The International Cessna 170 Association

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 also noticed the kangaroos would “take off” as I taxied past them . [no George ,kangaroos do not have wings]

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!

(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.

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 .