Proper Replacement Attitude Indicator for Venturi Vacuum?

[Fearless Tower]

My AI may need to be overhauled or replaced - seems to have a different concept of level than what I see out the windscreen in flight.

I was speaking to a guy at a nearby avionics shop about replacing it and he told me that modern vacuum AI's won't work properly with the venturi vacuum. Can anyone confirm or refute this claim?

If it is correct, where can I find a suitable replacement AI?

Thanks,
Andrew

[busav8or]

Andrew, if you need a rebuild, these guys are one of the few I found that will rebuild the older "AN" type gyros. $300/instrument. I haven't used them yet, so I can't vouch for them, but most other places wouldn't touch them!

http://www.flyaqi.com/gyro.htm

Joe

[GAHorn]

Your instrument shop is misinformed.

Quality Aircraft Instruments in Wichita did my AN gyro for $245, three years ago.
AN gyros require 3.5 inches of vacuum....modern gyros require 4.5....but a healthy venturi
system of dual gyros will work just fine....and if you have"super" venturiis you'll need a regulator.
I have THREE gyros...two older ANs and one modern horizon....all powered by two super venturiis
and they are all up and running before 300AGL.

[Bruce Fenstermacher]

George has the correct info. Old gyros 3.5, new style 4.5 min. I ran tow modern gyros with one 4" venturi on my first 170 and if worked OK most of the time for a VFR airplane. Any time I wasn't near 100 mph there wasn't enough vacuum to make them run reliably for long so prolonged slow flight could cause them to precess. While I intended to, I never got around to installing a second 4" venturi (dual system) or replacing the one 4" with a super venturi.

I attributed my good fortune to having a decent gyro performance despite low vacuum to the fact that the gyros were practically new when I bought the plane. But they still worked as well 10 years later.

I always recommend Keystone Instruments, Lock Haven PA. If they can't fix it or won't fix it, no one will and last I talked to them they'll pretty much crack open anything.

[Fearless Tower]

All - thanks for the info.

Can someone 'splain the dual or super venturi systems that you guys are referring to? This is my first plane with a venturi system.

My plane has two external venturi tubes mounted on right side of the aircraft (I believe the original venturi was installed on the left, but I recall seeing a 337 that detailed the removal and reinstall on the right side).

Does the fact that there are two venturi tubes indicate that I have a dual system and if not, how can I tell?

Thanks,
Andrew

[3958v]

You have a dual system probably with standard venturis. I have a simlar system with a modern attitude gyro and it works fine. Before you do to much you might want to check the inlet filter or screen to make sure it is not clogged with lint or debris. I had that happen on one I remove after purchasing a new one. After cleaning the screen a friend installed it on his plane and it lasted several more years. Bill K

[Bruce Fenstermacher]

Andrew the venturi system was as option. Many planes were delivered without any but they were quickly added. So you are as likely to find them anywhere as your are to find them in the same place.

The '48 IPC doesn't show any installation at all. The 170A IPC shows a single venturi mounted on the right side of the cabin with no regulator. In the B model IPC a dual venturi is shown again both mounted on the right side with a regulator. I'd guess that some had one venturi added per the A model IPC. Then a second one added on the left side when more capacity was wanted. I've seen them high and I've seen the low including on the belly. I've seen the dual setup plumbed together then to each gyro and I've seen them plumbed one venturi per gyro.

The standard venturi is an AN5807-1. It is designed to pull 4" of vacuum though they tend to pull just a pinch more. Adding a second one in parallel doubles the volume but not the pressure though the pressure is likely to rise a bit more. The super venturi being marked by Spruce and Wag Aero are made to a different AN standard and pull more vacuum. Spruce markets theirs as pulling 8" and Wag as 9". The super venturi looks slightly different in the bell area but you would have to be a venturi aficionado like gahorn and I to notice. If you have either of the super venturi you will also need a regulator as anything over 5.5" is to much vacuum for any gyro system.

[GAHorn]

I have a few pics but not the complete project. (I'll need to take some and post them, but here's a few, at least.)

My panel, before my new modification. These two vacuum gyros ran off a dual venturii setup using standard venturiis, as per the IPC.
Notice the two blank instrument spots on the far left end of the panel. This is where I moved the T&B and installed a CDI, after I installed a new gyro in the T&B's location directly in front of the pilot.

