Attention all ballisticians

[alinwa]

If 1% variation in a bullets BC number is significant let me know whose bullets you are using that are half that.
Waterboy

I think I made a boo boo here All's I did was muddy the waters.

I think All Keith was pointing out is that a bullet "loses" almost 1% of it's theoretical BC because it's flying a little sideways.... equilibrium yaw. Keith can correct me if I'm mis-reading his intent.

I was just noting that this occurs wind or no wind. I furthermore think that this is often the difference between "calculated BC" and actual BC as measured by folks like Henry Childs. And NOW by Berger bullets and others, thanks To Bryan Litz. (And of course Henry who first brought the differences to light.)

"Losing" a little BC is unimportant..... but random fluctuations would be important. As would a loss of BC in the wind because then you would be losing vertical as well as horizontal in a switchy wind. I don't believe this to be the case. Well made and/or well sorted bullets shoot tabletop flat at yardage.

sorry, my bad

al

 

[bsl135]

Bryan,

My point is that paper BC (and even effective BC) changes when that bullet points into that wind. While that COG is continuing on its flight path the COP is turned into the wind which presents a different form factor as it travels forward.

Tony,
The BC does not change when the bullet points into the wind.
When flying thru a crosswind, the bullet is angled to the LOS, but that's irrelevant. What's important is how the bullet is oriented in the airflow. Stability dictates that the bullet's axis is aligned with the airflow. This being the case, the bullet is not presenting even a little of it's side to the airflow. It is presenting a little crooked to an observer looking down the LOS, but again, that's irrelevant.

As I mentioned in a previous post, and has been discussed since, there are reasons for a bullet to present at an angle to the airflow (it's called 'angle of attack'). Reasons for this are:

1. Muzzle blast disturbance settling out.
2. Bullet precessing to point it's nose into a crosswind.
3. Yaw of repose (aka equilibrium yaw). This is the yaw that results from the bullet's trajectory bending as a result of gravity. The bullets' nose points to the right at an ever increasing angle.

The first two disturbances settle out very quickly, within ~10 to 20 yards or so beyond the muzzle. Whatever minor angle of attack the bullet would see in that first 10-20 yards will not have a big effect on the entire trajectory since the bullet flies with zero angle of attack for most of the way.
An exception is (as Al points out) in 'switchy' wind conditions. If the bullet has to constantly re-orient its axis to align with the changing wind vector, its axis is constantly disturbed as it seeks the new orientation.)

Let's revisit the ~.8% 'effect' for the .50 caliber bullet flying with 1/2 degree of yaw. Remember that the yaw is quickly damped and the bullet flies with near zero (much less than 1/2 degree) yaw for most of the way. In other words, the .8% effect is not present for most of the flight.
The plots showing the drag increase as a function of yaw don't imply that the bullet actually does fly with that much yaw normally.

All 3 of the above reasons will only cause the slightest angle of attack under normal circumstances, like < 1/2 degree.

I think the biggest challenge with this discussion has to do with understanding the relative magnitudes. As Kieth pointed out, some of the 'facts' are secondary effects, which means they do happen, but really have no practical consequence.

I'll break out the 6-DOF simulation to illustrate some of the effects with numbers and plots.

Stand by.

In the mean time, have a look at this article which was published in the latest Varmint Hunter regarding measuring BC and the potential effects of yaw on drag. Keep in mind, the level of initial yaw assumed in the simulation is a guess. I have no idea how realistic it is to get such high levels of initial yaw in reality, the simulation was only run that way to show how much it would take to explain the difference in measured BC over 100 yards. My gut tells me that the level of yaw that's simulated in the article is way way more than normal, and there's some other error responsible for the disagreement.

-Bryan

 

[bsl135]

OK,
Here are the simulation details:

.30 caliber 155 grain Sierra MatchKing (2155), 3000 fps, standard sea level atmospheric conditions.

I ran 3 simulations with 3 different wind conditions and plotted the resulting angle of attack (AOA) that the bullet sees as it flies thru these different conditions.

The first condition is no wind. The AOA is shown in blue. You can see that there is some AOA that grows slightly, approaching 0.02 degrees at 1000 yards. This is a lateral AOA, which is known as the yaw of repose, which is responsible for spin drift.
In this no-wind condition, the bullet reaches 1000 yards in 1.6067 seconds, with 1166.9 fps remaining velocity which accounts for the extra drag induced by the small yaw of repose.

