known SFM Performance and Inaccuracies

[Curly]

Problem: The AI controlled MiG-15bis has unrealistic performance.

Cause: Some of the aerodynamic characteristics of the SFM exceed those of the real aircraft.

If we compare the SFM data in MiG-15bis.lua to the aerodynamic data from the real technical manual for the MiG-15bis.  

https://www.digitalcombatsimulator.com/en/files/2365583/  

Some discrepancies  are apparent.  For example, the DCS MIG-15Bis SFM has 200 kgf more thrust than the real MiG-15 bis at low speed.

 

 

This is because the thrust values for the SFM are based on bench tests of the VK-1 Engine and not on installed thrust data. The installation losses are given in Figure 84.

 

 

The manual confirms this and notes figure 80, reflects these losses.

 

 

Since thrust available defines many of the performance parameters of the aircraft. It is important that the SFM thrust parameters match this data.

Using the data from the technical manual, I’ve updated almost all of the SFM of the MIG-15bis to more closely match the real aircraft. Feel free to try it out, I’ve placed a link below. The mod contains extensive notes. I cite a source for almost every change I’ve made to the SFM too.

Link to Mod:

https://drive.google.com/drive/folders/19cQXcBNfOdnoJkxlY2BFdUO3Zc77KESm?usp=sharing

 

 

I’ve tested and validated the performance of this mod in a few different ways too.  The max velocity at various altitudes, climb rates as given in table 5 of the technical manual. The SFM also matches the level acceleration data in figures 46.  

 

Below I’ll discuss some of the changes I’ve made to the SFM to more closely match the historical data below.

Almost all of the aerodynamic table was changed to better match the source data.

Drag:

The drag polars Cx = Cx_0 + Cy^2*B2 +Cy^4*B4

Were changed to better match figures 55 and 54.  A new data point was also added at mach 0.86. Which is important because the lift and drag become nonlinear here due to mach effects.

 

LIFT:

Cy max as function of Mach was changed to match Figure 185.

 

The current values in SFM are too high, resulting in too much lift. The SFM Cy max values are drawn below in red,  The mod’s Cy max are drawn in the green line.

Cy Alpha was also changed to more closely match the actual aircraft.

 

Cy0, the zero AOA lift coefficient, was set to match the lift polar in figure 51.

 

Aldop was set to the AOA corresponding to CY max  from fig 185 and Cy alpha at that mach.
AOA = ((1/Cya)*Cy+((Cy0*-1)/Cya))

 

Engine:
Some additional changes were also made to the SFM engine model.
Dpdh_m was changed to more closely match the loss of thrust as a function of altitude.

Thrust Kgf At Alt = Thrust Base (Kgf)-(dpdh*(Alt m*0.001)

 

Above, the thrust available at 5000 meters for SFM is drawn in red. The mods values are drawn in blue. At low speeds the mod more closely matches real aircraft. The mod also changes to the idle rpm and thrust specific fuel to more closely data in table 19.

 

Roll Rate: Omxmax, the max roll rate was changed to match Figure 40:

 

 

Other Changes and Notes:

V_opt = 367 / 3.6, -- velocity for L/D Max, Mach .3 at 0 m =367 TAS Kph Page 24 . Setting V_opt to high means the AI does not have enough excess thrust to maneuver. causing it to only do high speed loops.  V_cruise = 1.31*V_opt  .Def of Vopt from translation of Russian Aircraft Design book.  https://archive.org/details/DTIC_AD0741485/page/62/mode/1up
 
  g_suit   =  0.0, -- This Aircraft does not have a G suit, Default is 0.35, AI will sustain 6g turns all F-86 = 0.7

  -- Performance Limiting Factors of AI SFM are AI Level, Cy Max, drag, Aldop, and g_suit, Vopt

 

CAS_min     = 50,       -- This is Close Air Support time on station in Minutes. How long (the AI) will LOITER TIME,

 

WEAPON CHANGES

There are also some changes to the weapons parameters.  I plan on doing a separate post on these changes. One of the most impactful of these changes was changing the value

Tbl.effective_fire_distance to 600m to reflect the data on the 23mm and 37mm cannon uses as
presented in Table 18  in The Flying Techniques and Combat Use of the MiG-17

 

 

 

Which gives the effective ranges when firing at fighter aircraft as 180 to 550 Meters. The previous value was 1km and resulted in a sniper bot.

There are also changes to dispersion values, shell and fillers weights. These changes were made to match the data from Soviet Manual on Aircraft projectiles.

https://disk.yandex.com/i/YVj0XI6O3Mu4LM

 

That just about covers all the changes I’ve made to the MIG-15bis SFM.

While these changes are not a miracle fix for the AI or the SFM. I feel the do represent a significant improvement to the AI MIG-15bis  making the AI MiG-15 a much more realistic and engaging opponent.  

I believe this project illustrates that significant improvement to the AI performance can be made if one takes the time to realistically tune the SFM parameters. With a spreadsheet and an understanding of the SFM parameters it’s possible to create much more realistic SFM AI opponents.  

