C170B Vacuum Pumps

faustrj · Post by **faustrj** » Thu Dec 02, 2004 3:25 am

I'm interested in installing a dry vacuum pump in my 1952 C170B. I've been doing some research on the subject so I don't have to pay my A&P. Here's a letter I just sent to my A&P:
---------------------------------------------
Here's what I've been able to find so far:

SA10126SC (1 OF 2)SA10126SC (1 OF 2)
Installation of Aero Advantage ADV200 series vacuum pump per Aero Advantage Drawing Number ADVMSI
Amended
2003102310/23/2003

SA2162NMSA2162NM
Installation of the Precise Flight stand-by vacuum system, Model SVS III.
Amended
2003110711/07/2003

You may already be aware of the fact that Aero Advantage has gone out of business. See this article from the the ABS web site. The Aero Advantage web site, http://www.aeroadvantage.com/, says their assets have been sold to an outfit called the Phoenix group. Last update to the web site was 7/22/04, so I'm not sure when the ADV200 or equivalent will be available for purchase. I've sent a note to the Phoenix group to inquire about that.

I should be meeting some fellow C170 pilots at the Christmas party we're going to in Georgetown this weekend. Maybe I can pick up some more information first hand. In addition, the C170 web site says there are two vac pump, one vac system (emergency), and one vac system (standby) STC's available for $2 per copy. I've requested copies and will pass them on when they arrive.

Would it be silly to put a vacuum gauge on the current venturi to see if it's pulling sufficient vacuum before attempting to proceed with the engine-driven vacuum pump? It might be good to have a suction gauge even if I never get a pump, but there may not be an STC available for just the gauge.

Why don't you just plan on removing the vacuum pump from the current to-do list. I'll update you when I learn more about vacuum pumps for C170's.
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Anyone have any comments, questions, or lessons learned about vacuum pumps?

Thanks,

Rich Faust
1952 C170B, N2449D, SN 20601

zero.one.victor · Post by **zero.one.victor** » Thu Dec 02, 2004 6:51 am

If you've got a vacuum system, whether it's powered by venturi- or vacuum pump- powered, I would think it should have a suction gauge. Shoulda been part of the original vacuum system/gyro installation.
what I like about venturi's: they don't fail. What I don't like about vacuum pumps: they do. Especially dry pumps.
Unless you're running a D model engine, the only vacuum pumps I've seen for the C-145/O-300 are mounted in front of the #5 (?) cylinder-- belt-driven off a pulley mounted behind the propeller. Ugly, & not so great for cooling. You'd need to replace or hack up the nosebowl, too.
I have heard of a vacuum pump that piggybacks off another accessory ( the generator maybe?) but never seen one.
Stick with the venturi's.

Eric

Post by **GAHorn** » Thu Dec 02, 2004 7:03 am

I have (and have sent to hdqtrs) the drawings for the alternator/piggy-back vacuum pump. (There is insufficient room between a gen and the firewall......this unit piggy-backs onto an alternator.)
The problem I see with such a system is: VACUUM systems and ELECTRICAL systems are ordinarily expected to back up for each other. If you lose the vacuum ...the electric gyro (T&B usually) backs up for it. If you lose electrics....the vacuum gyros do the work.
But with the piggyback system....loss of the alternator drive....causes the failure of the alternator (generated electricity) AND the vacuum!

jrenwick · Post by **jrenwick** » Thu Dec 02, 2004 2:27 pm

The Precise Flight SVS was mentioned in this thread. I have experience with one of these, in a Cessna 182. It consists of a shuttle valve connected to the intake manifold, so that if the vacuum pump fails, the instruments still get some suction from the manifold. Instrument performance after pump failure is inversely related to manifold pressure, so it's strictly an emergency system to keep your instruments working while you descend to VFR conditions or a precautionary landing -- but it worked very well for me at low cruise power settings. If you have a venturi instead of a vacuum pump, this system would solve the problem of getting the gyros spinning before takeoff. You'll get excellent vacuum while warming up and taxiing, lose some during runup, then switch to the venturi just prior to takeoff (don't forget, because full throttle reduces the standby vacuum to almost nothing).

I asked Precise Flight about this, because their installation instructions (which you can download from http://www.preciseflight.com) only talk about connecting it to the vacuum pump -- not a venturi. I asked them if the STC (which covers C170s) would cover installation in a venturi system as well. They said it had been done, and is STC'd. This could make a venturi system a little more suitable for IFR flight, but you still have to worry about icing! I offer this for what it's worth... I haven't installed it on my 170, but maybe someone else here has.

Best Regards,

John

zero.one.victor · Post by **zero.one.victor** » Thu Dec 02, 2004 6:23 pm

There have been numerous comments on these threads about venturi's icing up. I'm a VFR guy, so have very limited experience in clouds & icing conditions,but I've never seen an iced-up venturi-- never talked to anyone who had one ice up on them either. Is this a valid concern? If your venturi's ice up, you can bet your pitot will also. I would guess that your flying surfaces won't be far behind.
If you try to make a connection between carb icing & venturi icing, I don't think it's valid--I think the carb ice occurs from the temp drop taking place due to the evaporation of fuel at the carb venturi, not due to the acceleration of airflow thru the venturi.

