Range Report Super-stabilizing to avoid transonic upset?

[dbooksta]

We know that bullets tend to suffer a degradation of accuracy when they slow through transonic speeds.

Does a superstabilized bullet -- say, something with a Miller Stability factor above 2.5 -- avoid or reduce the effects of transonic upset?

 

[Tactical]

Re: Super-stabilizing to avoid transonic upset?

Interesting question. No expert here but here is what I have found shooting 155 Scenars.

Now at 600 yards a 1x12 twsit works great but I get better groups at 1000 yards with 1x11.25 twist, so my theroy is it takes more twist as the bullet slows. The 155 scenar is a weird bullet as its very long and seems to like lots of twist where the other 155s out there seem fine with 12 or even 13 twist

 

[gstaylorg]

Re: Super-stabilizing to avoid transonic upset?

Agreed; interesting question. My guess would be that transsonic stability has more to do with the shape/BC of the bullet than its spin rate because an insufficient rate of spin can clearly generate significant instability issues even when the projectile is supersonic. If I understand it correctly, projectile linear velocity slows much more rapidly than does its spin. So the question stated in a slightly different way would be, "Would a faster twist barrel allow a projectile like a 168 SMK, which I believe are known for their transsonic instability issues, to be stabilized out to longer distances than they would be from a slower twist barrel?"

 

[bohem]

Re: Super-stabilizing to avoid transonic upset?

The bullet's ability to go through the sound barrier and remain stable/predicatable has a lot more to do with the bullet design than the rate of twist.

As the bullet slows during flight it retains a very high spin rate compared to the corresponding forward velocity and so as any of these rounds approach the sound barrier they will have high static stability values, many will be over 5.0 yet only SOME bullets will transition well.

Taking a bullet and shooting it from an 8tw instead of a 10tw will help if the bullet is "borderline" but it isn't going to get a poor shape through the sound barrier on twist rate alone.

Case in point:

M193 bullets are stable from a 14tw rifle but fall out of the sky at 0DA around 550yd because they approach Mach 1.2 and have major issues.

I've shot them back to back from a 14tw and a 7tw. The 7tw will have doubled the rotational rate that they come out of the barrel for the same velocity and yet I still couldn't hit a 600yd target with them repeatably.

 

[jwoolf]

Re: Super-stabilizing to avoid transonic upset?

I'm no expert either but, I know there are some here but, I'd bet it's spin and bullet design.

You can stabilize a bullet with a slow twist if you shoot it fast enough. ( up to a point )

Think of it this way with twist rates... The exit velocity determines the spin along with the spin rate... the bullet isn't in the air that long and won't slow down in spin that much for it's flight time.... Getting the bullet to stabilize for the exit velocity would be the trick ( again, I'm no expert )

I imagine it's a total balance that has to be achieved... along with the right bullet design.

You can spin so fast you can tear the copper jacket right off a bullet... don't spin fast enough and, you get crap too. ( that's why I don't design bullets... LOL )

bohem said it well, I think.

 

[200 20x]

Re: Super-stabilizing to avoid transonic upset?

If somebody figured out how to defeat transonic upset they must be keeping it a secret. All the best small bore ammo (22 rimfire) is subsonic and those guys would love a few more fps.

 

[Skigolfmike]

Re: Super-stabilizing to avoid transonic upset?

Supposedly, the Berger 175 OTM 308 bullets are transonic stable in a 11.25 twist barrel. However, they were designed by a rocket scientist.

 

[gstaylorg]

Re: Super-stabilizing to avoid transonic upset?

Increasing velocity adds only a very, very small effect (usually less than 3%) within the normal ranges of MVs found in precision rifles. The reason for this, as shown in Don Miller's corrective formula for muzzle velocity, is that the correction involves an inverse cube function. For example, going from 2800 fps to 3000 fps, which is a pretty significant increase in MV, only buys you a little over 2% increase in stability factor.

 

[dbooksta]

Re: Super-stabilizing to avoid transonic upset?

That's worth noting. If you want to significantly enhance bullet stability you'll have to use shorter and/or denser bullets, or faster barrel twist rates.

 

[7mmSTW]

Re: Super-stabilizing to avoid transonic upset?

