Variable twist rate barrel for projectile stability testing (not gain twist)

[subscriber]

The question about how slow a twist rate will work with a given projectile does not have a clear answer.  Experience reported by people using different twist barrels often seems contradictory.  This is probably because of differences other than twist rate, that are not tightly controlled.  For instance;

How "tight" the projectile fits in the bore;
How well the projectile was centered on loading;
Pressure profile and beneficial or harmful in-barrel projectile distortion;
Taper in the bore that flares at the breech, or towards the muzzle;
Barrel whip and its potential effect on projectile yaw as it leaves the muzzle.

Now, this recent discussion about twist rate with BBTs got me thinking about how to test different twist rates, without changing all the other aspects of the system.  That is, without purchasing a wide enough range of quality custom twist barrels to test:  https://www.gatewaytoairguns.org/GTA/index.php?topic=161381.msg155797251#msg155797251

Proposed Concept:
Choose a heavy barrel blank from a reputable manufacturer, in the most popular caliber.
Precision cut the chamber configured for airgun slugs, with a long "self aligning" cone.

This barrel would be mounted in a stiff horizontal cradle, with precision rolling bearings near the breech and muzzle.

The idea is to be able to drive a nominal twist barrel, such as 1:12 rotationally; in either direction.
By increasing the barrel rotation in the direction of twist, the projectile fired from a 1:12 barrel could achieve the spin of a 1:10 or faster.  By driving the barrel backwards, a 1:14 or slower twist can be simulated.

Now, a projectiles fired from a 1:12 barrel at 1000 FPS revolves at 60,000 RPM.  This would be very fast if the barrel had to be spun at that speed.  The actual barrel rotational speed to simulate 1:10 or 1:14 would be 10,000 RPM; forwards, or backwards respectively.  This is fast in the grand scheme of things, but not fast for a precision lathe; or even a compared to a car engine cam shaft.

The air delivery system will be by means of a sturdy static mounted breech, with retractable loading probe.  Breech sealing can be achieved by means of a loose fitting Teflon O-ring between barrel and breech.  Or,  the breech can use a close-fitting, non-contact labyrinth seal.  If that idea seems full of holes, consider that a revolver cylinder barrel gap has an actual gap.  Yes, it operates at much more than 3000 PSI, but the typical "flash gap" is over 0.004".  Also, we are not trying to test efficiency here; only achieve a given projectile velocity and spin speed.

The rear of the barrel can be relief cut by 0.01", so only a narrow ring close to chamber diameter is in close contact with the breech.  This way, if there is slight contact, the friction won't be significant.  The breech face could be made of brass and a drop of grease applied to tolerate close, but low load contact.

Anyway, the above set-up should enable the same barrel to be used for tests, where the velocity and effective twist rates can be varied independently.   It would be purely a lab testbed.  A project that someone such as Lloyd could put together, should the motivation exist.

Now, you might argue that the two bearing sets should both be near the breech, to simulate a free floating barrel.  I would argue that a two foot long cantilever section, spun at 10k RPM without a "tail stock" support would be inviting trouble.

The barrel could be driven the way shafts are spun for balancing; with a belt, or by using a compressed air nozzle acting on a toothed or bladed wheel.  A reliable means for measuring the barrel spin rate should be used; so as not to exceed the rated bearing speed.  While a well made barrel should have its bore concentric with its OD, that should be verified before configuring the bearing diameter, as barrel blanks are usually turned to size, thus ensuring concentricity.

[rsterne]

Interesting concept.... the practicality of it, I'm not so sure.... 

Bob

[UnderPressure]

IMHO a good place to start would be the .50 caliber used by the Seneca Dragon Claw.  This way you could test various arrows as well as .50 slugs.

Not sure what the twist rate of the Seneca Dragon Claw is but it should be a very slow one. Maybe 1 in 36" which translates to 20,000 rpm at 1000 FPS/ 13,600 rpm at 680 FPS/ 10,000 rpm at 500 FPS.

With that slow twist you should be able to operate with a relatively slow barrel rotation rate in the beginning.

[rsterne]

Well for sure, the slower the twist rate you are exploring, the lower the RPM needed to vary it.... If the goal is accuracy testing (and why else would you do it), the whole rig needs to be portable to set up outside.... and it has to be perfectly concentric or the spinning of the barrel will throw the bullet off course at the instant the bullet exits (lateral throwoff).... in the same way an imperfect bullet would react with the CG offset from the C/L....

Bob

[Brian W Cook]

I have been wanting to try that for years with a smooth Teflon lined steel barrel in 9mm .   Modern brushless motor drives could wind out pretty quick .   

