Venturi Tubes

AdamZ

AdamZ

Touchdown! Greaser!
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Adam Zucker
A while back there was a 172 for sale over on Red and I just saw an add for a Sundowner and each had venturi tubes. When someone on Red mentioned something about them someone responded that if you have a venture tube powering your suction instruments you don't need a vac pump and thus there is no vac pump to fail.

It got me to thinking that in IR training we train for instrument failure and vac pump failures etc. So why then did the aviation industry switch to vac pumps from venturi tubes? I thought perhaps at lower appraoch speeds the tubes were less effective but I've heard thats inaccurate.

So what gives if a venturi tube follows the K.I.S.S principle then why the switch to vac pumps?
 
they wanted operating instruments before takeoff.
 
I have a 2" venturi tube on the belly as a backup for the vacuum pump. It works much better than the engine suction type backup since you do not have to throttle back to get vacuum. The only drawback is that it does not work on the ground. But one minute after takeoff you have enough gyro speed. The 2" provides enough vacuum for two gyros at 140kts, which for Mooneys works pretty well.

José
 
AdamZ said:
Ah makes sense, So I guess prop blast is not enought to get it going.

Yes it is, it requires about 30 seconds at full power.

how much drag could a tube cause?

Not much I couldn't see any difference with or with out them on my 170.

I'd imagine they could heat the venture tube as well as the pitot but this also makes sense.
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They could but why?

the reason we went to vac pumps is simple, we the public wanted more instruments in our panels, that required the Gyros be smaller, The old AN1 style does not require as much vac (3.5 -4 Hq) as does the newer small gyros (4.8-5.2 Hq) and the venturies can't deliver the pressures to operate them properly at low speeds.
 
AdamZ said:
Ah makes sense, So I guess prop blast is not enought to get it going.
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Correct, at runup RPM, you can at least get an indication that the venturi is providing suction, but you really don't know if the gyros themselves are working properly until you are committed to flight.

It is definitely less than ideal to take off in low IFR conditions with a venturi system.

Venturi vacuum systems were pretty common in GA airplanes into the 50's. Alot of early GA engines (like the C-145/O-300 A) won't even accept an engine driven vacuum pump.
 
Fearless Tower said:
Correct, at runup RPM, you can at least get an indication that the venturi is providing suction, but you really don't know if the gyros themselves are working properly until you are committed to flight.

It is definitely less than ideal to take off in low IFR conditions with a venturi system.

Venturi vacuum systems were pretty common in GA airplanes into the 50's. Alot of early GA engines (like the C-145/O-300 A) won't even accept an engine driven vacuum pump.
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Yes they will there are 4 or 5 STC to place a van pump on a C-145/0-300-A
 
Fearless Tower said:
Venturi vacuum systems were pretty common in GA airplanes into the 50's. Alot of early GA engines (like the C-145/O-300 A) won't even accept an engine driven vacuum pump.
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That was one of the features of the "Skyhawk" option package introduced on the 1961 C-172B. "Skyhawk" equipment included an O-300-D engine with vac pump. The base 172 had the O-300-C without a pad for the pump, so if you wanted gyros you needed venturis. It continued that way until Cessna switched to the Lycoming O-320 with the 1968 172I.
 
Kinda seems like the venturi should make a come-back as a backup vacuum source, though... cheap, works, simple... hmm...

It'd probably cost $2000 a pop after "certification" costs, though. Sigh.
 
denverpilot said:
Kinda seems like the venturi should make a come-back as a backup vacuum source, though... cheap, works, simple... hmm...

It'd probably cost $2000 a pop after "certification" costs, though. Sigh.
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My 2" tube cost me $50 from aircraft Spruce and the local FSDO had no objection on the field approval. Keep in mind that venturi tubes are not new to aviation and has been used for decades in the past.

José
 
I own a 1957 172 lyc 180hp conversion vacuum pump. It is set up IFR and flown IMC. I have the venturis set up as the back-up vauum source using a Precise Flight back up system control on a Field approval. We test it every year at annual works great.
 
OBTW the XX" is not the size of the unit, it is how much Hg the venturi will pull.

my needle ball requires a 1/2" venturi.
 

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I WANTED to play with a back up system on my IFR AA1B just to see if it would work.

This was around the time Precise Flight was having problems with their system.

IMPO a 11" or 14" venturi, under the cowl and situatied close to an exhaust pipe is the simplist and least expensive vaccum backup possible. Like a heat muff you need to keep the airflow going thru it to keep it cool during non-use but air's going thru the cowl anyway right? You could use the inlet for the heat muff for testing if you were so inclined...:wink2:

Add a low vac light and a E/P switch (or a simple cable) and you have an on-the-fly backup at a low cost.

Burt Rutan was right (ain't he always?), "sometimes engineers create problems just so they can solve 'em".

Once you've passed the pressure point at liftoff and removed the iceing problem I don't see a downside to Venturi's being a backup...

Not enough moving parts prehaps???

Chris
 
Hg = Inches of mercury.

Measurement of how much it sucks.

Like a Dyson. ;)
 
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