The International Cessna 170 Association

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

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.

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.

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 ...

This may be helpful....

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

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?

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.

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.

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!

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.