Aerodynamic Jump

[afrench]

I’m still wrapping my head around AJ but think I understand what’s going at a very basic level.

My questions are:

What can be done to reduce the distance of AJ?

Would reducing the release pressure at the muzzle at exit have any effect on this? For example, if powders with two different burn rates produced similar accuracy and velocity, would the faster powder of the two have a reduced AJ because of reduced pressure at exit?

Would muzzle brakes and similar muzzle devices help protect the bullet for a portion of the AJ distance?

 

[dcali]

The big factors are bullet design & quality, twist rate, and chambering/ammo alignment. More twist = more jump. Better bullets = less jump. Doing whatever you can to keep the bullet going into the bore straight will also help eliminate jump. In-bore tipping is basically creating the same thing as poor bullet balance. Large muzzle disturbances can also exacerbate jump, but I don't know how much that matters. My intuition says that a plain muzzle would be optimal, as it would reduce asymmetries that could cause tipping. But that's just speculation.

That said, sometimes you want to trade ballistics for a little jump. You reduce jump with short, stubby, solid, perfect bullets. But will they shoot at long range? Nope. So you give up a little jump to use those long, sleek VLD's. Because at long range, drag matters more than dispersion (jump). At short range, you might make the opposite call. There's a reason point blank benchrest bullets tend to be short, stubby and obsessively concentric. And there's a reason nobody knows or cares what their BC's are.

Similarly, Bryan Litz has published a convincing case that a little more spin can reduce drag a small amount. Again, you're trading BC for increased jump. Which factor wins depends on your goals. (At long range, BC wins hands down).

Sorry that that's not more specific, but almost everything we do in accuracy work is, at it's core, an effort to eliminate jump. It all boils down to perfect balance and straightness.

 

[seymour fish]

N Tex, IIRC there have been discussions re: lower muzzle pressure being of benefit in a theoretical sense over the yrs. Don't recall any hard data, but perhaps someone will chime in. With a theoretically perfect bullet, perfect crown, perfect gas escape all around the bullet at release, it might not matter at all, yet common sense leans the other way. Re: some protection from wind at the moment of AJ, kinda funny, as had this exact discussion yesterday with a very fine shooter up your way. Again, common sense might dictate that protection of the bullet at its most vulnerable stage (if that can be agreed upon) would be beneficial. Perhaps "protection" could include stripping away the pressure jet forces on the bullet base over the perhaps 1' after muzzle departure, as the bullet jumps and attempts to stabilize,which would seem beneficial if done well. Perhaps benefit could be seen at 25 yds, cheap and dirty, examining bullet hole shapes as J Ohara has eluded to. Many such devices receive attention of experimenters in rimfire. The Vais muzzle brake could be modeled to see if it qualifies, as it didn't hurt accuracy, and some claim some additional benefit. Enough speculation. We need 1) the smart guys with the math to chime in 2) the voices of experience to provide both anecdotal evidence and facts. Seymour

 

[DTubb]

Why not just use a reticle that accommodates the effects of crosswind jump (aero)?
http://www.davidtubb.com/index.php?route=information/information&information_id=10
Dtubb

 

[jcampbellsmith]

Why not just use a reticle that accommodates the effects of crosswind jump (aero)?
http://www.davidtubb.com/index.php?route=information/information&information_id=10
Dtubb

David

Thanks, is this the link? http://www.davidtubb.com/index.php?route=account/download/free&download_id=30

Regards

JCS

 

[DTubb]

Should be a good start.
Do you have an iphone? more stuff available..

 

[jcampbellsmith]

Should be a good start.
Do you have an iphone? more stuff available..

David. Thanks, wouldn't touch an iPhone with a barge pole though. Regards JCS

 

[Bryan Litz Ballistics]

There seems to be some confusion here on terminology.

Aerodynamic Jump (AJ) is usually discussed as being the vertical component of cross wind deflection. This is sometimes mistakenly referred to as crosswind jump. In a crosswind scenario, the aerodynamic jump is a systematic and predictable function of bullet stability and crosswind speed.

All mention of bullet imbalance and misalignment being related to dispersion is correct. However, the mechanism by which a bullet disperses from these things is called lateral throw-off. It's an inertial (mass properties) thing, not aerodynamic. So in the case of no crosswind, if you fire an imbalanced or 'off-axis' bullet, it will exhibit lateral throw-off at the muzzle which will establish some yaw. During the several precession cycles it takes to dampen the yaw is where AJ occurs as a result of the bullet being launched with some yaw rate.

