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Thread: Venturi Location (inst vacuum, not carb)

  1. #41
    BC12D-4-85's Avatar
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    Quote Originally Posted by Gordon Misch View Post
    Good food for thought Gary.

    Thanks --
    Well Gordon we are what we eat.

    Gary

  2. #42
    skywagon8a's Avatar
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    Quote Originally Posted by Gordon Misch View Post
    It's very likely though that the poor flow is due largely to the long length, probably about 8 feet total, of 1/4" tubing from venturi to gage.
    1/4" tubing? Is that what it was before? It's been a while since I looked at one of these but that sounds too restrictive for the volume of air needed to turn the gyro.
    N1PA
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  3. #43
    Gordon Misch's Avatar
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    1/4" is what the Piper print specifies. However, it specifies hose, not aluminum tubing. So my run of 1/4" aluminum tube is more like 3/16 on the inside. A quick check last night suggested that its capacity is reduced significantly, like by 3-fold. And my venturi location requires a longer run than stock. Double-trouble.
    Gordon

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  4. #44
    skywagon8a's Avatar
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    1/4" hose is ID. 1/4" aluminum tubing is OD. A 3-fold capacity difference would likely be a minimum. I await your test results.
    N1PA

  5. #45
    PerryB's Avatar
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    Handsome young fellow you got to pose with the plane...

  6. #46
    BC12D-4-85's Avatar
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  7. #47
    skywagon8a's Avatar
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    Good one Gary. Looks like I found a schematic somewhere which specifies 1/4" OD in post #7. Even so that seems small. It does require some forward airspeed in order to function. In my opinion 100 mph is about minimum for good results.
    N1PA

  8. #48
    Gordon Misch's Avatar
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    A 3-fold capacity difference would likely be a minimum.
    Yeah, likely closer to four-fold, based on the same I.D. ratio for water.
    Gordon

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  9. #49
    Gordon Misch's Avatar
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    Quote Originally Posted by BC12D-4-85 View Post
    Egg splattered all over face. And shirt. And.......

    Thank you!
    Gordon

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  10. #50
    BC12D-4-85's Avatar
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    Hey 'Tis the Season' for a reason. At least you have some daylight and a fun project. We in Fairbanks are looking for something to keep us occupied during the 19 hours of darkness.

    BTW venturis suck!

    Gary

  11. #51
    Gordon Misch's Avatar
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    S'posed to, you mean - - Haha!
    Gordon

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  12. #52
    Gordon Misch's Avatar
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    Well, bit the bullet and drilled new holes in the boot cowl in accordance with the Piper drawing. I guess worst case will be two cover-patches on the boot cowl instead of one, and rip the whole mess out as Denny and Mike so subtly suggested.
    Gordon

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  13. #53
    Gordon Misch's Avatar
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    Quote Originally Posted by PerryB View Post
    Handsome young fellow you got to pose with the plane...
    Kinda skinny though - - -
    Gordon

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  14. #54
    BC12D-4-85's Avatar
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    Use original hole for male 120V electrical socket. Plug in drop cord for engine and cockpit heat plus small battery tender. Add LED to confirm power ok. Many around here like that.

    Gary

  15. #55
    Gordon Misch's Avatar
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    Clever. I'm no longer having to deal with Ak temps, but I'll give it some thought. Thanks - -
    Gordon

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  16. #56
    aviationinfo's Avatar
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    Gordon you are the smartest guy I know, but the 1930's are calling and they want their venturi back. Come and join me in the 1960s---put a vacuum pump on that bad boy. Then when the 1960s call you we'll consider the 1980's. Baby steps...

    I note well that your avatar is a picture of a horse-drawn farm implement!!

  17. #57
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    Quote Originally Posted by aviationinfo View Post
    Gordon you are the smartest guy I know, but the 1930's are calling and they want their venturi back. Come and join me in the 1960s---put a vacuum pump on that bad boy. Then when the 1960s call you we'll consider the 1980's. Baby steps...

    I note well that your avatar is a picture of a horse-drawn farm implement!!
    Gordon, I consider you a personal friend and enjoy your company so I hope you don't hate me now, but when I read that I laughed so hard I think I pee'd a little!!

  18. #58
    Gordon Misch's Avatar
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    Ok sir, just wait till I find my rolodex file of snappy comebacks. Besides, my whiskey compass works and I use it and furthermore what could be more lonely than a lonely anachronism?