There is an article on the 300 Norma by Todd Hodnett in the last issue of SNIPER where he talks of running a 1:7.8 twist barrel on a .308 for better long range/transonic stability.

 

[rizla]

Re: Super-stabilizing to avoid transonic upset?

If a bullet is spinning too fast it can actually become over-stabilized. When the bullet begins to slow and drop due to gravity the nose of the bullet will ordinarily drop as well, so the bullet continues to be pointing in the direction it is flying. In an over-stabilized bullet the nose is much less willing to move in this way and continues to point in the direction it was initially fired in despite the bullet losing altitude. Therefore relative airflow over the bullet is no longer exactly head on but is instead hitting the ogive, so the bullet effectively becomes a lot less aerodynamic which results in reduced accuracy as well as a quicker decrease in velocity.

 

[dbooksta]

Re: Super-stabilizing to avoid transonic upset?

That's a problem for artillery; not so much for man-portable rifles where <span style="font-style: italic">total drop</span> within maximum range is still measured in inches (or, at extremes, feet).

 

[rizla]

Re: Super-stabilizing to avoid transonic upset?

While you are correct in saying it's more of a problem for artillery, it can also be a factor to consider with man-portable rifles. A .338 Lapua Mag for example can display drops at 1000 yds of over 80 yards, not feet. And at the extremes a 750gr .50 BMG shooting 1700 yards will drop more than 300 yards. I would consider that enough drop to cause me concern if my bullet started flying in a style akin to that of an airliner about to land.

 

[dbooksta]

Re: Super-stabilizing to avoid transonic upset?

You must be confusing units. I just ran ballistics on a 300gr .338 MatchKing with muzzle velocity of 2700fps. It falls below the sound barrier after 3.3 seconds at over <span style="font-weight: bold">1 mile</span>, at which point it has dropped 120 feet (or 40 yards) <span style="font-style: italic">assuming a horizontal shot</span>.

If you held for a target at 1 mile your bullet would leave the muzzle about 4 degrees nose up to the horizon. Peak trajectory out to 1 mile would be about 45 feet. At 1 mile it would be falling 60fps while still moving forward at 1000fps. Even in this extreme case, whether it is still slightly nose up seems like a third- or fourth-order consideration.

Of course twist rate affects spin drift, but I've never heard super-stabilization mentioned as a practical factor even in .50BMG ballistics.

 

[waste_knot]

Re: Super-stabilizing to avoid transonic upset?

I thought transonic upset is more of a pressure wave shift affecting the center of pressure. I think i'll look that up.

 

[dbooksta]

Re: Super-stabilizing to avoid transonic upset?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: dbooksta</div><div class="ubbcode-body">Of course twist rate affects spin drift, but I've never heard super-stabilization mentioned as a practical factor even in .50BMG ballistics. </div></div>

Possible exception: I was reviewing some old .50 literature and found a 1993 article by Eric Williams (<span style="font-style: italic">Fundamentals of Bullet Design: A New Bullet Designed by PRODAS</span>) where he mentions that super-stabilized bullets take longer to "go to sleep" (i.e., for any nutation or precession to damp out). But I don't know how significant those moments of instability are in state-of-the-art ballistics, or in smaller calibers.

 

[Graham]

Re: Super-stabilizing to avoid transonic upset?

What is all this talk of super-stability and over-stability? Either a bullet is stable, or it isn't.

 

[dbooksta]

Re: Super-stabilizing to avoid transonic upset?

Read up on gyroscopic stability and the stability coefficient (typically denoted by Sg): There is a spin rate that barely stabilizes a bullet, denoted by Sg=1. Anything above that is technically "over-stabilized," though in practice it's necessary to spin a bullet to Sg = 1.5 or 2.0. "Super-stabilized" would be bullets spinning well in excess of 2.5 or 3.

 

[Graham]

Re: Super-stabilizing to avoid transonic upset?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: dbooksta</div><div class="ubbcode-body">Read up on gyroscopic stability and the stability coefficient (typically denoted by Sg): There is a spin rate that barely stabilizes a bullet, denoted by Sg=1. Anything above that is technically "over-stabilized," though in practice it's necessary to spin a bullet to Sg = 1.5 or 2.0. "Super-stabilized" would be bullets spinning well in excess of 2.5 or 3. </div></div>Hmmm.....

