Target position in HUD does not match target position in Tacview

[Stickler]

I initially thought this was an aircraft-agnostic Tacview problem (see post below) but now I'm not so sure anymore.

Compare the following screenshots and .acmis taken from an attack by an F-14 and an F-16 against the same target.

F-14.acmi

F-16.acmi

In both the F-14 and F-16, the target is on the -25° degree line on the in-game HUD and the pitch of the aircraft is approximately -25° as indicated on the HUD, the F2 view and in Tacview.

However, in the F-14.acmi, the target is located on the -30° degree line, while in the F-16.acmi it is located on the -25° line.

The same mismatch between the in-game HUD and the Tacview HUD view also occurs in the Mirage F1. I have not tested other aircraft.

I am at a total loss how this is even possible. If it was a wrong HUD trim, surely the F-14's pitch as indicated by the aircraft symbol would not be consistent between HUD, F2 view and Tacview.

Any ideas?

Edited April 7, 2023 by Stickler

[RustBelt]

You’re confusing the F-14’s adjustable horizon guide to the Tacview datum line. 

There’s paralax on the F-14’s hud to the horizon. Welcome to the pacman days of avionics.

[Stickler]

Thanks for the reply, but I don’t understand what you‘re saying and how exactly this explains what I’m seeing. Could you elaborate?

[RustBelt]

Tacview doesn’t know where the eye line is. It doesn’t correct for the fact that the pilots eye looking through the HUD is like 4-6 feet above the ADL centerline of the plane. 
 

And with the Tomcat the horizon reference is just an arbitrary setting the pilot sets just as a visual guide. So it’s also not lined up straight because the pilot kind of looks down through the hud to the horizon. 

tacview is looking out from the center of the plane straight along the longitudinal axis. Which for the tomcat is like right above the valley between the intakes.

[Stickler]

It's getting a little bit clearer now, but I have not been able to fully grasp the problem.

Consider the following screenshots showing the same situation in-game and Tacview:

Both DCS and Tacview correctly show the aircraft's pitch to be -40° (this matches the "raw" pitch shown in the F2 view, screenshot not included).

Looking at the math of the situation:

F-14 altitude = 1944 ft MSL

TGT altitude = 11 ft MSL (not shown)

ATL = 1933 ft

Slant range F-14 to TGT = 2628 ft

Ground range therefore (Pythagoras) = 1780 ft

The target position angle (TPA) with reference to the physical horizon is then calculated as:

TPA = arctan(1933/1780) = (-) 47.35°

 

This is pretty much exactly the TGT position as shown in Tacview.

 

The F-14's HUD has the target beneath the (approximately) 43° line. Since the HUD shows the pitch correctly as per the F2 view, I'm going to assume the target really is at 43° with reference to the pilot's eye position.

 

If we calculate backwards to find the pilot's eye position altitude (ATL), we can say:

ATL = tan(43°) x Ground range = 0.93 x 1780 ft = 1660 ft

 

The pilot's eye position is therefore 273 ft (1933 ft - 1660 ft) below the aircraft considered as a point in space.

 

With the aircraft having a vertical extension of 16 ft total, how is that possible? What am I doing/thinking/calculating wrong?

[Callsign JoNay]

Forgive my ignorance, but isn't this all explained by the default position of the HB DCS Tomcat's HUD pitch ladder being trimmed ~5 degrees below the horizon, as well as the distance between the rungs on the ladder not being linear?

[Stickler]

14 hours ago, Callsign JoNay said:

Forgive my ignorance, but isn't this all explained by the default position of the HB DCS Tomcat's HUD pitch ladder being trimmed ~5 degrees below the horizon, as well as the distance between the rungs on the ladder not being linear?

OK, I think I've figured it out thanks to the first part of your comment.

I was departing from the assumption that you could derive target placement and pitch from the HUD ladder simultaneously. Due to how the HUD works, this is incorrect. You can only ever get one or the other with a specific HUD trim.

This means that, for example, if you calculate a 30° dive attack with a 33° initial target placement (ITP), provided you fly the geometry perfectly with a default 5° down HUD trim, your apparent ITP at the track point will instead be 28°. Confusingly, you'll now need to set 30° instead of 25° pitch on the HUD to reach bomb range at the planned release altitude.

I have not read anything about the rungs not being linear nor can I confirm this by testing. Using the ELEV LEAD knob and MAN mode as a reference, there are 86 to 88 mils between the rungs, mostly 86-87. While this means that the HUD calibration is slightly off (89 mils would be most accurate), IMHO this is close enough for government work.

So summing up, the problem is solved on my end. Still a weird HUD for A/G employment, but then again it wasn't designed for that purpose.

[Callsign JoNay]

3 hours ago, Stickler said:

I have not read anything about the rungs not being linear nor can I confirm this by testing.

I regret saying that part, because now that I think about it, they might be linear in A2G mode, just offset about 5-deg down like I said, unless trimmed otherwise.

But in a mode like A2A they act a little different. For example you will see the 30-deg rung in your HUD when you're near level flight. Obviously that can not indicate a true 30-deg as viewed from that attitude, and if you begin pitching up towards that rung it starts to run away from you as you chase it. Your wings symbol will eventually intercept it at around 30-deg of pitch though.