School me on Venturi horns
Moderators: GAHorn, Karl Towle, Bruce Fenstermacher
- mmcmillan2
- Posts: 161
- Joined: Sat Jul 04, 2020 9:30 pm
School me on Venturi horns
My 170B came with a belt drive vacuum pump to run my big old AI and a newer style DG. I don’t really care for the mechanical pump vs a normal Venturi and only fly VFR. Classic look is actually preferred.
After todays in-flight chucking of the pulley adapter I’m thinking about going back to the original style horn(s). One half of pulley is missing, and it looks like it slipped for a little while. I made a 1 hour local flight and noticed some slight variations in the vacuum indicator. Then a loud noise and no vacuum pressure.
Univair has a super Venturi that may be able to run both gyros. Any help with this would be much appreciated.
After todays in-flight chucking of the pulley adapter I’m thinking about going back to the original style horn(s). One half of pulley is missing, and it looks like it slipped for a little while. I made a 1 hour local flight and noticed some slight variations in the vacuum indicator. Then a loud noise and no vacuum pressure.
Univair has a super Venturi that may be able to run both gyros. Any help with this would be much appreciated.
170B owner, KCFD, CFI(I), ATP Multi
- mmcmillan2
- Posts: 161
- Joined: Sat Jul 04, 2020 9:30 pm
- Bill_Green
- Posts: 27
- Joined: Thu Jul 02, 2020 12:53 am
Re: School me on Venturi horns
I have a couple of venturi's taking up space in my hangar, and would be happy to send you one if you like. Send me a private message with an address and I will get it in the mail to you.
Bill
Bill
Re: School me on Venturi horns
I am a fan of venturiis and operate 90% VFR and occasional “light” IFR. (I use clearances to ascend to VFR and descend to VFR/non-prec. appchs.)
There have been plenty of discussions about the pros/cons of various vacuum systems. In my experience the venturi system SUCKS…. and since that’s what it’s supposed to do….that’s GOOD!
1- It is reliable. If you’re flying…it’s sucking. There are those who fear icing conditions. That’s also good. This airplane is prohibited from being flown in icing conditions. If you get into icing conditions…LEAVE it….and use your back-up-system like FAA and Cessna intended (the needle/ball/airspeed) to stay right-side-up until you get out of that condition you’ll swear will never happen to you again.
2-It has ALL THREE of my gyros up-and-running before I get 200-feet above the runway on take-off. ( I don’t do “zero-zero” takeoffs in single-engine airplanes. In fact, I quit doing zero-zero takeoffs in ANY airplane after the one I did from a private ranch-strip with no runway markings in a Hawker-jet and discovered that after 60 kts all the little pebbles in the runway-asphalt appeared as streaks coming at you and was of zero-use in determining if you are headed for the side of the runway or not. After I heard “V-One Rotate!” …. that was the last time I’d ever perform a zero-zero takeoff in the last 30 years of my professional career.)
Therefore, I don’t need my gyros to be running until after takeoff….and even tho’ I’ve departed with “clearance void” times on an IFR flight-plan with an 800’ overcast sky…I’ve got plenty of attitude information before entering the clouds. (And why anyone would fly single-engined airplane in serious, wide-spread, low-IFR conditions is a wonder to me. I’d like to be able to descend to VFR and see which pasture to select if more important things than venturiis fail.)
3-Venturiis require almost No Maintenance…other than a pre-flgiht inspection for condition, security, cleanliness …. and confirming in-flight that adequate vacuum exists. Some folks have experienced low-vacuum with venturiis….. I believe most are due to improper installation and/or improper INSTRUMENT-system maintenance. Vacuum system integrity, plumbing-condition, and filter changes are still necessary…. But you’ll never lose vacuum due to a failed vacuum-pump or suffer expensive gyro-instrument repair-bills when the carbon brushes in “ modern” pumps disintegrate between 300-500 hrs as designed. (The old Pesco “wet” pumps never did that…. so if you have a vacuum pump… I advise trying to find/keep the old-style “wet” pump system.)
