Altimeter Questions

[corn322]

What is the purpose of the two triangles indicated in the image?

[Frederf]

They represent the difference in pressure altitude between the original QNH and the current setting.

 

Example, you spawn on a 1,234m airfield where QNH is 751mmHg. Your altimeter will show 1,234m and triangles will show 0m. If you set the pressure adjustment such that the altimeter shows 1,134m (100m less) then triangles will show 100m.

 

In your picture the triangles are showing about 940m while your altimeter is showing 70m. This suggests your real altitude is 1010m.

[corn322]

I was in the Batumi parking lot. According to the mission editor, the altitude there is 10m.

[Jonne]

They represent the difference in pressure altitude between the original QNH and the current setting.

 

Example, you spawn on a 1,234m airfield where QNH is 751mmHg. Your altimeter will show 1,234m and triangles will show 0m. If you set the pressure adjustment such that the altimeter shows 1,134m (100m less) then triangles will show 100m.

 

In your picture the triangles are showing about 940m while your altimeter is showing 70m. This suggests your real altitude is 1010m.

 

Sorry, but I have to correct you. The triangles show the difference of the altimeter setting to 760mmHg (standard) converted to m. In this particular case, the altimeter is set to 767mmHg therefore the difference is -60m (as on the outer triangle).

 

Now looking at the screenshot more closely the inner triangle seems to be incorrect, or maybe there is a function in settings above 760mmHg that I do not know.

[Frederf]

Sorry, but I have to correct you. The triangles show the difference of the altimeter setting to 760mmHg (standard) converted to m. In this particular case, the altimeter is set to 767mmHg therefore the difference is -60m (as on the outer triangle).

 

Now looking at the screenshot more closely the inner triangle seems to be incorrect, or maybe there is a function in settings above 760mmHg that I do not know.

 

No, put a MiG-21bis down in the editor and set QNH to 750mmHg and look. The triangles' zero position is not relative to 760. Maybe you're thinking of the L-39.

[foxbat155]

In real world MiG-21's barometric altimeter exist in two variants: WDI-30 and WDI-30K. Only difference between them is the command marker ( from Lazur system ) on the inner scale in that second one. Any others markers not exist, and here in DCS MiG-21 we should have WDI-30, because type 75B aircrafts don't have GCI. Here the picture of real WDI-30K:

 

 

[Frederf]

WDI or VDI. I don't know what altimeters have the outer marker or both. L-39 has VD-20.

 

So if VDI-30 is not VDI-30 (K) then LNS can remove both index triangles and make it the real instrument without any worry of programming behavior.

[corn322]

Maybe you're thinking of the L-39.

I went and looked through the L-39 manual to see if it had any information about this. It says: "Pressure correction indexes for landing at high altitude airfields, where pressure is less than 670mmHg. Indexes are moved by the knob."

 

 

The manual points to arrows 1 and 2, which I think is an error. I think arrow 2 is for alignment of the barometric pressure.

 

 

Then I noticed something about the image foxbat155 posted.

The alignment between 800m mark and 25km mark. Compare images:

 

 

Then I saw the numbers in the pressure scale. They increase going clockwise in all images except the one from our game Mig-21.

What's going on here?

[Frederf]

How it is working in L-39 DCS module is that both indexes are working to show some kind of height similar to how the two hands of the normal altimeter function. When the outer index orbits once the inner index moves from 0 to 1. When the outer index orbits again the inner index moves from 1 to 2. Let's examine notable settings, altitudes, index heights for a standard pressure day at Batumi 11m when worked from its most CCW turning to most CW.

 

[TABLE=head]Pressure | Altitude | Index Height | Notes

---|-10000|10330|Physical CCW limit of knob

---|-10000|8180|Physical limit of altimeter hands

---|-3340|3120|Blocking shield fully covers window

670|-1065|1050|Lower limit of marked scale "67"

737|-255|265|Blocking shield disappears to right

759|0|11|QFE

760|11|0|QNH

782|250|-240|Blocking shield appears at left

790|340|-330|Upper limit marked scale "79"

---|710|-710|Physical CW limit of knob, blocking shield covers left 10% of window[/TABLE]

 

The MiG-21 indexes are working similar but not relative to 760 but whatever is the passed on by the mission as the "QNH" static weather pressure.

