3 1/8" Gyro instruments and a venturi?

[PilotDAR]

A maintainer has asked for my help in confirming the correct configuration of a certified plane. This plane is being restored after a number of years inactive. It is presently equipped with 3 1/8" sized gyro instruments, and a venturi instead of a vacuum pump. (The engine cannot be equipped with a vacuum pump). In the documented configuration of this type that I have, if there are gyros, they are electric. There is no parts catalog for this aircraft, and the original drawings I have, which are not a complete set, do not include any information on a vacuum system of any type, thought that does not mean that drawings might not have existed, as I know that my set is incomplete. There is zero manufacturer's support for this aircraft type any more.

My old tribal knowledge said that 3 1/8" gyros required a vacuum pump rather than a venturi, as their gyroscopes were a smaller diameter, and needed the higher vacuum produced by the pump. Venturis were only compatible with the older and larger [gyro] "AN" instruments. I have reviewed the Cessna parts catalogs over the years, and I find no combination of 3 1/8" gyros, and a venturi in the same applicability.

Is there any knowledge out there on the possibility that 3 1/8" gyros are compatible with a venturi, rather than a vacuum pump? If there is, I would try to follow it to substantiating data, and work toward assuring the existence of approved data for this installation, otherwise, the system probably has to be removed.

[Posthumane]

I can't comment on the specific gyro's you're using, but my plane has a venturi* and produces about as much vacuum in cruise as a similar vacuum pump equipped aircraft. I typically see over 4.5" in cruise. Vacuum on climbout is lower, typically 3.5-4" due to the lower speed, but it's still enough to run a modern attitude indicator and heading indicator.

*technically my plane has two venturi tubes, but it was discovered at my first annual that only one of them is actually connected.

[CID]

This takes me WAY back. Back when people started putting newer smaller air driven instruments in their airplanes, they didn't always pay attention to the capacity of the venturi system. It's not necessarily because smaller gyros need more pressure, its just because most of them were designed for the higher pressure of vacuum pumps and could be installed without the little regulators that you had to put on some of the older instruments when you connect them to higher pressure.

So it's really a matter of matching the instrument to the supply. You wouldn't install a 14 volt electric gyro in a 28 volt aircraft. At least not without some sort of a adapter.

Even if the venturi provides good pressure in cruise it's not a great idea to wait until you're off the ground to have your gyros spun up and stable. Way back when, there was talk of limiting aircraft with venturi vacuum to VFR but a lot of that angst went away as it became less of a problem and the regulator turned their focus away from the class of aircraft that typically use venturi vacuum.

[Posthumane]

I should mention that my aircraft is vfr only. I would never consider Venturi's on an IFR aircraft for exactly the reason you mention CID. Although my gyros appear stable on climbout and during continuous circuits, they do take some time to settle down on the first departure. I can help settle then faster by sitting on the runway for 10-20 sec at full power since my Venturis are in the propeller slipstream, but I don't think it gets then up to full speed.

[SuperchargedRS]

The Stinson I had for years had a standard issue vacuum DG and AI running off a medium sized Venturi, worked just great.

[jamesel]

It is my understanding that the "size" of the various venturi tubes is the amount of vacuum produced instead of the physical size. Aircraft Spruce advertises 2, 4, & 9 inch venturi tubes. I believe the 2 inch will run a T & B, a 4 inch will run a A/H or DG, and a 9 inch will run both a DG & a A/H.

In practice, I have flown Champs with a 2 inch running a T & B, Cessna 170s/172s with 3 venturi running T & B, A/H, & DG, or 2 venturi tubes running the DG & A/H, as well as Stinsons running a DG & A/H with a single large venturi tube. All worked well once the aircraft was up to speed, usually they were the large A.N. type. I have flown a mix of the old and new (mostly the small A/H with the Hockey Puck DG), and some with both gyros being the 3 1/8" and there seemed to be adequate suction.

Unfortunately, I am unaware of any suitable paperwork/certification data. That said, I would think the 9" venturi would provide more than enough airflow for a single smaller gyro.

regards
jamesel
(edited to make a touch more sense)

[crazyaviator]

A superventuri aught to run a modern DG and A/H. I once installed a generic 12 v motor coupled to a vacuum pump and bolted it to the firewall. I used it only when needed and I believe it took between 15 to 20 amps to drive.