Inadequate Vacuum

N8249A · Post by **N8249A** » Sun Mar 13, 2005 6:07 pm

I have dual venturis, One on each side. My vacuum indicates 3.8 to 4.0
routinely. My currrent instruments require more so my DG just precesses constantly. When the aircraft was redone they put in a filter and all new hoses. Would an additional venturi help? This is a c-145. Can I put a vacuum pump on it?. I'm only driving a DG and AH. Thanks, Marty

doakes · Post by **doakes** » Sun Mar 13, 2005 6:23 pm

Thoughts Marty,
Before you embark on more Venturi, I think that your system is not producing enough for both. Da!

What I mean is this one venturi should run one insturment. I have the same problem. I have one venturi, it runs the one gyro, but does not have enough vac to run my DG. So at this time I have not DG. I also do not want to put another Venturi either. My solution, I do not have one that is usable. I have a c-145 too and there is not a good way to put on a vac system.

I think that if you do some checking and run one venturi for one instrument and one for the other it will work. If your system is fixed so that both venturi's are combined for vacumn then something is wrong. There should be enough for both.
Only my thoughts.

Post by **Bruce Fenstermacher** » Sun Mar 13, 2005 7:41 pm

Marty

In a perfect world you should know if you have the stock venturi, then each one will pull just 4" at its rated speed. A little more if faster and less if slower. Now you have 2 of them but that doesn't double the vacuum, it doubles the volume. It's not a perfect world so with 2 venturis plumbed together you may see 4.5". But the point is you won't see 8"

The old AN style gyros where designed to run on lower vacuum than todays modern instruments so your 1- 4" venturi would run one AN gyro, 2 ventures would run 2 gyros.

Modern Gyros need better than 5" of vacuum so even at their best one 4" venturi will not run one modern gyro correctly.

Now depending your particular bearings in your gyros they may run pretty well or not so well.

What can you do. The first thing is to check the accuracy of your vacuum gage. Then I'd check the way it is now plumbed against the IPC and change it if it is not the same. How new is your DG? Maybe it's shot.
I'd look at what fittings are used to plumb it. It makes a difference. Fittings designed for vacuum systems cost much more but they are designed to move air more efficiently. This is critical when you are dealing with minimum vacuum like we are.
Lastly there are larger ventures available from Spruce and Wag Aero. Talk to your IA before going this route about legality issues.

There are lots of 170s running one and two venturis successfully. I have 1-4" venturi running 2 modern gyros and they work pretty well. I wouldn't take the plane in the clouds though as the DG does precess a little more than I like and plan at some point to improve the situation.

I've never seen or heard of anybody using 3 venturis.

You can't install a vacuum pump on a C-145 because for one there is no place to mount it. There might be some belt driven STC'd pump set up for the front of the engine you could look into but I think they are for older 172s. If you had a 0-300D there would be a vacuum pump pad but doubt if you'll change the engine just to get it. There also seems to be a cool little foreign made alternator which has a vacuum pad on it but getting approvals to do the alternator conversion could prove to be difficult.

This subject has been talked about a lot here so if you haven't already do a search on venturi and vacuum and Im sure you'll find more to read.

N8249A · Post by **N8249A** » Mon Mar 14, 2005 12:51 am

Thanks guys, I have the larger (Super?) venturies and they were plumbed to a regulator and then to both instruments. They are modern instruments and were overhauled. My belief is that the peolple who put this together did not understand the capacity of the veturies. My AI did also say that I should plumb each venturi to one instrument.

I think I will seriously consider the old AN instruments. Thats what my dad's 170B had but I still remember them precessing.

Post by **Bruce Fenstermacher** » Mon Mar 14, 2005 1:19 am

Wait a minute Marty. If you have 2 super venturis you should be able to run 2 gryos of the prop wash. OK may not the prop wash but you shouldn't have any trouble pulling 6" or more vacuum. I wouldn't re-plumb any thing till you adjust your regulator to allow more vacuum. I think your looking for around 5.5" at cruise. If you cant' get that somethings wrong with the plumbing.

doug8082a · Post by **doug8082a** » Mon Mar 14, 2005 1:45 am

If you really do have 2 super venturi's then you have MORE than enough capacity for your instruments. Check your regulator setting and filters. I have a single super venturi and it's more than adequate for my modern AH & DG.

zero.one.victor · Post by **zero.one.victor** » Mon Mar 14, 2005 1:45 am

The remarks about the precessing DG's take me back. Years ago, the DG in my Cessna 150 was going gunnysack on me. It started out precessing, then it went to a slow constant turning, finally it got to where it whirled like a dervish whenever the engine ran! It was sort of hypnotic, it really drew your eye and you couldn't keep from looking at it-- sorta like a bird looking into a snake's eyes. It was very disorienting- and this was VFR! I can imagine what it'd do to you in the soup! I covered it with an INOP placard until I could afford to have it overhauled.
BTW, me & several people I know have had instruments OH'd by Rudy Aircraft Instruments in Rudy OK. Quick,cheap,good service. They advertise in TAP & GA News.