Another reason some folks have had poor experiences with venturi system is odd installations. Cessna placed venturiis on the right side of the fuselage. I believe they did testing and found that the ideal location. (See the IPC) Some folks have them “split” with one on the left and one on the right….and it’s been reported that the one on the left does little-to-no-good. I even know of one FAA Inspector who owned a 170-B and his venturiis were mounted on the belly…aft of the cooling-air exit, behind the lower “lip”…. in some mistaken belief that the warm air exiting the engine comp’t was a good thing…. (apparently ignoring the fact that warm air is less-dense than cool air and would provide less vacuum…. but also placed where considerable contamination and degradation from oil-blow-by kept them filthy.
4- My original venturiis are available cheap, if you want them. The only reason I replaced them was because of my addition of a 3rd gyro…a modern “pictorial” A.H. which requires more vacuum than the old A.N. types and the fact that I’d be burdening the system with more gyros than intended with the standard system. I’m pleased with the result. (A.N. gyros require approx 3” of vacuum while 3-1/8” modern “pictorial” gyros need 4.5” - 5”. If you have that kind of vacuum and your instruments don’t work properly with a half-inch less than spec…get them repaired or overhauled!)
If you are the least bit worried about it….install the so-called “super” venturiis in the original IPC-described manner and be done with it. Keep in mind that venturiis are described by their capability. A 4” venturi is not 4” long…. it is designed to produce 4” of vacuum., etc. The small 2” venturi was intended to run an air-driven T&B (and sometimes used to evacuate a “relief-tube”). The so-called “super venturi”….is a 9” venturi and designed to produce 9” of vacuum. That description is a measurement taken from a closed system and does not allow for actual air-flow losses. My dual 9” “super” venturiis produce 5” at cruise…and 4.5” in the pattern…because I had to install a vacuum-regulator to bring the system down from 7” which would over-work my gyros. (see the article I posted in a link below)
As already stated, I have TWO art. horiz’s and a WW2 AN-D.G. Cessna installed back in the day…
The vac pump you presently have is an obsolete and no-longer supported mod/STC. It also req’d a change of cowling nose-bowl from original, but that’s not likely to cause any grief for you….leave that alone…Just wanted you to know.
The following posts/threads might prove useful for your project:
viewtopic.php?f=39&t=6800&p=147650&hili ... is#p147650
viewtopic.php?f=5&t=15377&p=143059&hili ... on#p143059
viewtopic.php?f=24&t=7955&hilit=2nd+horizon
Installing dual venturiis might find this post/thread helpful:
viewtopic.php?f=24&t=9901&p=88023&hilit ... zon#p88023
There have been plenty of discussions about the pros/cons of various vacuum systems. In my experience the venturi system SUCKS…. and since that’s what it’s supposed to do….that’s GOOD!
1- It is reliable. If you’re flying…it’s sucking. There are those who fear icing conditions. That’s also good. This airplane is prohibited from being flown in icing conditions. If you get into icing conditions…LEAVE it….and use your back-up-system like FAA and Cessna intended (the needle/ball/airspeed) to stay right-side-up until you get out of that condition you’ll swear will never happen to you again.
2-It has ALL THREE of my gyros up-and-running before I get 200-feet above the runway on take-off. ( I don’t do “zero-zero” takeoffs in single-engine airplanes. In fact, I quit doing zero-zero takeoffs in ANY airplane after the one I did from a private ranch-strip with no runway markings in a Hawker-jet and discovered that after 60 kts all the little pebbles in the runway-asphalt appeared as streaks coming at you and was of zero-use in determining if you are headed for the side of the runway or not. After I heard “V-One Rotate!” …. that was the last time I’d ever perform a zero-zero takeoff in the last 30 years of my professional career.)
Therefore, I don’t need my gyros to be running until after takeoff….and even tho’ I’ve departed with “clearance void” times on an IFR flight-plan with an 800’ overcast sky…I’ve got plenty of attitude information before entering the clouds. (And why anyone would fly single-engined airplane in serious, wide-spread, low-IFR conditions is a wonder to me. I’d like to be able to descend to VFR and see which pasture to select if more important things than venturiis fail.)
3-Venturiis require almost No Maintenance…other than a pre-flgiht inspection for condition, security, cleanliness …. and confirming in-flight that adequate vacuum exists. Some folks have experienced low-vacuum with venturiis….. I believe most are due to improper installation and/or improper INSTRUMENT-system maintenance. Vacuum system integrity, plumbing-condition, and filter changes are still necessary…. But you’ll never lose vacuum due to a failed vacuum-pump or suffer expensive gyro-instrument repair-bills when the carbon brushes in “ modern” pumps disintegrate between 300-500 hrs as designed. (The old Pesco “wet” pumps never did that…. so if you have a vacuum pump… I advise trying to find/keep the old-style “wet” pump system.)
