artificial horizon question

[rupertjl]

This might be more of an overall vaccum question in general but my artifical horizon sometimes gyrates when flying. Has anyone ever seen this condition? It almost acts like it's tumbling, other times it's fine. My VSI, airspeed, and altitude indicator all seem to function properly at all times. Annual is coming up in July, I thought I'd at least remove the unit and check the lines for any problems or blockages. Just fishing for possible problems anyone has seen before. The obvious one is that I need the unit overhauled.

any help would be appreciated!

v/r,
Jud

[lowNslow]

The VSI, airspeed, and altimeter operate of the pitot/static system not the vacuum system. Your vacuum gauge should indicate 4.5 - 5.5 inches in cruise. If not there is a problem with the vacuum system, check your filter, regulator, connections, etc. If the vacuum system is indicating normally, then you are probably looking at an overhaul of the gyro.

[blueldr]

Be advised that the old style AN gyros need only four inches of vacuum for proper operation. The newer style attitude indicators and heading gyros should have five to five and a half inches. If you don't have a vacuum gage, that'sthe first thing to install.

[Bruce Fenstermacher]

Both Carl and BL are correct. But using a venturi specially a single venturi you may not see 4.5. I see 4.25 at best. I know that is low.

I've seen horizons tumble like you describe. After assuring the source of vacuum is sufficient you'll probably be getting it overhauled.

[GAHorn]

I have a gyro overhaul manual that specifies 3.5" Hg for AN gyros, and 4.5" for modern 3 1/8" pictoral gyros.

[Dward]

I don’t intend to hijack your thread Jud, but I have a similar question that I think one of these sages might be able to help with. I have a late model artificial horizon that seems to function properly but my DG rotates slowly clockwise. The vacuum gage is in the middle of the green. Is this DG in need of overhaul or could there be a restriction somewhere? I don’t have any history on this setup because they were installed in the panel but non functional when I bought 26D. Any advice would be appreciated. Thanks

[GAHorn]

Dave , it's most likely time for overhaul. Once the bearings begin to drag, they slow the rotor down below rated rpm and the gyro will precess excessively. Of course, "middle of the green" is subjective. Is it about 4.5"?

[Dward]

George, thanks for asking. I just had to go fly last evening to check my vacuum gauge . Vacuum seemed to stay between 5" and 5.25". But I was mistaken, the gyro rotates counter-clockwise. If I was in the southern hemisphere would it go the other way .

[GAHorn]

Is your gyro a modern, pictorial, 3-1/8" instrument? Or an older AN style?
(Too much vacuum will also create precession, especially on older gyros. On newer gyros it creates unnecessary wear.)

[Dward]

George, it is the newer style 3 1/8".

[Trng wheel (N2625U)]

I have a '63 172 with a venturi. A couple of years ago it was only sucking about 3 inches on the vacuum guage. The AI was fine but the DG was lagging quite a bit. The local A/P told me to check the filter & hoses. I replaced the filter with no change. When I checked the hoses I found a pinhole in the hose from the AI to the DG. In my bird the suction runs from the venturi - AI - DG- vacuum guage with hoses from AI & DG to the filter. Wrapped it up with tape and it works fine. I replaced the hose later. I found a screw that was too long very close to the hose and replaced it with a shorter screw. I assume that was where the leak began.

[33alfa]

Just a quick note on the vacuum issue. I keep hearing about how much vacuum it takes to run a gyro and the real important factor is being left out. Flow. It is possible to 5 inches of vacuum but to little flow to operate a gyro at the speed it needs to run properly. Most vacumm gyro's need a certain amount of flow as specified in the maint manual for operation. Anyway a flow check is what i would have done. It will tell you if you have a restriction in your system. Ie a kinked hose. Good luck. Aircraft Instrument Repair. Any Instrument Questions Give us a call. Advice is still free here.907-332-8366 Or email to airinstrepair@acsalaska.net. Roger or Pat

[Bruce Fenstermacher]

So Roger or Pat, what you are saying is something I've always wondered. Some 170s have 2 venturis doubling the volume but not the pressure. Do you think running gyros on the lower pressure, 4.25 lets say, but twice the volume might work as well as less volume at higher pressure?

[cessna170bdriver]

The pressure vs flow is a function of the gyro. In other words, at any given pressure drop across the gyro, the gyro determines how much flow there's going to be. Raising the pressure drop (vacuum in our case) across the gyro causes flow to increase, but not necessarily in a linear manner. If you force more flow, the pressure drop across the gyro will rise. This is similar to the way an electrical device determines how much current it will draw.

Different gyros will have different characteristics. I don't have any of the flow specs for any gyro, but it's entirely concievable that an old AN gyro will require more flow at 3.5 inches than a "modern" vacuum gyro might require at 4.5.

Think of a venturi as a "battery" for a vacuum system. If one will not provide sufficient flow, adding another in parallel probably will.

Miles

[33alfa]

Bruce i would say No to your question. The newer style gyros do require more inches 4.5 to 5.5. I guess i should make it clear that what we see when these issues pop up is that when a line gets kinked or a restriction shows up the regulator gets adjusted to compensate the problem. Another thought is that i think there are two different venture sizes. I have two on my 170 one for the DG and one for the VG.