So the people who say that one can over-stabilize a bullet
http://bisonballistics.com/articles/barrel-twist-and-bullet-stability
Are the same people who, when they say that, simply mean that bullets can be spun faster than the minimum required to stabilize them.

Put another way, if by the over/under stable standard there is such a thing as an 'over-stabilized' bullet, then that same bullet will also become 'under-stabilized' because it will lose stability faster as velocity decreases.

 

[dbooksta]

Re: Super-stabilizing to avoid transonic upset?

As I said earlier, we do know that over-stabilizing a bullet exacerbates gyroscopic sources of error (like lateral throw-off). I don't know if those moments are significant for modern match-grade bullets in "good" loads and rifles, and I would appreciate any data on that question.

Of course aerodynamic effects like jump and spin drift are also accentuated with higher spin rates, but they're predictable and can be incorporated into any decent exterior ballistic model. And they're a small price to pay if the higher spin rate increases transonic stability.

 

[Helmet]

Re: Super-stabilizing to avoid transonic upset?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: dbooksta</div><div class="ubbcode-body">We know that bullets tend to suffer a degradation of accuracy when they slow through transonic speeds.

Does a superstabilized bullet -- say, something with a Miller Stability factor above 2.5 -- avoid or reduce the effects of transonic upset? </div></div>This is from Applied Ballistics For Long-Range Shooting By Bryan Litz, page 145.

"If a bullet has marginal dynamic stability at transonic speeds, it's sometimes possible to compensate with faster twist. It's a similar situation to the bullet overcoming its basic static instability with adequate gyroscopic stability. If the bullet has a dynamic instability, you might be able to fix it by spinning the bullet faster, making the axis rigid enough to overcome both the static and dynamic instability of the bullet. It's easy to calculate the spin rate required to overcome static instability at the muzzle (supersonic speed). However, it's extremely difficult to calculate how much spin rate is required to overcome a dynamic instability caused by transonic flight."
Bryan Litz goes on to explain why it would be difficult to predict how much spin would be needed.
The answer to your question is yes, more spin can reduce the effects of transonic flight. However, how much extra spin you would need for a certain bullet would require experimentation as it can't really be predicted.

 

[tobias]

Re: Super-stabilizing to avoid transonic upset?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: dbooksta</div><div class="ubbcode-body">
Possible exception: I was reviewing some old .50 literature and found a 1993 article by Eric Williams (<span style="font-style: italic">Fundamentals of Bullet Design: A New Bullet Designed by PRODAS</span>) where he mentions that super-stabilized bullets take longer to "go to sleep" (i.e., for any nutation or precession to damp out). But I don't know how significant those moments of instability are in state-of-the-art ballistics, or in smaller calibers. </div></div>

I don't understand this. "Stable" means the amplitudes of the nutation and precession decrease. How can a "super-stabilized" bullet take longer to go to sleep? For me that's a contradiction.
Where can I find the article?

 

[Helmet]

Re: Super-stabilizing to avoid transonic upset?

I agree. Bullets "fall asleep" because they have marginal stability to begin with and gain stability as they slow down. Technically that's because the "overturning aerodynamic torque"(air resistance) decreases as the speed decreases, but the "rigidity of the spinning mass"(spin) changes very little throughout a bullets flight.
A highly stabilized bullet should be stable from the beginning unless some other factor from faster spinning is causing the instability.

 

[dbooksta]

Re: Super-stabilizing to avoid transonic upset?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: tob</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: dbooksta</div><div class="ubbcode-body">
Possible exception: I was reviewing some old .50 literature and found a 1993 article by Eric Williams (<span style="font-style: italic">Fundamentals of Bullet Design: A New Bullet Designed by PRODAS</span>) where he mentions that super-stabilized bullets take longer to "go to sleep" (i.e., for any nutation or precession to damp out). But I don't know how significant those moments of instability are in state-of-the-art ballistics, or in smaller calibers. </div></div>

I don't understand this. "Stable" means the amplitudes of the nutation and precession decrease. How can a "super-stabilized" bullet take longer to go to sleep? For me that's a contradiction.
Where can I find the article? </div></div>

The article was in <span style="font-style: italic">Very High Power</span> magazine, and I found it reprinted (poorly) in Dean Michaelis's <span style="font-style: italic">Complete .50-Caliber Sniper Course</span>.

