The International Cessna^® 170 Association

History

My 170A had two 4" venturis mounted on the right side of the fuselage in front of the door and as best I can tell as Cessna illustrates it in the IPC. The venturis where however not plumbed as the IPC indicates, that being both tubes tied together, then through a Peco regulator, then to the AH with a T to the DG and the vacuum gauge of the DG. Instead my installation had each venturi feeding each gyro separately, The AN DG was feed direct from one the pictorial modern AH was feed from the other through a Peco regulator. Vacuum was measured at the regulator.

On my old system I never saw more than 4.25" of mercury on the gauge and this was in a dive. I also now know the Peco regulator was fully closed meaning it never regulated the vacuum. This regulator, and perhaps all of them regulate by having the vacuum pull open a waste gate held closed by spring pressure. The spring pressure us adjusted so the gate opens at a set vacuum. But I digress. Most of the time the vacuum was 4" or below if under 100 mph. I know understand I have know idea what vacuum the AN DG was seeing and can assume the AH was seeing less than what the vacuum gage read. Still the AH and DG seemed to work OK at 100 mph or above.

Gyros and Venturi

With out getting into the complicated world of moving (sucking) air which I only have cursory knowledge, AN gyros are designed to run on 4" (± a little) and a given volume. A 4" venturi is designed to pull 4" of vacuum at 100 mph and more volume than one gyro requires but not twice the volume. In reality a 4" venturi pulls a little more than 4" as I experienced. So in simple terms having a 4" venturi for each AN gyro should work fine so long as the plumbing is not overly restrictive. So it stands to reason 2 -AN gyros, a AH and a DG would require 2 - 4" venturis and that was the Cessna solution.

Enter the modern pictorial gyro which is designed to work a vacuum of 5" ±.5" of mercury. You can see that a 4" venturi can not pull the minimum required vacuum in inches of mercury for this gyro to work correctly but it will work, specially when the gyro is in good shape. And it probably works OK because the volume of air is more important that the vacuum.

Yes it is not enough to have enough vacuum pressure, each gyro must have enough volume to work and I've been told by the good folks at Keystone Instruments, who should know, volume is more important. Two 4" venturis tied together does not double the vacuum pressure but it does double the volume potential. Notice I said potential because in the end if your plumbing is restrictive it could be the limiting factor.

What to do

For a long time I've wanted to improve the vacuum system on my first 170 with one venturi and now my second which has two. I often wondered if one Super Venturi or 9" venturi (some market this as an 8") would have enough volume for two gyros, as it is rated at 9" of vacuum it has plenty of vacuum. As I'm removing the paint from my aircraft what better time to buy new venturis so I don't have to remove the paint from the old. I found an article by Tony Bingelis that says it will supply enough vacuum for two gyros, he did not specify what type of gyro.

So I bought and installed just one 9" Super Venturi thinking I could always buy and add a second one. While I was at it I took the time to remove and replace all the plumbing in the aircraft. In doing so I eliminated all but one 90º degree fitting, the most restrictive, instead using polyethylene tubing for larger graceful bends. The plumbing runs from the venturi through the Peco regulator then straight to my modern AH gyro with a T to my AN DG. I did this with the thinking the AN gyro requires less vacuum and with the 90º T it would receive less than the AH. The vacuum gauge is plumbed from the back of the AH so it is reading the vacuum this gauge is seeing.

So far with the new setup on climb out at 80 mph the vacuum gauge reads 4.5", .25" more than the best reading I ever saw with my original setup of two 4" venturis. At 100 to 105 mph I have the vacuum regulated to 5". My plane cruises at about 105 to 110 mph so this is about right. In a dive of about 125 mph the vacuum climbs to nearly 6". I have tried to regulate this down but then I see a loss of vacuum performance at the lower end. This doesn't really make sense and I'm thinking my regulator might be bad.
So far both gyros seem to be happy and work just fine. I'm holding off buying a second 9" gyro. Time will tell if I need it.