I removed the two venturiis...and here's what the right side looked like with them removed.

Here's what the inside looked like when I mounted the two new "super" venturiis. Notice the manifold (newly added) which plumbs together the two previous gyros with the new one, (two of the PolyFlo tubing come from the old gyros,...the third PolyFlo tubing brings the new "modern" gyro into the system.) All three gyros get their vacuum from the manifold, which you can see is plumbed to the dual venturiis. The last (far-left) port of the manifold has a blue, aluminum "barb" to which will be installed a vacuum regulator to "spill" the excess vacuum generated by the "super" venturiis.

Here's a pic of the manifold after the regulator is attached to it. In the background can be seen two gyro filters. One was already existing and served the two AN/old gyros. The other serves the new "modern" gyro. The reason I did it that way was twofold: 1-the AN gyros filter had been recently serviced at last annual and, 2- the new gyro's warranty requires a fresh filter, so I didn't want to connect it to an existing filter and also I needed a receipt for a new filter coinciding with the installation date.

Here's a view of the modified, left-side area of my panel. Note the changes: 1- additional, new "modern" 3-1/8" gyro horizon placed where the T&B previously resided, ...2- T&B relocated to the left end of panel (actually a replacement T&B with a "Failure" flag provision,.... 3- a newly installed Course Deviation Indicator (just to fill an unused instrument hole in the panel, hoping to add some dead weight to slow the plane down so I didn't have to operate at reduced power, which gave me cold oil temps when in formation with green 170's.)

Sorry...bad lighitng and focus...

Why did I do this mod? Mainly because I frequently fly light-IFR and tired of looking over in front of my co-pilot to observe the Artificial Horizon, (gave me a "crick" in my neck and, due to facing half-right while travelling forward, a touch of spatial-disorientation when making turns and holding patterns), and secondly, I wanted another attitude gyro in case the other one "tumbled" or took the day off in the clouds. If I'd simply moved my existing horizon, I'd have had two problems: 1-the 3-5/8" AN gyro is larger than the 3-18" holes in front of the pilot's yoke making such a re-locaton difficult to make pretty, and I'd have an ugly, non-standard-sized hole to fill where it had vacated. (This second thought is what actually prompted me to realize that I had an opportunity to have two attitude gyros, providing excellent "back-up" capability.)
I considered an electric until I contemplated their Cost, and Reputation for unreliability... then it dong-ed on me that with venturiis.... I would not likely suffer from the usual reason different power-sources are desireable for attitude gyros... If I'm flying...the venturiis are sucking and no vacuum pump failure is possible.
The system works beautifully. By the time I'm retracting flaps on takeoff, all gyros are up-and-running... in fact, the modern gyro is faster to be "on-line" than the AN gyros, which need to be manually un-caged. The Sigma Tek attitude gyro has a "cage" knob, but it's not for parking the gyro. It only is a "fast erect".... of which I have prior experience.

Here's the page from the 170B IPC which illustrates the original factory setup.

Click to ENLARGE...Click a second time to further ENLARGE

Here's what my log entry says, Posting it so as to provide parts-list information for anyone wishing to obtain similar parts like the manifold, etc.etc.
"Relocated existing T&B indicator to left side of panel adjacent to Airspeed Indicator No change of elect. or wt/bal.
Installed secondary gyro horizon, Sigma-Tek Model 5000M-1, PN 23-501-035-5, SN T98479F, weight 1 lb, in instrument panel at sta. +15, using Parker PolyFlo tubing, and fittings, Replaced Cessna floating instrument shock mounts, PN 0411059-1, on new nylon bushings.
Replaced dual AD15-000 4-inch venturiis with dual AD16-000 8-inch venturiis (Standard Parts, no wt chg) and added New Airborne 2H3-12 Vacuum regulator and Rapco Central Filter assy PN1J7-1 using Aeroquip 306 hose and vacuum manifold PN 04-00206. 1 lb at sta +13.