The "contradiction" may stem from an abuse of the word "stable." Based on the same article my understanding is that the damping coefficients of nutation and precession are a function of bullet design, and Williams didn't cite any direct relationship between spin-stability and those coefficients. So we could say a bullet is "stable" because it has high yaw damping coefficients (which help it go to sleep faster), or we might say it is "stable" because it is spin-stabilized (which prevents it from tumbling, but which may actually reduce damping rates of gyroscopic motions).

I think this is easy to picture if you think of a bullet with a large imbalance between center of mass and center of spin: The faster it spins, the more lateral throw-off it experiences on leaving the barrel and the longer it will take to damp out that precession.

Another example you can observe just spinning a toy top or gyroscope is that both moments persist while the gyro is spinning above its minimum stable speed, and they don't strongly decay until the gyro slows towards its minimum rate of stability (i.e., Sg = 1.0, right before the top is about to fall over).

 

[dbooksta]

Re: Super-stabilizing to avoid transonic upset?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: ArmchairElite</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: dbooksta</div><div class="ubbcode-body">We know that bullets tend to suffer a degradation of accuracy when they slow through transonic speeds.

Does a superstabilized bullet -- say, something with a Miller Stability factor above 2.5 -- avoid or reduce the effects of transonic upset? </div></div>This is from Applied Ballistics For Long-Range Shooting By Bryan Litz, page 145.

"If a bullet has marginal dynamic stability at transonic speeds, it's sometimes possible to compensate with faster twist. It's a similar situation to the bullet overcoming its basic static instability with adequate gyroscopic stability. If the bullet has a dynamic instability, you might be able to fix it by spinning the bullet faster, making the axis rigid enough to overcome both the static and dynamic instability of the bullet. It's easy to calculate the spin rate required to overcome static instability at the muzzle (supersonic speed). However, it's extremely difficult to calculate how much spin rate is required to overcome a dynamic instability caused by transonic flight."
Bryan Litz goes on to explain why it would be difficult to predict how much spin would be needed.
The answer to your question is yes, more spin can reduce the effects of transonic flight. However, how much extra spin you would need for a certain bullet would require experimentation as it can't really be predicted. </div></div>

Ah ha -- flagged for first answer!

Now the follow-up: Can a good 6 DoF simulator like PRODAS model transonic instability factors as a function of spin stability, or is transonic upset such a noisy process that it can't be practically modeled or estimated?

 

[Helmet]

Re: Super-stabilizing to avoid transonic upset?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: dbooksta</div><div class="ubbcode-body">Ah ha -- flagged for first answer!

Now the follow-up: Can a good 6 DoF simulator like PRODAS model transonic instability factors as a function of spin stability, or is transonic upset such a noisy process that it can't be practically modeled or estimated? </div></div>Bryan Litz:
"1.You can't accurately predict the amount of instability the bullet will have during transonic flight.
2.You can't accurately predict how much the spin rate of the bullet will have decayed by the time it reaches transonic speed."

I don't know anything about simulators, but without being able to feed the correct information into it I doubt you would get anything worthwhile back out of it.
Unfortunately it wouldn't be realistic for someone to do this experiment as it would not only take a multitude of barrels with different twists, but the results would be specific to one bullet. Quite an undertaking for one man.
If you just spun the crap out of a bullet it would definitely increase your odds of spinning it enough to accomplish your goal. That's a pretty basic solution though. It would definitely be better to spin it the correct amount so you didn't unnecessarily add to the consequences increased spin.
I hope this helps you. If you figure out a genuine solution to this please let me know. I would be happy to add a little spin to my next barrel even if it was specific to one bullet.

 

[tobias]

Re: Super-stabilizing to avoid transonic upset?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: dbooksta</div><div class="ubbcode-body">I think this is easy to picture if you think of a bullet with a large imbalance between center of mass and center of spin: The faster it spins, the more lateral throw-off it experiences on leaving the barrel and the longer it will take to damp out that precession.
</div></div>

The lateral throwoff is the tangent of an angle and can't be damped out. It is true that a faster spinning bullet will experience a higher lateral throwoff, but the effect is to change the direction of the trajectory.

