[NOT BUG]Cabin Altitude - Differential Pressure Texture

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flapman
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[NOT BUG]Cabin Altitude - Differential Pressure Texture

Post by flapman »

Hi all, a quick observation from my last flight (before the sim crashed :roll: )

Image

My understanding is that the green band (with red mark) is attached to the C (Cabin) needle. The red mark should be on the opposite side of the green band, thus indicating the maximum differential pressure between A (Aircraft) and C (Cabin). As it functioned on this last flight (my first real long flight in the Connie) the red mark always stayed behind the C needle pointer.

This is the default cockpit textures from A2A, and the same depiction is found on p. 121 of the manual.

Thanks,
Last edited by flapman on 27 Jul 2017, 11:02, edited 1 time in total.

Gypsy Baron
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Re: Cabin Altitude - Differential Pressure Texture

Post by Gypsy Baron »

he red mark should always be 'behind' the 'C' needle. If the 'C' needle goes into the red, you are
exceeding the max differential pressure.

The green/red band moves with the "A" needle, the aircraft altitude.

Paul

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flapman
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Re: Cabin Altitude - Differential Pressure Texture

Post by flapman »

I guess I should clarify what "behind" means,

It appears that the C needle is superimposed on the Red line. I do not see them moving in relation to each other. If your suggestion is the case, then my aircraft is at maximum legal differential pressure.

Can you post an image showing a cabin which is substantially below max differential pressure, or which the max differential pressure is substantially exceeded (basically any image where the C needle and red line are not touching. :?:

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Nick - A2A
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Re: Cabin Altitude - Differential Pressure Texture

Post by Nick - A2A »

Here's the gauge with the aircraft on the ground at high altitude (i.e. unpressurised) flapman.

Image
flapman wrote:It appears that the C needle is superimposed on the Red line. I do not see them moving in relation to each other.
In your example, if you flicked the cabin dump valve switch from 'auto' to 'open', the 'C' needle should have moved clockwise rather quickly! :wink:

With the aircraft parked on the ground at sea level, the gauge indicates the 'C' needle in the red which is, of course, nonsense. Presumably this is a mechanical limitation of the real gauge and that the red end of the arc is prevented from swinging back below the 0 feet position. Graphically, something seems a little funky with the counterweight but perhaps it's a counterweight for the moving arc itself, not the 'A' needle?

Cheers,
Nick

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CAPFlyer
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Re: Cabin Altitude - Differential Pressure Texture

Post by CAPFlyer »

The counterweight is attached to the "A" needle, which is the top needle, and needs to be balanced because of its mass and it's height so it doesn't interfere with the operation of the "C" needle. The safe operating range arc is the bottom of the stack and operates in concert with the "A" needle, but it is not a linear motion. It is self-balanced with extra material on the part of the arc opposite the arm that is not modeled in the A2A gauge, but present on the real thing. There's a similar gauge (I believe out of a B-29) in the parts supply with the CAF in Midland that I got a chance to look at a few years ago, shortly after the B377 release, and like many gauges of the late 1930s and early 1940s, it's a pretty ingenious bit of engineering, especially for a time when they didn't fully understand how a lot of things worked at altitude and more especially, had no real ability to simulate altitude beyond simple vacuum chambers (high altitude wind tunnels hadn't been built quite yet).
Image

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Nick - A2A
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Re: Cabin Altitude - Differential Pressure Texture

Post by Nick - A2A »

Thanks for the info on the gauge construction Chris - I tried without success to find a photo of the real thing. :) The counterweight certainly looks like it should be part of one of the needles; when I say "graphically, something seems a little funky", I mean this...

Image

...When the 'A' needle is registering less than around 8,000 feet it becomes 'detached' from the counterweight.

This image also shows the 'C' needle superimposed over the red section of the arc. However, the cabin certainly isn't over-pressurised; it's just that the green and red arc hasn't moved off its stop at 0 feet, so to speak. Not sure if this a limitation of the real instrument which A2A have reproduced, or if it's a little glitch in the gauge programming.

Cheers,
Nick

flapman
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Re: [NOT BUG]Cabin Altitude - Differential Pressure Texture

Post by flapman »

Thank you for the responses everyone! I have edited the thread title as this is obviously working correctly.

Sorry for making the assumption that something was wrong, we've really delved into the depth of the simulation of this single instrument.

From the discussions and images posted it appears that there are 3 elements to this instrument (as described by CAPFlyer): an 'A' needle, a 'C' needle, and a 'Diff. Pressure PSI' needle.
(thank you for posting images, I was in career mode and didn't want to play with pressurization/dump the cabin with passengers on board).

If you look at all three images in this thread, I noticed that the counterweight is always opposite of the center of the green arc. I think this counterweight actually balances the moving differential pressure arc.. presumably because it has the most material and will be unbalanced by the greatest amount. It's really a strange gauge design, and I've never seen anything like it (this is my excuse for assuming it was broken :mrgreen:)
It appears that so long as the differential pressure is not exceeded, the red light will be just below (in terms of indicated altitude) the C needle. This doesn't give the FE any warning his planned cabin altitude will exceed the diff. limit (but he has the pressurization chart). But in Nick M's example this isn't the case. Perhaps this is to indicated how low the cabin altitude can be lowered before the limit is reached.

The fact that this gauge is more complicated than modern designs would make sense, as this is one of the first pressurized airliners ever produced.

Thanks again.

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CAPFlyer
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Re: [NOT BUG]Cabin Altitude - Differential Pressure Texture

Post by CAPFlyer »

In your picture Nick, it seems the A2A gauge has the counterweight attached to the arc, which is the bottom arm. Which is interesting, but certainly possible that the gauge I looked at, while appearing the same, was not. Again, back during this time, there were certainly more manufacturers of instruments, so it's entirely possible to have two gauges look externally identical but not be.
Image

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Nick - A2A
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Re: [NOT BUG]Cabin Altitude - Differential Pressure Texture

Post by Nick - A2A »

Yes, counterweight attached to the moving arc was how I first interpreted it, but it does look a little bit odd if so. What I find interesting about the instrument is that the whole moving green arc with its red bit is essentially static and meaningless until we climb to about 8,000 feet AMSL, whereupon the 'A' needles 'catches up' with it and they starts to move in unison. And this is the flight level at which the placard advises we start pressuring the cabin. :)

The VFE (being a diligent chap and keen to impress) starts pressurizing as soon as we take off I believe. :)

Nick

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