Venturi location

[minton]

minton, first of all you are giving yourself more credit than you deserve. You couldn't ground my plane if you tried.

Second, you asked me a question in a public forum and get defensive when I actually reply? I apologize...I'll try and learn to just go ahead and let you spread your gospel unchallenged from now on. It may take me awhile to adjust though, so please have some patience.

You asked me to explain the rational of having to move the static port to the opposite side that the venturi was located on. So...do you agree that the Cessna Acc Kit document shows the venturi and static port on opposite sides of the fuselage? Mine sure is. I think there are a few other 170s (regardless of which side the venturi is on) that have their static ports on opposite sides from their venturis too. (Hint: there is the background for my statement).

In case you missed it, my only declaration was/is don't have your static port on the same side as the venturi. I think you're fishing for me to declare that it's OK to put your venturis anywhere you want, but I've never/and won't say that.

I've always enjoyed your input on this board, think you're a good guy, and you're probably a heck of an AI A&P, but you're having issues seeing the forest for the trees here. I forgive you though, as there seems to be a lot of times on this board that some really smart folks forget that their opinions of what is/is not approved on these old airplanes is much less meaningful than an airworthy airplane signed off by an AI, and in compliance with their local FSDO. My aircraft happens to be one (of many others) that have the venturi on the left...don't lose too much sleep over it OK?

According to some other folks that have venturis on both sides (GASP!!!!) the left side ones don't work as good...that should make you feel better, right?

Absolutley not fishing for anything. Sure wish that I was though. This winter has been the pits!!

I don't get upset by this stuff just enjoy the back and forth towards the goal. I would never ground an airplane. George sez I can't

[russfarris]

Both my 170 and my current '57 172 have the venturi mounted on the left side, added years after manufacture on a 337. Static port is still on the left, and both aircraft are/have been IFR certified. Both work very well. I'm not following what the issue is supposed to be.

Russ Farris

[Bruce Fenstermacher]

Russ, there may or may not be an issue with the venturi on the same side as the static port.

From memory we had a forum member who had a venturi on both sides and he discovered that the left venturi didn't seem to work as well. He asked if we might know why. I don't recall anyone could explain it but in the conversation of course we pointed out the Cessna installation and it's relationship to the static port. Some thought was that the venturi may on the same side may effect the static port. I don't recall anything but speculation came out of that discussion.

[cowboy]

FWIW, my 1950 'A' model came from Wichita with one 2" Venturi on the left side and the static port on the right. It has since been upgraded, first with an additional 4" Venturi above the original 2" in the early '60s and then the 2" replaced with an additional 4" about 5 years ago. While taxiing I see 0.5 to 0.7 inches with 1000 rpm and around 4.4 inches in flight.

A therory about higher vacuum with the venturi on the right side than the left, at least while on the ground. Given the three point attitude and the pitch of the prop blade, the blade on the left side is acceding and taking a relatively small bite of air, where the blade on the right side is taking a much bigger bite. If my therory is correct the amount of air moved through the venturi would be notably higher on the right side, thus making the venturi on the right more effective.

[GAHorn]

FWIW...
A therory about higher vacuum with the venturi on the right side than the left, at least while on the ground. Given the three point attitude and the pitch of the prop blade, the blade on the left side is acceding and taking a relatively small bite of air, where the blade on the right side is taking a much bigger bite. If my therory is correct the amount of air moved through the venturi would be notably higher on the right side, thus making the venturi on the right more effective.

The same is true of aircraft in high power, low speed climb....when vacuum would otherwise be
low, as descending aircraft would presumably have plenty of speed and vacuum.
I believe that is why Cessna mounted/illustrates them on the right.
(Just because an airplane " left wichita" with one mounted on the left...doesn't mean Cessna did
it....Yingling did lots of alterations for new owners prior to delivery in accordance with custom
orders.

[minton]

I Emailed my brother in law in hopes of him pulling the rabbit (Or a pooring written paper) out of the hat as he is a aeronautical engineer.

Questions presented:

1) What factors come into play when determining the optimum positioning/location of a gyro instrument drive venturi on a single engine aircraft.

2) When subjected to prop wash. (Direction of rotation, RPM, airspeed, angle of attack, etc.

