Vacuum System

[dkalwishky]

I own a '56 C172 with dual venturis. A month or so ago I started a thread about suction and my gyros not working properly.

I had a suction gauge installed and found that inlevel cruise flight I would pull 7". We decided to put a releif valve in the system so I could bleed out some of the excess pressure.

With the valve in place I am only getting 3" but my Gyro's are working better than they did before.

Here is a link to a graphic I did of the original system:
http://www.kalwishky.com/images/Origina ... System.jpg

Here is a link to the new system:
http://www.kalwishky.com/images/New%20V ... System.jpg

Any ideas would be appriciated on why I've lost so much suction. Do I need to put the releif valve near the filter instead of off the venturies?

Dave

[Bruce Fenstermacher]

Here is the key Dave. What size are the venturis? If you have stock venturis they where designed to develop 4" of vacuum. If you have stock venturis IMHO there is no way you where getting 7" from the one venturi you where measuring as you have it depicted.

With the stock venturis in parallel like your second drawing you still theoretically wouldn't get more that 4" of vacuum but you'd have twice the volume. Any restriction would make that less so you'd be seeing 3"
It's been reported here by some that with stock venturis plumbed like your second drawing will get more than 4" say 5"

Now if you have a super venturi sold by Aircraft Spruce and Wag Aero, they are design do develop nearly 10" of vacuum

If this is the case then You probably did get 7" with the first plumbing and either you are using the wrong type of regulator or it's not working. If your venturis are capable of developing more that 6" you should be able to regulate it down to the 5" or so you want.

[GAHorn]

Here is the key Dave. What size are the venturis? If you have stock venturis they where designed to develop 4" of vacuum. If you have stock venturis IMHO there is no way you where getting 7" from the one venturi you where measuring as you have it depicted.

With the stock venturis in parallel like your second drawing you still theoretically wouldn't get more that 4" of vacuum but you'd have twice the volume. Any restriction would make that less so you'd be seeing 3"
It's been reported here by some that with stock venturis plumbed like your second drawing will get more than 4" say 5"

Now if you have a super venturi sold by Aircraft Spruce and Wag Aero, they are design do develop nearly 10" of vacuum

If this is the case then You probably did get 7" with the first plumbing and either you are using the wrong type of regulator or it's not working. If your venturis are capable of developing more that 6" you should be able to regulate it down to the 5" or so you want.

According to the venturi blueprints:
The so-called "4-inch" venturi .. the original one for our 170's...is actually a AN5807-1 venturi. (AN5807-1 is an Air Force-Navy Aeronautical Standard for a double-throated venturi.) It's horn is 10" long from leading edge to trailing edge with an outside diameter of 3-3/16" and a trailing edge radius of 7/32". The inner tube has an O.D. of 1" and an I.D. of 5/8", with an overall length of 3". The drilled orifice supplying vacuum to the inner cone is 11/16". It is designed to produce 4" in Hg at 100 mph in a std atmosphere. It will produce more than that at denser atmosphere's or higher speeds (or if installed directly in the propeller slip-stream.) (Standard AN attitude/directional gyros only require 3.5" to meet their performance standard while Turn gyros only req'd 2".) It was declared an Inactive design 15 Feb 1950, and a Cancelled design after 4 Dec 1970. There is no superceding standard.
The problem many people have with them is they have replaced their AN gyros with modern 3-1/8" "pictoral" gyros....the type with pretty grid-lines and blue skies painted on them, or with swept-wing jets depicted on the glass faces of the compasses. Those more modern types are lighter weight rotor designs which require 4.5 in. Hg to operate reliably, so a venturi which does not produce a minimum of 4.5" continuously may allow the pictoral designs to slow down and become "lazy". The solution is to go to the so-called "super" ventruis in those cases, unless more expensive routes are chosed such as pumps and/or electric designs.
Personally, I like the good-ol' antique AN/venturi systems on rounded tailfeathers ...and all-glass Multi-Function Displays with laser-ring gyros on modern aircraft. None of that out-dated pictoral stuff for me!

Country tune:....(Older gyros,...
younger wimmen, ...
more w h i s k e y.........!)

[dkalwishky]

Here is the key Dave. What size are the venturis? If you have stock venturis they where designed to develop 4" of vacuum. If you have stock venturis IMHO there is no way you where getting 7" from the one venturi you where measuring as you have it depicted.

I revised my drawing as I made a few mistakes on it. I am going to measure the venturies tomorrow to see if they are the 4" or the 9" supers. Even if they are 4" and are working in series it might pull alot more vacuum than 4".

Today I went out again, we moved the value from it's present location to be near the filter, so now it's on the other side of the system. When I test flew she pulled over 11", the dial nearly spun one complete revolution of the dial! I had to completly remove the valve (in flight) to get the suction down to 7".

At this point we are going to remove the valve idea and just put in a vacuum regulator, Rapco sells one for $140....

Dave

[dkalwishky]

Thanks Geroge. I do have the newer style Gyros in the plane.

Is there any where I can obtain the "blueprint" of how the system should go together with what I have?

I have two newer gyros, two venturies and a suction gauge. The shop is going to order a Rapco Regulator for $140 and install that.

Dave

[GAHorn]

The gyros should not be run in series, but in parallel. The earlier airplanes ran the gyros in parallel, but the later airplanes had them in series.
The B-model illustratred parts catalog (IPC) has an excellent sytem schematic depicted on pages 132 thru 135. The serials above 25372 used the series system. Both systems relied upon each gyro having it's own, factory equipped filter. New systems should use a central gyro filter, in my opinion. In the series system, that filter should be placed at the far end of the system.

[Bruce Fenstermacher]

Dave please let us know how your getting that much vacuum. Perhaps you have 2 Super Venturis.

[dkalwishky]

Nope, two 10" venturies. Properly hooked up two 4" venturis should be able to deliver 8".

The regulator is in and will be installed in the plane in a few days. I'll report back then.


Dave

[blueldr]

If a 4" venturi is designed to pull 4" of vacuum at 100 MPH, I can't immagine how two 4" venturis could be hooked up to pull 8" of vacuum. It seems like it would be like two air compressors that each produced 100 PSI. How could they be connected to produce 200 PSI?

[dkalwishky]

If a 4" venturi is designed to pull 4" of vacuum at 100 MPH, I can't immagine how two 4" venturis could be hooked up to pull 8" of vacuum. It seems like it would be like two air compressors that each produced 100 PSI. How could they be connected to produce 200 PSI?

I have no idea. I can only tell you what the gauge tells me

Dave

[Bruce Fenstermacher]

I agree with BL. Don't think it can be done.

The 8" venturi sold by Wag Aero is 10" long according to their catolog. The Super Venturi sold by Spruce pulls 9" and is 11-1/4" long.

So you still could have two or at least one 8" venturi from Wag Aero.