Dual Venturis

[cessna170bdriver]

The subject of dual venturis is getting a bit off the topic of porting and polishing of the induction system, so I'm breaking out this discussion to a separate topic.

Dual venturies are a good, adequate system and shouldn't require any backup, in my opinion.
Best regards
George

Even though the SVS is a backup system, my most common use is to get the gyros spooled up prior to departure. The run up will provide an inch or two of vacuum, but not near enough to really get things going. I select the SVS soon after engine start and get the gyros spinning, set the DG and make sure the AI is going to erect before I even taxi. All I have to do prior to takeoff is cross check the DG to the compass, and I'm not fiddling around on the runway getting it set.

Also, I'm not sure it make any real difference, but i feel better bumping along on the ground with the gyros spooled up rather than sitting dead in their gimbals. The same goes for keeping the gyros spooled up in the pattern prior to landing. Even though I've logged over 3000 landings, most can be classified as "arrivals" and I just feel better if the gyros are up to speed at touchdown(s) .

'98C may have some room for improvement as far as venturi performance at lower airspeeds. I've heard others say they get 4 inches or more from their venturis, even at pattern speeds. Some claim to be able to get the gyros to spool up on run up. When I'm slowed down to the 70-80mph range, the vacuum is down to 2 to 2.5 inches, and even a full power run up will give me less than 2 inches of vacuum. I have the modern style RC Allen AI and DG, and they get lazy anywhere under 3 inches.

I've changed to the 8-inch venturis with very little improvement. One thing that is different on '98C as opposed to many other 170's is that there is one venturi on each side of the airplane as opposed to both on the right as shown in the IPC. They have always been this way, as there is no evidence of a second venturi ever having been on the right side. Just as a test, I have plugged the venturis one at a time, and found that the left one will not pull adequate vacuum by itself, but the right one by itself is nearly as good as both together.

My questions: With the slipstram rotating around the airplane as it does, could the venturi on the left be at a disadvantageous angle of attack? The left venturi does "whistle" at idle on the ground, indicating that the airflow may may be somewaht across the face, rather than straight through(?) I might be amenable to moving the left venturi to the right if I had some assurance of an improvement. Is there an installation drawing that gives exact dimensions for locating the venturis?

Miles

[GAHorn]

Miles, my dual venturi installation was done at original build by Cessna, and I can send a pic if you wish.
As for the discussion of vacuum systems...
The modern, 3 1/8" gyros you have (also known colloquially as "pictorial" gyros) require 4.5 in. Hg to meet their specs. The original AN series gyros only required 3.5 in. Hg. This is one reason that venturiis may be satisfactory on some airplanes but not on others....due to the types gyros actually installed. (I have original AN types. "Bumping" along the ground...they are "caged". Of course, that's not normally available to the pictorial types.)
Your own experiment with left VS right side mountings is the best evidence I've ever read as to why Cessna put them on the right side. I know some "after-market" installations put them on both sides, and one that you and I both know (owned by a certain auctioneer) has them mounted on the belly, presumeably to operate in a heated environment. This is an imagined advantage, I'd ...er....imagine. (Warm air exiting the lower cowl might be less likely to contain moisture or otherwise ice up, but iced-up venturiis are one's least problem in icing conditions. The airframe is going to build ice much faster than venturis I think. Crashing right side up or not is not much comfort to me and assumes that an iced-up airframe is capable of being controlled... something that is an oxymoron at best. In any case, warm air is also less dense and will provide less vacuum, and the air exiting the lower cowl is laden with oil droplets/mist and will rapidly reduce the effectiveness of venturiis, IMHO.)
I've always thought that a simple, brass, valve (like sometimes used on Piper/Aeronca's for fuel) would be a good way to use engine vacuum to spool up gyros when taxying out, then switch to venturis for flight. But when I fly IFR... I never fly with less than 500 ft. overcast and 1 mile viz anyway. And my venturi-fed gyros are up and running long before I enter the clouds. (I'd not fly a single engined airplane unless I could descend to at least "special" VFR before hitting the ground anyway, so there's plenty of time to get 'em up and running before entering IFR conditions.)
When I win the lottery I'm going to install an electric 3 1/8" horizon as a backup in one of my spare holes. (This'll be right after the IO-360 and the Garmin 530 goes in.)

[zero.one.victor]

Miles, you talk about spinning up the gyro's prior to takeoff, and keeping them spun-up in the pattern prior to landing.
You must fly a lot of IFR? Or is this gyro spin-up a concern for VFR flight?
I myself am strictly VFR, and in fact removed my venturi's gyro's and vacuum system a few years ago. Much less likely now to tempt fate by going into the clouds "just long enough to break out underneath".
I often see people install or maintain a "full gyro panel" in their VFR airplane, when it doesn't have enough radio gear to even consider IFR op's. And they don't usually spend enough (or any) time practicing under the hood in case they do inadvertantly go into the soup- which is usually their reason for the instruments. Even using a hood, there are enough visual cues in your periferal vision (even when you're not trying to cheat!) that make it a whole lot easier than real IFR conditions. Lotsa people don't realize this, and kid themselves that flying into the clouds wouildn't be that big a deal-- after all, "I got in 5 minutes of hood time doing my BFR a couple years ago".
Let's be careful out there.

