quote:
Originally posted by Ackley Improved User:
I'm impressed with Bryan Litz. I've read his books and his response is what I thought it would be - very thoughtful, yet diplomatic.
Originally Posted by BryanLitz View Post
Guys,
Here's a post I made in another forum on this subject; most of the points address questions that have been discussed here.
In the video, Hornady observes the BC to drop at long range. In response to that...
It's well established and understood that BC's are velocity dependent based on the comparison of a bullets drag to the standard model (G1, G7, etc). A bullet that's perfectly stable and not melting in flight will have it's G1 BC fall off as it slows down; that's just normal for all modern LR rifle bullets, not just those with plastic tips.
Also, there are numerous explanations to the observed convex drag curves. This paper (http://www.arl.army.mil/arlreports/2010/ARL-TR-5182.pdf) is a full aerodynamic work up the government did on the M855 round showing its dynamic instability, pitching/yawing, etc. In other words, some bullets fly with what's called 'limit cycle yaw', which is a coning motion that acts sort of like a trim angle to keep the bullet in equilibrium. Flying at a small coning angle adds drag which, depending on the damping exponents, can result in a convex drag profile. One aspect of bullet design that is known to have an effect on the magnitude of limit cycle yaw is boat tail design. Steep BT's tend to fly with larger limit cycle yaw, the 168 SMK being the most popular example of a bullet that exhibits dynamic instability at supersonic speed. The whole line of Nosler Ballistic Tips and Accubonds have steep BT's, as well as many of the Hornady Amax and Vmax line. The steep BT's on these bullets could cause convex drag curves.
The amount of limit cycle yaw a bullet has depends a lot on its gyroscopic stability, which is tied to twist rate. The Hornady paper doesn't say the barrel twist used for the testing. If, for example, the 7mm 175 Hornady and the 7mm 175 Nosler LRAB were both fired from the same twist barrel, it could just be a matter of the Nosler not getting fully stabilized and flying with larger limit cycle yaw angles which creates the convex drag curve etc. I've measured this very thing (higher drag and lower BC's from the muzzle) for bullets fired with marginal stability. The Nosler LRAB's in particular are longer bullets that require faster than standard twist to stabilize.
Another strange thing about the 'melting tip' theory and the convex drag curves is that the drag curves are shown to be convex beginning at the muzzle. They talk about the tips melting in flight, at long range, for heavy high BC bullets that maintain higher speed for longer flight (vs. a varmint bullet that slows down quickly). That makes sense, but then why are the drag curves convex beginning at the muzzle? It seems to take no time at all for the tips to 'melt' and affect the drag.
Setting aside the 'melting tips' theory for a moment; consider the positives.
Hornady has come out with a new line of high BC, heavy for caliber bullets which should be good for long range.
They are providing G7 BC's for these bullets. Based on Hornady's measurement of G7 BC's of some Berger bullets matching my measurements nearly identical, I'm guessing that the G7's Hornady is putting out for their new bullets are very accurate.
I'll continue exploring the melting tip theory vs. other theories that fit the data.
-Bryan
Originally Posted by BryanLitz View Post
Thanks for posting that Q&A paper. It directly addresses all of the points in my post.
There is one question that isn't addressed very well, and that is the issue of the drag jumping so high, so fast.
Consider Figure 1 in the link (http://www.hornady.com/assets/files/...HeatShield.pdf) The drag of the old tip is immediately higher than the heat shield tip. Hornady explains this is due to the radar picking up the track 40-50 yards downrange. The plot shows that by this point (40-50 yards) the drag is already different by about 5%. Assuming the tips are the same shape, this implies that the old tip melted enough in 40-50 yards (0.06 seconds) to already be different by 5%.
By the time the bullet slows to ~Mach1.85 (~2065 fps) the bullet is about 400 yards downrange and the drag is now different by more than 10%. At this point the bullet has flown for only 0.52 seconds, and the old tip has already deformed enough to increase it's drag by more than 10%.
By my rough calculations, a tip would have to increase in diameter by about 0.050" to make 10% more drag if that's the only thing going on. Measuring one of the old Hornady tips from an SST bullet, the tip is about 0.040" in diameter, and the base of the plastic tip is like 0.100". So in order for this plastic to produce 10% more overall drag for the bullet, the tip would have to melt from 0.040" to ~0.090", when the metal part is 0.100". In other words, it would have to nearly melt entirely off, all in 0.52 seconds.
