Vacuum Relief Valve Adjustment

[DaveF]

I'm not surprised the leaf blower didn't work because in flight the air velocity through the inner venturi is faster than airplane flies. The trailing flange on the outer tube causes extra suction to pull air through the inner venturi. Read more about venturis in this excellent 1937 NACA research paper. They really studied this stuff back in the day. I was most interested to see that venturi tube pitch and yaw have almost no effect on suction performance.

Your connection sounds right to me. At least, there's not much more you can do. You're going to lose a few tenths of an inch of suction no matter what you do, given air friction in the hoses and elbows and tees. Use the largest hose you can and the fewest elbows. Yes, you could replace hard 90s with gradual curved fittings, but those things are ridiculously expensive. My system drops about 0.2" of suction between the regulator and the AI at max suction.

AC43.13-2B, chapter 11 says AI plus DG draw about 3.7 cfm of air, that one foot of 3/8" ID hose carrying 3.7cfm will drop about 0.1" Hg, and that each 90 ell is equivalent to about 6" of 3/8" ID hose. Oddly, AC43.13 calculates pressure drops for hoses in terms of their OD and wall thickness, not hose ID.

[NateRydman]

I've been enjoying your adventures with the Venturi's. I have my own question; my current set up is a 9" venturi, a relatively new Heading Indicator and a Turn & Bank. There was no vacuum relief valve in my system as my parts catalog calls for. The part # is 0513120, (according to my parts catalog for a 170A). I'm coming up empty on locating one and am interested in a modern regulator replacement. Does anyone have an up to date part number (or would this be an STC?) for such a thing?

[blueldr]

My '52 model had two 4 inch venturis and seemed to do just fine. I never did think that the C-170 made a good airplane for flying "Hard" IFR. It just didn't seem to be enough airplane. That was, perhaps, a result of having flown mostly large airplanes such a transports and bombers that were well equipped for hard IFR.
A descent thriugh a lower layer with a reasonable ceiling underneath was an OK deal, or a departure through an upper layer that had known VFR above was also OK if there was enough VFR after take off to be sure the gyros were up to speed. I never had enough confidence of gyro speed provided by the take off roll. My airplane was not equipped for precision approaches.

[Bruce Fenstermacher]

Nate, we missed your earlier question. You should probably have a relief valve or regulator as likely your system is pulling more that 5.5" of mercury which is more than your gyros are rated for. Do you have a guage to tell you how much vacuum is being created? If you do and it does note exceed 5.5" then you don't need a relief otherwise you do.

Most new regulators are an arrangement meant to be used with a pump on the engine and so they have a fire wall mount and a larger attachment fitting. You can't use one of these. Most others have a 5/8" hose nipple which is still to big so you have to adapt it down.

Here is a selection: https://www.aircraftspruce.com/catalog/ ... o2h312.php

As you can see they get spendy real quick. I looked around at fly marts and found mine for $5

[DaveF]

There are lots of vacuum regulators for sale on that popular auction site. I've bought a couple for around $50 each. They're mostly the same design and differ only in the size of input and output hose sizes, for example, 5/8" in, one or two 3/8" out, straight, angle, etc. The dash number calls out the different models. Airborne and Rapco have info on the web.

[DaveF]

In addition to this excellent 1937 NACA research paper, here's another good paper on venturis and other suction sources. Its by Charles Stark Draper, founder of the MIT Instrumentation Laboratory, now known as the Draper Labs. http://cybra.p.lodz.pl/Content/6351/AER_56_6.pdf. Interesting result from this paper is that venturis are inefficient -- two venturis consume about 1.5hp at 100 mph IAS.

[DaveF]

I'm not getting rid of my venturis, I'm just interested in the research. I spent enough years preparing for vacuum pump failures and am glad to be done with that. I'd install an electronic instrument before I went back to a vacuum pump.

[NateRydman]

Clearly I don't check back on my own posts too often, as you can tell this hasn't been my highest priority as more than a year has gone by...


To go back to your question Bruce, if I fly fast downhill I can pull 6+ inches of vacuum. Level flight is almost 6 at 10,000 MSL (which is required to go anywhere here in Utah) and higher vacuum when I'm at lower altitude.

So if there is a modern replacement to the vacuum relief valve I'd like to know what to get. If a functioning old one can be had that would be fine too. Without knowing what I'm looking for, I'm just guessing at this point and for the $$$$ at the Spruce link you put in above, I'd assume not have to experiment.

[Bruce Fenstermacher]

Nate, I wouldn't be to worried about at 6". Apparently your not.

I would just keep my eye open for a used relief valve that has fittings at each end you think you can fit into your system. The one I found I paid $5 for. Most people have no idea what they cost new and figure it's not worth much.

[mongo2]

Nate, just a question on how you have it plumbed.. I have (2) 9" venturi's with no regulator, and I can barely make 3-4". I had a regulator but recently removed it to see what results I'd get. It improved , but slightly..
What size line do you have to your T or manifold? And what type of filter setup are you using...? I'm thinking I may have a leak
somewhere but everything is new and tight, hoses, fittings clamps etc.... No tape or sealer on the fittings at the instruments though, as well as the gauge.

[Bruce Fenstermacher]

Actually Nate, that's a got question Mongo asks. I also have a single 9". At 1500ft MSL I get 5.5" and my modern pictorial horizon and AN directional gyro work fine. However at 5000 ft MSL even with 5.5", there isn't enough volume and the horizon will stop working properly. I've had in it mind for several years to add a second 9" venturi to double the volume.

[Bruce Fenstermacher]

Actually Nate, that's a got question Mongo asks. I also have a single 9". At 1500ft MSL I get 5.5" and my modern pictorial horizon and AN directional gyro work fine. However at 5000 ft MSL even with 5.5", there isn't enough volume and the horizon will stop working properly. I've had in it mind for several years to add a second 9" venturi to double the volume.

[GAHorn]

The manner in which the system is interconnected plays a part in this issue.
Here's the method recommended by my instrument references book:

gyro%20project.jpg (26.97 KiB) Viewed 31701 times

However, finding a relief valve with three inlets .... or if using an existing valve with only one inlet..may be problematical.

A simpler method, IMO, is to use a manifold and individually connect each gyro and the relief valve directly to that manifold. This is what I did in when replacing 4" venturiis with 9" units and adding a second horizon (see this discussion for pics): http://www.cessna170.org/forums/viewtop ... +venturiis

[ghostflyer]

My vacuum system has the same figures that Bruce got but my Bat and ball instrument only runs on [needs] 1.5 ins of vacuum. So it has its own small 4 inch Venturi. I haven't put in a relief valve yet as everything works well. Venturi,s do not wear out or shalf,s shear , and are also not affected if the prop is turned backwards etc. we removed the venturies off a early cessna 172 recently and fitted a Garmin G5. The difference in speed per RPM and fuel flow ,"NONE".