    And Perry, amigo, I didnt realize you were THAT old!����
    Last edited by Gordon Misch; 11-27-2018 at 03:00 PM.
    Gordon

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  19. #59
    Gordon Misch's Avatar
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    Well, the venturi is moved to the location specified in the Piper drawing that Musket shared. Removed 2 or 3 ft of 1/4 alum tube and replaced with about a foot of 1/4 Tygon Tube. Test flight tomorrow.
    Gordon

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  20. #60

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    Just make sure you're not starting out slow when you decide to trust it.

  21. #61
    Gordon Misch's Avatar
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    Quote Originally Posted by 46 Cub View Post
    Just make sure you're not starting out slow when you decide to trust it.
    For sure, thanks! Got a line of wx moving thru right now so test flight will be a little later.
    Gordon

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  22. #62
    S2D's Avatar
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    Seems to be the preferred spot.

    If it doesn't work out, I have a larger one around here somewhere I'll give you. It should produce enough vacuum. Click image for larger version. 

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    Sent from my E6810 using Tapatalk
    I may be wrong but that probably won't stop me from arguing about it.

  23. #63
    Gordon Misch's Avatar
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    Looks experienced, too!
    Gordon

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  24. #64
    wireweinie's Avatar
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    Quote Originally Posted by S2D View Post
    Seems to be the preferred spot.

    If it doesn't work out, I have a larger one around here somewhere I'll give you. It should produce enough vacuum. Click image for larger version. 

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    Wow! Durable! Everything BUT the venturi looks bent.

    Web
    Life's tough . . . wear a cup.

  25. #65
    Gordon Misch's Avatar
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    Well, I'm bummed. It think the venturi is fine because with a strong turn the needle jumps over. But when the strong turn is reduced to standard rate the needle settles on the doghouse momentarily, then drops to 1/3 or 1/2 of the correct deflection. Same when entering standard rate from straight and level gradually, the needle only moves a little. This gage has 7 years of being inop in the plane, after overhaul by Keystone. I'm thinking maybe its works (gimbals and such) might have vibrated in one position so long that they're compromised. Thoughts?
    Gordon

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  26. #66
    skywagon8a's Avatar
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    Hook up a vacuum cleaner to the exhaust end of the venturi. Then move the plane back and forth in the hangar.
    N1PA

  27. #67
    wireweinie's Avatar
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    Has this setup ever worked? It sounds like the venturi works fine. You get instrument operation in turns? Sounds like it's still 'shaded' by the cowl door. Try another style venturi or put a spacer under it to move it out closer to clean air.
    Take a blower chuck and shop air and blow air into the venturi from about two feet away. If the instrument works, the venturi is wrong or located in a bad place.

    Web
    Life's tough . . . wear a cup.

  28. #68
    Gordon Misch's Avatar
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    Thanks guys. I THINK the venturi is fine now, but the instrument is acting like it's "sticky". Good needle deflection in strong turns but "stuck" needle in gentle turns. I think I'll pull the instrument and the venturi out, and play around with vacuum cleaner wind by the bench. Any experience with instruments acting like this with known sufficient vacuum??
    Gordon

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  29. #69
    BC12D-4-85's Avatar
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    See my post 37 and yes it may have become compromised. Remove and inspect the instrument. If suspicious have it overhauled. That's assuming you now have the required vacuum displayed on that gauge and an assumed flow of air.

    180* turn in 60 seconds once in the doghouse and coordinated. If not then I'd suspect the gyro rotation speed. Too late a 180 turn (>60 sec) or no doghouse = more vacuum. Too soon = less with flow needle.

    FWIW there's adjustable ball bearing preload at both front and back of instrument the gyro cage is suspended on. Also bearings for the gyro. Plus a dampening pot and spring to smooth and adjust the gyro's precession. If any of that needs adjustment or gets compromised though wear and the instrument is noisy.......

    Gary

  30. #70
    skywagon8a's Avatar
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    Other than a dirty filter the old vacuum T&B is basically trouble free. Like the energizer bunny.
    N1PA

  31. #71
    Gordon Misch's Avatar
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    Quote Originally Posted by wireweinie View Post
    Has this setup ever worked? It sounds like the venturi works fine. You get instrument operation in turns? Sounds like it's still 'shaded' by the cowl door. Try another style venturi or put a spacer under it to move it out closer to clean air.
    Take a blower chuck and shop air and blow air into the venturi from about two feet away. If the instrument works, the venturi is wrong or located in a bad place.

