[2.9.5.55300] SUU-23 Gunpods not boresighted along FRL

[DSplayer]

The HB F-4E manual states that the "the guns are boresight along the Fuselage Reference Line. Thus, with the optical sight in air to air mode, the rounds will fall in accordance with reticle center, just as the main cannon- only with the natural offset of their respective pylon position, thus allowing for effective natural gunnery, if only affected by a moderate increase in resulting CEP."

 

However, as of 2.9.5.55300, the SUU-23 gunpods are actually not boresighted to the FSL like the internally mounted gun.

 

 

[trev5150]

Likely performing accurately. IRL gun pods of this type and era were wildly inaccurate and used mainly as area weapons fir strafing large swaths of ground, for example prepping HLZ’s ahead of heli born assaults. 

[DSplayer]

3 hours ago, trev5150 said:

Likely performing accurately. IRL gun pods of this type and era were wildly inaccurate and used mainly as area weapons fir strafing large swaths of ground, for example prepping HLZ’s ahead of heli born assaults. 

However inaccuracy doesn't mean that it shouldn't be completely boresighted to a different reference line which is above the sight which is in direct opposition to what is stated in the manual. This can be seen most prominently with the wing mounted gunpods.

[trev5150]

Boresight does not coincide with the reticle, which changes position with mode and mil settings. It’s only a mechanical alignment to the Fuselage Reference Line. It does not compensate for bullet drop or flight path. 
 

Fix the PEBKAC issue. 

[KlarSnow]

The FRL is 2 degrees (35 mils) above the nose guns boresight, per the -34 for the jet. So the manual is currently correct for how the gunpods are boresighted.

[DSplayer]

3 hours ago, KlarSnow said:

The FRL is 2 degrees (35 mils) above the nose guns boresight, per the -34 for the jet. So the manual is currently correct for how the gunpods are boresighted.

Ok that would mean the manual's language of "the optical sight in air to air mode, the rounds will fall in accordance with reticle center, just as the main cannon- only with the natural offset of their respective pylon position, thus allowing for effective natural gunnery" (https://f4.manuals.heatblur.se/stores/guns.html#external-employment) is incorrect then right as the rounds do not currently fall in accordance with reticle center, just as the main cannon?

Edited May 23, 2024 by DSplayer

[Smyth]

I notice that the gunpod shoots (at high point of arc) just above the caged pipper, which should be RBL. The nose gun shoots significantly low.

The F-4C/D/E NNWD manual says that the nose gun is harmonized to caged pipper (RBL = -2deg from FRL) at 2250ft. In that case I would expect the high point of the arc to fall just above the pipper, like the gunpod currently does. Perhaps the gunpod is correct here, and it is the nose gun that is wrong?

To eliminate some variables, I confirmed the caged reticle is at least in the right place (-35mil) from the un-depressed A/G position which should be FRL.

Pod:

Nose:

Edited June 5, 2024 by Smyth
clarified description

[DSplayer]

1 hour ago, Smyth said:

I notice that the gunpod shoots (at high point of arc) just above the caged pipper, which should be RBL. The nose gun shoots significantly low.

The F-4C/D/E NNWD manual says that the nose gun is harmonized to caged pipper (RBL = -2deg from FRL) at 2250ft. In that case I would expect the high point of the arc to fall just above the pipper, like the gunpod currently does. Perhaps the gunpod is correct here, and it is the nose gun that is wrong?

To eliminate some variables, I confirmed the caged reticle is at least in the right place (-35mil) from the un-depressed A/G position which should be FRL.

Pod:

Nose:

 

Just a thing of note for anyone reading this, the SUU-23s on the outer wing pylons shoot significantly higher than both the internal gun and centerline gunpod when the 1F-4C-34-1-1 also mentions the harmonization of the gunpods to 2250ft.

[Smyth]

On 6/5/2024 at 2:03 AM, DSplayer said:

Just a thing of note for anyone reading this, the SUU-23s on the outer wing pylons shoot significantly higher than both the internal gun and centerline gunpod when the 1F-4C-34-1-1 also mentions the harmonization of the gunpods to 2250ft.

