drag, twist, and transonic flight.

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Kurt
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Re: drag, twist, and transonic flight.

Post by Kurt » Tue Apr 20, 2021 8:01 pm

I have trouble at the 900 and at the pig line.
My scores are higher on the 1000 than the 900 and I have a hard time cleaning the pigs but I clean the turkeys more than the rest.
There has to be something going on at the 900 but the holes are round going through the paper as well as the 800 and 1000.
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powderburnt
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Re: drag, twist, and transonic flight.

Post by powderburnt » Tue Apr 20, 2021 8:03 pm

Interesting slow motion video of sonic shock waves from a high velocity bullet and sonic shock waves off of a subsonic bullet with explanations.
https://www.youtube.com/watch?v=BPwdlEg ... A&index=52
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Distant Thunder
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Re: drag, twist, and transonic flight.

Post by Distant Thunder » Tue Apr 20, 2021 8:39 pm

Powder,

What that does not show about the sonic shock waves off of a subsonic bullet iis that those shock waves move forward on the bullet as it slows down and eventually end up on the nose of the bullet. At that point I think the shock wave would be something like fletching on an arrow only at the front. That's probably not good.
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Don McDowell
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Re: drag, twist, and transonic flight.

Post by Don McDowell » Tue Apr 20, 2021 8:56 pm

When you look at these videos, one thing that does pop into mind, what affect if any does the driving band have on the shockwaves? And the flow over the bullet base, maybe beveled bases aren't a bad thing for bullet flight, but a real pain with the gas cutting, which screws the whole deal...
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MikeT
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Re: drag, twist, and transonic flight.

Post by MikeT » Wed Apr 21, 2021 11:44 am

DT,

I was thinking that the subsonic bullet would loose the shock waves as it slows down, because the shock waves are actually supersonic air moving over the fat part of the slow moving bullet.

Keep on hav'n fun!
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Re: drag, twist, and transonic flight.

Post by powderburnt » Wed Apr 21, 2021 3:53 pm

Mike, I agree. The bullet is subsonic and the compressed (displaced) air is sonic and the shock wave will subside. I wonder what it looks like at that moment of transition from sonic to subsonic and what effect it has on the bullets course if any. It seems in rimfire that subsonic works best at shorter ranges of 100 yards and less with the flat nose match ammo doing the best. When I go to 175 yards and beyond the slightly higher than sonic(1125 fps +) works best with the more traditional round nose. But maybe that's just me.

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Don McDowell
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Re: drag, twist, and transonic flight.

Post by Don McDowell » Wed Apr 21, 2021 4:07 pm

As picky as 22 rifles are about ammo when it comes to long range I'm not sure you can judge much by them. My wife's lowall is a classic example of that. Man folks swear by center x and midas, and some like SK, put anything lapua or sk in that rifle and you just as well be shooting a 410 with 0 shot, that 410 will deliver better patterns.
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bruce m
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Re: drag, twist, and transonic flight.

Post by bruce m » Wed Apr 21, 2021 4:43 pm

zack,
with regards your discussions with dan.
it seems i had similar discussions with him.
he certainly was of the mind that keeping the centre of mass as far forward as possible is beneficial to stability.
in facton his original money nose sphere at the front of the nose was meant to be 55% to 60% the diameter of the groove diameter.
this allowed more weight to be forward than some pointier bullets.
modern baco money bullets are examples of reducing this diameter.
he was also working on reducing drag on the nose at the same time, in order to keep the centre of pressure as far to the rear as possible, in essence to shorten the overturning moment.
not only does having a fatter nose move the centre of mass forward, it also puts more mass at a greater diameter which improves stability as well.
the delemma is that it also moves the centre of pressure forward which in turn moves the centre of pressure forward, then lengthening the overturning moment.
this requires a bit of playing around to find the best balance.
the other thing with more mass more forward is that there will be more nose upset, potentially shortening the ogive and driving b.c. down due to increased drag.
dan was moving into the elliptical nose zone toward the end.
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Re: drag, twist, and transonic flight.

Post by bruce m » Wed Apr 21, 2021 5:15 pm

as we have discussed previously, there are 2 types of bullet stability, dynamic and gyroscopic.
gyroscopic can be calculated and a number assigned to the level of stability called the stability factor, often referred to as Sg.
however dynamic stability can override this, and is not fully understood.
gyroscopic stability will be accurate in a vacuum.
dynamic stability seems to be affected by external forces like airflow, air density, and other things.
the best way to deal with dynamic stability problems is to find a bullet that has less of them, but for the same bullet just keep increasing the gyroscopic stability until the problem goes away.
dons description of the pilot in the bullet is a good one, in terms of that pilot and not the pilot behind the butt can at times be in control.
wind deflection can be calculated and controlled to a greater or lesser degree by the shooter, depending on experience and how the tongue is held between the teeth.
when we have stable bullets we have more latitude in how we hold our tongue.
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desert deuce
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Re: drag, twist, and transonic flight.

