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LOW SUCTION/artificial horizon drift (EDIT: primarily cold start)


Jakey-Poo

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After talking with several members of the community (more knowledgeable than myself) the common consensus for the problem with the artificial horizon drifting constantly is due to low suction in the vacuum system.  After comparing this to readings on the suction gauge during flight this checks out.  The problem then becomes why is the suction consistently below operational range even while in climb power?  Is there a fix for this that I have not stumbled across, or is this a bug?

P-47 suction 2mp.png


Edited by Jakey-Poo
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The vacuum-based attitude systems on these aircraft were never able to maintain a stable reference for the entire flight even when working perfectly.  Really the best a pilot could hope for was a somewhat stable horizon reference during a cloud layer penetration when absolutely necessary to land.  The expectation was the pilot would have been flying straight and level for enough time before penetration for the vacuum to provide a somewhat reliable reference during the time the pilot had no visual horizon.

 

Important to remember these fighter aircraft were only marginally IFR capable by today's standards.

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21 minutes ago, Nozzle said:

The vacuum-based attitude systems on these aircraft were never able to maintain a stable reference for the entire flight even when working perfectly.  Really the best a pilot could hope for was a somewhat stable horizon reference during a cloud layer penetration when absolutely necessary to land.  The expectation was the pilot would have been flying straight and level for enough time before penetration for the vacuum to provide a somewhat reliable reference during the time the pilot had no visual horizon.

 

Important to remember these fighter aircraft were only marginally IFR capable by today's standards.

Is it normal then for the suction gauge to be consistently below the operational range, and I'm just tweaking?  I figured there was something that I was missing or operating incorrectly to have these results be the norm.

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KvbAEv7.png

Looks like at least  3.8" is required to proper operation of gyro-horizon .

I also has impression that gyro-horyzons drifts a bit too quick, not only in P-47 other warbrids as well. I expect to loose gyro coordination after sudden maneuvers, but i have this problem in level flight as well.


Edited by grafspee
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System specs: I7 14700KF, Gigabyte Z690 Aorus Elite, 64GB DDR4 3600MHz, Gigabyte RTX 4090,Win 11, 48" OLED LG TV + 42" LG LED monitor

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Oddly, I find the P-51 and Spitfire to have fairly reliable gyros. They drift after hard maneuvering, but given a few minutes of stable flight they sort themselves out. The P-47's, however, is a functioning alcoholic. Some consistency needs to be applied here.

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13 hours ago, grafspee said:

 

Looks like at least  3.8" is required to proper operation of gyro-horizon .

I also has impression that gyro-horyzons drifts a bit too quick, not only in P-47 other warbrids as well. I expect to loose gyro coordination after sudden maneuvers, but i have this problem in level flight as well.

 

 

9 hours ago, Nealius said:

Oddly, I find the P-51 and Spitfire to have fairly reliable gyros. They drift after hard maneuvering, but given a few minutes of stable flight they sort themselves out. The P-47's, however, is a functioning alcoholic. Some consistency needs to be applied here.

 

After flying the P-51 this morning I'm getting more and more the feeling that this has to do with inadequate suction in the vacuum system in the P-47, as opposed to the gyro itself.  While in the P-51 I had no problem keeping the suction gauge within the operational range (although admittedly on the lower side), and, I would assume correspondingly, had no problem with the ADI during "normal" flight.  This is a completely different experience than the Jug.  If this is modeled accurately, and the jug had particular problems with the vacuum system, then fine.  I'm just looking for info.


Edited by Jakey-Poo
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  • 8 months later...
  • Jakey-Poo changed the title to LOW SUCTION/artificial horizon drift (EDIT: cold start only)

After further testing in Instant Action missions where the suction gauge is reading within normal operating parameters, it’s pretty obvious at this point that the artificial horizon acting up is a symptom of inadequate suction in the vac system.  I’m going to be testing more with the compass, but haven’t noticed anything obvious. 

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  • Jakey-Poo changed the title to LOW SUCTION/artificial horizon drift (EDIT: primarily cold start)
  • 5 months later...
  • 3 months later...

Here are two screenshots, taken in flight after cold start, Channel map:

1) Max. continuous power in climb, suction gauge needle is barely over the bottom red line

max cont climb.jpg

2) Max economy in almost level flight, suction is at or slightly below bottom red line

max econ level.jpg

Shouldn't it be firmly in "green" zone even at idle power?

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  • 2 weeks later...
On 11/16/2022 at 6:40 AM, cw4ogden said:

This is still a problem.  Tend to agree with the original poster, the poor gyro is linked to low vacuum.  
As is, the plane would be on a red X, awaiting the crewchief to get his duct tape. 
 

 

If you or anyone else knows how, and would be willing to post a track file for this, it would help me out a ton - I’m computer illiterate; I just fly imaginary airplanes.  This got flagged for missing info, and I’m assuming per NL’s response above that the track files are what they need. 

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  • 1 year later...

P-47D-40 suction on the ground.trk

P-47D-40 suction climb 2700 RPM.trk

Here are two short tracks with P-47D-40 (hot start on parking and in air, I haven't noticed a difference with cold started plane in these tests). At 900 RPM suction stays at about 3 in Hg, with RPM increase it starts to rise (beginning at around 1100 RPM) and gets to about 3.8 (low red line on the gauge) and stops there, never rising above low redline. One test was done on the ground, other - in air at military limit (2700 RPM, 52'' MP), both on Caucasus with "DCS default" weather. Same results were obtained with P-47D-30.
There was an outlier case where suction rose to 4,2 in Hg in flight at high altitude (see screenshot) (P-47D-30 early, Normandy, clod start at Azeville ALG)...

Edit: I've got 4.2 in Hg of suction again with D-30 early on Normandy after an air start, this time at low altitude:

P-47D-30 early suction 4,2.trk

Even if vacuum pump has 1:1 engine-pump RPM ratio (I don't know real ratio, but kinda doubt it is 1:1), it should still provide adequate pressure difference at 2500+ RPM.

Also, aren't external tanks pressurized by vacuum pump exhaust?


Edited by Aernov
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