IFR Capable 170's

[Pdogace]

Thanks, I am really liking the HSI, those boogers are expensive to buy these days. I like the layout a lot. It's good to see what other guys have done.

[hilltop170]

I finally was able to get the picture to upload, it has taken several tries because of the size limitation.

I would like to get a new plastic overlay someday with the current configuration instead of stick-on labels but I still like the original one too.

An Aspen will fit nicely in the center two instrument holes without having to modify the T-yoke. It would give you an HSI and lots of other goodies.

N1715D Instrument Panel

[nippaero]

Question? How much vacuum suction does a venturi provide? When you have dual venturi's, are they connected together or dual lines to a "T" somewhere? Are dual Venturi's required for IFR because of redundancy or do you just have two because of the vacuum required by multiple instruments?

[Bruce Fenstermacher]

A 4" venturi, the stock model found on 170s, will pull 4' of mercury at it's design speed. They actually pull a little bit more but I've not seen much past 4.25". These are made to an AN standard so if you dig around the internet, someplace close to the end, you will find the government specifications for it. Look for Venturi Tube AN 5807-1 Type B-4.

The fine art of vacuum and venturi systems is somewhat of a black art. Most know little about them and how to set them up best.

The venturi system in our 170s was a option. Many where not installed at Cessna but at the dealer after. So you will find venturi installation runs the gamut. Cessna did have installation drawings and we do have copies of them. But like most things worth will to see, you got to know what your looking for and then it's hard to find in the details of the various SBs and SKs. Most people just look at the IPC to get an idea.

To answer your question short and sweet, the dual venturi installation is what you got when you ordered an IFR package from Cessna.

A dual venturi system still will only pull about 4 to 4.25" of mercury. They are T'd together. But I theorize there is twice the volume.

Our forum collective experience is varied. The reason being is modern gyro's require 5" of mercury to run properly and many 170s have two modern gyros or a mix of old and new. Old AN gryos run on 4" of mercury. Then you must consider the condition of each gyro being driven.

Some folks have had no issue with the gyro(s) they had with 1 - 4" venturi. Most will find it lacking in someway however. Many have found the dual 4" venturi system completely adequate even when driving new gyros. I have found that a single 8-9" venturi also called a super venturi, seems to be adequate for my mixed bag of gyros so long as I'm under 5000ft. The same system higher and on occasion the modern horizon just tumbles like someone cut the suction line. Most that install the 8-9" venturi(s), install 2 of them. I was being cheap. I intend at some point to install a second. I've not heard a complaint from a person with dual 8-9" venturi system. One caveat. A 8-9" venturi system requires a regulator. A vacuum relief really. The Cessna dual 4" system also calls for a regulator but it is not really needed specially if you have 2 modern gryros.

[gfeher]

There are a LOT of interesting threads on this forum about the venturi's, including how to plumb them. Here's a recent one that has a link to an early NACA study of them: http://cessna170.org/forums/viewtopic.p ... sa#p120337

[DaveF]

Here's Cessna's description of a dual venturi installation.
http://cessna170.org/forums/viewtopic.p ... 053#p78987

It calls for both venturis to pull on a common manifold, which is regulated by a single regulator. From there you run one hose to each instrument. That's basically how most people do it. Depending on the realities of mounting everything, you could tee the venturi outputs together into a single hose that runs up to the instruments, and then tee to each. But be careful about pressure drops in the hoses. Use the largest hose diameter and the fewest elbows and tees. Like all plumbing jobs, it's conceptually simple but bogs down quickly in hose and fitting sizes.

George Horn wrote up a nice description of his installation, but I can't seem to locate the thread right now.

[gfeher]

I think this might be the thread you are referring to Dave: http://cessna170.org/forums/viewtopic.p ... orn#p88042

Or this one: http://cessna170.org/forums/viewtopic.p ... orn#p70452

(As I said, there are a lot of them.)

[Pdogace]

Nice panel Richard, thanks for posting.

[Avee8or]

I agree with the thrust of the comments in this thread--a 170 is fine for "light" IFR is properly equipped, and it is far better to legally punch through a layer to VFR than to scud run.

