GCSC and INS steering error
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One question that’s come up is what determines the alignment of FPM and GCSC? Is it the comparison of airplane heading to some calculated desired heading? Or is it comparison of azimuth component of velocity vector to desired ground track? A modest look into it suggests that BMS is doing the first one and the real airplane is doing the second one.
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Last I looked at this, BMS used track, not heading, for the GCSC, which is correct behavior. Has that changed?
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Looks like heading to me.
I’m guessing the extended line is also steering error and not “bearing” so crosswind steering would still be 12 o’clock even though your bearing to destination isn’t directly ahead.
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Thats unfortunate. I recall the conversation coming up back in 4.32, and I thought it was resolved for 4.33. Ill try find the thread.
And even older: https://www.benchmarksims.org/forum/showthread.php?14472-Hey-all-2-questions&p=203403#post203403
haaa…if GCSC is the tadpole i.e. the small circle in the hud etc…then there is indeed a bug…when correct heading is taken , diamond , tadpole and fpm shall be aligned and the indicator on the heading tape shall indicate the nose heading then (because you can’t see neither fpm, tadpole or diamond in extreme situations).
=> Bugtracker
Oh, on rereading the thread it seems it WAS fixed.
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Wings level, steady flight the current issue is invisible in BMS. In the case of zero AOB, heading error and velocity error are interchangeable. The issue comes in when when there’s some bank and AOA such that heading error and velocity error are different numbers. In the picture above my heading error is zero but my path error is ~10°. GCSC should be positioned appropriate to this 10° error.
The flight manual says that it’s “bearing” but that’s for the stick monkey operators. -34 describes it later in better detail. There’s an equation there with the words “ground track.” Additionally I don’t think the little line off the circle cue is bearing either but a 360 representation of the same value which drives the GCSC-FPM positioning. By the way the “W” indicator on the MFD corner uses this same GC steering error and suffers the same issue where it shows centered based on heading error instead of track error.
Ancillary note diamond and GC steering diverge, and should, over long distances. The GC steer is more complicated math than the math that places the diamond so if your waypoint is 4000nm away you’ll see they don’t line up. That’s normal and BMS does it too.
Drift cutout centers the FPM in the HUD only. Remember that GCSC lateral displacement is displayed relative to FPM regardless of where FPM is. It’s not like GCSC is drawn out in world-space like the TD box. The only oddity when at zero GC steering error is that while FPM and GCSC are aligned, they’re pointed upwind instead of toward the diamond. In one video I found that it appeared that full deflection GCSC placement is 10 degree steering error (another HUD tape from perhaps older plane it looked like 5 degree). Because full deflection is about 3 degrees lateral in HUD draw space there’s a sort of parallax effect as you turn where the GCSC marches from limit to limit across the HUD through about 6 degrees total while the world out the window (also diamond) displaces 20 degrees.
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The FPM and GCSC shouldnt be pointed upwind. The FPM and GCSC should be pointed at the diamond, and the nose be upwind.
If the diamond isnt drawn accurately at long distance, thats something I dont know about. If the diamond -is- drawn perfectly in position, then the GSCS should point to the diamond. Line of sight IRL is a great circle.
Wings level, steady flight the current issue is invisible in BMS. In the case of zero AOB, heading error and velocity error are interchangeable.
Thats… not correct. Thats only correct in the case of nil wind. Your actual track across the ground, which is where the FPM is pointing, is what the GCSC should be looking at. In crosswind, thats a different spot to the aircraft heading, where the nose is.
Perhaps thats an indication of where the anomalous behavior is, if it only happens with wings banked?
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FPM, GCSC, and gun cross should all be pointed upwind at same azimuth with drift cutout enabled.
I’ll explain what I mean by interchangeable. Imagine the following situation:
Wind from the north
A. Calculated track to destination 270
B. Current ground track 272
C. Calculated wind-corrected heading 275
D. Current heading 277Real plane is track error, B-A. BMS is doing heading error, D-C. In this case they are both +2. With no bank then B-A = D-C and it cannot be determined which supplied the +2.