[subscriber]

it has to be perfectly concentric or the spinning of the barrel will throw the bullet off course at the instant the bullet exits (lateral throwoff)

A lack of concentrically will cause massive vibration at such high RPM, so this must be a precision made instrument.  The idea is to reduce system variability, rather than increase it.

The question is if one might be better equipped for such an exercise with one FX Impact, and a range of their dainty X-twist barrel liners, made with whatever twist your heart desires.  That is, assuming FX can be commissioned to help.  Everyone that has an Impact seems to report surprisingly good accuracy from it, considering the thin inner liner, supported at just a few O-rings...

The goal is to narrow the gap between emphatic statements and calculations about a given slug needing a certain twist barrel, or else; and the actual results people experience in the field.  Perhaps the status quo is just fine:  Try different slugs and see if your barrel likes them. Forget warnings about "there be dragons" at a given twist rate; until you have seen them yourself...

[UnderPressure]

This thread mentions some very high spin rates (1 in 12" and 1 in 6.7") for two vanes used on Arrows:

https://www.crossbownation.com/community/threads/crossbow-arrow-spin-rate.96360/

So perhaps there is a good amount of room for improvement in air rifles that shoot arrows via rifled barrels.

[subscriber]

Russ,

I am no bow shooter, but am flabbergasted by the high spin rates the guys at that link are using.  I was always under the impression that arrows only require just enough spin to null out irregularities in circular mass distribution, and in shaft and head aerodynamics.

My gut suggests that very high spin rates on arrows would increase drag at the fletching and slow them down faster.  Also, overdriving a long thin shaft that has mass and aero-force anomalies may actually cause it to kink, due to centripetal forces.  Something like the video below. 

That said, I am all for proving things out by actually trying them; rather than relying on common wisdom (up to the limits of safety).

[Mole2017]

I had to read it twice to realize I had misunderstood the idea, which leads me to suggest what I first thought you were doing:

Why not find a barrel of suitable size and nominal twist rate, clamp/chuck one end in a sufficiently rigid frame and twist the other end, bracing against same frame, to achieve slower or faster twist rates?**

It won't do everything, but within the limits of the materials it might doable. Just need a quick check from someone what kind of torsional deflection we can get on a tube before it deforms. Or for that matter, maybe let it deform anyway...

No guarantee the bore won't change in dimension, surface quality or performance, for that matter. But, hey, maybe it saves some work?

**one difficulty I see already is guaranteeing a uniform twist at the exit. Maybe you might have to ream out the exit end so you can get past that section in the "clamp". For example, as shown here:

[subscriber]

Interesting idea, David.

I suspect it would take a lot of torque to wind a barrel up by 60 degrees.  Also, that doing so would probably yield soft airgun barrels.  That said, this might be a way to alter the twist permanently.  Or to create a rifled barrel from scratch - see last paragraph.

As an experiment about 10 years ago, I took a three foot long 1/2" square mild steel bar and tried to wind it up to create a rifling guide.  I gave up due to not having a pipe wrench long enough to do it by hand.   Now, if I had been brave enough to try this in a large lathe, geared to the lowest speed, I might have succeeded. 

Anyway, I repeated the hand wound exercise with 1/2" 6160 aluminum, and just managed to get the effect I wanted.  Except that the bar did not stay straight and the twist was not uniform...

I am sure that a barrel could be wound up "easily", if heated to a red heat, but it would probably affect the bore dimensions; especially after the scale had been removed:


Now, taking a hollow hex tube and winding it up "cold" after annealing, might be a crude way to make a rifled polygonal barrel.  I would want to wind it up inside a round steel tube to help stabilize it...
For instance:  https://www.mcmaster.com/9063t31

[subscriber]

Here is a working improvised helical cutting arrangement for a manual mill; dubbed the "Norris Chuck".
Would be even better with a toothed belt and pulleys:

[musquat]

What if the barrel was encased in a larger diameter sleve to keep it straight as it was torqued? Like a 7/16 OD .22 barrel inside a 1" OD sleeve?

[subscriber]

What if the barrel was encased in a larger diameter sleve to keep it straight as it was torqued? Like a 7/16 OD .22 barrel inside a 1" OD sleeve?

That would help stabilize it.

[rsterne]

Use a 1" OD sleeve with a 0.44" ID to keep the 0.435" barrel straight as you torque it?.... I still think the ID of the barrel would change, however....