The second condition is a constant 10 mph crosswind. AOA for this condition is shown in red. You can see that when the bullet emerges from the barrel, it has an initial angle of attack of around .28 degrees (atan(14.667/3000) = .28). With the stability of this bullet, a first max yaw of ~.36 degrees is reached within a couple yards. The average yaw level then decays to <0.05 degrees by 100 yards.
In this wind condition which induces such AOA, the tof and remaining velocity are identical to the no wind condition all the way to 1000 yards. This is the point I set out to illustrate with the 6-dof program from the beginning: although wind does induce a small transient angle of attack, it's a second order effect, and does not change the effective BC of the bullet enough to make a difference of even 0.1 fps of retained velocity at 1000 yards.

The third condition simulates a (very abrupt) switchy wind condition. The wind condition is: 10 mph crosswind with a complete direction reversal at 500 yards. The direction reversal forces the bullet to realign it's axis to point into a new wind vector. The magnitude of the initial AOA upon entering the new wind direction should be equal to the atan of the crosswind speed (essentially a 20 mph change) divided by the bullets remaining velocity at 500 yards: 1974 fps. atan(20*1.667/1974) = 0.85 degrees. This is exactly the initial AOA at 500 yards. The AOA jumps from 0.85 degrees to just over 1 degree in the first max yaw after 500 yards, then settles to about 1/10th that value in the next 100 yards.
So what is the effect on tof and retained velocity of the extra drag induced by that big change? At 1000 yards, the tof is 1.6068 seconds, and the remaining velocity is 1166.7 fps. That's 0.0001 seconds greater tof and 0.2 fps less retained velocity compared to the prior two wind conditions.
This last case goes to show that the simulation is adding induced drag from AOA, and it also shows how little of an effect it has on the effective BC of the bullet.

This second image is the same data as above, zoomed on the y-axis.

The real challenges of shooting thru wind have much more to do with the real nature of wind than the second order effects on the bullet. In other words, if you have vertical induced by wind, it has more to do with vertical wind currents, or the vertical component of wind deflection than some minuscule effect of the wind lowering the bullet's BC.

-Bryan

 

[alinwa]

holy cow

hot diggity dawg

and Halle-freakin'-luia!!!!!

I think we've achieved some sort of concensus in this thread! Finally!

Thanks Guys.... and a 'special thanks to Gene Beggs Da' Man who keeps his hand in the pot.........................stirring...........

You Rock Gene


al

 

[mks]

Vertical wind?

OK,
Here are the simulation details:

-Bryan

Bryan,
The simulations are awesome. Could you possibly run a comparison of the pitch of the same bullet with and without vertical wind? Even though Al says no, I am still curious whether vertical wind increases lift, overturning moment and, therefore, pitch. Seems like it might also increase yaw through the gyroscopic effect.

Thanks so much for your participation in this discussion.

Cheers,
Keith

 

[alinwa]

Bryan,
The simulations are awesome. Could you possibly run a comparison of the pitch of the same bullet with and without vertical wind? Even though Al says no, I am still curious whether vertical wind increases lift, overturning moment and, therefore, pitch. Seems like it might also increase yaw through the gyroscopic effect.

Thanks so much for your participation in this discussion.

Cheers,
Keith

Whaddaya' mean "Al sez no"?????? I believe that gravity induced "vertical wind" DOES produce a lift force. If it didn't there'd be no right-hand drift.

A'gain with the communication breakdown eh...... in the end we may all have been in agreement the whole time???


mebbeso NOT ..... but closer than we sometimes think.

al

 

[mks]

Whaddaya' mean "Al sez no"?????? I believe that gravity induced "vertical wind" DOES produce a lift force. If it didn't there'd be no right-hand drift.

A'gain with the communication breakdown eh...... in the end we may all have been in agreement the whole time???


mebbeso NOT ..... but closer than we sometimes think.

al

Al,
I was inexact. What I think you said was that vertical wind doesn't cause pitch. But if we agree that it causes lift, then how can it not increase overturning moment, which causes pitch and yaw?

Cheers,
Keith

 

[bsl135]

A pure vertical wind???

Keith you are a FA-REEEEEAK!

OK, I'll run it. My hunch is that the effects of wind, gravity and spin drift will remain completely independent. In other words, the effect of the vertical wind will be identical to a horizontal wind, just rotated 90 degrees.

Gimmie a minute...

 

[bsl135]

Simulation conditions same as above.
Additional variables that are relevant to the trajectory are:
Sight height = 1.5 inches
Total bore angle = 36 MOA (~33 MOA above a 100 yard zero)
Target range = 1000 yards

The values above were held constant.

