J
J. Valentine
Guest
How do I go about measuring and calculating an Rt / R head shape ratio for the ogive on my bulllet ?
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C
Cheechako
Guest
J
What is Rt / R ???
ray
What is Rt / R ???
ray
J
J. Valentine
Guest
It is a measure of an ogive to see how Secant it may or may not be with reference to a perfect Tangent ogive . If the ogive is perfectly Tangent the value is 1 , if it comes out greater than 1 it is somewhat Secant by that ratio amount .
Some ballistic programs ask for the Rt/R ratio . However I don't know how to work it out .
Just because some maker says a bullet has a 10 cal Tangent ogive does not mean it's a perfect Tangent ogive.
I think that if I could measure the actual bullet ogive and then divide it into the stated perfect Tangent it produces the ratio. Like if they say a bullet should be 10 cal Tangent ogive and I measure a10 caliber ogive then 10 -:- 10 = 1 a Perfect Tangent. If I measure say 9.99 that is 10 -:- 9.9 = 1.0101 slightly Secant .
That is as much as I understand and am very happy to be corrected as I may have some parts wrong.
Some ballistic programs ask for the Rt/R ratio . However I don't know how to work it out .
Just because some maker says a bullet has a 10 cal Tangent ogive does not mean it's a perfect Tangent ogive.
I think that if I could measure the actual bullet ogive and then divide it into the stated perfect Tangent it produces the ratio. Like if they say a bullet should be 10 cal Tangent ogive and I measure a10 caliber ogive then 10 -:- 10 = 1 a Perfect Tangent. If I measure say 9.99 that is 10 -:- 9.9 = 1.0101 slightly Secant .
That is as much as I understand and am very happy to be corrected as I may have some parts wrong.
I think it's a great question.
Would love to find math that could predict it based on a few points of measure(datums).
Maybe this question would get more response where BC matters: 600 / 1000 Yard Benchrest
I asked Bryan Litz where he got his radius values(for his book) and he said he measured the noses mechanically, and it was CNC program calculated for him.
I'd like to do the same with a laser mics and milling table, but I still need the x-y math.
Would love to find math that could predict it based on a few points of measure(datums).
Maybe this question would get more response where BC matters: 600 / 1000 Yard Benchrest
I asked Bryan Litz where he got his radius values(for his book) and he said he measured the noses mechanically, and it was CNC program calculated for him.
I'd like to do the same with a laser mics and milling table, but I still need the x-y math.
Butch Lambert
Active member
Sounds like a case of needing to watch your windflags closer.http://benchrest.com/images/smilies/wink.gif
Butch
Butch
Actually
If the ogive is slightly secant, the number has to be less than 1. If it calculates to 1, it is tangent, but if it calculates to .5 it is completely secant. Another words, .98 is slightly secant. Remember the direction of the ratio, that the smaller number is divided by the larger. A secant ogive radius is a larger radius than specified. If the specified ogive radius is 8, but you measure it at 8.2, then it is slightly secant 8/8.2 =.9756, being less than 1 so slightly secant. The problem is accurately measuring the ogive radius. I would use a dial indicator to map out the curvature in steps (digitize), then lay the points into autocad, then fit an arc to the mapped out points, and then let autocad list the radius. Keep in mind that most bullets that are considered to have a tangent radius almost always have a slight blending radius, meaning that they are not a true tangent, but slightly secant instead.
Michael
I think that if I could measure the actual bullet ogive and then divide it into the stated perfect Tangent it produces the ratio. Like if they say a bullet should be 10 cal Tangent ogive and I measure a10 caliber ogive then 10 -:- 10 = 1 a Perfect Tangent. If I measure say 9.99 that is 10 -:- 9.9 = 1.0101 slightly Secant .
That is as much as I understand and am very happy to be corrected as I may have some parts wrong.
If the ogive is slightly secant, the number has to be less than 1. If it calculates to 1, it is tangent, but if it calculates to .5 it is completely secant. Another words, .98 is slightly secant. Remember the direction of the ratio, that the smaller number is divided by the larger. A secant ogive radius is a larger radius than specified. If the specified ogive radius is 8, but you measure it at 8.2, then it is slightly secant 8/8.2 =.9756, being less than 1 so slightly secant. The problem is accurately measuring the ogive radius. I would use a dial indicator to map out the curvature in steps (digitize), then lay the points into autocad, then fit an arc to the mapped out points, and then let autocad list the radius. Keep in mind that most bullets that are considered to have a tangent radius almost always have a slight blending radius, meaning that they are not a true tangent, but slightly secant instead.
Michael
Last edited:
J
J. Valentine
Guest
I think it's a great question.
Would love to find math that could predict it based on a few points of measure(datums).
Maybe this question would get more response where BC matters: 600 / 1000 Yard Benchrest
I asked Bryan Litz where he got his radius values(for his book) and he said he measured the noses mechanically, and it was CNC program calculated for him.
I'd like to do the same with a laser mics and milling table, but I still need the x-y math.
I know Bray Litz knows how to do it . He uses a small mini lathe and a dial indicator and takes diameter sizes at measured lengths reference points along the ogive curve . Then he enters those diameter changes and reference points into a program that analyzes the curve. Unfortunately I have not been successful in raising Bryan or maybe it's a trade secret . I understand that.
That's as much as I understand on that. Also very happy to be corrected .
J
J. Valentine
Guest
If the ogive is slightly secant, the number has to be less than 1. If it calculates to 1, it is tangent, but if it calculates to .5 it is completely secant. Another words, .98 is slightly secant. Remember the direction of the ratio, that the smaller number is divided by the larger. A secant ogive radius is a larger radius than specified. If the specified ogive radius is 8, but you measure it at 8.2, then it is slightly secant 8/8.2 =.9756, being less than 1 so slightly secant. The problem is accurately measuring the ogive radius. I would use a dial indicator to map out the curvature in steps (digitize), then lay the points into autocad, then fit an arc to the mapped out points, and then let autocad list the radius. Keep in mind that most bullets that are considered to have a tangent radius almost always have a slight blending radius, meaning that they are not a true tangent, but slightly secant instead.
Michael
Thankyou " mturner" that is excellent . I can see what you are saying after reading it a few times. That has helped me no end . What you are saying is very similar to what Bryan Litz said he does. Thanks a million for clearing up my mistake .
Last edited by a moderator:
J
J. Valentine
Guest
Sounds like a case of needing to watch your windflags closer.http://benchrest.com/images/smilies/wink.gif
Butch
Trouble is the wind flags will not make my bullets and as much as I talk to them they never answer back other than to do a lot of finger pointing .
V
Vibe
Guest
I think I'd find an Optical Comparator to measure them with. With 3 points along the ogive curve you can determine what the ogive radius is, and where the center of that radius is. If it's truly tangent, it will be located 90° from the beginning of the ogive, end of the straight wall - if it's secant, the center of the ogive radius will be farther to the rear than the radius beginning.
M
mks
Guest
I think it's a great question.
Would love to find math that could predict it based on a few points of measure(datums).
http://www.riflebarrels.com/articles/bullets_ballastics/bullet_weights.htm
http://en.wikipedia.org/wiki/Nose_cone_design
Here are a couple of sources for the equations. If the profile is known to be tangent, then you only need one measurement at a known distance forward of the ogive to calculate the ogive radius (one equation, one unknown). If the profile is secant, then you need two measurements to find the radius and the setback of the center of the curve (two equations, two unknowns).
Cheers,
Keith