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 Post subject: Vacuum system doubts
PostPosted: Sat Apr 21, 2018 8:42 pm 
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Airman Basic

Joined: Sat Jun 04, 2011 5:05 pm
Posts: 6
Location: Bytom
Hi,

I'm writing an article explaining "suction" or "VAC" gauge and I've been testing my P3D aircraft trying to collect some data on how they simulate vacuum system and its nuances.

I really like how C172, 182 and Comanche simulate VAC, and with these little issues I mention they are still the best (and by far!) among the planes I've tested. But I have some questions and observation (I think I noticed a little bug in an unusual engine failure scenario).

My questions:
- Is it really possible to get gyros spinning with windmilling prop at low RPM?
- Does VAC indication vary so much with RPM changes?
- Shouldn't I see L VAC R annunciation in C182 when VAC drops below 3 inHg?
- Shouldn't I see some problems with instruments on C182 with VAC below 3 inHg? (or even below 4.5inHg)?

Observations and questions:
Cessna 172
Suction at 500 RPM is 4.3 inHg. It rises to 4.5 inHg at takeoff power and then rises to 4.6 inHg as the RPM increase with speed on takeoff and then goes down to 4.5 inHg at cruise settings. Just what I expected from watching some videos showing C172's panels on the runup, landing, etc.

Engine failure scenario (for whatever reason - for this test I just cut out the fuel). At normal glide speed, with windmilling prop (300-400) RPM VAC gauge shows 4.4inHg. Nothing really changes and my vacuum-driven instruments work well.

Then I check another scenario - I stop my propeller. Suction drops to 0. Great! I have a gyro flag on my Attitude Indicator and my Heading Indicator doesn't work anymore. So then I check if I can get it all to work with just the wind turning my prop. I increase speed to 110 knots, the engine starts turning, then gets to 1000-1100 RPM and the suction gauge again shows 4.5. Vacuum driven instruments are functional again. This is great! Almost.

The only thing I noticed when I did multiple tests was that the logic of suction dropping to 0 is based on speed and prop RPM instead of prop RPM only. I was able to almost stop the prop (it was turning once every couple of seconds) with suction gauge reading 4.3inHg. Only when my airspeed dropped even lower the VAC gauge showed 0.0, L and R VAC annunciator flashed and my instruments were gone.

To sum it up. I have some doubts about vacuum simulation in the C172 with windmilling prop. Shouldn't suction decrease with RPM far below 1000? Unfortunately - I found no sources to confirm this either way.

There is also clearly a small bug with the scenario I described - 4.3 inHg suction with 0 RPM at the speed of 40 knots. There is no way to get any suction with the engine not turning as the pumps are engine driven.


Cessna 182
At idle (650 RPM) VAC gauge shows 2.7 inHg (it doesn't show values that low, I took this value from the tooltip)
At "low idle" (high altitude airport - 430 RPM) VAC gauge shows 1.9 (again - tooltip)
VAC L or R light is not flashing in either scenario - shouldn't it?

When I increase the power suction gets to 4.5 inHg by 1150 RPM and raises to 5.3inHg at takeoff power.

I did my "engine failure" test here too. It was quite simple. Fuel was cut at cruise speed and then I slowed down to 80 kias (4.3inHg) and then to best glide (70 kias at 2600lbs) where I read 3.7 inHg and 950 RPM with windmilling prop. At 58 kias (best glide at 2100lbs) I was at 2.6inHg and 680RPM. No VAC indication, instruments were ok.

Instruments were still fine at 45 kias (1.9 inHg and 480RPM, no VAC indication). I actually got to 38kias (full flaps, full back pressure on the elevator, -2000ft/min descent) and both vacuum-driven instruments were ok and the prop was windmilling.

Only after spinning and some crazy aerobatics did the flag on Attitude Indicator go up and L VAC R lights light up. But at this point, I could not get my instruments back without starting the engine. I increased speed (110 kias) and VAC went up to 5.3inHg, but both instruments were out (gyro flag visible, Heading Indicator dead). Starting engine solved the problem (with no change in suction).

My doubts (yes, I know it's a different plane and a different engine):
- should there be such a change in suction when RPM changes (it's unlike C172 and PA24)
- Shouldn't there be L VAC R indication when suction drops below gauge scale?

Bug:
- vacuum-driven instruments not working when suction restored (logic depends on engine combustion that has nothing to do with the vacuum pump)

Piper Comanche
4.2inHg at idle - 4.6 inHg at takeoff power (4.7 at max RPM inflight).

Engine out, windmilling at 70 kias (83 mph / 800 RPM) - 4.4inHg
50 kias, -1000ft/min, full flaps, stall warning, prop not turning - 4.3 inHg
Intentional spin (probably speed dropped to nearly 0) - 0.0 inHg + flags on both vacuum-driven instruments
Prop rotating again at 100kias (120mph) - 0.0 inHg + flags on both vacuum-driven instruments
140kias (160mph), engine at 2250 RPM - 0.0 inHg + flags on both vacuum-driven instruments
Combustion (mixture back to rich to start the engine) - 4.6inHg + flags on both vacuum-driven instruments for a moment, and then out and instruments ok.