Bob

[Mole2017]

The sleeve idea might help some. The key is to be able to keep control of the ends relative to each other. We had a machine in the teaching labs that looked like a lathe in some respects, except that the "tailstock" could clamp the end of a bar and hold it stationary while the chuck end twisted a bar until it broke. They'd look like candy cane sticks afterward, but always straight. The funny part was that it had a hand crank available; it would wear out even the football players to twist a longish bar until it broke.

Tension would help too, but it would be built into a machine like that torsion tester. No additional tension needed like when you are twisting wires together.

[UnderPressure]

Russ,

I am no bow shooter, but am flabbergasted by the high spin rates the guys at that link are using.  I was always under the impression that arrows only require just enough spin to null out irregularities in circular mass distribution, and in shaft and head aerodynamics.

My gut suggests that very high spin rates on arrows would increase drag at the fletching and slow them down faster. 

I am not a bowshooter either.

Here is some information on something called "FOC". (A high "FOC" means an arrow has a higher proportion to weight in the front.)



https://www.nockout.com/understanding-arrow-foc/

High FOC arrows will shoot straight but lose trajectory (nose-dive) faster than arrows with less FOC. Shooting a low FOC arrow will gain back trajectory (flat shooting farther) but will be unstable and less accurate in flight.

So at this level there is a balance between accuracy and trajectory occurring.

I'm assuming that increasing arrow speed beyond 300 FPS will also increase drag (for the fletching linked in reply #6), but possibly allow a lower FOC to  be used.

[subscriber]

An arrow is a bit like a plane with its wings removed.  The tail can still steer (or keep it straight), but there is no lift.  I can see that with a larger FOC, the downward moment is increased; and that the arrow is not just dropping under gravity, but being "steered" down by the fetching.

I can also see that if the CG is too far back, the "self righting" or straitening tracking moment is small.

I wonder if arrows that spin more than a few RPM have any stability from that...

[UnderPressure]

Russ,

I am no bow shooter, but am flabbergasted by the high spin rates the guys at that link are using.  I was always under the impression that arrows only require just enough spin to null out irregularities in circular mass distribution, and in shaft and head aerodynamics.

My gut suggests that very high spin rates on arrows would increase drag at the fletching and slow them down faster. 

I am not a bowshooter either.

Here is some information on something called "FOC". (A high "FOC" means an arrow has a higher proportion to weight in the front.)



https://www.nockout.com/understanding-arrow-foc/

High FOC arrows will shoot straight but lose trajectory (nose-dive) faster than arrows with less FOC. Shooting a low FOC arrow will gain back trajectory (flat shooting farther) but will be unstable and less accurate in flight.

So at this level there is a balance between accuracy and trajectory occurring.

I'm assuming that increasing arrow speed beyond 300 FPS will also increase drag (for the fletching linked in reply #6), but possibly allow a lower FOC to  be used.

This video explains the difference between Aerovane II (1 rotation in 12" at 300 FPS) and Aerovane III (1 rotation in 8"* at 300 FPS):



Based on his explanation of Aerodynamic Elasticity Memory effect the angle of attack of the vanes should decrease as the lift increases thus reducing drag.

*In reply #6 I wrote that rotation was 1 in 6.7" for AV III at 300 FPS.....I was mistaken....it is 1 in 8" for 300 FPS.

[subscriber]

Thanks Russ,

I had no idea that arrow fletching was that sophisticated.  So, the effective helix angle of the arrow increases as its velocity drops, down range.  Or, more to the point, the effective helix angle is reduced by vane flex early on in flight.

By the way, there is an archery gate on this forum; if you want to compare notes with bow shooters:
https://www.gatewaytoairguns.org/GTA/index.php?board=235

[UnderPressure]

According to this post by Firenock (the makers of aerovane) Aerovane 3 was predicted to have 350% more rotations than Aerovane 2 in their test with a 400 FPS crossbow.

https://www.crossbownation.com/community/threads/crossbow-arrow-spin-rate.96360/page-2#post-1162866

Theoretically, base on ideal and design only Av3's ability to adjust the angle of attack by itself via Aerodynamic Elasticity Memory effect and wing-let approach to generate more lift, it should spin 350% more than AV2, base on high speed camera results, we find out it is only 150% more than AV2 which is still quite significant. This is what we have observed out of a 400+ fps crossbow arrow at 420 gn. With Blazer at 1.5 degree offset it is about 8-12 turns from 60- 80 yards, AV2 is doing close to 120 turns, while AV3 exceed 300 revolution in that same distance.

350% more than 120 turns is 540 turns which over an average of 70 yards comes out to be 1 turn every 4.67".

At 400 FPS 1 turn in 4.67" turns out to be 62,000 RPM.

I'm surprised by that, but perhaps they will eventually achieve this with next version of Aerovane?