Vacuum/Suction gage fittings

[sreeves]

I was under the panel rearranging a couple things and noticed the back of my suction gage has the one small flexible line that goes to the back of the attitude indicator and there is a second fitting on the back of the suction gage that appears to be open. No plug or line in that second fitting.

I've been thinking about that and it is probably correct the way it is but wanted to check with anyone with a good knowledge of the vacuum system.
My suction gage reads about 4.25" to 4.5" inflight at cruise and both gyros appear to work normally. I might try to boost the suction a little by adjusting the regulator. BTW, my aircraft has two venturis. I don't know which size they are. I am guessing they are a foot long, maybe a little less. The airplane is in the hangar about 5 miles from where I write this, so I don't know what suction they are capable of.

Thanks for your advice...free as it may be

[Bruce Fenstermacher]

Show us a picture of the venturi. But likely, the venturis are rated at 4" of Mercury. Your already getting more that they are rated that being the case and this is typical. The second opening on the back of the gauge needs to be open otherwise no air could be sucked through the lines over the gyro air motor to turn it. Of course today that opening would go to a filter.

[nippaero]

Stephen,
Mine is the same way. If it’s the small UMA gauge, the line labeled “P” is connected to a vacuum source like your instrument. The other line labeled “V” is a vent and is ok left open. It vents to normal atmospheric pressure.

You can adjust your regulator by trial and error to see if you can get the vacuum up a little. Give it a turn or two in either direction and go for a trip around the pattern. I adjusted mine for the middle of the green arc in normal cruise flight.

[DaveF]

Most suction gauges have two inputs and the gauge reads differential pressure. One input is connected to a port on the attitude indicator, so it sees the amount of suck applied to the instrument. If the other gauge input is open then the gauge is reading instrument vacuum relative to cabin pressure. The problem with that setup is that if your system inlet filter clogs you'll read full suction, even though the actual pressure drop across the instrument is zero because no air is flowing. Most factory installations connect the other gauge input to a tee on the filter output. That way the gauge shows the true pressure drop across the AI, which is what it cares about. This is probably what Bruce just said ...

[pdb]

This may be helpful....

https://www.eaa.org/en/eaa/aviation-com ... ent-system

[DaveF]

Good article, thanks Pete.

I'm interested that Tony said to mount the venturis on the left side of the airplane. I thought Cessna mounted them on the right side, and assumed that was based on testing?

[Bruce Fenstermacher]

Cessna did mount them on the right side. Why we will never know. The static port is on the left side. Someone, I think Miles, reported that his left venturi had little vacuum compared to the right. I think he have them both mounted on the right during the rebuild.

[GAHorn]

It is common knowledge that the downswing side of the propeller is more efficient than the upswing side (as proven by P-factor studies) and the main reason I can imagine that Tony suggested the left side is he had a bran-pharrt while writing about VW powered experimentals, despite his specificity about them. Another reason might be pilot-boarding considerations.
Cessna put it on the right side of the airplane likely for that (P-factor) reason, is my guess. Miles' experience seems to confirm that choice.

[n2582d]

Of course, for maximum effectiveness place your venturis downstream the exhaust pipes. Higher velocity exhaust gas results in higher vacuum and automatic anti-icing. Just think how great this would work with the Benham or Bartone pipes!

IMG_0478.jpg

[GAHorn]

Gary, that's a great photo.

I actually have seen another 170 with that same setup and wondered how the owner thought it worked. I suppose the owner/installer was thinking that warm air exiting the lower cowling might keep them from icing up. (I suppose an equally-valid argument could be made that the less dense warm air (not to mention the oil-residue from a typically-leaky 145 Continental) reduced their effectiveness.)

I have no idea how they expected the rest of the airplane to operate in such unapproved conditions. The airplane had/has no heated pitot. No heated windshield. No airframe anti or de-icing, not to mention the engine intake and propeller. The 170 is not approved for icing for much more serious reasons than the vacuum system. The last thing I'd worry about would be the venturiis icing up. It'd be more important they'd already be at max efficiency if such were encountered.