-------------------------------------------------------------
The first shot was modeled with no wind, as a baseline. The shot impacts 1000 yards:
+4.51" high, and 9.64" right of the x-axis (LOS)

The 9.64" is the spin drift, the +4.51" is just because 36 MOA elevation isn't exactly a perfect zero.
-------------------------------------------------------------
Now, with a 10 mph, 9 o'clock crosswind, the point of impact is:
-0.12" low, and 116.53" right

So the vertical effect of the crosswind is to deflect the bullet down 4.51" + 0.12" = 4.62".
The horizontal effect of the crosswind is: 116.53" - 9.64" = 106.89"
-------------------------------------------------------------
With the 10 mph wind coming straight out of the ground, the point of impact is:
+111.89" high, and 14.23" right

So the vertical effect of the up-wind is 111.89" - 4.51" = 107.38" (compared to 106.89" for the horizontal effect of the crosswind)
The horizontal effect of the up-wind is 14.23" - 9.64" = 4.59" (compared to 4.62" for the vertical effect of the crosswind).
-------------------------------------------------------------

Here is a plot of the 3 bullet impacts at 1000 yards on an x-y grid:

Analysis:
The up-wind deflects the bullet up in the same way a crosswind deflects a bullet to the side. Gravity is still present, and still acts on the bullet the same as it does in a no wind scenario.

As for a bullet getting 'lift' from it's nose pointing above the flight path in a no-wind scenario.... no. The nose doesn't point above the velocity vector, it points to the right. The 'lift' is to the side, not in the vertical plane.
(This last comment isn't illustrated by the simulation in any way. I'm just addressing it because I thought it was commented on, or then again, I may have misunderstood the statement that was made about lift.)

-Bryan

 

[tobybradshaw]

Thanks to all who have contributed to this multi-thread discussion -- it has been very illuminating.

Two remaining issues (of minor importance in the grand scheme of things):

1. Bryan, what do you make of Sierra's murky statement that bullets "turn to follow" a crosswind? http://www.exteriorballistics.com/ebexplained/5th/43.cfm Most people here have concluded that whoever wrote this for Sierra is claiming that the bullet's nose points to the left in a right-to-left (3 o'clock) crosswind ("following the crosswind"), which is flat-out wrong, and a little eye-popping considering the source.

2. Re: the Sierra major boo-boo above. Boyd, send me your mailing address. I still owe you a box of bullets because I have to admit that "experts disagree," and I'm willing to pay up!

Toby Bradshaw
baywingdb@comcast.net

 

[Old Gunner]

Lift is a matter of a partial vaccum or low pressure area on the curved surface of an object oposite the wind or air stream. Wind doesn't actually push anything thats not flat.
Wings generate lift by the airflow over the wing rather than under it.

The Bernoulle effect on a rotating ball in flight produces lift by creation of a tiny bubble of greatly reduced pressure. Thats why golf balls fly so far, and the principle is used with the Airsoft BB guns to increase range.
A unpatched musket ball rolls along the barrel, and through the air and the effect is reversed, that shortened the range of the smooth bore muskets.

To carry this further, a clindrical bullet spun by rifling should generate a low pressure area above and to the side opposite to rotation. Shockwave of supersonic flight would disrupt this low pressure area, but as the bullet fell below 1100 fps at longer range the effect would tend to first overcome then counteract gyroscopic bullet drift.
It was noted with the early Springfield that bullet drift went to one side up to a certain range then dropped off and went in the opposite direction from that point on.

A flat base bullet would have a low pressure area directly behind it, and directly in line with airflow, this should turn it into the wind.

 

[bsl135]

Toby,
I think you're right, the description of crosswind effects given by Sierra's technicians is incorrect.

The start of the problem is where they say the nose of the bullet turns to follow the wind. Actually, the nose of the bullet turns the other way. They build on the mistake by discussion the resulting vertical deflection as being opposite of what it really is. They say:

Barrel Twist /Crosswind.....Crossrange.....Vertical
................Direction.....Deflection.....Deflection
RH /L to R Right Upward
RH /R to L Left Downward
LH /L to R Right Downward
LH /R to L Left Upward

Which we know is reverse of real life. IF the bullet nose turned to follow a crosswind, the vertical deflection would be as they describe, but the nose actually turns the other way, so the vertical deflection is opposite.

My statements are supported by Harold Vaughn's Rifle Accuracy Facts, pgs 195-199.

