The Cessna 120-140 Association Forum

Guys:
I'm working on a plane with a 9" venturi powering a DG and Artificial Horizon. They're plumbed as in the top diagram. Suction gauge shows 6" +/-. The DG precesses badly and 2 different AH's are "inop". I'm thinking the lower diagram would be better. Any expert opinions?

Mine is not the answer you are looking for but ... my airplane has as its single attitude instrument a T&B indicator which works well off of one small venturi. Preferring silence to the noise of a gyro spinning when I shut down, I leave it disconnected.

You're a big help! (If it's disconnected, how do you know it works well? Or, do you mean it "meets expectations"?)

I know it works well because I connect it up in anticipation of my annual inspection because that's the way my AI wants it, and I disconnect it sometime after. I bought the airplane to enjoy VFR flying only and do my IFR flying in something else much less fun. Like I said, mine was not the answer you would be looking for. Just thought I'd inject a bit of humor into the thread. Sorry if it falls flat for you.

John

Suction instruments are normally plumbed in parallel with each instrument having its own unobstructed inlet line. Some have a gang filter some are individually filtered.
Your alternate diagram is correct.
You may not have enough suction as a 9 inch venturi for two large gyros is marginal. I doubt that you will even need the regulator. Try it and see.
Joe

I was able to run a DG and AH off of a super venturi in my old plane (Bellanca Cruisair) since you couldn't run a vacuum pump off of a Franklin engine (not enough "holes" in the accessory case). The instruments were hooked up in parallel with no regulator. However, since I did fly the plane IFR, I had the venturi ice up twice (I am a slow learner!) so I moved it to a position in back of the exhaust. This pretty much eliminated the icing problem, but it actually reduced the amount of suction I could pull. While it was enough to drive the instruments (barely) I finally just removed the whole vacuum system and installed electric gauges.
Larry

6863 wrote: ↑Fri Aug 17, 2018 12:13 pm Just thought I'd inject a bit of humor into the thread. Sorry if it falls flat for you.

No, I got the joke. Guess my joking response is what fell flat...

8175 wrote: ↑Fri Aug 17, 2018 3:22 pmThe instruments were hooked up in parallel with no regulator.

So, the second diagram, minus the regulator?

I seem to recall some diagrams of vacuum systems in old Aircraft Spruce catalogs, or maybe Univair, but I can't seem to find one in my archives. Does this strike a chord with anyone? Anyone have more old catalogs than I (I just found one for Alexander's...)

6641 wrote: ↑Fri Aug 17, 2018 12:45 pm Suction instruments are normally plumbed in parallel with each instrument having its own unobstructed inlet line. Some have a gang filter some are individually filtered.
Your alternate diagram is correct.
You may not have enough suction as a 9 inch venturi for two large gyros is marginal. I doubt that you will even need the regulator. Try it and see.

Just got some more info from the previous owner. The system worked in it's current (series) configuration until he had a "hard landing" at which point the horizon quit. I (actually he) acquired a used (unknown) horizon which I installed and it doesn't seem to work, either. At this point I'm tempted to have the original overhauled, I just don't want to do that then find it still doesn't work...

To add insult to injury, the panel space is very limited, and I have actually tried to install 3 different used unknown gyros. The third one fit, but...