 

[tobias]

Re: Super-stabilizing to avoid transonic upset?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: dbooksta</div><div class="ubbcode-body">Now the follow-up: Can a good 6 DoF simulator like PRODAS model transonic instability factors as a function of spin stability, or is transonic upset such a noisy process that it can't be practically modeled or estimated? </div></div>

Not sure about that, but I don't think that any 6 dof program can model anything in the transonic region. The problem is there isn't any reliable data. The involving coefficients (esp. Magnus moment coefficient) are hard to measure even in the supersonic region.

 

[dbooksta]

The Myth of Mach 1 Turbulence

John Whyte, a scientist at Arrow Tech Associates (producers of PRODAS) was gracious enough to address this question. His short answer: "[T]he assertion that transitioning Mach 1 causes any type of disturbance (i.e. cg swerve) for a symmetric projectile shape ... is a myth."

Jeff Siewert, their small caliber expert even produced a whitepaper two years ago on "The Myth of Mach 1 Turbulence." Its introduction reads:

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">There are a whole pile of gun pundits that are more than willing to tell you that there is something inherently mystical about the flight of bullets that have to decelerate through the local speed of sound (Mach 1.0). Turbulence, shock waves, flow transition from laminar to turbulent flow, etc. are all widely listed as the great bug-bears of “accurate” projectile flight once a bullet which left the gun at supersonic speeds slows down through the speed of sound and reaches the intended target below the speed of sound. While it is true that bullets making this transition are more sensitive to cross winds or changes in cross winds than bullets that remain entirely supersonic or are wholly subsonic, there’s nothing more sinister lurking in the weeds here than rapid changes in drag coefficient with Mach number.</div></div>

John Whyte further elaborated:

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">Nutational/precessional disturbances for spin stabilized projectiles are driven by subsonic Magnus Moment performance. Nutational or Fast Arm Instability is extremely undesirable as it can cause angular motion well over 15 degrees AOA and is not repeatable round to round and thus can cause extreme round to round hit variation. However Fast arm instabilities for a typical small arms shape (Say a 7.62 M80) are virtually impossible since the combination of physical properties and aerodynamics require a very large AOA before the fast arm dynamic instability threshold is crossed. Precession/Slow Arm limit cycling typically occurs subsonically but will not affect dispersion since every single round fired will exhibit the exact same limit cycle and the increase in drag due to the limit cycle can be adjusted for in the fire control.</div></div>

 

[dbooksta]

Re: Super-stabilizing to avoid transonic upset?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: tob</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: dbooksta</div><div class="ubbcode-body">Now the follow-up: Can a good 6 DoF simulator like PRODAS model transonic instability factors as a function of spin stability, or is transonic upset such a noisy process that it can't be practically modeled or estimated? </div></div>

Not sure about that, but I don't think that any 6 dof program can model anything in the transonic region. The problem is there isn't any reliable data. The involving coefficients (esp. Magnus moment coefficient) are hard to measure even in the supersonic region. </div></div>

Fortunately our government has spent a lot of money acquiring these data. To quote John Whyte again:

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">I do not know if you are familiar with Spark ranges. Basically it is 85m-200m long tunnel with Orthogonal screens mounted to the walls and ceilings that as a projectile flies past a flash source (hence the name Spark range) fires and photographic equipment takes a picture of the projectile’s shadow on the screens. This captures the X,Y,Z, position, spin and pitch and yaw of the projectile are captured with extreme accuracy (The ARL Small caliber range can measure within .4 millimeters and .05 degrees) from this data the true aerodynamics can be derived. Prior to 1972 this aerodynamics were derived via Linear Theory which has some limitations due to some of the assumptions. In 1972 a 6DOF analysis routine was introduced after the initial linear theory routine and substantially increased the accuracy of the aero coefficients derived to where Spark Range coefficients for ballistic shapes became the most accurate source for ballistic coefficients. The reason for this is that the 6DOF completely recreated the motion seen in the spark range to within the noise of the test equipment. The 6DOF used for this analysis is the same 6DOF within PRODAS.</div></div>

 

[MitchAlsup]

Re: Super-stabilizing to avoid transonic upset?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: dbooksta</div><div class="ubbcode-body">We know that bullets tend to suffer a degradation of accuracy when they slow through transonic speeds.