At this point, the mad scientist part of me reached for a lighter Some quick internet research indicates that a standard butane Bic lighter has a flame temperature of 3000-4000 degrees F. So I light up the tip of the Hornady SST bullet I just measured. Holding the plastic tip in the hottest part (top) of the flame, it took about 5 seconds before any noticeable deformation began to occur. According to Hornady's original research paper (http://www.hornady.com/assets/files/...al_Details.pdf) the stagnation temperature on the bullet tips is under 1000 degrees F for a velocity of 3000 fps.
So, How can the tips nearly melt completely off in 0.52 seconds at in-flight temperature of under 1000 degrees F, when it takes a bic lighter more than 5 seconds to begin melting the tip at 3000 degrees F?
There is the matter of aerodynamic force being applied to the tip, which is absent in my lighter test. Based on the bullet losing about 150 fps in the first 100 yards (0.119 seconds), we can calculate the average force to be:
F=M*a
F=(140/7000/32.2)*(150/0.119)
F=0.78 pounds
So there's roughly 0.78 pounds of aerodynamic drag (force) acting to slow the bullet over the first 100 yards on average. Only a fraction of that force is applied to the actual tip. Could that small amount of force, combined with the heat, result in the tips deforming in such a short time?
Going back to figure 1, we see that the difference in drag is maximized at around Mach 1.85 (~2065 fps), but then something strange happens. The drag of the old and new tip bullets converges back together again. By the time the bullet has slowed to Mach 1.25 (~1395 fps, somewhere around 950 yards), the drag of the two bullets is nearly the same, with the new and old tipped bullets matching to within about 2%.
How is it that a tip melts enough to produce more than 10% difference in drag by 400 yards, can 'heal' itself further downrange and fly with nearly the same drag as a 'non-melted' tip by 950 yards?
Several places in Hornady's Q&A document, they mention that all of the old tips melt (as well as all other brands). But the heat shield tips are only used on long range bullets because the effect only matters when you're shooting past 400 yards. I don't see this going over well with all the guys who've been shooting Amax bullets at long range. Hornady is now saying that yesterdays long range match bullet (the Amax) is now limited to 400 yards, and you need the new ELD-X bullets to hit anything past 400 yards. That's simply not consistent with the observations many have made shooting long range with Amax bullets.
The questions about tips melting are interesting, and still not entirely settled for me. Not having been present for the testing, I don't have any strong alternate theories to explain the drag data their publishing. I'll reiterate the positives of the ELD-X line as: heavy for caliber bullets intended for long range, and represented with G7 BC's which appear to be accurate. Introducing more options in this class is a good thing, regardless of the questions about plastic tips.
-Bryan
Bullseye,
You make some good points. I'm not trying to invalidate Hornady's work with a bic lighter, but some of their explanations are difficult to accept in light of some common sense observations.
Bic lighters aside, Amax bullets and other tipped bullets have been flying well at long range for many years. By flying well I mean:
Flying with BC's that are consistent with the shape of non-deformed tips, and
Flying with trajectories that are predictable with these BC's.
All of a sudden, a discovery is claimed which suggests that trajectories are not predictable, and BC's are lower than they should be, which goes against years of direct observation by many shooters and my own measurements.
In order to accept the claim of 'melting tips' as reality, we have to:
A) accept that the testing and doppler radar data that Hornady is presenting is accurate (free of excessive measurement error). This is the part that is actually contrary to common observation.
B) accept that the melting tip theory explains why the data indicates performance that's different from what we know it to be.
I've been measuring BC's for bullets for many years. Plastic tipped bullets of all calibers have very uniform BC's, which are consistent with the mass, caliber and un-deformed shape of the bullet. For me personally, it will take more evidence to believe this data, and the conclusions being drawn from it.
To be clear, I'm not saying they're wrong, but there are good questions to ask.
-Bryan
I in a ham and egger style said this in another thread suggesting we take two bullets of same weight, caliber,and initial bc measurement and design or at least two G7 style projectiles and you would have thought I shot someone's dog.
I never said the Eld was incorrect; just that the bullet's geometry and weight were more important than the tip material. The real critique being Hornaday was more busy playing with such gimmicks or gadgets than making Dangerous Game Ammo. Again, the tip being a gimmick or gadget.
