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Posted (edited)

Issue 1

All RL 34-1 available on the internet state that the gunsight can be depressed up to 245 mils below the FRL. This is confirmed by the game manual at LCOSS - Heatblur F-4E Phantom II. In the game, while the knob can be turned so the mil depression setting shows 245, the gunsight actually only depresses up to 225 mil. Any more depression in the window will not cause the sight to depress further.

depression.png

 

Issue 2

Compare the following screenshots:

Sight setting Cockpit view Tacview
90 090_close.png 090_close_tacview.png
225 (close) 225_close.png 225_close_tacview.png
225 (far) 225_far_cockpit.png 225_far_tacview.png
179 179_cockpit.png 179_tacview.png

In the "90" case, I would expect Tacview to show a depression of 90 / 17.45 = 5.2° or 90 / 17.78 = 5.1°. Tacview shows 5.0°.

In the "225 (close)" case, I would expect Tacview to show a depression of 225 / 17.45 = 12.7° or 225 / 17.78 = 12.9. Tacview shows 12.0.

In the "225 (far)" case, I would expect Tacview to show a depression of 225 / 17.45 = 12.7° or 225 / 17.78 = 12.9. Tacview shows 13.0.

I am going to assume that the second discrepancy is due to parallax.

These results seem to indicate that Heatblur's F-4E uses 1° = 17.78 mil, therefore adopting the NATO-mil definition. This is supported by \DCS World OpenBeta\Mods\aircraft\F-4E\UI\BombingTable\main.js, which contains "const FACTOR_DEGREE_TO_NATO_MILS = 17.777778".

However, in the "179" case, Tacview shows a depression of 10.3°.

10.3 x 17.78 = 183, which is not the sight setting that will put the BTR under the sight. That setting is 178 or 179. Incidentally, calculating 10.3 x 17.45 = 180, which is not exact, but at least closer than 183.

This makes me wonder whether the chosen factor is actually 1° = 17.45 mil after all. This would make sense since all RL 34-1 available on the internet assume that 1° = 17.45 mil, which seems to indicate that the RL F-4E's gun sight was also calibrated using this definition.

mil.png

What is Heatblur's intended degree/mil ratio? Is the gun sight calibrated to that setting throughout its entire range? Could the above slight discrepancies be related to the first issue (225 vs 245 max depression)?

Edited by Stickler
  • Like 3
Posted (edited)

AFAIK this is correct behavior. The HUD reticle is limited vertically to move between +14.4 and -225.1 mils.

The reason you can select a higher depression than that is that there are also other factors that can displace the reticle vertically in certain delivery and HUD modes. For example, it can be either relative to the fuselage reference line or to the radar boresight line, as well as compensate for INS pitch and/or drift.

With a certain mix of those, it is possible that the "default" undepressed position of the reticle is above 0 and then the full range of 245 mil depression can have a visible effect.

I'll try to dig out the reference doc, cheers.

Edited by Zabuzard
  • Like 1
Posted (edited)
On 8/5/2024 at 9:47 AM, Zabuzard said:

I'll try to dig out the reference doc, cheers.

That would be appreciated. Even with your mil figures I wasn't able to find a reference to these numbers in the sources available to me or on Google.

BREAK BREAK

Did some extensive testing using my own bombing calculator (the in-game calculator not providing usable results) and my hypothesis is that Heatblur's F-4E gun sight automatically compensates for parallax error.

parallax.png

Reference the attached track.

With a release speed of 472 KTAS, 27° dive angle, 1917 ft ATL release altitude, -0.9° AoA and 0.9 G (expected G for a 27° dive angle is 0.89), calculated bomb range is 3121 ft with a sight angle of 66 mil (1° = 17.45 mil).

Note that the actually required sight depression to superimpose the sight over the target (as shown by the left crater position after impact) is 61 mil, thus the calculated setting (which includes parallax correction) minus the correction itself.

One might assume that this is due to inaccuracies in my calculator's ballistic model, but in this case it's not. Compare the data points visible in the .trk (need to activate the frame counter) with the values from my calculator:

Time index KTAS/ALT (F2) KTAS/ALT (calc)
261.414 (release) 472/1932 472/1932
262.414 479/1556 479/1544
263.414 486/1149 485/1137
264.414 493/712 493/700
265.414 501/245 501/234
265.785 (impact) 505/25 504/14

Note that the lowest altitude shown in the F2 view prior to (and at) impact is 25 ft even though terrain elevation is 16 ft. I therefore assume that at impact, the bomb's actual altitude is 16 ft and the "25" is a measuring error due to an insufficient update rate of the F2 view.

In conclusion, I'm assuming that the bombing calculator is correct since it provides speed and altitude results observable in the game.

What I find very strange, then, is that the parallax correction must be factored OUT of the calculated sight setting to obtain the correct setting. It is as if the sight in the game "knows" the parallax correction required and automatically applies it, and if you then apply the parallax correction manually, it will have been applied twice. This is not immediately plausible for a sight which to my knowledge does not receive input from the INS in DIRECT and A/G mode.

Alternatively, the above may be explained by a virtual co-pilot situated among the bombs (or at the logical location of the aircraft considered as a point in space, which seems to coincide mostly with the bombs' location) looking at the target through another sight. What this virtual pilot sees through the virtual sight is then transmitted to the cockpit for the actual pilot to view.

I have noticed the same discrepancy (calculated sight setting WITHOUT parallax correction is the one that provides the correct results, not the setting WITH parallax correction) with other release parameters, but the above example was deliberately constructed to showcase the issue.

I'm not necessarily asking that this issue be fixed (although that would IMO improve realism), but a confirmation that we have a sight that internally and automatically compensates for parallax (or which does not need to be compensated for parallax due to the "virtual pilot" construct above) would be helpful to preserve my sanity.

parallax.trk

Edited by Stickler
  • Like 2
Posted (edited)
On 8/7/2024 at 8:40 AM, Stickler said:

That would be appreciated. Even with your mil figures I wasn't able to find a reference to these numbers in the sources available to me or on Google.

The team and SMEs werent able to dig it out anymore. Cant say for sure at the moment if its correct or not without reference. Ive put the investigation into the backlog for when theres more time and we will in the meantime go for a compromise that raises the limit to 250 mils.

Edited by Zabuzard
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