3) What comes into play if a static port is near by.

Who knows we might get the some useful information from him

[jrenwick]

Well, we have a couple of good data points that I find persuasive for putting the venturis on the right side:

1. Asymmetric thrust -- the right propeller blade generates more thrust than the left at high angles of attack (slower airspeeds). This has to mean the airflow over a right-mounted venturi would have more velocity than over a left-mounted one, at times when you might need it most;

2. Actual measurements by members with a venturi on each side, showing that the left venturi produces less vacuum.

I can easily imagine engineers (not just Cessna's) favoring the right side just for reason #1, and maybe never even testing how left-mounted venturis perform.

Given that, from a manufacturing and maintenance point of view it would be natural to put the static port on the opposite side from the venturis -- not for performance reasons, but just because the plumbing behind the venturis takes up quite a bit of space, and the opposite side is a less congested place to put the pitot-static hoses.

[cowboy]

(Just because an airplane " left wichita" with one mounted on the left...doesn't mean Cessna did
it....Yingling did lots of alterations for new owners prior to delivery in accordance with custom
orders.

Which is located in "Wichita". Gotcha! (Georgenote: I din't dispute Wichita. I merely pointed out that didn't mean that Cessna factory did the installation. You din't "gotcha" me.)
Not saying that the location of the venturi on the left is "correct", just saying that the first reference of venturi's in my logbooks is the addition of the second one on the left.

[minton]

The factors to which aeronautical engineers and the fliud dynamics guys consider when engineering placement of static ports and venturi's are the following (+ or-) .

Static port location:

Basically they place them in the most quiet spot on the fuselage or other airframe positions based on airflows, chances of icing or other contamimation and steady "real" ambient air pressure. If a venturi is located nearby the venturi can influence the ambient air pressure through either the compression of air in the venturi areas or turbulence caused around and behind the unit. This causes errors in altimeters, vacuum guages, air data sensors used in autopilots and altitude reporting units and VSI's

Venturi placement/location:

This relates to airflow vectors driven by prop rotation (Stand off from airframe skin to venturi centerline, airframe angle of attack, engine RPM, propellor blade angle and direction of rotation). Forward airspeed can mitigate/resolve some airflow issues. Disturbed air caused by turbulance, icing and/or extreme angles to relative wind can actually cause the venturi to stall in extreme cases. Being that the right hand engine rotation on C-170's and large prop blade angles (to which RPM has relationship) at the inboard propellor blade stations both tend to redirect the relative wind in a positive relationship to the venturi when the venturi placement is on the right side of the fuselage. These exhisting conditions render the airflow not to be able to progress to that stall extreme. The issues relating to placement of the veturi on the left effect things in an inverse manner.

There is probably more to the story but thats all I can remember from our conversation.

I think many members have touched on many of these points giving me confidence in intuative thought. (Right brain thinkers) Most pilots excell in those areas!

I hope that explanation by brother in law helped to clarify the conversation.

[blueldr]

Back in the olden days when I kept my airplane, an L-5G, on Weeks Field in Fairbanks, AK., one of the operators there had a cut-out valve on his exhaust that, when selected, routed the exhaust through a venturi to run up his gyros before take off. I thouht that was a pretty good idea. Once the gyros were up to speed and the instruments stabilized, they would hold until air speed on the regular venturis provided the vacuum needed to maintain them. He told me he got the idea from when he was a fire fighter and one of the fire trucks had that kind of a system on an auxillary pumping motor used to suck up water to fill the tank in the truck. The Vacuum provided sucion to prime the pump.

[MoonlightVFR]

My 54. B model came with one venturi

In 1982 I was doing instrument training in this A/C. Instrument approach practice ADF. And the DG. Would begin to wander slightly as we slowed. I made the course adjustments but this concerned the. CFII greatly. He researched the subject and it appeared that a DC3 on the field had two (2) venturri. Because of wanderering DG.

I installed a super venturri and that helped greatly . It took two to get the job done at slow landing speed. both right side.


On the left side said a/c. Has a small venturri just for the. ( ???? ) For years I kept a small 20. Inch baseball bat in the plane. Open the left window and tap the small Venturi to dislodge ice. Never had to use it for that purpose.


Regards