Cleo Bickford was a radio-operator on B-17s and he and I once discussed it. He said the only time he saw ice while in a B17 the venturi continued to work fine. I asked him how he knew and he replied that it was on his mind since the venturi drove the autopilot gyro and the pilot was scaring him by using the autopilot. (For those not familiar, using AP in ice has long been discouraged in aviation because it can hide deteriorating flying-control effectiveness and mask AP trim-changes. It's not a recent discovery. WWI Ace Eddie Rickenbacker was a pax (he was also president of the co.) on an Eastern flight which crashed in the south due to icing. He blamed the pilot's use of the AP opining that pilots should always hand-fly so they could "feel" the ice. He had all APs disabled on all Eastern aircaft for the rest of his tenure as president.
Of course, Eddie was goofy in some ways.... he also had the circuit breakers pulled and disabled the weather radars on all Eastern B-727s because he believed pilots should be paying attention so they can "sense" thunderstorms. FAA finally put an end to that silliness with regulation requiring detection equipment on airliners.... but airline pilots used to walk past Eastern crews and place the backs of their hands on their foreheads and wave their forefingers before their eyes (indicating to the Eastern crews they should be "sensing weather"... in a mockery of what was called "Rickenbacker-Radar".)

Ahh, the good ol' days.

I knew you guys wanted to know all this.

[n2582d]

I'm in the process of planning my vacuum and pitot/static system plumbing. Also learning how to use QCAD. I have two sets of venturis, one pair 10" long -- referred to as 4" venturis, and one pair of 11.25” long venturis. AC Spruce says that each of the smaller ones is sufficient to power a DG or a gyro horizon. They say one of the 11.25" long venturis (referred to as a 9" super venturi) "pulls enough vacuum to operate a gyro horizon, directional gyro & turn/bank." What sucks (no pun intended) is that the mounting holes for the super venturi are narrower than those of the original10 inch long 4" venturi; upgrading will require a skin patch in the boot cowl.

My question for other A&P's has to do with legality. My journey from following the IPC to my own design has involved several steps. Reasons for modifying the original design include cost, weight, effectiveness, and ease of installation. First, on the pitot-static system, the IPC uses 1/4" aluminum tubing linked together with AN fittings. It's common practice to use Nylaflow or Polyflo lines and fittings on aircraft these days. I don't see that either of these meet any sort of AN, NAS, or MS standard. But, as they seem to be the standard, that's what I was planning on using. That was until I ran across various vendors like SteinAir, EDMO, and Midwest Panel Builders offering push-to-connect fittings. Brands like Nitra Pneumatics, CDC Pneumatics, and SMC Pneumatics offer these fittings for a fraction of the cost and 10X the ease of installation over aluminum tubing with corresponding fittings. So, as I rationalize that these push-to-connect fittings are just as legal to use as Nyloseal or Polyflo for the pitot-static system, it wasn't much of a jump to think about replacing the vacuum hose and aluminum tubing on the vacuum system with push-to-connect fittings.

IMG_1746.jpeg

My drawing uses 1/2" lines from the venturis to the vacuum relief valve, two 3/8" lines from there to the DG and Sperry F3 attitude indicator, and 1/4" lines for the T&B and suction gauge. I've also added a central filter which is connected with 3/8" Aeroquip 306 or Stratoflex 193 hose because the filter housing has hose nipples. I wanted to install straight 1/4" NPT fittings into the venturis but the minimum bend radius on 1/2" nylon tubing is around a 4" radius which would intrude too far into cockpit. I'm thinking the 1/2" diameter hose will offset the sharp 90º bends there. The Cessna drawing is similar in that area with 3/8" aluminum tubing.

Anyway, feedback appreciated. Does this need DER approval with a field approval/major alteration? Minor alteration with a logbook entry? UnSat -- Use AN fittings with aluminum tubing or Aeroquip 306 or Stratoflex 193 hose?

KQ2 Vacuum System.pdf

[Bruce Fenstermacher]

I think I'd have to be convinced it is not a minor alteration.

There are two "Super Venturi" or were two sold. One was a 9" the other a 10". I think Wag Aero sold the 10". I installed a 10" on my 170 replacing 2 original 4 inch venturis. I don't remember having to patch anything but it's been a long time since I did this.

[GAHorn]

When I replaced my (original cessna-installed) venturiis with “super” venturiis from Spruce..they were an exact-match footpirnt.

I wonder why you are connecting the suction gauge to the Horizon case …as well as the Horizon filtered air line?

I would think it more useful to sample the D.G. case.