Oh wow... I didn't think to test that. Wing cannons are actually shooting close to FRL (un-depressed pipper), while centerline pod is shooting right at RBL (caged pipper) and nose gun around a degree under that.

This all seems very odd. Some clarification from HB would definitely be appreciated.

[sirius]

As I understand it, fuselage reference line (FRL) coincides with A-G reticle set to 0 mils and all the guns should be boresighted to FRL.

After some testing I get that the different guns are boresighted accordingly:

Internal: ~50 mils

Fuselage mounted: ~24 mils

Wing mounted: ~0 mils

So, internal (nose) and fuselage mounted gun are not boresighted correctly.

[Smyth]

11 hours ago, sirius said:

As I understand it, fuselage reference line (FRL) coincides with A-G reticle set to 0 mils and all the guns should be boresighted to FRL.

After some testing I get that the different guns are boresighted accordingly:

Internal: ~50 mils

Fuselage mounted: ~24 mils

Wing mounted: ~0 mils

So, internal (nose) and fuselage mounted gun are not boresighted correctly.

The nose gun is definitely supposed to be at RBL (-2deg from FRL).

What the F4C/D/E section on -E nose gun says specifically is "The nose gun is harmonized with the optical sight for an optimum firing range of 750 yards (2250 feet). During harmonization, the sight is depressed 2.0 degrees from fuselage reference line."

Note that it shouldn't be shooting exactly parallel to the RBL, but zeroed to the sight at a set range (just like a rifle).

F-4C manual is less clear about gunpod vertical harmonization, but F-4B Mk4 gunpod was also harmonized to RBL, so it's hard to imagine the SUU-23 pointing anywhere else.

[sirius]

On 6/8/2024 at 3:41 AM, Smyth said:

The nose gun is definitely supposed to be at RBL (-2deg from FRL).

What the F4C/D/E section on -E nose gun says specifically is "The nose gun is harmonized with the optical sight for an optimum firing range of 750 yards (2250 feet). During harmonization, the sight is depressed 2.0 degrees from fuselage reference line."

Note that it shouldn't be shooting exactly parallel to the RBL, but zeroed to the sight at a set range (just like a rifle).

F-4C manual is less clear about gunpod vertical harmonization, but F-4B Mk4 gunpod was also harmonized to RBL, so it's hard to imagine the SUU-23 pointing anywhere else.

After a closer look at the F-4 in side view with gunpods mounted it definitely looks like the fuselage mounted gun is tilted down about 1 degree and internal is down 2 degree.

Wing mounted guns looks level with FRL.

I guess this is a feature and not a bug after all. 3 different guns and 3 different settings for the reticle. What a mess!

 

[sirius]

3 hours ago, Cobra847 said:

The Phantom in a nutshell..

Thank you for bringing this old bird back to life in the virtual world.

With all its quirks and special features.

[Smyth]

9 hours ago, Cobra847 said:

The Phantom in a nutshell..

Can you please clarify specifically if you are saying this is intended behavior (gunpods zeroed in different locations)? Is it documented somewhere? Thanks.

[DSplayer]

All the gunpods and internal guns not being zeroed to the same rough location seems extremely counterintuitive and would definitely make A/A and A/G gunnery difficult as it would require different amounts of lead for A/A and different amount of sight depression for A/G depending on what guns are being used (especially for the wing mounted guns vs the centerline guns). Looking at the 1976 GAF T.O. 1F-4F-34-1 (which should be functionally extremely similar to our F-4E in this regard), it includes only 1 sight depression table for both the wing mounted gunpods and nose gun (centerline gunpod can't be mounted on the F-4F) as well as an extremely similar gun dispersion/harmonization chart to T.O. 1F-4C-34-1-1 with the only difference being the 5 mil for the nose gun as the nose gun has a specific density of 80% of rounds impacting in a 5 mil cone vs the 8 mil for the gunpods. 

There should be a slight upward angle for the wing gunpods on the F-4 to account for parallax but it definitely seems way too angled upward to be of any use.

Edited June 10, 2024 by DSplayer

[DSplayer]

Can a follow up or further context be provided on this topic? @Zabuzard

[Zabuzard]

Its on the list. We will investigate and fix any issues and inaccuracies. And then report back on the findings or provide explanation if something is intended. Cheers

[Stickler]

I can contribute the following to this discussion. All data was collected in 2.9.7.59263.

