Venturi suction vs. IAS

[DaveF]

My airplane has a single venturi driving a "modern" AI and DG, with no regulator. At 6000 DA and 120 IAS I get about 4.5"Hg suction and about 3.5" at 100. I rarely see less than 5000 DA around here, so I'm wondering if I should install a vacuum regulator for times that I do go to sea level.

What do you guys see? Will I get more suck at low altitude, even at the same IAS?

[lowNslow]

Dave, I think with the modern gyros you need an additional venturi, preferably two 8-9 in. vacuum ones which will also require a regulator. The new style gyros should have 5 - 5.5 in. vacuum for proper operation. With this set up I am getting just under 6 in. vacuum in cruise and below 80 knots it drops to just under 5 in.

[Bruce Fenstermacher]

Dave,

Karl has it right. You actually are not seeing enough vacuum as it is, you would not want to restrict it in anyway. At sea level with the same setup as you have I've barely seen 4.5" even in a high speed dive and I wouldn't anticipate you would either. I don't have a regulator and have no need for it, you won't either.

I always intended to replace my single 4" venturi with an single 8 or 9" and then I'd need a regulator as Karl points out.

[GAHorn]

According to my gyro overhaul book, "modern" gyros require 4.5 and AN gyros require 3.5". A regulator will not "restrict" vacuum...it will "relieve" excessive vacuum....which you do not have and will not likely experience even at low altitudes, so I agree that you do not need a regulator.

I would recommend a dual venturi setup...or a "super" venturi setup with regulator, for modern gyros.

[Bruce Fenstermacher]

A regulator will not "restrict" vacuum...it will "relieve" excessive vacuum

Yes I guess I said that the wrong way thinking about a valve that might reduce pressure with restriction. Sometimes we put finger to keyboard before we engage the brain to think about what should be typed.

[GAHorn]

A regulator will not "restrict" vacuum...it will "relieve" excessive vacuum

Yes I guess I said that the wrong way thinking about a valve that might reduce pressure with restriction. Sometimes we put finger to keyboard before we engage the brain to think about what should be typed.

It's so rare that your brain is disengaged ... it is a "relief" to see it's re-engaged.

[DaveF]

My venturi is already "super", so I'm a little surprised I don't get more suction. Maybe the gauge (some kind of automotive 0-30") is inaccurate at low levels, because the DG holds heading and the AI stays upright even when I'm doing touch-and-goes. I'll probably install a second venturi with the regulator my mechanic has offered me. It's cheaper than a vacuum pump, and the whistle of two venturis would definitely chase the deer off the runway.

[Bruce Fenstermacher]

Dave if you already have a super venturi something seems wrong. How do you know you have a super venturi?

[blueldr]

If my memory is working properly, a super venturi should be able to run both a directional gyro and an attitude indicator simultaneously. My airplane had two venturis, but they only had the ability to run one AN instrument each. It was essentially a matter of volume of air per instrument.

[DaveF]

I know it's an 8" venturi because I bought it. It's got the big flange on the trailing edge.

If you guys think I should be ok with one super venturi driving two modern instruments then I'll get under the panel and check for leaks. One more inch of suction would be nice. But like I said, the instruments seem to work well, it's the gauge that isn't happy.

[hilltop170]

......Maybe the gauge (some kind of automotive 0-30") is inaccurate at low levels...........But like I said, the instruments seem to work well, it's the gauge that isn't happy.

Dave-
Before you spend a bunch of time and money on other stuff, take that 0-30"Hg automotive gauge out and get an aircraft 3-7"Hg gauge and see what it says. Gauges usually are most accurate in their mid-ranges and the current gauge is at the extreme low end of it's range. Plus, who knows how well it's calibrated. Your system is probably working just fine, I bet the gauge is your problem.

[GAHorn]

If my memory is working properly, a super venturi should be able to run both a directional gyro and an attitude indicator simultaneously. My airplane had two venturis, but they only had the ability to run one AN instrument each. It was essentially a matter of volume of air per instrument.

I don't know how in hell I got two comments in here!

Maybe it's because you have TWO venturiis.

(I deleted the second copy of the comment before I recognized the opportunity to "rib" you, bluEldr! This error can occur when the internet-connection seems slow to post your new msg...and you "click" on the msg a second time in an effort to get it "sent". The internet thinks you want it posted twice.)

Richards words are right-on-target! I'd like to add that if you install a proper, calibrated gauge and you STILL find your super-venturi to put out less-than-expected vacuum.... then consider the possibility that you have blockage in tubing...or that a rubber hose has a collapsed inner-liner.

[Kyle]

Dave,

Funny you should ask... In preparing to install my new panel which has a vacuum driven Century 360A HSI , I wanted to know if my Venturies would pull enough suction. I looked them over and was unable to get any info off the data tags which I had hoped would list their capacity.
By the way, my left side Venturi ran the DG, the right venturi ran the artifical horizon. The left venturi has a regulater in the line.

I disconnected the line to the regulator and fasioned a small tube / hose combo that went straight to a 2-10" suction gage. The readings were as follows:

Climb at full throttle

60 mph / 3.8"
70/ 4.0
80/ 4.2
90/ 4.7
100/ 5.0
105/ 5.3
110/ 5.5


Decent power pulled back

70 mph/ 2.2
80/ 3.0
90/ 3.8
100/ 4.5

My conclusion was that the higher readings on T/O are due to air forced thru the venturi by the prop at high RPM. The decent has less prop wash, so the smaller numbers seem logical. The century unit calls for about 4.5 - 5.4 + or -, and if you fly an ILS at 90, it should be fine considering the instrument won't spool down to a lower gyro rotation speed instantly anyway. And in my plane, it's not intended for tight, low ceiling approaches anyway - my minimums need to be higher.

As a side note, I flew a 180hp 172 the other day, equipped with a vacuum pump and upon retarding the throttle to idle abeam the numbers, the low vac warning light came on and the small 1" gage had gone all the way down to about 2. So those instruments were not getting the full benefit of high suction during the landing either. Is that normal for you folks flying with pumps, AND does my logic work? Your thoughts....

Kyle

[HA]

Kyle, the 172 with the vacuum pump should be able to hold normal vacuum with the power pulled back, likely the pump is pretty worn out (inside clearances).

George's advice to check your vacuum hoses for inside disintegration is right on the mark. with these old airplanes the age of the hoses and the high probability that nobody has ever checked them means it's a good idea to replace those fossils.