BTW the inches a venturi is designated as is not the tube length. The 9" Super Venturi is only about an inch longer than the 4" and the tube design looks nearly the same in shape except the 9" exit bell is bigger in diameter. It has the same mounting design and hole placement as the 4". Most folks wouldn't know the difference unless they where side by side and the trained observer couldn't tell from 30ft.

Bruce,
Had the same experience with the 2 4" venturi's, couldn't get enough vacuum to run AN style DG & AI. Replaced with one 9" and see 4.25" at 105 mph, 4" at 80 mph, works great!
Scott

Here's my take on this subject:

The in-Hg developed by the venturi, as seen by the gyro, will be the same regardless of volume. It is the INlet to the gyro which regulates volume. (If 4" is applied to the gyro....and the INlet is increased in size (this would require a change of nozzle within the gyro, not something we ordinarily can accomplish.....but follow along for the theory)...if the INlet is increased in size so as to allow additional volume....then Either the vacuum will be reduced to some point Less than 4" of Hg....OR...the venturi will be able to handle the increased inlet volume and still produce a full 4" of Hg.
IN other words....if you've still got 4"of suction as seen by a gage attached to that gyro...then the volume requirements of the gyros will still be met (since 4" equals 4" and the actual volume is controlled by the gyro inlet.) Another way to think of it is this:.... 5 cubic yards of air at 4" of Hg....or 10 cubic yards of volume at 4" of Hg....STILL equals 4" of Hg. If the increased size of the inlet allowed so much air into the system that it affected venturi performance....then the suction gauge would show a Lower Suction.

If you want to run two gyros on one venturi...then the best solution is NOT to use a "T" between the gyros*.... the best solution would be to run the gyros in Series....with only ONE Inlet supplying air (coming from your gyro filter) to the AN DG.....(which means the entire DG case has 4" of Hg vacuum within it's body/case....and then the DG Exit goes direct to the Inlet of the AH (not to the venturi via a "T")...then the plumbing goes to the regulator then to the venturi. The suction gauge should be mounted on an extra fitting on the AH case, thereby measuring the available vacuum inside the AH case.

*A "T" between the gyros offers a couple of opportunities for failure. 1- Additional pnuematic fittings/nuts introduce more opportunities for leaks, 2- such leak will reduce available suction to the DG. However, a direct line from DG supplying air to the AH...if it develops a leak...still allows full vacuum to be developed within the AH (the most important gyro of the pair.)

gahorn wrote:The in-Hg developed by the venturi, as seen by the gyro, will be the same regardless of volume.

Yes but if the volume inadequate the gyro won't work. For example lets say you used 1/32 tube to plumb your system. The gauge would still read 4" but the volume of air that could move through that tube wouldn't run the gyro.

gahorn wrote:If you want to run two gyros on one venturi...then the best solution is NOT to use a "T" between the gyros*.... the best solution would be to run the gyros in Series...

This was a system I considered. The problem I see is the INlet volume is limited to that volume that can pass through the most restrictive gyro. And if the INlet of the first gyro gets blocked, both gyros fail. It is true the system could probably be done with one less 90. The way I did it there are two lines from the filter, one for each gryo. And any resistance through either gyro doesn't effect the other. The weak link as you already pointed out George is you need to T the vacuum and in essence have limited the volume to that that can pass through the single line to the venturi.

Its probably the like 6 to one half dozen and neither system is better than the other.

Vacuum pump!

Vacuum pumps suck.


Anyone care to despite that?

If the properly sized plumbing is utilized then no additional restriction to flow should occur. The filter is intended to prevent any blockage of the inlet to the DG (or any other gyro to which it is directly connected), and you'll notice that the method I suggested is more like the method Cessna designed, the only difference in main plumbing being the attempt to use one super venturi in lieu of two 4" venturiis. Of course, in the proposed system there are other differences such as the use of 3-1/8" pictorial gyro horizon, and the fact that the original AN gyros used individual filtering.

In the thread about my 2nd gyro horizon project (http://www.cessna170.org/forums/viewtop ... on+project) I posted a diagram of the suggested method according to my old-instrument overhaul manual, which more closely resembles what you describe, Bruce.



It may be an unimportant matter, especialy if as many inefficiencies are removed as possible. I like your use of poly tubing.