English: Close-up of the forward part of a venturi tube mounted on the side of a light aircraft's fuselage. Venturi tubes were used on small aircraft as a means of providing suction for air-driven gyroscopic instruments such as the artificial horizon and directional gyro. The aft end of the centre tube is in line with the narrow part of the outer tube. A vacuum is created in the centre tube, drawing air through the circular slot that can be seen inside the centre tube behind the unpainted lip. Plumbing connects the slot to the gyroscopic instrument. Air from the cockpit is sucked through the instrument, spinning the gyroscope rotor.
The drawback to using venturis is that the aircraft has to be moving quickly to generate the airflow through the venturi necessary to create a vacuum, which means that the instruments will not work until after the aircraft takes off. Modern light aircraft are fitted with engine-driven pumps to provide suction. These tubes are photographed installed on a Piper Colt.