Another reason some folks have had poor experiences with venturi system is odd installations. Cessna placed venturiis on the right side of the fuselage. I believe they did testing and found that the ideal location. (See the IPC) Some folks have them “split” with one on the left and one on the right….and it’s been reported that the one on the left does little-to-no-good. I even know of one FAA Inspector who owned a 170-B and his venturiis were mounted on the belly…aft of the cooling-air exit, behind the lower “lip”…. in some mistaken belief that the warm air exiting the engine comp’t was a good thing…. (apparently ignoring the fact that warm air is less-dense than cool air and would provide less vacuum…. but also placed where considerable contamination and degradation from oil-blow-by kept them filthy.
4- My original venturiis are available cheap, if you want them. The only reason I replaced them was because of my addition of a 3rd gyro…a modern “pictorial” A.H. which requires more vacuum than the old A.N. types and the fact that I’d be burdening the system with more gyros than intended with the standard system. I’m pleased with the result. (A.N. gyros require approx 3” of vacuum while 3-1/8” modern “pictorial” gyros need 4.5” - 5”. If you have that kind of vacuum and your instruments don’t work properly with a half-inch less than spec…get them repaired or overhauled!)
If you are the least bit worried about it….install the so-called “super” venturiis in the original IPC-described manner and be done with it. Keep in mind that venturiis are described by their capability. A 4” venturi is not 4” long…. it is designed to produce 4” of vacuum., etc. The small 2” venturi was intended to run an air-driven T&B (and sometimes used to evacuate a “relief-tube”). The so-called “super venturi”….is a 9” venturi and designed to produce 9” of vacuum. That description is a measurement taken from a closed system and does not allow for actual air-flow losses. My dual 9” “super” venturiis produce 5” at cruise…and 4.5” in the pattern…because I had to install a vacuum-regulator to bring the system down from 7” which would over-work my gyros. (see the article I posted in a link below)
As already stated, I have TWO art. horiz’s and a WW2 AN-D.G. Cessna installed back in the day…
The vac pump you presently have is an obsolete and no-longer supported mod/STC. It also req’d a change of cowling nose-bowl from original, but that’s not likely to cause any grief for you….leave that alone…Just wanted you to know.
The following posts/threads might prove useful for your project:
viewtopic.php?f=39&t=6800&p=147650&hili ... is#p147650
viewtopic.php?f=5&t=15377&p=143059&hili ... on#p143059
viewtopic.php?f=24&t=7955&hilit=2nd+horizon
Installing dual venturiis might find this post/thread helpful:
viewtopic.php?f=24&t=9901&p=88023&hilit ... zon#p88023
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
-
- Posts: 77
- Joined: Wed Feb 20, 2013 1:15 am
Re: School me on Venturi horns
I have a venturi with the hose and filter, assembly, for free, shipping on me if interested, PM me
Re: School me on Venturi horns
It just so happens one of our Members has JUST this minute contacted me offering to donate his venturiis and vacuum gage… I’ll refer him to this thread so he can ship them to YOU McMillan! OR contact him direct…one of our Directors, David Nelson in FL.
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
- mmcmillan2
- Posts: 161
- Joined: Sat Jul 04, 2020 9:30 pm
Re: School me on Venturi horns
Thanks for the replies. I’m going to proceed with the venturi horn route. Would a single 9” run my two gyros pictured in my original post?
I found some details in the Cessna 100 Service Manual.
I found some details in the Cessna 100 Service Manual.
170B owner, KCFD, CFI(I), ATP Multi
Re: School me on Venturi horns
I have two "super" venturis plumbed in parallel, running two modern (non-AN) gyros, with a vacuum regulator. I get 4" Hg suction on the takeoff roll and 5" at all speeds over 60 mph. I don't know how much I'd get without a regulator.
I used to have just one venturi but it wasn't enough to hold 4" at 70 mph on an instrument approach.
Other people do ok with one venturi. There seems to be a lot of variation in performance, maybe due to venturi mounting location, engine RPM, or altitude?
I used to have just one venturi but it wasn't enough to hold 4" at 70 mph on an instrument approach.