Does a superstabilized bullet -- say, something with a Miller Stability factor above 2.5 -- avoid or reduce the effects of transonic upset? </div></div>

To a first order, ariplanes that transition through the sonic barrier have similar stability issues. What happens is that the local flow around one part of the projectile goes sub-mach while the flow around the rest to of the projectile remains above mach. This creates a great deal of non-symetrical force on the projectile (see shock wave).

Since the bullet points itself into the wind, different wind conditions lead to different point of on the projectile body of this non-symetrical force.

As the bullet flies nose high (from the sideway view at long range) the top near the ogive-shank transistion is the last point of the super-mach flow.

 

[Helmet]

Re: Super-stabilizing to avoid transonic upset?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: MitchAlsup</div><div class="ubbcode-body">As the bullet flies nose high (from the sideway view at long range) the top near the ogive-shank transistion is the last point of the super-mach flow. </div></div>This makes sense to me. Did you read this and can you remember where?

 

[Country]

Re: Super-stabilizing to avoid transonic upset?

Spark range or dopler photo I am not sure which of military FMJ bullet flying slightly nose up .
You can tell it is nose up because of the unequal angle of the shock wave compared to the center axis of the bullet .
I think boat tails tend to do it more than flat base due to the extra drag of the flat base .

 

[The Mechanic]

Re: Super-stabilizing to avoid transonic upset?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Rizla</div><div class="ubbcode-body">If a bullet is spinning too fast it can actually become over-stabilized. When the bullet begins to slow and drop due to gravity the nose of the bullet will ordinarily drop as well, so the bullet continues to be pointing in the direction it is flying. In an over-stabilized bullet the nose is much less willing to move in this way and continues to point in the direction it was initially fired in despite the bullet losing altitude. Therefore relative airflow over the bullet is no longer exactly head on but is instead hitting the ogive, so the bullet effectively becomes a lot less aerodynamic which results in reduced accuracy as well as a quicker decrease in velocity.</div></div>
This ^^^
This is also the premise for the Cheytac's controlled spin or balanced flight. It engraves the bullet so the spin decays rapidly so the bullet will nose over for much more stable transonic flight just like a well thrown football noses down and goes farther.

 

[dbooksta]

Re: Super-stabilizing to avoid transonic upset?

Here's a shadowgraph of a .50-cal bullet right at Mach 1 -- from an interesting summary of BRL research by Bob McCoy.

 

[dbooksta]

Re: Super-stabilizing to avoid transonic upset?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: The Mechanic</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Rizla</div><div class="ubbcode-body">If a bullet is spinning too fast it can actually become over-stabilized. When the bullet begins to slow and drop due to gravity the nose of the bullet will ordinarily drop as well, so the bullet continues to be pointing in the direction it is flying. In an over-stabilized bullet the nose is much less willing to move in this way and continues to point in the direction it was initially fired in despite the bullet losing altitude. Therefore relative airflow over the bullet is no longer exactly head on but is instead hitting the ogive, so the bullet effectively becomes a lot less aerodynamic which results in reduced accuracy as well as a quicker decrease in velocity.</div></div>

This is also the premise for the Cheytac's controlled spin or balanced flight. It engraves the bullet so the spin decays rapidly so the bullet will nose over for much more stable transonic flight just like a well thrown football noses down and goes farther. </div></div>

Which is another way of approaching the original question: Do you get more reliable trajectories if your bullet transitions the sound barrier with a lower Sg and smaller Angle of Attack, or a higher Sg and higher Angle of Attack?

W.r.t. the Cheytac-style "controlled spin damping" I would also wonder if it significantly reduces ballistic coefficients? And do the spin-drag engravings produce turbulence that otherwise enhance or alter the standard aerodynamics we expect of bullets?

 

[The Mechanic]

Re: Super-stabilizing to avoid transonic upset?