Eric

[cessna170bdriver]

Miles, you talk about spinning up the gyro's prior to takeoff, and keeping them spun-up in the pattern prior to landing.
You must fly a lot of IFR? Or is this gyro spin-up a concern for VFR flight?
Eric

I don't fly IFR at all these days (some of the locals in the desert consider less than 10 miles viz to be IMC, so I've gotten lazy), but I still think it would be good skill to resharpen and keep sharp. I would recommend the instrument rating to any pilot, whether they think they would ever use it or not. If nothing else, it gives you a good awareness of the "system", and makes you a more precise pilot, always a good thing IMHO. Also, with the rating, you'll be more likely to do things legally and safely rather than "to tempt fate by going into the clouds "just long enough to break out underneath". "

I got my instrument rating in 1990, so in 1991 I installed a decent set of avionics, the SVS system, and "pictorial" gyros (the AN gyros were shot, and I wasted a lot of money trying to make them servicable). In those days, living in south Mississippi, there was more opportunity to exercise the rating. I never did fly "hard" IFR, mostly just getting up and down through high broken and overcast layers for a smoother ride. Also, we would often have "clear and one" mornings near the coast and it was handy to file to make an early morning departure legal. I've never taken off into less than 1000 OVC, and I've only ever shot one "for real" instrument approach, and that was on an instrument competency check with an instructor in the right seat. Nonetheless, I always felt better departing with the gyros already up and running; just one less thing to worry about before entering the clag.

I know some "after-market" installations put them on both sides, and one that you and I both know (owned by a certain auctioneer) has them mounted on the belly...

George, I've seen our "auctioneer's" installation, and it was my impression the idea of the location on the belly was not so much to take advantage of engine heat, but that it is further out from the engine centerline and could take advantage of the higher energy slipstream further out on the prop. I've never seen it, but have heard of venturis mounted on top of the cabin for the same reason. Up there, you wouldn't have the problem with oil fouling things up. All that said, if I decide to move my venturis, it will be to the Cessna-specified location, and I would appreciate the picture of your installation.

I've always thought that a simple, brass, valve (like sometimes used on Piper/Aeronca's for fuel) would be a good way to use engine vacuum to spool up gyros when taxying out, then switch to venturis for flight.

You'd probably need a second valve to isolate the venturis. Otherwise the manifold would be sucking on two open venturis as well as the gyros. The SVS solves this problem with a flapper valve arrangement that automatically takes vacuum from whichever source (manifold, pump, or venturis) that is providing the most vacuum. My theory as to the problem with sticking flapper valves (the cause of the AD) is that with an operating engine-driven vacuum pump, the flapper valve on the manifold suction side never gets exercised. With the venturis, mine gets exercised every flight, and I've never had a problem with the valve sticking.

I think a good system would be to have an SVS system with the vacuum pump on one side, and venturis on the other, with a valve to isolate the vacuum pump in flight to exercise/test the flapper valve. Along this line George, if you do win the lottery and get your IO-360, will you use the vacuum pump?

Miles

[GAHorn]

No. (Really.)

On that note, I've got a brand-new, never installed Sigma-Tec vacuum pump PN 1U128-006 (Spruce prices them at $400) I used to carry as a spare on my Baron. (Two motors, two vacuum pumps, and I still carried a spare...if that tells you how much I trust vacuum pumps.)
I'll discount it $100 for anyone who wants it. I'll pay shipping. (Price is $300, shipping included.)

[170C]

Miles, I am not familiar with the SVS system. Is this one that takes vacuum off the intake manifold? Educate this ole flatlander.

My '56 172 TD has one venturi on each side as your 170 does. I don't know if it was this way from the factory or not. A friend had a '57 172 and both venturies were on the right side, so I suspect mine got changed somewere along the line. I think the venturi on either side "looks" better than both on one side, but function wise I have no knowledge of what's best.

You mentioned something that has been a question of mine for 16 yrs. "venturi whistle". Anytime I approach 120 miles an hour (it actually happens once in a blue moon) I get a sharp whistle that I have always suspected came from one or both venturi's. Since I don't cruise that fast very often (thus ole pokey) it isn't a common occurence, but when it happens it is very annoying . I have threatened several times to tape up one or both venturi's to see if I could verify what the culprit was. Does yours whistle at higher airspeeds?

[doug8082a]

You mentioned something that has been a question of mine for 16 yrs. "venturi whistle". Anytime I approach 120 miles an hour (it actually happens once in a blue moon) I get a sharp whistle that I have always suspected came from one or both venturi's. Since I don't cruise that fast very often (thus ole pokey) it isn't a common occurence, but when it happens it is very annoying . I have threatened several times to tape up one or both venturi's to see if I could verify what the culprit was. Does yours whistle at higher airspeeds?