Also by Bob McCoy's Modern Exterior Ballistics, section 12.9: The Aerodynamic Jump Due To Crosswind.

Generally, Sierra's technical stuff is very good. I think Bill and Ted just made a mistake in this case, it happens. Who wants to tell them? Furthermore, who thinks they'll acknowledge and correct the mistake?

-Bryan

 

[Vibe]

Lift is a matter of a partial vaccum or low pressure area on the curved surface of an object oposite the wind or air stream. Wind doesn't actually push anything thats not flat.
Wings generate lift by the airflow over the wing rather than under it.

The Bernoulle effect on a rotating ball in flight produces lift by creation of a tiny bubble of greatly reduced pressure. Thats why golf balls fly so far, and the principle is used with the Airsoft BB guns to increase range.
A unpatched musket ball rolls along the barrel, and through the air and the effect is reversed, that shortened the range of the smooth bore muskets.

To carry this further, a clindrical bullet spun by rifling should generate a low pressure area above and to the side opposite to rotation. Shockwave of supersonic flight would disrupt this low pressure area, but as the bullet fell below 1100 fps at longer range the effect would tend to first overcome then counteract gyroscopic bullet drift.
It was noted with the early Springfield that bullet drift went to one side up to a certain range then dropped off and went in the opposite direction from that point on.

A flat base bullet would have a low pressure area directly behind it, and directly in line with airflow, this should turn it into the wind.

I have to disagree. The low pressure area does not provide the "lift" other than it provides an unbalanced reaction for the higher pressure side to push into. Without considering this, a perfect vacuum would only provide for an anemic 14.7psi suction hardly enough to decelerate our bullets at the rate that real life proves to happen.

 

[mks]

Thanks

As for a bullet getting 'lift' from it's nose pointing above the flight path in a no-wind scenario.... no. The nose doesn't point above the velocity vector, it points to the right. The 'lift' is to the side, not in the vertical plane.
(This last comment isn't illustrated by the simulation in any way. I'm just addressing it because I thought it was commented on, or then again, I may have misunderstood the statement that was made about lift.)

-Bryan

Thanks for running the case. I think it confirms part of what I expected - that the up-wind increases yaw. (Yaw isn't plotted, so I'm assuming the increased rightward drift relative to no-wind is caused by increased yaw.) And if there is more yaw, then this is evidence that there must be more pitch, too.
If there no pitch at all, then there would be no upward lift force at the CP to create a right-facing pitching moment vector that turns the right-hand spinning bullet to the right (see McCoy section 10.6). The pitch is really, really small, and the yaw is too, but they still have an important effect in the form of several inches of drift at 1000 yds.

Cheers,
Keith

 

[bsl135]

Keith,
I'm still not sure I'm tracking 100% with what you're looking for.

The total AOA reached in the upwind case is equal to the total AOA reached in the crosswind case.

In the crosswind case, there is an out-of-plane pitch induced which causes the vertical component of wind deflection (aerodynamic jump).

In the up-wind case, the out-of-plane is yaw, and it results in a horizontal component of the up-wind deflection.

Here's another article which may or may not be directly related to your concerns, but is relevant to lift.

A little background. The article was written in response to a .338 bullet manufacturer who was claiming performance for their bullets in terms of drop data, which implied a BC of over 1.1. One of the theories that was floated is that: perhaps the bullets have normal drag, but are generating lift which makes them drop less, and have a trajectory that implies a higher BC'. Well, I put it to the test by creating a 6-DOF model of the exact bullet, and testing it to see how much lift was actually generated over 1000 yards.

 

[mks]

Wind doesn't actually push anything thats not flat.
Wings generate lift by the airflow over the wing rather than under it.

Here is a typical pressure distribution on an airfoil. You can see that the air is both pushing (p>0) and pulling (p<0).

Cheers,
Keith

 

[mks]

Yep, they are really, really small

Keith,
I'm still not sure I'm tracking 100% with what you're looking for.

The total AOA reached in the upwind case is equal to the total AOA reached in the crosswind case.

In the crosswind case, there is an out-of-plane pitch induced which causes the vertical component of wind deflection (aerodynamic jump).

In the up-wind case, the out-of-plane is yaw, and it results in a horizontal component of the up-wind deflection.

Here's another article which may or may not be directly related to your concerns, but is relevant to lift.

A little background. The article was written in response to a .338 bullet manufacturer who was claiming performance for their bullets in terms of drop data, which implied a BC of over 1.1. One of the theories that was floated is that: perhaps the bullets have normal drag, but are generating lift which makes them drop less, and have a trajectory that implies a higher BC'. Well, I put it to the test by creating a 6-DOF model of the exact bullet, and testing it to see how much lift was actually generated over 1000 yards.

