# Vacuum venturi

Discussion in 'Instruments / Avionics / Electrical System' started by Dana, Apr 2, 2015.

1. Apr 2, 2015

### Dana

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Anybody using a vacuum venturi driven gyro on a slow plane?

My plane has (came with) a gyro turn and bank, driven by a venturi mounted under the fuselage, between the landing gear. It never worked, at speeds up to 80 kts. The instrument itself works (verified on the bench with a vacuum pump) but the venturi draws no vacuum. It wasn't plugged, but I took it off and cleaned it anyway, driving at 80mph in the car it produces some vacuum, but not the 2" the instrument requires (I haven't put it back on the plane yet).

From what I understand, they're rated at 100 mph, so a venturi that draws 2" of vacuum would only draw 1/2" at 50 mph, but being in the prop blast... I don't know. I shouldn't really need it in a plane like mine, but I had one in my T-Craft years ago and it saved my butt once when I waited a little too long to turn around in bad weather, so it's comforting to have.

Dana

2. Apr 3, 2015

### pictsidhe

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I have a feeling 50mph isn't going to give you 2"
Does it need much airflow? Intake vacuum is used for car brakes, you might have enough velocity in the intake runners for that to work at full throttle. Easy enough to try, just make sure you have a one way valve to keep nasties out of your gyro and the cockpit. Diesel cars have a small vacuum pump for their brakes, maybe 1lb in weight.

3. Apr 3, 2015

### pictsidhe

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Another thought. Those sort of things work on a pressure difference. If you somehow feed the high pressure side from a pitot tube and use the venturi for the vacuum side. You'll have several times the pressure drop of a venturi. It might make the difference.

4. Apr 3, 2015

### gtae07

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I almost hate to say it, but sell the vacuum gyro and buy a digital instrument (like a Dynon D1 if you don't have an electrical system, or maybe a GRT mini if you do). It'll weigh less and work better.

5. Apr 3, 2015

### Dana

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Even if I wanted to spend $1500 for something I don't really need, electronic instruments would just look wrong in an open cockpit biplane like mine. The T&B isn't something I would have bought and installed myself, but since it's already there it's worth trying, not too hard, to make it work. Pitot pressure at, say, 65 knots is only 0.2" Hg. Dana 6. Apr 3, 2015 ### TFF ### TFF #### Well-Known Member Joined: Apr 28, 2010 Messages: 11,647 Likes Received: 3,274 Location: Memphis, TN Most are on the side of the fuselage not bottom. Make sure it's not in a dead air spot. Also make sure it is plumbed correct. I think AC43-13 has some plumbing instructions. 7. Apr 3, 2015 ### Dana ### Dana #### Super ModeratorStaff Member Joined: Apr 4, 2007 Messages: 8,633 Likes Received: 3,014 Location: CT, USA Next time I fly I'll try it again (with the vacuum gauge) in the original location. If that's no good I'll perhaps hold it in my hand outside the cockpit just to see what happens. Dana 8. Apr 3, 2015 ### don january ### don january #### Well-Known Member Joined: Feb 11, 2015 Messages: 2,646 Likes Received: 1,070 Location: Midwest Dana if ya want I'll go wing walk for you and will try different areas of air flow. :gig: that way you can keep both hands on the stick. 9. Apr 3, 2015 ### aeromike49 ### aeromike49 #### Well-Known Member Joined: Nov 9, 2011 Messages: 53 Likes Received: 0 Location: Stanfield Arizona USA You can get a larger venturi - 4 inch - and then you could use a regulator for the 2 inches you need. That should work or just try the 4 inch venturi without the regulator to see if it is in the 2 inch range. 10. Apr 3, 2015 ### Pops ### Pops #### Well-Known Member Joined: Jan 1, 2013 Messages: 7,144 Likes Received: 6,034 Location: USA. A long time ago, I had a 2" venturi on an Ercoupe. The T&B would spin up on the take-off roll. My instructor would have me take off under the hood with just the T&B. Your T&B is either in the wrong place or there is a restriction somewhere. Dan R. 11. Apr 3, 2015 ### pictsidhe ### pictsidhe #### Well-Known Member Joined: Jul 15, 2014 Messages: 6,872 Likes Received: 1,851 Location: North Carolina Another idea, build a venturi into an exhaust header. A simple backwards facing tube would quite likely do the job thanks to the high gas speed. 12. Apr 3, 2015 ### Dana ### Dana #### Super ModeratorStaff Member Joined: Apr 4, 2007 Messages: 8,633 Likes Received: 3,014 Location: CT, USA Interesting idea... as long as your engine keeps running... Dana 13. Sep 12, 2016 ### Dana ### Dana #### Super ModeratorStaff Member Joined: Apr 4, 2007 Messages: 8,633 Likes Received: 3,014 Location: CT, USA Old thread, but I came across it today while searching for something else. A few weeks ago, I removed the venturi from its location under the fuselage and reconnected it inside the cockpit with enough tubing to move it around. I then flew, and held the venturi in various places outside the aircraft. Nothing... so I made a cover plate to go over the hole where the venturi was, and last week removed the instrument itself, saving a pound or so and a bit of drag. As I said, I really don't need it and so won't waste any more time trying to get it to work. I know the gyro itself works, and not much can go wrong with a venturi (I checked it's clear), so I conclude my plane simply isn't fast enough for it to work. Dana 14. Sep 12, 2016 ### Swampyankee ### Swampyankee #### Well-Known Member Joined: Dec 25, 2015 Messages: 1,398 Likes Received: 357 Location: Earth USA East Coast The front end of the venturi is going to see the same total pressure as the dynamic port in the pitot tube: p(static) + (1/2) density * velocity squared. One could, in theory, use the pressure drop in a venturi to measure airspeed. 15. Sep 12, 2016 ### Victor Bravo ### Victor Bravo #### Well-Known Member Joined: Jul 30, 2014 Messages: 6,099 Likes Received: 4,878 Location: KWHP, Los Angeles CA, USA There are apps for many smart phones that give you an (emergency, get me out of here) artificial horizon. Total installation cost: 6 linear inches of Velcro tape or a$5.00 car dashboard/suction cup mount. Drag: zero, plumbing: zero.

16. Sep 12, 2016

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