The International Cessna^® 170 Association

As those of you who know me know I have a '56 model 172. I would like to do my instrument training in the plane. I have noticed that the artificial horizon doesn't behave quite right. The instrument is only two years old so I was figuring I wasn't getting enough suction from the venturi. I have two venturis, one for the AI and the other for the DG. I have having a suction gauge installed right now so I can see what the venturi is producing, my suspicion is that it will be low. Since there is no regulator I'm not sure how I can bring the suction up.

For those of you flying 170's, are any of you doing any kind of IFR flying? Is there something I can do to get more suction? From what I can tell, the venturies produce proper suction at 120mph. I only do 110 - 115mph.

Any suggestions?

Dave

Well, FWIW, I don't fly IFR, but I do have a single Super Venturi installed which has a regulator and drives my AI & DG. I usually pull 4.5 - 5.5 on the suction gage. Never been a problem.

Remember, for those of us "in the know", they are venturis. For the uninitiated, they are "deer horns". After all, deep inside all of us is a little bush pilot struggling to get out.

Thanks Doug, my A&P calls them torpedo tubes

I will look into the "Super Venturi", what kind of cruise speed do you see?

Dave

I have a vacuum pump on the 170 but had a C140 for 8 years with a venturi . I ran an AI and DG off it . Cruise was @ 100 and I got @ 5 inches on the suction gage.

I'm usually cruising between 115-125 mph

I'm showing about 5" on mine and I'm cruising 100mph.

Dave, are you sure that you don't have a regulator? It can be a small device with a sorta knurled screw for adjusting. Check out all your vacuum lines under the panel & the side panels. It could be that since there is no suction gauge, there was never a regulator. But check out all the lines first.

Eric

I fly my 170 IFR when the occasion calls for it, and the conditions are suitable, i.e. no heavy convective weather or icing forecast. I have the super nine inch venturi with a regulator, pulls about 5.0 hg at 120 mph 5,000 and below. The venturi system is reliable and safe for IFR, properly set up.

What kind of equipment does your airplane have? Russ Farris

Dave, just another thought before you tear everything apart. Have you checked all the filters. There should be system filter, and some gyro instruments have there own filter as well.

Dave as Karl said check the filters but don't be surprised if you don't have any. Older aircraft such as ours didn't have them specially with the big AN type gyros.

The most you may find is a fine metal mesh screen in the inlet of the instrument. Remember back then there was a large stock pile of surplus AN gyros available cheap so not much thought was put into making them last longer. You just replaced the gyro with a new one when it went bad.

When my attitude indicator started acting funny I removed it and found dirt and dust acumulated in the inlet port. After digging out the junk it went on to work fine again. Since I the statute of limitations has expire I have to say that my ventures did not stop working when i encountered freezing drizzle many years ago. However I must say that I do not gaurantee that yours won't nor do I recommend finding out.

Thanks for all the advice guys, I really appriciate it. To adress some of the questions:

1. My static system was rebuilt two years ago.
2. We are going to look at the filter next week.
3. I have a two year old Directional Gyro, bought it rebuilt from Kelly Instruments.
4. I have a two year old Artifical Horizon, bought it rebuilt from Kelly Instruments.
5. I have two 4" venturies, if I don'get get enough suction from these I wil look at a 9" ventury.

Also, Russ, does you 9" drive one of both of the instruments?

Dave

As to the comment about if the venturi were to ice up, you'd already have a lot of other problems related to ice: not necessarily. A venturi will ice up REALLY fast in almost any icing condition, much more quickly than a surface, for example. So, don't count on a venturi keeping your vacuum system functioning in any kind of icing. Its like a pitot static system in some ways: Doesn't take much ice to bring things to an apparent standstill.

Mike Vivion

Mike: I'm not so sure on the venturi icing before the airframe. In theory this might happen, but there are more variables than just the pressure and temp drop across the throat.

A few years ago, while solid IMC with the temp just above freezing, I picked up a fairly significant load of rime ice, which was unforecast. My escape route was the fact that warmer air was below, up to almost 45 degrees! The entire windshield iced over, to about 1/16 to 1/8 thick. ATC immediately cleared me down to a warmer altitude; the entire encounter lasted about ten minutes. At no time did I see any change on the suction gauge, and you'd better believe I was watching it closely!

Pre-war DC-2s and DC-3s, which flew many hours in horrendous icing had venturi driven gyros (actually half-flush venturies with the fuselage skin right behind the cockpit windows.) The C-47/C-53 was the first to introduce vacuum pumps, along with electric fuel boost pumps, since the original DC-3 had the hand wobble type, if you can believe it.

Ideally the venturi should be heated, but practically speaking I don't think it's that important. Too bad at this late date someone like NASA won't spend the time to determine what the icing characteristics of these instrument venturis are. Mike's opinion and mine are anecdotal.

Dave, my nine-inch venturi runs an RC Allen ADI and a Standard Precision full face DG easily, 5.0 HG at 120 mph. Russ Farris

Thanks Russ. The speed is something that I wonder about, you cruise at 120mph, I only do 110mph, must be that trianing wheel hanging off the nose Anyway, it will be interesting to see how much suction I pull when I get back into the air, hopefully this week.

Dave