My 170 has a vacuum pump so not worried about ventures. HOWEVER--my long standing radio shop refused to do an IFR cert on the airplane as it does not have an alternate static port. (They installed the radios and ADSB in/out and then made this determination). How many of you have faced this issue? I can think of numerous 172's and other light aircraft that I have had IFR certified without an alternate static source. While probably wise to have, given the little use I would make of the IFR capability, and the unlikelihood that I would ever actually use the alternate source, I am resisting spending the $$ and looking for a new shop!

(My other ride is FIKI and I have never needed to use the alternate source--now that I have said that, I'll need it next winter!)

[DaveF]

No IFR cert because no alternate static source? That is ridiculous and not in conformance with the regs. He's not approving your airplane for flight in IMC, he's certifying that your static system, altimeter, and Mode C altitude reporting work, which is just one required element for IFR flight.

My transponder/static cert guy once told me that my airplane could not be flown IFR because it didn't have heated pitot. I had to pull out the FARs to prove him wrong. But alternate static is particularly ridiculous because you'd have to be in some pretty heavy ice to need it.

[rnealon1]

Reviving a thread, I am pondering the venturi/vacuum system in my aircraft. I have dual venturis, one on each side of the fuselage. They run the AH and DG; turn coordinator is electric. There are 2 suction gauges mounted one above the other. I have not found any documentation of the installation, but did locate the installation instructions on the forum.

Both instruments are functional by the time I look at them after takeoff; the AH is erect by the time I lift off. In flight both gauges indicate about 5.2" hg.

I was curious if the 2 gauges correspond to the 2 venturis, and if so which is which. I experimented by reaching out the window to block the back of the left venturi while the engine was run up, but the suction decreased on both gauges.

Also curious if losing suction from one venturi would cause the other to take up the slack, but have not conducted an in flight experiment on this yet.

Any thoughts on this are welcome.

Bob

[Bruce Fenstermacher]

Bob, I was going to suggest you do what you did. Block one off and see what happens. It seems by having two guages that someone installed your system as two completely separate systems. But if one being blocked affects the other, this can not be the case. You will have to look under the panel and follow the hoses and see how it is all connected. If they are tied together, you don't need two gauges.

[n2582d]

Bob,
In the pile of junk I got with my project was an Airborne check valve manifold, p/n 1H5-4.

According to Parker these are to be inspected annually on units over five years old and replaced after 10 years. I wouldn’t be surprised if you find one of these tucked away behind your panel. If so, the check valve(s) are not working if you continue to show a vacuum when the corresponding venturi is blocked.

Click to Enlarge

If approval wasn’t an issue, I’d try routing one side to the engine manifold, and the other side to your venturis. That would permit gyros to be spun up as you taxi out. A problem with this idea might be that tapping into the intake manifold would cause those three cylinders to run leaner, similar to having a vacuum leak there. A moot point as there is no approval for such a modification.

[rnealon1]

Thanks for the replies; I will crawl up in there and see what I can find. Will also try blocking off one venturi for a test flight.

Bob

[cessna170bdriver]

If approval wasn’t an issue, I’d try routing one side to the engine manifold, and the other side to your venturis. That would permit gyros to be spun up as you taxi out. A problem with this idea might be that tapping into the intake manifold would cause those three cylinders to run leaner, similar to having a vacuum leak there. A moot point as there is no approval for such a modification.

Actually I have an STCd system to do just that, the SVS system from SafeFlight. It has a manifold with check valves functionally similar to the one you picture except there is only one inlet from the instruments. One outlet goes to my dual venturis, and the other to the base of the 1-3-5 intake manifold through a manual shutoff valve. I had the manifold modified with a 3/8” NPT pipe boss, as the existing 1/8 primer boss is way too small for the required flow. The system is intended as a backup vacuum source, so there is also a required vacuum switch to drive a light warning of loss of the primary source.

The boss I had installed on the right intake manifold replaces the primer boss near where the manifold attaches to the sump, so it doesn’t seem to inordinately affect the cylinders on one side over the other. I do get about a 100 rpm rise at idle due to the additional airflow with the system on, but the engine maintains its normal smoothness. I leave the system on most of the time so I have he gyros powered on the ground. When the throttle opens on takeoff, the manifold vacuum goes away, but the venturis quickly take over, so im never really without vacuum. About the only time I shut it off is sometimes when I’m a little high on final to get the engine down to a true idle.