All work accomplished in accordance with AC 43.13-1(b) . Test flown. Inst VACuum adjusted to 5 in. Hg. Ops ck OK. "

Addendum: The Sigma Tek gyro I ordered has a caging-feature, intended for those who perform aerobatics to provide them a simple/fast means fo erecting the gyro. Fearful of not having sufficient vacuum to bring the gyro into-service prior to entering clouds, I ordered that model. It was a waste of money ($300 extra) because the Sigma Tek gyro is erect and running before actual lift-off. I have never had to utilize the caging/erection feature, regardless of the gyro's displayed position at the beginning of the takeoff roll. If I had it to do again...I would not order that feature and save some bucks.
BTW, Sigma Tek has/had a promotional offer to sell you a new gyro for $500/exchange. I had an old junk gyro from the Baron I used to own, so I sent that to them for the core. The difference between the cageable and the non-cageable gyro is almost $300. ... therefore I advise against the model I used in this project as unnecessary expense. (buy the cheaper, non-cageable version.)
Hope this helps.

[Fearless Tower]

Thanks for the info.

I found the 337 last night and it sounds like the airplane was indeed delivered without a vacuum system. The 337 describes the initial installation of AI and DG plus two 4" Venturis and assorted piping. Nothing was removed. This was done in '63.

I think I'll have the avionics shop check the filter screen like Bill K suggested and see if that improves things. Otherwise I'll probably end up sending my old AI off for overhaul or exchange.

Interesting note on the variation in piping for the dual Venturis. I'll have to have the shop check to see how they plumbed mine - I don't think it was specified in the 337.

[Bruce Fenstermacher]

Many of the old AN gyros don't have or didn't have a filter as we are accustom today. Just an inlet screen. Don't be surprised it you don't find a filter.

[GAHorn]

The old AN type gyros had a filter .... just not the type of filter known today as a "central" filter. They have a flat filter-body on their aft sides which contains a paper "disc" filter. It consists of two metal screens seperated by a paper element made up of two sheets of brown, porous paper (about the same consistency as a paper coffee filter), and they were all held together by a staple. Yes, an ordinary office-stapler look-alike staple, driven thru the three elements. It was about 2-inches in diameter and 1/8th inch thick.
The body of the gyro filter was usually held together with screws, and a pipe-threaded hole for a fitting was also provided. Many of the gyros had no fitting installed.... they merely allowed cabin air to enter thru the threaded hole... pass thru the disc-filter... and then continue in to the gyro itself.
You can see the filter-body in this picture, posted in the TradeMart the other day.

AN Gyro Filter body.jpg (31.44 KiB) Viewed 17979 times

The blue, angled AN-fitting is threaded into the disc-filter body. Notice the four small machine-screws which hold the flat disc-filter body together. Removing those screws allows the old filter to be discarded...and a new installed... then the gyro is ready for further use.
Admittedly...it wasn't much of a filter...but it was more than only a screen.

[blueldr]

So Gahorn and his Sigma Tec gyro both have "Fast Errect" capability, At least the gyro is humble enough to keep from bragging about it.

[GAHorn]

I wasn't bragging... I was only mentioning that I have experience with that function.

(Which reminds me of the Beech 99 I once flew in commuter airline service. One of the birds had an air-driven gyro that suffered on the fast turn-arounds we had at some outlying airports, so someone had taken a pencil and written upon the instrument panel adjacent to the gyro a warning to other crews who might operate the airplane:
"Slow to erect on fast turn-arounds"

About a week later someone had written another message beneath:
"Who isn't?"

[busav8or]

Hi George,

Would a dirty filter on the AN type instruments cause a low vacuum reading? I don't see anywhere in my logs that they have ever been changed. Do you know of a source for new filters?

Thanks,
Joe

[GAHorn]

A dirty filter usually results in the opposite... a borderline-higher (or nearly-normal) vacuum reading... with sluggish gyroscope. Keep in mind that the suction source, either venturi or vac/pump, will continue to draw air from the gyro, which see's incoming air as the motive force to spin the rotor. A blocked/dirty filter will reduce that motive air. But the continued efforts of the source will be relieved by any regulator, so little, if any, change in vacuum readings result.

Here's a typical vacuum system to help illustrate the point. Keep in mind that in THIS illustration, each gyro has it's own filter at the back of it's case, where the air enters the gyro:

gyro vacuum schematic.jpg (26.97 KiB) Viewed 17955 times