vacuum venturi minimum speed?

wby0nder

wby0nder

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Matt Michael
I'm considering installing a vacuum operated turn coordinator and have been advised that at the lower speed range of my Fly Baby, say climbing at 55mph, the 2 inches of vacuum needed to run 1 gyro instrument may not be provided by a "2 inch" venturi. I have a 4 inch venturi but it's really huge. I'd rather have the cute LITTLE 2 inch venturi hanging on my plane but I can't find the specs anywhere to verify the minimum design speed of these venturi's

Anyone happen to know the minimum speed for a 2 inch venturi to produce 2 inches of vacuum?

Thanks,
Matt
 
Matt, how about a PCEFIS instead? Solid State Attitude indicator, completely battery powered with six hours' endurance.....
 
bbchien said:
Matt, how about a PCEFIS instead? Solid State Attitude indicator, completely battery powered with six hours' endurance.....
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Sounds great but I bet it costs more than my airplane is worth.:D

Actually, I kind of like the retro approach. It fits my airplane's style
 
jwriteclub said:
Allow me to briefly plug my employer.

http://dynonavionics.com/docs/EFIS_intro.html

Although it may be worth more than your flybaby :)

Alternatively, install the venturi, get a differential pressure sensor and go fly a bunch of profiles and see how it works.

~ Christopher
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I suspect that the pressure difference would be the same in either size venturi, the real difference being the available airflow. IIRC a two inch is sufficient on most any airplane for driving a single gyro, the four inch would likely be required to drive two gyros off one venturi. You could probably use two 2" venturis each powering one gyro.

And you don't need to test any of this in the air, just hang them out the window of a car.
 
To follow up on Lance's comment, just how many gyros do you have? All you mentioned was the TC -- do you also have AI and/or DG, or it the TC the only thing you need powered?
 
Ron Levy said:
To follow up on Lance's comment, just how many gyros do you have? All you mentioned was the TC -- do you also have AI and/or DG, or it the TC the only thing you need powered?
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Planning only the TC. My "research" so far suggests that these venturi's were rated at 100mph. So, my instrument guys contention that a 2 inch venturi on an airplane climbing at 55 or 65 mph just isn't going to create 2 inches of vacuum may in fact be true. The gyros spin but at a sub-optimal speed giving sluggish though seemingly correct indications. I'm kind of inclined to believe him as he owned a C-140 for decades, has hundreds of various aircraft instruments around his house, and was a second stage engineer on the saturn V rocket. Our local Rocket Scientist.
MM
 
Your 2" venturi is rated at 100 mph, but your Turn Co-ordinator is a .5 Hq instrument. it will spin up on the prop blast only.
 
lancefisher said:
I suspect that the pressure difference would be the same in either size venturi, the real difference being the available airflow. IIRC a two inch is sufficient on most any airplane for driving a single gyro, the four inch would likely be required to drive two gyros off one venturi. You could probably use two 2" venturis each powering one gyro.

And you don't need to test any of this in the air, just hang them out the window of a car.
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The venturi vac systems that used the old AN style gyros required 4.5 Hq to run the gyro, the new style gyro requires 4.8-5.2 Hq to operate.

On my 170 I installed 2, 11inch venturis, and 2 new style gyros, and it required 3 minutes at 100+ MPH to spin them up to stablization.

a 2" venturi is a any venturi that will develop 2 Hg at rated speed.
a 11" Venturi is any venturi that will develop 11Hq at rated speed.

2' & 11" is notthe phisical size of the venturi. it is what the vac pressure will be at rated speed.

I knew you knew that, but many folks don't
 
NC19143 said:
Your 2" venturi is rated at 100 mph, but your Turn Co-ordinator is a .5 Hq instrument. it will spin up on the prop blast only.
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I'm thrilled to be communicating with someone who actually knows something about this!

To correct my earlier post. What I have is actually a Turn and Slip and it's an old one. I was told it required 2 inches of Hq vacuum. The venturi I have is a Sperry 640600, supposedly 4"Hq. It's about 11 inches long, 3 inches dia.

The airplane I want to install this in climbs about 55 and cruises between 75 and 100 mph
 
wby0nder said:
I'm thrilled to be communicating with someone who actually knows something about this!

To correct my earlier post. What I have is actually a Turn and Slip and it's an old one. I was told it required 2 inches of Hq vacuum. The venturi I have is a Sperry 640600, supposedly 4"Hq. It's about 11 inches long, 3 inches dia.

The airplane I want to install this in climbs about 55 and cruises between 75 and 100 mph
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On the instrument some place, there is a tag that tell what the Hq requirement is.

Mine is an old one too, but has a tag inthe back telling Hq requirements.
and my little brass venturi will spin it OK
 

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jwriteclub said:
Allow me to briefly plug my employer.

http://dynonavionics.com/docs/EFIS_intro.html

Although it may be worth more than your flybaby :)

Alternatively, install the venturi, get a differential pressure sensor and go fly a bunch of profiles and see how it works.

~ Christopher
Click to expand...

[thread hijack]

You work for Dynon? Cool. I've got over 200hrs behind the 'old' D-10. In fact, I did my IR training/checkride behind it last fall/winter.

Nice flying and useful instrument! My ONLY concern is DG drift. I haven't tried to research it at all yet, but it seems that the drifts from time to time while not being flown - to the point that the 'MAG ADJ' doesn't have enough span to be able to correct it. Do G-loads cause problems with the solid-state DG or is it a calibration issue?

[/hijack]
 
yea i think the hanging it out the window of your car test is a good idea.
 
Matt,

The turn and slip in my 140 always seems to be ready to go as soon as I get off the ground. I'm not sure what my system vacuum pressure is, but I have a small-size venturi.

We could do some slow flight sometime and see how it performs. I've always wanted to be a test pilot. This is my chance!!!

Matthew
 
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