revolutions per minute debate

[goodgrouper]

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GG, I'm surprised to hear that you found conflicting info. Classical physics is well understood.

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Ok. I had some more time to research this today and I believe I found why the information conflicts. One report looks at the actual measurement of the RPD's in relation to the RPM's, velocity, and distance. The other report is actually looking at just the spin itself independent of the other vectors.
In other words, this is how I interpret them:
The bullet CANNOT accelerate it's actual RPD's downrange. However, it appears that it does because our method of measurement involves a measurement of distance and time and since the bullet is decelerating but keeping it's RPD's, it is revolving 1 time in a shorter distance-thus making it LOOK like it is accelerating it's RPD's.

Now, there is some very slight decrease in actual bullet revolving due to the imperfections in the skin of the bullet like my link states, but it is so slight that it can only be measured in decimal points way into the 8th or 9th place.


So what I get from this is that the bullet's twist imparted to it by the riflings is a constant for all practical purposes throughout it's traj. The way we measure it changes because it is slowing down throughout it traj. In other words, if we give the actual twist of the bullet a value of "Y", and we don't associate "Y" with any other method of measurement (like velocity, deceleration, or rate of twist) the bullet will still be maintaining "Y" at 1000 yards.


Tell me if you think this may be why it seemed like the info was conflicting.

[ricka0]

GG - you've got it figured out.
I'd be a bit careful quantifying the declaration of RPD (I use Rev Per Foot in my graphs) - 8 or 9 decimal places might be understating it. The translational deceleration is at least in the thousands of times higher but I'm not sure it's billions of times higher. ( It might be, I'm hoping someone can provide a reference to this ).

I looked at some sub-sonic flow papers but they used square plates or tubes to test theory vs. measurement (besides we don't care about sub-sonic)
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the actual measurement of the RPD's in relation to the RPM's, velocity, and distance

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I've posted Buf Bob's data of RPF (RPD) vs Vel and RPD vs Distance at RPM_RPD be sure to select "Sheet2" on the bottom right of the page.

[goodgrouper]

I agree, RPF's would be a more accurate distinction.

[ricka0]

GG, just because your right doesn't mean your pals will accept it. Do you feel you have adequate evidence?

When will we know how MOAG does after it goes sub-sonic? I know an awesome custom rail builder if you need one. ( $70 )

[Richard338]

I think everyone has a good grasp of what's going on now.
I agree with Big bore that the rate of rotational decay has probably been understated. However, my intuition is that it is slightly higher than what has been guessed at. We would just need to look at the output of a well-parameterized 6DOF calc. I'll have a quick look at McCoys book on this.

[Buffalobob]

The rate of decay is probably higher for low BC bullets than antbody has admitted. The extreme example would be a wadcutter or huge meplat bullet like the 45-70 Hammerheads. The front rotating plane (meplat) will encounter a significant resistance to spin from the perpendicular contact with air molecules. A very long ogive bullet will have almost no perpendicular surface for air to encounter and reduce spin, infact the long ogive will cause the air to move parallel to the surface so there is very little "normal" (perpendicular component) force and the air flow over the bullet is only interacting with the skin rotating under it.
The rifling groove on the skin create a very complicated interaction that could reinforces the spin and at the same time decays the spin. Number and shape of these grooves is important.

Think about the grooves this way. These groove are impressed into the bullet at an angle equal to the twist of the rifling, so they are not parallel with the axis of the bullet. If the bullet was slowly moving forward through a thick substance such as jello and the bullet had no spin, the grooves would cause the bullet to develop a spin in the jello. This is the same principle as a wood screw which rotates as it goes into the wood.

Of course this induced spin comes at the price of resistance to forward motion because air is impinging on one of the sidewalls of the grooves

The top edges of the grooves create a mechanical brake as has been mentioned before because as they rotate they are hitting the air that is flowing back along the bullet.

[Buffalobob]

Big bore

You sure you don't want to move to the East and start a career in water quality computer modelling.

That some nice graphics.

The crossing of the lines on graph 2 is due to the way Sierra uses step BCs to calculate their velocities. I used their manual, number 2, because it is easier to keep a book open and read velocities than to keep switching screens.

I have made another calculation on a 308 180 gr round nose versus a 308 180 SMK both fired from a 1:8 twist and both at 3000fps. It is really different but more what one expects.

I will post it in a few minutes