Given that I do not have the doppler radar data on them but I did have someone that did do some of the testing, it was explained to me that the engraving would act like a screw in the air actually pushing due to intertia created by spin and that the slowing was accomplished by a forward push like one of those hand helicopter blade toys you spin in your palms and it flies up but slows down as drag on the blade hits the air. As to turbulance I would have to think that at most you may have some cavitation but as long as it is equal it may just act to reduce the effects of drag. Don't know other than the results of VERY long shots without keyholes of any kind. They had a GREAT design that suffered from VERY poor business practices.

 

[TiroFijo]

Re: Super-stabilizing to avoid transonic upset?

Check Bryan Litz's comments on LRB "controlled spin" theory:

http://www.snipershide.com/forum/ubbthre...231#Post2386231

 

[tobias]

Re: Super-stabilizing to avoid transonic upset?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: dbooksta</div><div class="ubbcode-body">Which is another way of approaching the original question: Do you get more reliable trajectories if your bullet transitions the sound barrier with a lower Sg and smaller Angle of Attack, or a higher Sg and higher Angle of Attack?
</div></div>

Why does a higher Sg lead to a higher angle of attack?

 

[dbooksta]

Re: Super-stabilizing to avoid transonic upset?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: TiroFijo</div><div class="ubbcode-body">Check Bryan Litz's comments on LRB "controlled spin" theory:

http://www.snipershide.com/forum/ubbthre...231#Post2386231 </div></div>

That's a good thread. And particularly applicable to this thread is Bryan's statement on the effect of super-stability:
<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">
Also, a bullets nose won't point higher late in the trajectory if it's more stable, it points more to the right (for a right twist barrel). This is called yaw of repose, and is the well understood mechanism behind spin drift. </div></div>

Bryan also notes a long discussion of this topic over here: http://www.usrifleteams.com/lrforum/index.php?showtopic=13415&st=0

 

[dbooksta]

Re: Super-stabilizing to avoid transonic upset?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: tob</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: dbooksta</div><div class="ubbcode-body">Which is another way of approaching the original question: Do you get more reliable trajectories if your bullet transitions the sound barrier with a lower Sg and smaller Angle of Attack, or a higher Sg and higher Angle of Attack?
</div></div>

Why does a higher Sg lead to a higher angle of attack? </div></div>

The higher the spin rate (which is proportional to Sg, a measure of gyroscopic stability) the higher the bullet's moment of inertia, and therefore the greater its resistance to pointing into oncoming air. As gravity changes the bullet's trajectory it takes the bullet longer to "nose in" to its ballistic arc; i.e., it maintains a higher "angle of attack" on its trajectory than if it were spinning more slowly and had less angular momentum to overcome.

 

[BryanLitz]

Re: Super-stabilizing to avoid transonic upset?

This old report would appear highly relevant to the current subject:

http://www.appliedballisticsllc.com/index_files/HATS_Report.pdf

It was originally written to investigate an outrageous BC claim, but there's lots of cool 6-DOF modeling stuff in there related to trajectory effects (spin drift and drop) at various levels of stability.

-Bryan

 

[tobias]

Re: Super-stabilizing to avoid transonic upset?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: dbooksta</div><div class="ubbcode-body">
The higher the spin rate (which is proportional to Sg, a measure of gyroscopic stability) the higher the bullet's moment of inertia, and therefore the greater its resistance to pointing into oncoming air. As gravity changes the bullet's trajectory it takes the bullet longer to "nose in" to its ballistic arc; i.e., it maintains a higher "angle of attack" on its trajectory than if it were spinning more slowly and had less angular momentum to overcome. </div></div>

The higher the spin the higher the stability meaning the amplitudes of precession and nutation are damping out. I would assume the yaw (and pitch) of repose is small compared to the yawing and pitching of a typical bullet even for long range. So a high spin rate (Sg) should lead to a small angle of attack.

 

[tobias]

Re: Super-stabilizing to avoid transonic upset?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Bryan Litz</div><div class="ubbcode-body">This old report would appear highly relevant to the current subject:

http://www.appliedballisticsllc.com/index_files/HATS_Report.pdf

It was originally written to investigate an outrageous BC claim, but there's lots of cool 6-DOF modeling stuff in there related to trajectory effects (spin drift and drop) at various levels of stability.

-Bryan </div></div>

I noticed that the yawing and pitching motion is damped very fast. Why? What are the initial complex yaw and yaw rate?
Thanks.