I get the same thing, except only on the ground at idle or near-idle. It disappears during runup and while flying. I wonder if it is, in part, a function of the type of venturi as well as mounting location. I have a single 8" super venturi on the co-pilot's side about 1/3 up from the belly of the plane. I agree, it's annoying, but for me it is shortlived. Once I'm in the air, the noise is gone.

[cessna170bdriver]

Miles, I am not familiar with the SVS system. Is this one that takes vacuum off the intake manifold? Educate this ole flatlander.

Actually I was a flatlander too when I installed this system. (ever been to south Mississippi? Doesn't get much flatter than that ) Yes, the SVS is the system that takes vacuum off of the manifold. It's difficut to describe without a picture, but it's actually a fairly simple system. I had a 3/8" AN840 hose nipple welded to the aft side of the 1-3-5 intake manifold just outboard of where it bolts to the sump. From there a vacuum hose is routed through a bulkhead fitting on the firewall to a shutoff valve controlled by a knob on the panel. From the shutoff valve a hose leads to one side of a "shuttle valve", and a hose from the venturis routes to the other side. Both the manifold side and the venturi (or vacuum pump if that is the primary vacuum source) side of the shuttle valve each have a flapper-type check valve causing which ever side has the strongest vacuum to open to a common port, which is connected through a regulator or needle valve to the vacuum port on the gyros.

My '56 172 TD has one venturi on each side as your 170 does. I don't know if it was this way from the factory or not. A friend had a '57 172 and both venturies were on the right side, so I suspect mine got changed somewere along the line. I think the venturi on either side "looks" better than both on one side, but function wise I have no knowledge of what's best.

According to an old 337 in '98C's records the venturis and gyros were installed not too long after delivery, but not at the factory, and whoever did it put a venturi on each side, which is different from the way it's depicted in the IPC. There are no empty holes to show that both venturis were ever on the same side.

You mentioned something that has been a question of mine for 16 yrs. "venturi whistle". Anytime I approach 120 miles an hour (it actually happens once in a blue moon) I get a sharp whistle that I have always suspected came from one or both venturi's. Since I don't cruise that fast very often (thus ole pokey) it isn't a common occurence, but when it happens it is very annoying . I have threatened several times to tape up one or both venturi's to see if I could verify what the culprit was. Does yours whistle at higher airspeeds?

Like Doug's, mine only whistles at idle. I can reach out and cover the end of the venturi with my hand and the whistling will stop. I've actually had to demonstrate this to a couple of Lycomians who told me the noise was my alternator belt. Next time Ole Pokey is feeling spunky and you get the whistle, you can reach out cover the end of the venturi on your side and see if it stops. I can't say I've seen it in writing, but it is my understanding that Vne on a C170 is 160 mph indicated with the windows open or closed.

Miles

[zero.one.victor]

Miles, I hate to admit it but I shot from the hip in my post re: weight at the tailwheel having a big effect on CG. I guess I was too lazy to whip out my W&B sheets & calculator and do the math. Glad you took the time to do it & straighten me out.
You describe welding a nipple to the intake manifold for your aux vacuum system. I seem to recall that on mine, the primer line goes into the 2-4-6 manifold at this same location. Don't have my C-145 IPC handy, but I would think that the two intake manifolds are the same. Therefore, there should be a tapped hole into the manifold there-- but mounted on cyl's 1-3-5, it'd be on the forward side. Seems like it'd be easier to tie into that & elbow down than to weld a new fitting onto the manifold on the opposite side?

Eric

[cessna170bdriver]

Miles, I hate to admit it but I shot from the hip in my post re: weight at the tailwheel having a big effect on CG.

Eric, actually I thought the same thing. I just wanted to do the math to prove it to myself. I was also surprised at the answer. When you go to the TCDS though, there is several pounds difference (and a small difference in the distance from datum) among the different approved tailwheels. Evidently, Cessna did the math a long time ago.

You describe welding a nipple to the intake manifold for your aux vacuum system. I seem to recall that on mine, the primer line goes into the 2-4-6 manifold at this same location. Don't have my C-145 IPC handy, but I would think that the two intake manifolds are the same. Therefore, there should be a tapped hole into the manifold there-- but mounted on cyl's 1-3-5, it'd be on the forward side. Seems like it'd be easier to tie into that & elbow down than to weld a new fitting onto the manifold on the opposite side?

Eric

Eric, you're correct in that the two intake manifolds are identical. They are both drilled and tapped for 1/8" pipe fittings. Normally on our engines, the 2-4-6 side has the primer nozzle installed there, and the 1-3-5 side is plugged. This hole would be adequate as a tap for a manifold pressure gauge (no flow), but way too small to get enough flow to run the gyros. I don't remember the flow required, but it's several CFM per gyro, enough to raise the idle RPM by a hundred or so. I chose a 3/8 fitting because that is the size of the fitting on the shuttle valve. I had the fitting welded in rather than drilled and tapped because there isn't enough "meat" in the manifold wall to allow tapping. I bought a used manifold to modify, so I can remove the system and reinstall the original manifold if I should ever want or need to.

Miles