Eagle Dynamic could also do a lot to improve the situation by better explaining the SFM parameters. Many of the notes in various SFM files date back over a decade and are from unofficial sources, some are misleading others are simply incorrect.

If you read this far, thanks for taking the time to do so. If anyone wants it,  I will share the google sheet I used to help develop this MOD. Be forewarned though, it’s a hot mess and some of the units are in …Gasp! American units of insanity.

I attached a track of a big dog fight mission, sabres V Mig15's down low. I know it's need for the report.

Thanks for taking them time.

 

 

MIG 15 Big Gun Fight.trk

[AeriaGloria]

This is fantastic! You found some amazing sources I haven’t seen before!!!! BRAVO BRAVO!!!

Do you think you or I could make a separate report on dispersion for main player aircraft? Right now dispersion for 37mm is something like 13.6 mil for 80% circle. Your source says it ranges closer to 5-6. Also,  NR-23 I think currently has 5.6 mil dispersion for 80% circle, but your source says 9.6 mil!

 

Would swap rolls of the guns, making 37mm the accurate one and NR-23 the wider spray, which makes sense considering it’s rate of fire 

Edited June 12, 2024 by AeriaGloria

[Curly]

5 hours ago, AeriaGloria said:

This is fantastic! You found some amazing sources I haven’t seen before!!!! BRAVO BRAVO!!!

Do you think you or I could make a separate report on dispersion for main player aircraft? Right now dispersion for 37mm is something like 13.6 mil for 80% circle. Your source says it ranges closer to 5-6. Also,  NR-23 I think currently has 5.6 mil dispersion for 80% circle, but your source says 9.6 mil!

 

Would swap rolls of the guns, making 37mm the accurate one and NR-23 the wider spray, which makes sense considering it’s rate of fire 

 

I'm working on a post about the weapons right now.  

RE: The accuracy values of the weapons:  The DCS parameter for dispersion, Da0. Is equivalent to Range Error Probable or 0.6745. the standard deviation of dispersion (sigma). The radial dispersion values often given in western literature are also functions of the standard deviation of dispersion (sigma), assuming circular dispersion. 

The radius of the 80% circle is  = sigma * 1.7941 

As,  (sqrt(-2*ln(1-0.8))) = 1.7941

To convert the DCS value of dispersion, Da0, to the 80% radius of dispersion:

Da0 * (1/0.6745) * 1.7941 = The radius of 80% of the dispersion

or

The radius of 80% dispersion is = 2.66 * Da0.

With this method we can compute the 80% radius of MIG-15 weapons in DCS and from the manual as:

 

Projectile:	DCS 80% Radius	80% Soviet Radius
N-37 HEI	4.5219	2.2166
N 37 APT	4.5219	1.7733
23 mm	1.8620	3.1919
M61A1		4
50 Cal Aircraft		2.153

 

 

The increased dispersion of the 23MM cannons is probably going to improve the effectiveness of the weapons. Some dispersion is necessary to compensate for errors in fire control equipment and can reduce penalties for aiming error. To much Dispersion and the density of projectiles within the target drops. Which reduces the overall effectiveness of the system.

The Soviet practice was to maximize the effectiveness of the weapons for a specific size targets over an the effective range of the weapon system. See the image below.

The dispersion of the projectiles on the MiG-15 and 17 appear to optimized for their specific use cases. The 23MM being used to target fighters. The increased dispersion and rapid rate of fire would be ideal for engaging fighter sized targets  at close ranges in high G maneuvering.   While the 37mm would be more ideal for shooting at large non maneuvering bombers from long range.

 

 

 

 

 

Edited June 13, 2024 by Curly

[Squiffy]

Fantastic! I hope the SFM mods can be implemented. I bought the MiG-15 recently and need to set up my controller profiles. I wanted to see what dogfighting is like in the same ship. AI still seems to have an advantage but my old stick is worn and needs to be replaced.

Edited December 19, 2024 by Squiffy

[Katmandu]

Truly beautiful work! Dogfights vs another Mig and the Sabre of course now make a lot more sense, previously the AI Mig-15 was simply climbing out of trouble even against my PFM Mig-15, my Sabre was altogether reduced to running away and head ons... 

The mod crashed DCS for me until I removed everything but the SFM changes, maybe @Curly could have a look? Otherwise, the mod is simply supreme, it absolutely transformed my DCS Sabre

[Curly]

I've been keeping the MIG-15 Bis SFM mod updated locally. I never bothered to post the files because I didn't think anyone was really using it.

I've placed the mod on Git Hub so that anyone can easily inspect the changes. 

 

https://github.com/AeroBaseAlpha/DCS-MiG-15bis-SFM-Overhaul/blob/main/CoreMods/aircraft/MiG-15bis/MiG-15bis.lua

 

[Raven (Elysian Angel)]

9 minutes ago, Curly said:

I didn't think anyone was really using it

I was
I'm not really dogfighting MiG-15s all that much but I have it installed in my OvGME library along with other bug fixes and texture replacements
Thank you for sharing your updates!