    Web
    Well, it didn't work at all with the venturi in the old location.

    Now the needle swings to large deflection just fine, but seems sticky with small deflections.

    I removed the venturi and connected the shop-vac to it on the downstream end, as Sky suggested, and I get 7" Hg static.

    I've re-installed the venturi and plumbed it direct to the vacuum gage, i.e. instrument not connected. Tomorrow I'll fly that and see how much suction I get.

    If that tests ok I'll remove the instrument (big pain, cuz it's in there with elastic stop nuts that are hard to reach), do all Gary's post #37 stuff, and test it with the venturi on the shop-vac. At least that will be definitive.
    Gordon

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  32. #72
    BC12D-4-85's Avatar
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    How much suction did you get during test flight with T&B attached to system?

    If you can reach it just remove the screw-on air filter before the whole instrument. Then try again. Might just be the filter's constipated with dirt. If so disassemble and clean.

    The needle can jump a bit until a steady coordinated 3*/sec turn is established then should settle into the doghouse.

    Why not use NAS 487 grasshopper nuts for assembly?
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  33. #73
    Gordon Misch's Avatar
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    Quote Originally Posted by BC12D-4-85 View Post
    How much suction did you get during test flight with T&B attached to system?

    If you can reach it just remove the screw-on air filter before the whole instrument. Then try again. Might just be the filter's constipated with dirt. If so disassemble and clean.

    The needle can jump a bit until a steady coordinated 3*/sec turn is established then should settle into the doghouse.

    Why not use NAS 487 grasshopper nuts for assembly?
    I didn't have the suction gauge attached when test flying today. I figured if it worked well, then I'm done. And if it didn't - well, here I am.

    Unfortunately I can't reach the filter. Can barely see it. The T&B is between other instruments, unfortunately. And unfortunately I was ignorant about grasshopper nuts until I'd already done the stop nuts. But that's what'll get used in the future, for sure!
    Gordon

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  34. #74
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    Test flew today with the venturi connected only to the suction gage. It gives between two and three inches at 90 mph, and is rated for 2 inches. So I'm satisfied that the venturi location is now ok.

    Removed the venturi and instrument, and connected the venturi to the shop vac with instrument and suction gage connected. Adjusted the suction to 2 inches. The instrument still seems to have a hitch in it, between zero and standard rate turn. Checked the filter, it's clean. Tested again with filter removed, same result. So I'm now pretty confident that the instrument has some sticky something or another in its innards. Go ahead, laugh at my Rube Goldberg arrangement. Take what ya got and make what ya want!

    Click image for larger version. 

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    Gordon

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  35. #75
    BC12D-4-85's Avatar
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    Hey looks like a good bench test! Does it hitch in both R and L rotation flat on the bench? I assume you laid it on a 360* polar plot and rotated at 3*/sec? (Joke). There's bearings that can flat spot from steady wear and a piston/cylinder damper that might need refreshing. If you have the curiosity have it examined and recertified then test again.

    Or replace with another unit.

    Gary

  36. #76
    skywagon8a's Avatar
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    Quote Originally Posted by Gordon Misch View Post
    ..... connected the venturi to the shop vac with instrument and suction gage connected. Adjusted the suction to 2 inches.
    Disregard
    Last edited by skywagon8a; 11-30-2018 at 07:03 AM.
    N1PA

  37. #77
    hotrod180's Avatar
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    Hey Gordon, I've got a vacuum T&B decorating my hangar.
    You're welcome to try it.
    Good excuse for you to fly up there to Jefferson County for lunch or pie.
    But I don't even remember where I got it, so who knows what kinda shape it might be in.
    Cessna Skywagon-- accept no substitute!

  38. #78

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    I think maybe there's a T&B on my shelf as well, that you are welcome to.

  39. #79
    Gordon Misch's Avatar
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    Thanks much guys, I just might try to take you up on that!
    Gordon

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  40. #80

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    Just for the


    Just for the h--- of it I looked at N3667N and the venturi is in line line with the cowl hinge on the left side. This PA12 still has all its original parts and no mods, so I would guess that was where Piper put it.
    You can't get there from here. You have to go over yonder and start from there.
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