 

[Greg Langelius *]

Re: Super-stabilizing to avoid transonic upset?

I did some LR testing with a .260, 120gr Nosler BT's and a 1:8" barrel. The results were unexpected and took some time to figure out, if I've actually managed to do that.

Accuracy was moderately good (once enough additional elevation was dialed in). Drop was significantly (way) greater than what was calculated, and the impacts indicated a definite pitchup. All the impacts were oriented nose up and suggested the bullets were travelling around 20 degree nose up angle of attack relative to the line of the plunging trajectory.

Best I can figure, the bullets were super stabilized, and failing to reorient nose-on to the curving trajectory; causing an effective pitch-up relative to that curve that increaaed drag quite significantly, further accentuating the curve, further increasing drag, and so on, and so on. The bullets were travelling on a very exaggerated downward line, but still pointing in roughly the same direction as the bore axis, hence the elongated impact holes.

But for some reason, dispersion wasn't really a lot more than what one would expect from a heavier bullet that was being properly stabilized, so my guess is that wobble was not significantly more present with the lighter bullet than would have been there with the heavier.

Calculated velocity would have been marginally supersonic, but my best guess is that it had gone subsonic due to the unanticipated additional drag.

Greg

 

[dbooksta]

Re: Super-stabilizing to avoid transonic upset?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Greg Langelius *</div><div class="ubbcode-body">I did some LR testing with a .260, 120gr Nosler BT's and a 1:8" barrel. The results were unexpected and took some time to figure out, if I've actually managed to do that.

Accuracy was moderately good (once enough additional elevation was dialed in). Drop was significantly (way) greater than what was calculated, and the impacts indicated a definite pitchup. All the impacts were oriented nose up and suggested the bullets were travelling around 20 degree nose up from the line of the plunging trajectory.
</div></div>

At what range were these targets, and what was your muzzle velocity?

 

[dbooksta]

Re: Super-stabilizing to avoid transonic upset?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Bryan Litz</div><div class="ubbcode-body">This old report would appear highly relevant to the current subject:

http://www.appliedballisticsllc.com/index_files/HATS_Report.pdf

It was originally written to investigate an outrageous BC claim, but there's lots of cool 6-DOF modeling stuff in there related to trajectory effects (spin drift and drop) at various levels of stability.
</div></div>

Thanks Bryan!

Page 8 is noteworthy: Increasing twist rate increases spin drift (as expected) but also provides a tiny amount of lift -- presumably because the superstabilized bullet maintains a relatively nose-high angle of attack as it resists weathervaning into the (descending) ballistic trajectory.

(But, as suggested earlier, this "lift" effect is practically undetectable, being an order of magnitude smaller than spin drift: At 1000 yards the 6DoF simulation shows a 1:11" twist striking .31" high and 4.75" right; 1:9" twist strikes .45" high and 5.79" right.)

However note that in this example the 1:9" twist bullet has Sg = 2.0, which is, practically speaking, "normally" stabilized, not "super-" stabilized.

Bryan, could you run some 6DoF simulations for lift with a bullet running around Sg = 5?

 

[Greg Langelius *]

Re: Super-stabilizing to avoid transonic upset?

1000yd (Bodines). Velocity? An old Pejsa Spreadsheet I have on file for this bullet says 2900fps, but may not be directly related to this project. Best I can tell.

Please, no more questions on this, OK?

It was several years ago, and I did not keep notes on specific numbers, etc. For me the matter is closed, and I'd rather not get embroiled in any detailed discussions of the events I described.

Sufficeth to say, I decided that long distances, faster twists, and faster/lighter bullets weren't working for me. If I were to try the distance and lighter bullets again, I'd be trying it with a slower twist, and I'm not ready to buy that barrel just yet. These days, my shooting doesn't exceed 250yd, anyway.

Heavier bullets, suitible for the twist, work fine. Lighter bullets, on down to 95gr at velocities around 3500fps, work fine at distances out to 250yd. I have no indications of any sort of any abnormal conditions regarding my barrel.

I replied as a courtesy. I had tried what you're asking about and felt obliged to report my observations.

But it's a dead issue for me, Several years now, and I'm not interested in participating further in this matter.