Referencing the ballistics data in GAF T.O. 1F-4F-34-1, p. 1-162, shooting from the M61A1 and the SUU-23/A gun pod at 30° dive angle, 450 KCAS, 48.000 lbs gross weight, 4000 ft ATL with a target elevation of 0 ft MSL requires a sight setting of 59 mil.

The real F-4F would have had an AoA of 22.4 mil at these parameters according to the following chart (which is identical in all manuals I'm aware of):

Subtracting 22.4 mil from 59 mil makes a sight depression from flight path of 36.6 mil.

When recreating the above scenario in the game as precisely as possible, Heatblur's F-4E exhibits an AoA of -1.1° = -19.195 mil.

36.6 mil + (- 19.195 mil) = 17.4 mil, which I what I entered in the sight for the following tests.

Observe the result of the test firings:

SUU-23/A on the wings (direct hit)

SUU-23/A on the centerline (short)

M61A1 (even shorter)

I will not reproduce the screenshots here to preserve everyone's bandwidth (they are available on request though), but depressing the sight further by 1 degree to 34.8 mil causes a direct hit by the centerline pod, and depressing the sight by another 1° to 52.3 causes a direct hit with the nose gun.

IMO, what we can deduce from this is that Heatblur's wing pods are rigged at X°, the centerline pod at X-1°, and the nose gun at X-2°.

The question is: What is X?

To answer that question, it is noteworthy that NAVAIR 01-245FDB-1T contains two sets of sight angle charts for the Mk 4 gun pod (the Navy's equivalent to the SUU-16/23), one for a gun bore line at FRL and one for a gun bore line 2 degrees below FRL. When extracting the required sight setting X for similar flight conditions as above from the "2 degrees below" chart (only difference is a release speed of 450 KTAS, not 450 KCAS, normalization for gross weight was of course accomplished), the results are qualitatively the same: Direct hit with the wing pods at setting X°, direct hit with the centerline pod at X-1°, direct hit with the nose gun at X-2°. The data from the "at FRL" chart does not provide results observable in the game.

Under the assumption that DCS's ballistic model very closely approximates RL data, from this I deduce that Heatblur's rigging is as follows:

Wing pods: 2° below FRL

Centerline pods: 3° below FRL

Nose gun: 4° below FRL

The question then remains if this is realistic.

Unfortunately, I have not been able to find a clear answer for this.

On the one hand, the fact that there is only one chart for the M61A1 and the SUU-23/A gun pod in GAF T.O. 1F-4F-34-1, and that T.O. 1F-4C-34-1-1, p. 5-10, in a section applicable to all covered F-4 variants, states that "the gun firing tables are applicable to the SUU-16/A, -23/A gun pods and the F-4E nose gun" strongly suggests that the real F-4E had the same rigging for pods and the nose gun. This is also what would make the most sense tactically. However, "being applicable for" does not imply "being identical", since one could argue that a difference of max. 2° in rigging was not though of as warranting producing different charts, and that using a single chart was "good enough for government work".

On the other hand, T.O. 1F-4C-34-1-1, albeit in the section applicable to the F-4D at 1-62, also states the following:

In a section applicable to the F-4E, it states that:

This would indicate that the nose gun was rigged 2° below FRL, and the gun pods were rigged along FRL.

However, it seems that the bore line of the nose gun was adjustable e.g. based on the mission flown. See the following extract from the Navy Tactical Manual:

My conclusion is therefore that Heatblur's current design choice is not categorically unrealistic, however I do find it odd that the in-game maintenance crews would elect to rig all three gun choices differently. My recommendation would be to rig all guns 2° below FRL since that would be the rigging most players presumably expect, since it has the advantage mentioned in the above extract, and it is the rigging for which we have at least two RL sources containing valid sight settings. This might also help to alleviate the problem reported at

where it is found that the A/A gun solution does not provide sufficient lead for the nose gun, possibly because the sight computations assume a 2° depression whereas the nose gun is actually depressed 4° from FRL (speculating here).

Edited August 30, 2024 by Stickler