Bryan,
I was having second thoughts about whether the very small quasisteady pitch and yaw angles I was thinking of could even be found amongst the dynamic precession angles. But I think you have exactly such a graph on page 9 of your report. Yaw is about 0.008 degrees and pitch less than 0.001 degrees at 1000 yards. What I expect is that with up-wind both these angles will increase.

Interesting that you say that the total AOA is the same for up-wind and cross wind. I would have guessed that it might be greater for the up-wind case, since I wouldn't expect pitch to increase in cross wind as much as yaw increases in up-wind.
Fun stuff to think about.

Cheers,
Keith

 

[Boyd Allen]

Toby,
Admitting that you were wrong is sufficient. Go to the range. Shoot the bullets for me, and have fun. Oh, and if you can see any evidence of which way their noses are pointing, let us know.

 

[Old Gunner]

I have to disagree. The low pressure area does not provide the "lift" other than it provides an unbalanced reaction for the higher pressure side to push into. Without considering this, a perfect vacuum would only provide for an anemic 14.7psi suction hardly enough to decelerate our bullets at the rate that real life proves to happen.

I can see where theres some confusion, I'm not a scientist nor do I play one on TV.
The following is from a theory intended to refute the equal transit /longer path theory.

{The upper flow is faster and from Bernoulli's equation the pressure is lower. The difference in pressure across the airfoil produces the lift.} As we have seen in Experiment #1, this part of the theory is correct. In fact, this theory is very appealing because many parts of the theory are correct.

http://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html

Lift is due to the difference in air pressures between the top and bottom of the wing.
You can say the air under the wing pushes up, but only because of the partial vacuum above the wing, nature abores a vacuum so the wing tries to fill it.
Air simply hitting the flat underside of the wing would not produce lift if the pressure were equal on both upper and lower surfaces, any more than a wing would produce lift in an airless environment with vacuum on each surface.

A flat wing at a high angle of attack still produces lift by creation of a low pressure area above the wing, same effect but different cause, The wing still rises to fill the vacuum.
It may not be sucked upwards per se' but the effect remains the same, without the partial vacuum air pressure under the wing could not produce lift.


Think of it this way.
place a hardboiled egg, or egg softened by soaking in vinegar, on the mouth of a bottle, the opening smaller than the egg. try to push the egg into the bottle. result would be a smashed up (permanent deformation) egg with more of the egg on the outside than inside.
Then try this light a piece of paper abd drop it in the bottle then place the egg on the opening. When the flame eats up the oxygen creating a partial vacuum the egg slides smoothly into the bottle with temporay elastic deformation .

The energy is provided by atmospheric pressure but the partial vacuum creates a path for the energy.

I don't think I mentioned "deceleration", through the partial vacuum of a flat base induces drag.
To hold an unpowered craft like a life boat, or dismasted vessel headon into the current you'd rig a "sea anchor" a bucket like construction on a line secured to the stern. Induced drag from current pulls the stern turning the vessel to face the prevailing current and avoid breeching to the seas and capsizing.

With something as small as a bullet and with so much impetus and inertia, the effects of the factors I've mentioned are miniscule, but they may be a source of odd behavior in poorly designed bullets and at extreme long range, beyond the normal target ranges, such as the volley fire at 3,000 plus yards during WW2, or long range indirect fire at 4000+ yards.

PS as evidence that airflow over the airfoil is the prime element in lift, modern jet fighters have pylons for ordnance on the underside of the wing, huge amoounts of drag inducing and airflow disrupting equipment can be hung under the wings but the upper surfaces must remain uncluttered.


The question comes down to this, air pressure provides the energy, but the shape of the object it acts upon determines the result, Air pressure is no the determining factor, otherwise all objects it acted upon would react in exactly the same way.
The partial vacuum formed to leewards is the actuating causation.

 

[tobybradshaw]

Toby,
Admitting that you were wrong is sufficient. Go to the range. Shoot the bullets for me, and have fun. Oh, and if you can see any evidence of which way their noses are pointing, let us know.

Now if we can only get Sierra to admit that they are wrong ...

And I don't think I'm going to be any good at seeing the bullet's nose -- at last month's match I couldn't even see my bullet hole at 300yd and took what turned out to be a second shot on one of my targets (hence the "no X" score on the attached photo).

Toby Bradshaw
baywingdb@comcast.net