Post by desert deuce » Wed Apr 21, 2021 6:53 pm

Bruce:

Yes I remember Dan muttering about elliptical noses. I recall that he did share with me a prototype drawing of one. May still have it somewhere.
The fatter nose and weight forward is evident on both money moulds I have that he designed. Both are for 16 twist barrels as he specified.
AND, I suspect the Sagebrush Elliptical was an attempt at producing a similar or even same Dan concept elliptical projectile when looking at a drawing and then at the as cast product at the time. So much so I acquired one of those Sage Brush elliptical moulds. I do recall Dan saying he had contacted the makers of those moulds about bullet design.

I also recall he was toying with effects of finning at the base and the wisdom of thicker/wider or thinner/narrower base bands.

We even discussed base bands smaller in diameter than the forward bands, I forget his thinking on that but I have seen original old dead guys era moulds (late 1800's) that were both smaller in diameter in the base band and some with beveled bases as in the original Ideal Postel.
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Distant Thunder
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Re: drag, twist, and transonic flight.

Post by Distant Thunder » Wed Apr 21, 2021 7:07 pm

MikeT,

Yes it will lose the shock wave(s) when it slows down enough. It's a little confusing to me just how this looks because the examples I have studied involved aircraft and not bullets. Aircraft have these shock waves as they are accelerating and these shock waves move from the front of the wing at low speed to the back as the speed increases. I assume that a bullet would then have these shock waves move from the backend of the bullet toward the frontend as the bullet slows down. It would seem to me that these shock waves would have a more detrimental effect on the bullet's stability when they are on the nose of the bullet, possibly increasing the drag at that point. I sure don't know and I haven't been able to find a source that would tell me if that's the way it is.

I'm out on this limb pretty far!
Jim Kluskens
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A new age of tyranny dawns in the United States! If anyone is brave enough to say so they're destroyed by the BIG Tech branch of the Federal Government for spreading lies! "Reeducation" camps to follow!

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Re: drag, twist, and transonic flight.

Post by bruce m » Wed Apr 21, 2021 10:58 pm

one of the ways to eliminate finning is to breech seat the bullet.
done with the airgap, the base of the bullet is already into the rifling.
particularly with bore diameter bullets, they just bump into the rifling with no finning at all.
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bruce m
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Re: drag, twist, and transonic flight.

Post by bruce m » Wed Apr 21, 2021 11:08 pm

jim,
i like you would just love a simple explaination.
the difference between an aircraft and a bullet is that the former has wings, which makes its situation much more complex.
part of that shockwave moving to the front of the bullet's nose comes as the pressure decreases on the nose, moving the centre of pressure within the bullet toward the base, thus shortening the overturning moment, which in turn increases stability.
this process stops at the bottom end of transonic, and the centre of pressure is slowly moved forward again as drag begins to increase again.
possibly this resembles in a small way the bullet hitting something, in which case all sorts of things can happen then.
combine that with irregular shock wave activity at around 900 yds :shock:
we just have to get the bullet through that with minimal disturbance possible.
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bruce m
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Re: drag, twist, and transonic flight.

Post by bruce m » Wed Apr 21, 2021 11:18 pm

related to the last post.
the 458 win mag tends to have twists around the 12" mark.
i am sure this is not required for flight, but in fact for when the bullet hits for example an elephant's skull.when this happens, suddenly the centre of pressure moves forward in an extreme way, increasing the overturning moment immensely.
yet that bullet must stay nose on to penetrate in a straight line, so they spin it harder to cope with the situation.
here could be an extreme version of what we need to do with our bullets to cope with a less extreme version.
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Kenny Wasserburger
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Re: drag, twist, and transonic flight.

Post by Kenny Wasserburger » Thu Apr 22, 2021 9:08 am

Finning can be a problem, have recovered many bullets on the mile range that exhibit this particular phenomenon, mostly grease groove bullets. My bore ride pp bullets sit .350 into the case, as I have a plethora of powder capacity. The wad stack, that I employ, I believe completely alleviates this issue. I have recovered many of my bullets from the mile range, none of them exhibit any finning.

I do believe that finning on any type of bullet, is definitely a contributing factor to down range stability. Almost every single finned bullet I have recovered, the finning is not uniform around the base, this has to contribute to stability issues.

I believe when not enough wad is employed, you will get finning, and more so with alloys less than 16-1.

Below are 2 of my NASA bullets that were recovered at 1 mile, and one of my 528 gr money bullets.[attachment=0]B850371C-9E3B-4D2C-AEC2-726DBD2EE3A6.jpeg[/attachment][attachment=1]A666F9D3-32DA-4F1B-8F4A-F4FAA82B4318.jpeg[/attachment]

Kenny Wasserburger
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