Eric

N8249A · Post by **N8249A** » Mon Mar 14, 2005 4:17 pm

Thanks again, I will try to adjust the regulator. There is also a new filter. My AI says that the venturi's might not draw enough through the filter.
Did the original installations have filters? Thanks, Marty

doug8082a · Post by **doug8082a** » Mon Mar 14, 2005 4:21 pm

My installation with the super venturi has a regulator and filter and has not been a problem. I consistently draw 4.5" - 5.5".

Post by **GAHorn** » Mon Mar 14, 2005 5:16 pm

Marty, you've now quoted your "AI" in two occasions which lead me to believe he's really not familiar with your airplane or with venturi systems (and may misunderstand gyros.)
First, the best way to plumb your vacuum system is exactly like your 170 Illustrated Parts Catalog shows it. In instrument qualified 170's, this was a parallel venturi system. see Fig. 76, page 132. (NOT one venturi for one gyro.)
Next, you should determine the health of your gyros themselves. If they are old, with failing bearings....then abnormal precession is a distinct indicator of failing gyros. It may have nothing to do with your vacuum system.
As time goes on and as these airplanes pass thru various owners, you'll find all kinds of unique modifications which may or may not meet the original design. I'd first recommend you bring the system back to original design (or it's equivalent if your airframe did not start life as an IFR qualified airplane.)
Dual standard venturis installed as the IPC shows should be capable of operating two vacuum AN style gyros in flight. They should even be capable of operating healthy 3-1/8" gyros (modern pictorials) if the system is healthy and set up properly. (Notice that the early B-models did not use the AN horizon but used the 3-1/8" horizon.)

mrpibb · Post by **mrpibb** » Tue Mar 15, 2005 6:30 am

Marty I'm running dual venturis, I have my regulator adjusted to pull 5" in cruise flight. I'm running a modern DG and GH and have enough volume as to have my gyros starting to spin up during warm up and taxi, I'll get 1.5 to 2" during run up and 3.5" static rpm. My first annual after my purchase while under the panel I removed all the sub standard hoses and re plumbed per the ipc and replaced all the slip on hose and clamp where possible with AN flared fittings. My gyros spin so fast now that I have adverse yaw when I turn!!

Here is a link to some shots of the venturi side of my ship

http://www.sandhillaviation.com/wonderlust.html

Vic

Post by **GAHorn** » Tue Mar 15, 2005 3:19 pm

Hey, Vic! NICE lookin' airplane!
Marty, I should have mentioned that my B-model (with the later panel/venturi setup as illustrated in Fig. 77 of the IPC) gives (unregulated)over 6"Hg of vacuum at cruise below 3K' MSL, 5" Hg at 5K', and a bit over 4"Hg at 7500'. (It does fall to 3.5"Hg or slightly less at 12K but the gyros still work just fine.) Prop blast during runup gives 1-2"Hg and the gryos are useable by the time 300' or so of altitude is gained on takeoff. (The restorer of my airplane (although he faithfully followed the IPC otherwise) installed a central filter and did not include the regulator, and I had to purchase one to regulate the vacuum to 4".) Therefore if my airplane is used as an example, dual standard venturis with a central filter will provide adequate vacuum for two air-driven gyros. (It just so happens I have the old AN-type gyros, but the system should also drive the modern 3-1/8" types as well.)

HA · Post by HA » Tue Mar 15, 2005 4:34 pm

I also have 2 standard venturis plumbed together, I added a central vacuum filter when I upgraded to new style gyros - I get around 4.8-5" at 5KFT running an attitude gyro, DG and turn & bank. One venturi on each side of the airplane to maximize thrust from all that interior air exhaust

Post by **GAHorn** » Tue Mar 15, 2005 4:48 pm

HA wrote:I also have 2 standard venturis plumbed together, I added a central vacuum filter when I upgraded to new style gyros - I get around 4.8-5" at 5KFT running an attitude gyro, DG and turn & bank. One venturi on each side of the airplane to maximize thrust from all that interior air exhaust

Hans, ...is your T&B vacuum? Not electric?

HA · Post by HA » Tue Mar 15, 2005 9:51 pm

yessir George, it's vacuum and old as heck. but it still works. along with my comm radio and loran, I'm definately VFR ready. I don't do IFR with this plane, that's for sure. Do enough of that in real life, know what I mean?

when I took over the maintenance of the plane (married into it

) the vacuum plumbing under the panel was galvanized steel pipe

. I replaced that with some aluminum line, and then updated the gyros as they died over the years and added the central filter to protect my investment but the T&B is still chugging along. must be my smooth-as-silk landings. or whatever. If it ever does quit, I will probably go to an electric turn coordinator for some redundancy.

I'm surprised that the system does put forth enough suction to run it all without the super venturis, but I tested the system with a vacuum tester from our shop and it's all good. so I won't mess with success.

Hans