Vacuum Powered Instruments - The Facts
- Venturi sizes are measured in inches
- This refers to the amount of suction measured in inches of mercury the venturi develops at 120 mph, not to the physical dimensions
- The faster you fly the more suction is developed
- Venturis should be mounted in the slipstream away from disturbed air
- If you are not sure where this is on your aeroplane, look for inspiration from similar types
- Connect with soft plastic hose
- Avoid constrictive bends
- An intake filter will prolong instrument life
- Filters will block with use and so require inspection
- Venturi powered gyros may not be effective until some time after take-off
- Venturi systems are not ideal for IFR flight because they can ice up
- A suction gauge will help to monitor the health of the system, but is not mandatory for VFR flight
- A 2" venturi will power a turn and bank indicator only
- A 4" venturi will power one vacuum horizon or a directional gyro
- A 9" venturi will power a horizon, a turn and bank and a directional gyro, although convention says that if you have a vacuum horizon, an electric turn co-ordinator is the usual combination