What It Looks Like

When An Airplane Has A Venturi Tube

Airplanes of old had features that you just don't see anymore-things like a hand crank to adjust the pitch of the prop (Globe Swift) and a cockpit bar to raise the landing gear by muscle power alone (early-model Mooneys).

The photo shows another gadget that has succumbed to progress. It's a pair of venturi tubes mounted on the fuselage of an early-model Cessna 172. A venturi tube performs the same function as the modern-day vacuum pump: It creates suction to drive gyroscopic instruments.

A venturi tube works on the same scientific law-known as Bernoulli effect - as an aircraft airfoil. The pressure in a stream of fluid is reduced as the speed of the flow is increased. When that fluid is air passing over the cambered upper surface of an airfoil, the result is lift. When it flows through a venturi tube, the result is suction.

As air passes through the narrow throat of the tube, its speed increases and its pressure drops. A small port inside the constricted area taps this pressure to create suction in a line connected to the port. The line is plumbed back to the gyroscopic instruments.

It's an elegantly simple way of using physics to drive an aircraft system. There are no moving parts to wear and no power source other than a slipstream is needed-and therein lies the problem. A venturi tube is ineffective until the aircraft gains enough speed to create a flow through the tube. That's not very comforting if you're taking off into instrument meteorological conditions. Also, the tube is prone to icing, and it generates additional drag.

The venturi tube has become a historical footnote in aviation, along with flying wires and hand brakes.