 

As you drop off the bottom of the scale <670 you can still set accurate QFE if you know what index height to use. It's not for very low pressure (<670 would be rare) but very low QFE mostly due to the high altitude of the airport. MiG-21 DCS can't leave the marked scale 67-79 anyway so there's little point in referring to index height. You also notice it doesn't gracefully handle negative values. Real bis manual tell to use QFE or at 1000m or above fields, QNH.

 

I don't know why there is a km index. How common would be an index height >999m be anyway? Maybe L-39 developer decided to use km index for this purpose by mistake. What could km index be used for on L-39 or other airplanes? Autopilot? ATC instructions? Reminder bug?

 

Yes, "5" index is fixed. It's still an index though. I guess technically it's used to set pressure. It's the fixed index that the moving scale is compared against. In this manner of thinking there are 3 indexes, 2 moving and 1 fixed.

 

Perhaps the gap between 30 and 0 is too large? I hadn't noticed but it looks like the alignment between the outer and inner scales starts aligned and gets progressively worse CW.

 

I also notice the 250/750 marks are different, having screws at the extreme ends and thus dark areas instead of going all the way to the edge.

[foxbat155]

WDI or VDI. I don't know what altimeters have the outer marker or both. L-39 has VD-20.

 

So if VDI-30 is not VDI-30 (K) then LNS can remove both index triangles and make it the real instrument without any worry of programming behavior.

 

 

Yea, should be VDI, I can't get used to english transliteration, letter V don't exist in russian and others slavic languages.

 

Yes we should have in DCS VDI-30 without any triangles markers.

[Seb71]

I can't get used to english transliteration, letter V don't exist in russian and others slavic languages.

You can't spell Vodka without a V.

[foxbat155]

OK guys, I found were is problem. Two seater variants of MiG-21 ( U,US,UM ) had different barometric altimeter - VD-28, and those in VD-28K variant have two markers, so looks like LN puted wrong altimeter into module. Photo:

 

 

 

 

 

 

You can be sure Seb71 that I'm able spell vodka by W.:megalol:

[Kang]

Nice find about the different altimeter models.

[Jonne]

The MiG-21 indexes are working similar but not relative to 760 but whatever is the passed on by the mission as the "QNH" static weather pressure.

 

And this is not how it is supposed to work, but you are right. I wonder if on the real altimeter you can zero it by pulling/pushing the knob, but there is for sure no magic automatic QNH input to the altimeter.

[Frederf]

I have no idea if it's possible to set the index home pressure or if it's permanently 760. It doesn't matter in the case of the MiG-21bis-ESN though as it should have VDI-30 with zero indexes.

[corn322]

The DCS Mig-21 manual itself lists "pressure altimeter VDI-30" on page 28, then describes "barometric altimeter (VDI-30K)" on page 53.:doh::helpsmilie:

[Kang]

My theory is that the real-life MiG-21bis someone in M3's team flies might be showing their age with some parts like these having been switched for some 'leftovers' from other variants or types over the years.

[-Rudel-]

Chosen by Beczl, any avionics in the cockpit, is from Beczl Studio years...before Cobra or myself took over the art.

[portman]

To make the confusion perfect: The MiG-21bis SAU at the Museums here in Germany all have the VDI-30 with one (!) index for LASUR-commands (deactivated in the SAU versions).

[Jonne]

To make the confusion perfect: The MiG-21bis SAU at the Museums here in Germany all have the VDI-30 with one (!) index for LASUR-commands (deactivated in the SAU versions).

 

I would have been suprised if there would not have been any part switches between the two versions. Either the museum put anything on they had, or the NVA only had one type of spares for their MiG-21bis.

[foxbat155]

To make the confusion perfect: The MiG-21bis SAU at the Museums here in Germany all have the VDI-30 with one (!) index for LASUR-commands (deactivated in the SAU versions).

 

Perfect explanation for perfect confusion: from about 9000 combat Mig-21 variants produced, only about 700-800 were Bis type 75b, so standard altimeter was VDI-30K because all that about 8200 aircrafts had some variation of Lazur system. At the service start all type 75b had VDI-30, but later during service many of them got standard altimeter VDI -30K ( because of malfunction, overhaul, service life expiry etc) . Both altimeters were functionally identical in main role, so no reason to not use K variant on type 75b.

[Frederf]

VDI-30 or VDI-30K I think we can all agree are perfectly acceptable substitutes. If LNS wants to go completely crazy then it can have the index present/absent by looking at the year of the mission date :p

 

In either case the outer index should be removed and the inner index either removed or deactivated.