Other people do ok with one venturi. There seems to be a lot of variation in performance, maybe due to venturi mounting location, engine RPM, or altitude?
Re: School me on Venturi horns
I have the same setup as Dave with two 9 inch venturis with a regulator and it works very well. With a slightly extended run up I can have the gyros working before takeoff if needed.
Karl
'53 170B N3158B SN:25400
ASW-20BL
'53 170B N3158B SN:25400
ASW-20BL
- Bruce Fenstermacher
- Posts: 10320
- Joined: Tue Apr 23, 2002 11:24 am
Re: School me on Venturi horns
To answer your question, yes one would run both instruments but it can be marginal. I always wanted a second one and you will to if you don't just get two right away.
CAUTION - My forum posts may be worth what you paid for them!
Bruce Fenstermacher, Past President, TIC170A
Email: brucefenster at gmail.com
Bruce Fenstermacher, Past President, TIC170A
Email: brucefenster at gmail.com
- cessna170bdriver
- Posts: 4064
- Joined: Mon Apr 22, 2002 5:13 pm
Re: School me on Venturi horns
Whether you mount one or two venturis, mount them in the exact factory location on the right side of the airplane. When I first got my B-model it had somewhat haphazardly located 4” venturis, one on each side. They were just barely able to run the original style DG and AH. Switching to 8” venturis didn’t help much. New modern gyros helped some, but still were outperformed by other 170’s installations. When I had the airplane rebuilt 2014-2017, I had both of the 8” venturis mounted in the factory locations (documented elsewhere in these forums), and I was amazed at the difference: 3-4 inHg during run up, and 4-5 early in the takeoff roll. In the interest of full disclosure, I also have an SVS manifold vacuum system, but it was disabled for the Venturi tests, and, it produces no suction at full throttle.
Miles
“I envy no man that knows more than myself, but pity them that know less.”
— Thomas Browne
“I envy no man that knows more than myself, but pity them that know less.”
— Thomas Browne
Re: School me on Venturi horns
Cleo Bickford had a good article on page 18 of the Fourth Quarter 2006 issue of The 170 News.
See also AK 7053-14 for dual venturi installation instructions and AK 7053-8 for single venturi installation instructions.
I can't tell by looking at your panel picture whether your turn and bank instrument is electrically powered or vacuum operated. Originally they were electrically operated and that's what I would recommend for redundancy. But, if it is vacuum powered, be aware that it should have a restrictor in its line to reduce the vacuum to 2". Google AN5829-2.
Oh, and don't forget to install a vacuum system test selector valve.
See also AK 7053-14 for dual venturi installation instructions and AK 7053-8 for single venturi installation instructions.
I can't tell by looking at your panel picture whether your turn and bank instrument is electrically powered or vacuum operated. Originally they were electrically operated and that's what I would recommend for redundancy. But, if it is vacuum powered, be aware that it should have a restrictor in its line to reduce the vacuum to 2". Google AN5829-2.
Oh, and don't forget to install a vacuum system test selector valve.
Gary
-
- Posts: 3481
- Joined: Sat May 06, 2006 6:05 pm
Re: School me on Venturi horns
The new digital instruments that replace vacuum instruments may not be all that much more expensive than converting over to venturis when its all said and done. If no venturis were ever installed on your plane no new holes in the fuselage would be required with the new digital stuff either.
Richard Pulley
2014-2016 TIC170A Past President
1951 170A, N1715D, s/n 20158, O-300D
Owned from 1973 to 1984.
Bought again in 2006 after 22 years.
It's not for sale!
2014-2016 TIC170A Past President
1951 170A, N1715D, s/n 20158, O-300D
Owned from 1973 to 1984.
Bought again in 2006 after 22 years.
It's not for sale!
- mmcmillan2
- Posts: 161
- Joined: Sat Jul 04, 2020 9:30 pm
Re: School me on Venturi horns
Looks like the original holes to me:
170B owner, KCFD, CFI(I), ATP Multi
- mmcmillan2
- Posts: 161
- Joined: Sat Jul 04, 2020 9:30 pm
Re: School me on Venturi horns
Well shoot, I got my hands on a venturi, but it’s 1.5” square mounting holes don’t match my existing 1.5”x2.0” holes (currently filled with rivets).
170B owner, KCFD, CFI(I), ATP Multi