," it's FALLING......it's got a direct "UP"-wind of near 11mph, more than the crosswind. and after the next second this has climbed to triple that. My question then is, IF an elongated bullet is "forced" into alignment with the apparent wind vector, how can it not feel the "wind" generated by it's "falling through the air and center on it?"

The key to all of this is a "properly stabilized" bullet will do just as you described. One that is spinning too fast for its mass will resist the change more effectively and end up in a nose high position to "keyhole" and loses accuracy. One that is not spinning fast enough will never damp out the initial forces to yaw and will also lose accuracy - both conditions of not properly stabilized bullets will also lose velocity faster since they are unable to achieve the minimum drag attitude. Even "properly stabilized" bullets will do this if launched at too high of an initial angle (Much steeper than what we would consider "normal").

CAN a stable bullet fly nose-high over yardage?

Only very slightly nose high - See the "Angle of Repose" section - it's a very small angle.
Improperly stabilized bullet can and do travel with a very large nose high attitude - in a stall so to speak.

I still can't wrap my head around the idea that a 3oz superball hits my head with twice the force of a 3oz lead sinker..... Maybe I'll go conduct some experiments

al

I expect a Youtube presentation of said experiments.

 

[Greg Culpepper]

Boyd,

If you mean that with increasing velocity that there is more drag and and greater rate of deceleration you are correct. As stated though, you could be understood to mean that over time (of flight) that drag increases and that is backwards from what happens.

Al's description of bullet behavior is from the point of view of its virtual, zero drag non-decelerating self. Its a lot harder for me to understand bullet behavior from a reference point on the ground than if I imagine I'm riding on the bullet and disconnected from the ground, running true in the air. Because of drag, the real bullet is backing down its own axis by comparison to its virtual zero drag evil twin. Try it. You'll like it. As Justin would say,"I guarouuuuntee".

Greg

 

[Vibe]

Vibe,

Of course drag varies with velocity. But if a drag model and the actual bullet fit each other, the observed cd will not vary from the predicted bc. In the interest of full disclosure, I confess I am talking WAY above my pay grade on this issue. But we do have honest to gosh rocket scientists on this board. Maybe Bryan will speak up.

Greg

I referred to the site this was on earlier - possibly an excerpt will be clearer.

For the precise establishment of BCs (or perhaps the scientifically better expressed drag coefficients), Doppler radar-measurements are required. The normal shooting or aerodynamics enthusiast, however, has no access to such expensive professional measurement devices. Weibel 1000e Doppler radars are used by governments, professional ballisticians, defense forces, and a few ammunition manufacturers to obtain exact real world data on the flight behavior of projectiles of interest.

Doppler radar measurement results for a lathe turned monolithic solid .50 BMG very-low-drag bullet (Lost River J40 .510-773 grain monolithic solid bullet / twist rate 1:15 in) look like this:
Range (m) Ballistic coefficient
500 -- 1.040
600 -- 1.051
700 -- 1.057
800 -- 1.063
900 -- 1.064
1000 -- 1.067
1100 -- 1.068
1200 -- 1.068
1300 -- 1.068
1400 -- 1.066
1500 -- 1.064
1600 -- 1.060
1700 -- 1.056
1800 -- 1.050
1900 -- 1.042
2000 -- 1.032


The initial rise in the BC value is attributed to a projectile's always present yaw and precession out of the bore. The test results were obtained from many shots not just a single shot. The bullet was assigned 1.062 for its BC number by the bullet's manufacturer Lost River Ballistic Technologies.

Measurements on other bullets can give totally different results. How different speed regimes affect several 8.6 mm (.338 in calibre) rifle bullets made by the Finnish ammunition manaufacturer Lapua can be seen in the .338 Lapua Magnum product brochure which states Doppler radar established BC data.[9]

 

[Greg Culpepper]

Vibe,

What this reported bc variation described in this chart means to me is that the drag model used is not a perfect fit to the bullet chosen. What am I missing?

"Measurements on other bullets can give totally different results." seems to make my point. Na?

Greg

 

[Vibe]

Vibe,

What this reported bc variation described in this chart means to me is that the drag model used is not a perfect fit to the bullet chosen. What am I missing?

"Measurements on other bullets can give totally different results." seems to make my point. Na?

Greg

The variation is independent of the model used, it's a real phenomenon. Measurements on different bullets would yield a different "average" BC - simply because those bullets would perform differently. But measurements on any bullets would also show these small variations simply due to the fact that the magnitude of the drag forces are dependent upon the velocity and the attitude - and the factors change during the flight.

 

[Greg Culpepper]

Vibe,

Take out the variable of yaw. Now is it not possible to fit a specific drag model to a particular bullet. Does the drag model G1 not perfectly fit the bullet it describes. Would not the bc of this bullet not be unvarying within this G1 model.

Seems to me that this is what mathematics solves.

Where is Brian when I need him?

Greg

 

[Vibe]

Vibe,

Take out the variable of yaw.

Yeah. Good luck with that. Though that is supposedly what makes a "Hummer" barrel.

Now is it not possible to fit a specific drag model to a particular bullet. Does the drag model G1 not perfectly fit the bullet it describes. Would not the bc of this bullet not be unvarying within this G1 model.

Seems to me that this is what mathematics solves.

Where is Brian when I need him?

Greg

All this may be true, but so what? It no longer describes any real world conditions, which was what we have been discussing. In the real world the BC is a levelized estimation - not the constant everyone seems to want to treat it like.

 

[alinwa]

Three points.

First, although the superball changes the momentum of your head twice as much as the lead sinker does (assuming both impact with the same velocity, and that the collisions are perfectly elastic or perfectly inelastic), the force experienced by your head depends on the RATE of momentum change, not the AMOUNT of momentum change. GIGO.


Ya lost me here...... GIGO I get.......but-but-but....

Second, the imprecision (and, in your case, length ) of verbiage is exactly why mathematical models are used by (apparently, the few) people who really want to understand what is going on in the universe.

Of course. BUT...... for every engineer who really understands his math there's one who DOESN'T and who just uses the formulas. I run into them every day. Structural engineers, civil engineers and geotechnical engineers in the building industry are thick as fleas on a donkey, good ones are scarce. Nummah h'ONE, blessed FEW folks are really concerned with what's going on in the universe........and secondly, blessed few folks have had opportunity to wade through the verbiage to GET TO the mathematical plane.

Third, mathematical/physical models can generate counterintuitive (but correct) predictions, verifiable by experiment. Your (or at least my) intuition tells you (me) that a moving bullet has the same mass as a bullet at rest, that your (my) watch runs at the same rate on the ground floor of your (my) house as the second floor, that a moving object will slow down unless force is applied continuously, and that a bullet is blown sideways by a crosswind. All of these intuitions are wrong, and have been shown to be wrong. All the words in the English language won't change that.

All true.

Toby Bradshaw
baywingdb@comcast.net

Using math alone can sometimes be an indication of either a lack of understanding, or a lack of interest in education..... condescension.



al