Bug:
- vacuum-driven instruments not working when suction restored (logic depends on combustion rather than restoring engine RPM)


Still - the best simulation of vacum systems I saw in P3D :)


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 Post subject: Re: Vacuum system doubts
PostPosted: Sun Apr 22, 2018 2:55 am 
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A2A Mechanic
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Joined: Sun May 26, 2013 5:03 am
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Calypte wrote:
My questions:
- Is it really possible to get gyros spinning with windmilling prop at low RPM?
Yes, it would be possible. They often work acceptably when low-idling on ground as well. In many circumstances, the windmilling RPM is higher than that, and can be all the way into the governing range (=no change in steady state RPM when engine is shut down).

Calypte wrote:
- Does VAC indication vary so much with RPM changes?
It certainly depends on the installation. In case of 182, the vacuum indicator is after at least some amount of hosing. The pressure would be approximately constant at regulator valve, and elsewhere it would depend on the flow.

Image

Calypte wrote:
- Shouldn't I see L VAC R annunciation in C182 when VAC drops below 3 inHg?
Yes.

Calypte wrote:
- Shouldn't I see some problems with instruments on C182 with VAC below 3 inHg? (or even below 4.5inHg)?
That is a possibility, yes. It would be an interesting test to try self-contained gyro units in test bench to see when they stop indicating correctly. Electric gyros may spin for several minutes after having lost their power at speed good enough for somewhat reliable indications, vacuum gyros not so much.

Your other observations indicate some sort of wrong dependencies in the simulation. Then again, I understand A2A mostly uses default model for the vacuum system, pending custom rework - unless my recollection is outdated!

-Esa


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 Post subject: Re: Vacuum system doubts
PostPosted: Mon Apr 23, 2018 8:21 am 
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A2A General
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Joined: Wed Feb 11, 2004 12:55 pm
Posts: 15263
Location: USA
Calypte,

- Is it really possible to get gyros spinning with windmilling prop at low RPM?
Yes. The vacuum pump is tied to RPM

- Does VAC indication vary so much with RPM changes?
Not much except for very low RPM operations

- Shouldn't I see L VAC R annunciation in C182 when VAC drops below 3 inHg?
Our annunciation lights turn on below 2" and off above 3"

- Shouldn't I see some problems with instruments on C182 with VAC below 3 inHg? (or even below 4.5inHg)?
Yes, but I'm not sure exactly what behavior is modeled off hand. Don't read too deeply into the vacuum physics behavior in the current released version of Accu-Sim as a complete overhaul of this system is in the works. And keep in mind, vacuum behavior isn't related to the airplane but rather the vacuum pump attached to the engine which can be different kinds. And they behave differently especially very low RPM's.

Scott.

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 Post subject: Re: Vacuum system doubts
PostPosted: Fri Apr 27, 2018 7:21 am 
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Airman Basic

Joined: Sat Jun 04, 2011 5:05 pm
Posts: 6
Location: Bytom
AKar, Scott - Thanks for the clarification. :)


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 Post subject: Re: Vacuum system doubts
PostPosted: Mon Apr 30, 2018 3:35 pm 
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Senior Airman

Joined: Sat Dec 13, 2014 11:38 am
Posts: 217
Location: Vancouver, BC
Some extra details:

The vacuum pump is a positive-displacement device. It moves the same volume per rotation regardless of how fast it is going. The total volume moved is proportional to the speed of the engine.

The pressure control valve maintains the proper pressure by bleeding in whatever air is required to keep the upstream vacuum within a specified range.

The suction gauge is generally connected to the directional gyro. It measures the pressure difference between the housing of the directional gyro and the cabin of the aircraft, which is also what the vacuum relief value is working on. In some aircraft the vacuum relief valve is mounted in the engine compartment or in other places and in this case the gauge is measuring something slightly different. Some gauges have two lines and are measuring the vacuum between the instrument housing and some other location in the system, such as just after the air filter. So specific subtle behavior of the gauge requires some knowledge of what it is actually hooked to and where the control valve is.

The actual flow rate through the gyro instruments is dependent on the filter system, the hose lengths, the size of the instruments, type of gyro, mass of the gyro, condition of the bearings and erection vanes, and so forth. The instrument housing accepts the air discharged by the gyro vanes and this is then sucked out. Whatever flow this is then made up by the pressure relief valve, and the total flow is delivered to the vacuum pump.

Old instruments (AN style) have much larger gyros than modern ones. These use more air and have inlet filters right on the housing sucking directly from the cabin (no remote mounted common filter). In the even of a slow vacuum failure these will topple over sooner and act in sluggish manner. In the event of too high a vacuum these will act in a very "live" way.

Modern gyros need less air and may work even at very low rpm as the vacuum pump fails.

William

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 Post subject: Re: Vacuum system doubts
PostPosted: Tue May 01, 2018 5:23 am 
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A2A Mechanic
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Joined: Sun May 26, 2013 5:03 am
Posts: 3853
Excellent write, William. I must say I noticed your blog, some rather cool stuff there. Fascinating material on Piper Autocontrol.

-Esa


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