Eric

doug8082a · Post by **doug8082a** » Thu Dec 02, 2004 7:11 pm

I too, have very limited experience in icing, but it would seem to me that if you reached the point where the venturi iced up, you probably have much more significant icing problems elsewhere on the airframe. I would think ice would build up to an unsafe level on the prop and wings before it became an issue on the venturi.

Am I off the mark here?

jrenwick · Post by **jrenwick** » Thu Dec 02, 2004 8:02 pm

I have never flown with ventuis in icing conditions, so I don't know how common venturi ice might be. I only mentioned it because it seemed to me an obvious drawback to using venturis in IMC (not to mention unheated pitot tubes). I once had some airframe ice on a 182RG, and I got out of it OK by descending out of the clouds and landing. I would hate to have my gyros fade away in that situation.

John

faustrj · Post by **faustrj** » Fri Dec 03, 2004 1:11 am

Thanks for the comments and information. Sticking with the venturi and going with the standby vacuum system sounds the best so far. I live in Texas and don't expect a whole lot of winter flying or even flight in hard IMC for that matter. My intention is to be able to fly above broken cloud layers (a conservative personal minimum) until I have much more experience.

I think I should at least get a vacuum gauge. Thanks again for all your feedback.

Rich
1952 C170B
N2449D
20601

blueldr · Post by **blueldr** » Fri Dec 03, 2004 5:13 am

When considering a standby vacuum system utilizing engine intake manifold pressure as a source, keep in mind the following:
You must reduce your manifold pressure to 4 1/2 to 5 1/2
inches below ambient in order to provide the vacuum required to drive the gyros. What is the ambient atmospheric pressure at five thousand feet?--- or higher? How well will your airplane fly at about twenty inches or less? Especially if it is carrying some ice.
It might work for you fellas down in the flatlands, but it sure in hell wouldn't be worth a damn crossing the Sierras at a minimum altitude of twelve thousand feet. Try honking off four inches of MAP at twelve grand and you will beat a crowbar to the ground.
The system would probably work pretty good to spin the gyros up prior to take off, as stated in one of the above comments, but then I'd rather rely on venturies than a dry pump.

N1478D · Post by **N1478D** » Fri Dec 03, 2004 2:46 pm

Rich, you might want to do a search on this subject. Rudy flies IFR on venturies, and Russ and George might also. There have been some very informative posts over the last 2 or 3 years on this subject. Glad you are starting off with those kind of personal minimums, I sure would also.

AR Dave · Post by **AR Dave** » Sat Dec 04, 2004 6:31 pm

My vacuum system was installed 2/15/78 per STC SA18EA. I have the complete Handbook Installation Instructions, 337's, and everything if someone needs it. This is by Airborne Mechanisms. Looking at the drawings in front of me now, it's a fairly simple piece of equipment. This one hasn't failed in 27 years, so I wouldn't stress over that too much and I have flown in considerable icing quiet often. The best thing about it, is the compliments on how cool it looks along with the open nose bowl PN 0552002-72 replacement. ERIC!

zero.one.victor · Post by **zero.one.victor** » Sat Dec 04, 2004 10:16 pm

Dave, that's the belt-drive pump, mounted in the nosebowl area? I could come up with a few adjectives for the way that mod looks, but (no offense

), COOL ain't one of them! However, it's probably the easiest way to have a vacuum pump, unless you've got a spare O-300D laying around the hangar. If you like it, or can live with it, that's what counts.
Cool is a subjective thing, I guess. I ran some black hubcaps on mine for a while, I thought they looked cool but everybody said they were ugly. So go figure.
Does anyone on-board here have the vacuum pump mod where it's piggybacked onto the alternator? What model/capacity alternator does it use? I have a 60-amp Ford/Cessna alternator, and it doesn't seem like there's much room between it & the firewall.

Eric

jrenwick · Post by **jrenwick** » Sun Dec 05, 2004 12:26 am

blueldr wrote:When considering a standby vacuum system utilizing engine intake manifold pressure as a source, keep in mind the following:
You must reduce your manifold pressure to 4 1/2 to 5 1/2
inches below ambient in order to provide the vacuum required to drive the gyros. What is the ambient atmospheric pressure at five thousand feet?--- or higher? How well will your airplane fly at about twenty inches or less? Especially if it is carrying some ice.

Your're absolutely right. More on this can be found on the Precise Flight web site, http://www.preciseflight.com/svsq&a.html. In particular: "the system is for emergency use only and is most effective below 8000 feet ASL." It's meant to let you descend out of IMC or complete an approach to a precautionary landing, not to continue your planned flight as if nothing were wrong.

Best Regards,

John

AR Dave · Post by **AR Dave** » Mon Dec 06, 2004 3:13 am

Good point! I'll consider the source

russfarris · Post by **russfarris** » Mon Dec 06, 2004 5:17 am

I considered installing the Precise Flight, but finally decided it wasn't worth the expense on a venturi-driven system. On a dry vacuum pump system, which is much more failure prone than the old deer horn, I probably would.

I don't launch into 200 and a half mile in my 170, so if the gyros aren't up to speed on take-off it doesn't matter. On my airplane both the ADI and DG seem to be working just fine by 500 feet, which happens to be my
personal ceiling for take-off into IMC. Properly set up, a venturi is all you need for the kind of IFR work a 170 can safely do. Russ Farris

The International Cessna 170 Association

C170B Vacuum Pumps

C170B Vacuum Pumps

Precise Flight Standby Vacuum System

Ice on venturis

SVS sounds the best so far...