IFR certified

[webmaster]

I have a single 8" venturi mounted on the passenger side. I power an AI and DG and am always able to pull 6" of vacuum (where I've set the regulator) in cruise. The instruments stabilize during the takeoff roll.

Dale

[lowNslow]

Wow I'll bet two 8" venturis will be way overkill. Do you have two standard venturis now? If so are the plumbed together as the should be or each to separate instruments?

I'd plumb both lines together and try one 8" with a 4" first and see how that works. The 8" should be enough by itself and the 4" just along for the ride. I'd just leave the 4" in place to plug the holes. What regulator are you going to use?

As for whether the venturi meets a AN standard or not does not decide whether the venturi is legal to install or not. But it might make the installation slightly less illegal. In other words one would have to determine whether changing the venturi it is a major or minor alteration and the venturi meeting a standard may be more acceptable than one that doesn't. Then at the end you need execute the appropriate paper work and once properly executed the installation becomes legal.

I don't have any venturis installed right now, but thought I'd use two 8" more for the volume of air moved then the extra suction, especially since I'll be using new style gyros. I'm have a Rapco 2H3-12 regulator which has two inputs (gyros) and one output (venturis). The 8" venturis are not much different size wise then the 4".
As far as the AN hardware, you are right, but since the Cessna drawings call for AN venturis I thought I'd use them just to LOOK more in compliance with the Cessna drawing even tho I'm using newer parts.

[lowNslow]

I have a single 8" venturi mounted on the passenger side. I power an AI and DG and am always able to pull 6" of vacuum (where I've set the regulator) in cruise. The instruments stabilize during the takeoff roll.

Dale

Dale, do you have the older AN style gyros or the new style?

[Bruce Fenstermacher]

As far as the AN hardware, you are right, but since the Cessna drawings call for AN venturis I thought I'd use them just to LOOK more in compliance with the Cessna drawing even tho I'm using newer parts.

Another possibility I hadn't thought about be cause we don't think of some items this way is Standard Parts. Standard Parts are those items made to a standard such as AN, MS and NAS to name a few. The stock Sperry Venturi is made to the AN5807-1 standard. I don't know it the 8" venturi sold by Wag Aero meets that standard, or can meet that standard, since pulls more vacuum. If it meets the AN5807-1 standard then it is a bolt on swap. If it meets another one of the standards then it is still a standard parts but not the same part Cessna used and someone would have to make a determination whether that change is major or minor. This in some ways could be compared to drilling out a worn bolt hole and used the next size up AN bolt.

The fact of the matter is that 4 out of 5 people don't know there are different venturis of the same physical size. And 4 out of 5 of those that do know there are different venturis could identify them on the ramp. This of course doesn't make them any more legal.

George, I can't recall if you have ever weighed in on the larger venturis and their status for installation on our 170s and if so what your opinion is.

What are the opinions of other mechanics here on the board?

[GAHorn]

Well, somewhere (I've made a quick look but didn't readily find it) I have a copy of the AN 5807-1 which was approved by the Army/Navy and later Air Forces, for the venturi. I obtained the document directly from the only manufacturer who makes the venturis these days and who claims it is their basis of production of their products (and they confirmed it is their product that Spruce, Univari, WagAero, etc. all sell.) I talked directly with the craftsman in California who spins the aluminum tubing into the venturi shape, and it is he who sent the document to me.
I didn't ask if the larger venturi had any basis of approval. He stated that it is simply a larger version of the AN 5807-1. When I find the document I'll post it here.

Addendum:

I don't know of any basis of approval for any venturi larger than this one, and the dwgs certainly are specific as to dimensions. Perhaps that's why Cessna specified two for the IFR installation.

Here is the specification:

[Bruce Fenstermacher]

Very interesting George. I may have to call and ask Wag Aero about it and see what they say. I need to do something about my system before I need to fly to a convention located at a high altitude.

[GAHorn]

According to WagAero the venturis are all "Code 5":

"Code 5; These items meet AN, MS, NAS or other Military Specifications or may be identified as Standard Parts as defined by the FAA. FAA definition of Standard Parts: A part manufactured in conformance with a specification established, published and maintained by a consensus standards organization, a government agency or a holder of an FAA Type Certificate (TC). The specification includes design manufacturing, and test and acceptance criteria, and identification requirements."

