investigating 50 Caliber Drag Values

[Curly]

The drag values for the 50 cal ammo look off.  There seems to be two sets of drag values. One for the world war 2 assets.

 

50Browning_APIT_M20.lua

cx = { 0.5, 0.61, 0.8, 0.17, 2 }

 

50Browning_API_M8.lua

cx = { 0.5, 0.61, 0.8, 0.17, 2 },

 

50Browning_AP_M2.lua

cx = { 0.5, 0.61, 0.8, 0.17, 2 },

 

50Browning_Ball_M2.lua

cx = { 0.5, 0.61, 0.8, 0.17, 2 },

 

Where the values are

 

-- Drag (Сx) = {
0.5 , -- Cx_k0 Cd0 at low mach ( M << 1) 
0.61 , -- Cx_k1 Peak Cd0 value 
0.8 , -- Cx_k2 steepness of the drag curve before the transonic wave crisis 
0.17, -- Cx_k3 Cd0 at high mach (M>>1)
2.0 , -- Cx_k4 steepness of the drag curve after the transonic wave crisis  
}

 

 

 

 

And, a second set of drag values for the 50 cal for the F-86 and possible the P-51 mustang. 

 

M20_50_aero_APIT.lua

cx = { 0.5, 0.61, 0.8, 0.27, 2 },

 

M2_50_aero_AP.lua

cx = { 0.5, 0.61, 0.8, 0.27, 2 }

-- Drag (Сx) = {
0.5 , -- Cx_k0 Cd0 at low mach ( M << 1) 
0.61 , -- Cx_k1 Peak Cd0 value 
0.8 , -- Cx_k2 steepness of the drag curve before the transonic wave crisis 
0.27, -- Cx_k3 Cd0 at high mach (M>>1)
2 , -- Cx_k4 steepness of the drag curve after the transonic wave crisis  
}

First thing to note is that entries for the world war 2 assets have a  peak drag value 40% lower than the older entries.

 

M2_50_aero_AP.lua

cx = { 0.5, 0.61, 0.8, 0.27, 2 }

 

Vs

 

50Browning_AP_M2.lua

cx = { 0.5, 0.61, 0.8, 0.17, 2 },

 

However both seem high compared to real world data on the rounds.

 

Source: https://apps.dtic.mil/sti/pdfs/ADA289645.pdf

 

And:

 

Source: https://apps.dtic.mil/sti/pdfs/ADA219106.pdf

Based on the charts the drag should possible look more like this

Cx = {.12, .375, .0, .35, .0 }

So a lower intial drag with a shallower rise, but a higher peak drag. With a much more gradual decrease in drag at high mach.

 

Edited June 30, 2021 by Curly
formatting

[Magic Zach]

bump!

[Brigg]

Hopefully these values will be adjusted accordingly when we get the new rounds and ammo belt configs. If not then bump

[Skewgear]

I wonder how much the drag curve data in the lua is fudged to provide roughly believable behaviour within the DCS flight physics engine. It's no good feeding it precisely accurate figures if the engine's calculations produce inaccurate outcomes.

Would be good to experimentally determine the ballistic arc of the 50 cal ingame and compare it with experimental real world data from similar barrel lengths, which is easy to come by for .50 BMG. I'm not familiar with how DCS handles muzzle velocity, having never poked around in the weapon luas, but I imagine that's defined somewhere.

This post is really quite eye opening about the gun heating and dispersion model as well. https://forum.dcs.world/topic/139404-super-slow-50-caliber-rounds/page/9/#comment-4976782

[Magic Zach]

On 6/18/2022 at 9:20 AM, Skewgear said:

I wonder how much the drag curve data in the lua is fudged to provide roughly believable behaviour within the DCS flight physics engine. It's no good feeding it precisely accurate figures if the engine's calculations produce inaccurate outcomes.

Would be good to experimentally determine the ballistic arc of the 50 cal ingame and compare it with experimental real world data from similar barrel lengths, which is easy to come by for .50 BMG. I'm not familiar with how DCS handles muzzle velocity, having never poked around in the weapon luas, but I imagine that's defined somewhere.

This post is really quite eye opening about the gun heating and dispersion model as well. https://forum.dcs.world/topic/139404-super-slow-50-caliber-rounds/page/9/#comment-4976782

You are able to open the ww2 weapons Lua and see the stats on each round type.  Muzzle velocity, for one, is too slow on average for all .50 rounds, for instance.  Aside from the drag curves also being very steep.

Makes me wonder though, if this was corrected how accurate might the gyro gunsights still be/not be?

[Skewgear]

49 minutes ago, Magic Zach said:

You are able to open the ww2 weapons Lua and see the stats on each round type.  Muzzle velocity, for one, is too slow on average for all .50 rounds, for instance.  Aside from the drag curves also being very steep.

Makes me wonder though, if this was corrected how accurate might the gyro gunsights still be/not be?

My point is that the sim engine itself may not produce true-to-life ballistics, and the inaccurate figures in the luas may be a dev attempt to work around a known problem. Not saying that's a great thing but without further experiments we don't know either way. The acid test is whether the mean point of impact in-game at a set distance is the same as the MPI in real life.

With some effort this can be worked out for a stationary warbird with its tail down. You'd need the starting elevation of the gun, though.

[NineLine]

I've asked someone to take a look. The F-86 issues as far as I am aware are related to overheating. 

[Yo-Yo]

On 7/4/2022 at 5:01 PM, Skewgear said:

My point is that the sim engine itself may not produce true-to-life ballistics, and the inaccurate figures in the luas may be a dev attempt to work around a known problem. Not saying that's a great thing but without further experiments we don't know either way. The acid test is whether the mean point of impact in-game at a set distance is the same as the MPI in real life.

With some effort this can be worked out for a stationary warbird with its tail down. You'd need the starting elevation of the gun, though.

No, your point is wrong.  DCS engine can accurate calculate trajectories even for large caliber artillery where the shell passes through very different atmosphere. The trajectories matches FT in  distance, max ordinate, time of flight, initial and terminal angles for the whole range of barrel elevation.
The point is that the data set that you see in the database works not this way some people suggest. By the way, 0.27 instead of 0.17 seems to be a typo from a programmer who copied the data, because there is no sign that it is an original set.
For small caliber projectiles drag is tuned to match time of flight and terminal velocity.