So there are multiple ways for AJ to occur; either from the bullet entering a crosswind, or as a reaction to lateral throw-off in a random direction.

-Bryan

 

[seymour fish]

Bryan, If AJ is defined as the vertical dispersion from a crosswind as you outline above, and we model for near-zero yaw at bullet exit, and an SG of 1.4 to 1.5, thus isolating the AJ, will there be a ratio of vertical/horizontal displacement which persists at long range ? Seymour

 

[Bryan Litz Ballistics]

No, the ratio changes.

Horizontal wind deflection builds with distance because the force is present for the whole bullet flight. So a .8 MOA deflection at 100 yards grows to a 14 MOA deflection at 1000 (not 8 MOA).

But the vertical component of AJ is an angular deflection, so it's linear with range. In other words, if AJ is 0.2 MOA at 100, it's 0.2 MOA at 1000 and all other ranges.

Because wind deflection is growing angularly with range, and AJ is constant with range, the ratio of the two is not constant.

Another way to view this is that the angle of deflection on the target face 'flattens out' at greater ranges. The 10 O-clock to 4 O-clock slant might be 20 degrees at 100 yards but only 5 degrees at 1000 yards because the horizontal component continued to grow but the vertical was fixed. This is another reason why AJ is more evident on short range (100 yard BR or shorter range rimfire): the angle is steeper at closer range.

-Bryan

 

[seymour fish]

Bryan, Thank you. Clear and concise. Exactly what I was looking for. Seymour

 

[dcali]

The terminology is confusing. My understanding is that AJ can be caused by a pure mass imbalance as well, even if there is no lateral throwoff present - when a bullet is dynamically out of balance, but statically balanced. Essentially, that when a bullet is freed from the confines of the bore, a dynamically unbalanced bullet will immediately tip due to a non-axial inertial axis, causing the AJ, even without wind present.

Where as lateral throwoff is caused by a bullet have an imperfect static balance. Again, no wind required.

Am I misunderstanding the situation - that both lateral throwoff and aerodynamic jump are likely present in some small degree for real bullets even in dead calm? And that aerodynamic jump will happen even with a theoretically perfect bullet in a cross wind?

 

[mman]

The terminology is confusing. My understanding is that AJ can be caused by a pure mass imbalance as well, even if there is no lateral throwoff present - when a bullet is dynamically out of balance, but statically balanced. Essentially, that when a bullet is freed from the confines of the bore, a dynamically unbalanced bullet will immediately tip due to a non-axial inertial axis, causing the AJ, even without wind present.

Where as lateral throwoff is caused by a bullet have an imperfect static balance. Again, no wind required.

Am I misunderstanding the situation - that both lateral throwoff and aerodynamic jump are likely present in some small degree for real bullets even in dead calm? And that aerodynamic jump will happen even with a theoretically perfect bullet in a cross wind?

You are correct. That is the way how it is explained by McCoy on p. 260. Actually McCoy explains it so well and going into detail that I should shut up and just suggest everyone to read the chapter 12. However I'm not that smart so I try to sum it up shortly:

Dynamic imbalance leads to aerodynamic jump while static imbalance leads to lateral throwoff.

These two components of dispersion can either compensate or reinforce each other depending on imbalance location relative to center of gravity. And as you said AJ can occur without lateral throwoff if bullet only has pure dynamic imbalance.

 

[M14AMU]

I've read this about 50 times now and I've got it down pat!!



Conclusions
The analytical approach for quantifying the eff
ect of a uniform sequen
ce of lateral square
impulses disturbing a projectile du
ring free flight is presented.
All the analysis was based on
projectile linear theory, which
produces simple closed form solutions for the assumed square
pulse disturbances. The swerving
motion caused by a single impulse (
1
) is modified by a
multiplying factor, equation 24, which acc
ounts for a uniform sequence, length K
m
+ 1, of
impulses. Changes in aerodynamic jump caused
by a sequence of lateral impulse forces are
shown to produce easy-to-understand additive
contributions to the usual aerodynamic jump.
Magnitude and phase angle changes that depend
on the sequence length and spacing are readily
obtained, which may prove useful guida
nce, navigation, and control.