Venturis

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Venturis

Postby ghostflyer » Thu Oct 15, 2020 1:04 am

Just have been reading some old posts on venturis and found it was a very interesting topic . My aircraft has fitted a dual venturis 9in and a single Venturi of 4in. This was the set up on the aircraft since I have owned [20 yrs] and haven’t any problems . The aircraft has a AH and a DG which has supplied vacuum air for both instruments . Both venturis are plumbed into the same manifold . Idle the vac gauge shows about 1.5 ins and both instruments are nearly up to speed On take off ,no issues what so ever . In cruise it’s about 4.5 ins . But there is no vacuum regulator . Both instruments were overhauled about 4 years ago and it appeared to be the first overhaul for both instruments . One of the large venturis is made of bronze . Now the third Venturi provides vacuum for the “bat and ball” turn indicator . It provides about 2ins of vacuum, this 3rd Venturi [4in] is also made of bronze . Manufacturer unknown. When COVID 19 restrictions are removed [when ????] I will be fitting a garmin G5 . However the small Venturi has another task which makes me very reluctant to remove it. It produces a high pitched whistling noise below 90 kts . I have flown over people at 500 ft and they hear it on the ground . Many have said I have a bad exhaust leak or exhaust gasket blown . It’s this 4in Venturi . Why?? I do not know BUT this high pitched whistle frightens off the kangaroos off the runways . Kangaroos in Australia are in plague numbers and eat the grass on the runways and drink water in the drains near the runways . They have become very used to the movement of aircraft . It’s a problem at many ALA,s. I noticed on some of my landings ,the kangaroos would move off the runway as I approached . After a couple of experiments to trace this noise [I could hear it in the cockpit ] I also noticed the kangaroos would “take off” as I taxied past them . [no George ,kangaroos do not have wings]
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Re: Venturis

Postby GAHorn » Thu Oct 15, 2020 5:37 am

The ‘Roos cannot hear the venturii..... It’s been widely broadcast to them that YOU David, ...are flying the airplane and they KNOW to get the hell out of the way! :lol: :lol: :lol:

(Seriously... are you CERTAIN they are actually “bronze” and not simply anodized...(or alodined)... in that color?

On further note.... the 4-inch venturi and the 9-inch venturi are referring to their capability to develop 4” and 9” of Hg/vacuum.... not their actual length. (The 4” venturi is 10-1/2” in length for example.)

It was Cessna’s intent to install the 4” venturi in a VFR airplane. The IFR kit (which utilized AN horizon and AN gyro-compass) used dual 4” venturiis. Keep in mind that until the 1950s an IFR equipped airplane only required a Turn and Bank as the “primary” gyro. The AH and DG were considered “high tech”... previously only needed to drive an autopilot such as in the B-17 and B-24 aircraft.

The early AN gyros require less vacuum (about 3.5”) than later 3-1/8” (so-called “pictorial” gyros) which require 4.5” vacuum.
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight.
An originality nut (mostly) for the right reasons. ;)
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Re: Venturis

Postby ghostflyer » Thu Oct 15, 2020 11:44 am

The 2 larger venturis are plumbed together while the smaller Venturi is plumbed only to the bat and ball. I am 90 % sure they are bronze as they are of a soft metal and coloured bronze. Plus the smaller Venturi has a very small stone dent in it. I am doing some paint touch ups and painting some stripes and rubbed some of the paint off the venturis as one is to be coloured black.
A kangaroo is something I do not want to hit . I have seen the result of a MT prop [fitted to a super decathlon] hit a kangaroo ,while the roo lost his head over it ,the prop looked like a sock with a heap of kindling wood in it. Engine tear down showed crankshalf damage and other engine damage .
I will put up with the whistling noise .
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