Ordinarily, the non-tumbling T&B should be powered separately from the Vertical gyro. (most are electric T&B matched to air-driven Vertical…. OR… air driven T&B with Elect V.G.. The purpose being so that a single power-source doesnt’ fail all attitude gyros.
(Another method to achieve the requirement for separate power sources would be to power the T&B from a separate, dedicated venturi. (Back in the day, T&B’s were powered by little 2” venturi of their own for this purpose. There may be an exclusion for certified non-tumbling attitude instruments…. it’s been awhile since I’ve dealt with this issue tho’.)

[n2582d]

Appreciate the feedback! I assume it's my new venturi that have the non-standard footprint. Weird.

IMG_1747.jpg

I elected not to go with the Schwien electric T&B because I've heard they are not very reliable and because I already have two vacuum-driven T&Bs. Don't tell the Convention judges but I had the instrument overhauler put the "Schwien" logo on the front of a Bendix T&B. But you have a good point that I'm taking an additional risk by having all my eggs in one basket when not having the gyro instruments split between being electrically and vacuum-driven. The previous owner tried to minimize this risk by placing one venturi on each side of the boot cowl and then plumbing them to a check valve manifold like you see on twin engine aircraft.

manifold.jpg

I suppose that would help if a bird took out one venturi, but if there's ice I assume both venturi (venturis?) go TU.

I replaced the 10 lb. horizontal card DG with a conventional Sigma Tek vertical card DG primarily so I could fit eight 3-1/4" instruments in the panel. A secondary reason was because the vertical card is so much easier to use. Flying with the Sperry F3 will be like reading an upside down book. Flying with the F3 and a horizontal card DG would be like reading an upside down book in the mirror.

Regarding the suction gauge vacuum source you've got a good point. I've studied a lot of Service Manuals, IPC's, and Cessna POH's. The plumbing runs the gamut. For the '52 170B the IPC shows it taps into the DG, on the later years the source is the artificial horizon.

C-170B IPC, Fig. 77.png

The 1976 C-172 and the 1978 C-152 POHs show this diagram:

C-172 Vacuum.jpg

I've thought about using the 1/8" NPT port in the suction relief valve for a suction gauge source. That would give me an overall indication of vacuum rather than just one instrument. My drawing currently shows that port being used for a low vacuum warning light switch. Some Cessnas have a selector valve where the pilot can check the suction at each gyro -- too much plumbing for me. One idea would be for the low vacuum switch to monitor the D.G. and the suction gauge monitor the A.H. or vice versa.

The Sperry F3 has a filter inside the back cylinder. I suppose this filter should be removed if I've added a central filter.

[GAHorn]

Gary, I take it that your suction-gauge has dual indicators? If so, that guage is likely intended for a muilti-engined airplane rather thana single.

The 170B diagram …(I know You know)…does not supply the gryos with separate inputs as you are planning. The vacuum in one gyro-case is the source for the other…. a simple method….but with a unique hazard in that an internal failure of one gyro will subject the other to debris…. or loss of vacuum should either gyro-case lose a gasket, or glass-seal, etc. (BTW, you can simplify your design with a “tee” at one gyro’s input to which the other is connected. This will dispense with one filter-to-gyro source-line.)

As for my earlier comment re: separate power sources (elect & air) for attitude gyros (T&B/VG)…. A Venturi-System is SO much more reliable source than any pump…. IMO your design removes the issue. After-all…… “if it’s flying….it’s sucking”.

There is an inordinate concern by some people about venturiis in Icing-Conditions.
Anyone operating a 170 in icing-conditions has a much greater problem than vacuum-source considerations.!

(Reminder to All: The 170 is not approved for flight in icing conditions. The venturiis will continue to operate normally long after the wing, flight controls, and especially the empennage are a greater concern.

( The placement of venturiis aft of the engine-cowl exit on the belly is probably done with icing-concerns….and that gives me Darwinian-imaginings… the contemplation of any idiot who is planning to fly this thing in ice. They should spend their time and money contemplating risk-reduction of much more-likely scenarios….like a Cockpit-Fire!
Any pilot who’s ever experienced serious icing conditions in their career will never drag a 170 through it. Treat Ice…. like you would treat impending engine or airframe failure! (After-all….. you have a lot better prediction-tools.).

Here’s a system-design which comes directly from an instrument-system school Textbook I have from the 1940’s: Hope it helps:

IMG_4713.jpeg