[GAHorn]

I"ve just talked to "Sandy" at WagAero, and she states that the 8" venturi does NOT meet AN 5807, but meets AN 5805 standard, and is approved by FAA. I have requested and have rec'd a copy of the letter of approval of all their venturis (2", 4", 8") as meeting the criteria as "Standard Parts", therefore they do not need PMA authorization. It is dated Dec. 16, 1997 and signed by Gregory J. Michalik, Senior Engineer, Chicago Aircraft Cert. Office., FAA.)

This should establish the legality of installing the 8" venturi upon airplanes,... in order to have an approved installation on a PARTICULAR airplane, the entire installation would have to meet FAR 43, of course. (Beware: the 8" venturi REQUIRES a regulator to avoid wearing out your gyros as it produces excessive vacuum. It is advertised as capable of producing sufficient vacuum to run pneumatic autopilots.)

[Bruce Fenstermacher]

George. If you look close at the AN5807-1 document you posted the critical dimensions of the bell of the venturi are missing. I wonder if a few strategically placed splashes of whiteout are how the manufacture is making a larger venturi they claim meets the AN standard?


Hmmmm. Sometimes I ask to many questions.

[GAHorn]

The document I posted is the one rec'd from ACS. The venturis sold by WagAero are mfr'd by Aero Fabricators, in accordance with AN5807/5805.
While I don't want to alter the document rec'd from ACS, I can supply the dimensions provided by WagAero if you need them.
The 8" (AN5805) exit is 3-3/4", and the 4" exit is 2-15/16". Otherwise the venturis outside dimensions are the same. Both inlets are 3.5".

[lowNslow]

I'd plumb both lines together and try one 8" with a 4" first and see how that works. The 8" should be enough by itself and the 4" just along for the ride. I'd just leave the 4" in place to plug the holes. What regulator are you going to use?

I was wondering what you were thinking with this arrangement. Are you running one gyro off of each venturi and two regulators or are you thinking of running them paralell to a single regulator? I would think would lose some suction by running these tied together.

[GAHorn]

Yeah, unless some sort of check valve were incorporated I suspect the 4" would backfeed. It'd be better to just plug the 4" if you intend to leave it "for show".

[Bruce Fenstermacher]

First George thanks for doing that leg work and getting to the bottom of this story for us. I'm not concerned what the measurements are. I just noted they where missing from the ACS document you posted. I'll be ordering a Wag Aero venturi for myself now that we have a handle on the legalities.

Karl this is my thinking and I certainly am no expert. First my system of a single 4" venturi works adequately at sea level operating two modern gyros. Adequately might be all it will do and there is no margin which is why it's not enough at altitude.

Adding a second 4" venturi will double the volume but won't appreciably raise the pressure. The added volume may be enough to make a difference and that is what Cessna did. But remember Cessna was running old AN gyros that didn't require the higher pressure.

OK Karl you have 2- 4" venturis. In my mind replacing one of them with the larger venturi will raise the pressure and according to others deliver enough volume for two new style gyros. I think the 4" venturi in parallel will do nothing to aid the 8" as long as the pressure of the 8" is higher as it should be. What I think will happen running the 4" and the 8" together is that the vacuum will be between 4" and 8" or 6".

I'm only suggesting you rather than remove it and have to fill the mounting holes to just leave it there intact. Of course the new venturi won't exactly match the old on so it that sort of thing bothers you you might just want to get 2 new venturis or remove the one old and fill the holes.

As as you know and George has pointed out you will also need a regulator once the vacuum exceeds the limits of the gyros or about 6" as I recall.

[djbaker]

Wenetz, I think you'll be happy with Sandhill. This year I had my first annual there. They found 41 items that had been overlooked in prior annuals. When I got the plane back it was like flying a new plane. The price was very reasonable. I know they do a through job.

[wenetz]

Jim, that's really good to hear! I have been on the phone with Eric for months now and he's doing an excellent job preparing for this project. I now I made a good choice but obiously, not knowing anything about Sandhill, has made me doubt in the past. But as I said, after months of conversations with Eric he has proven to me a very good knowledge on the airplane and I am really excited and looking forward for the arrival of my 170 in North Hampton, which should happen anytime next week. Thanks again for your post, I really appreciate you sharing this information with me.

Take care,
Pedro.