OK, a question for the techies that I’ve been pondering today.
Vacuum driven instruments, e.g. artificial horizon, direction indicator etc., can be operated by vacuum pressure generated by a venturi – let’s use the very early Hurricane as an example where the venturi was stuck just below the cockpit.
Later Hurricanes didn’t have this venturi which set me thinking – what are the alternatives for driving the instruments? Do you have a venturi hidden within the aircraft structure with an airstream ‘inlet’ or something? Or are the instruments operated by a vacuum pump of some sort – engine-driven or electrical?
This may well be a silly question but I am more used to the later electrically driven instruments so it’s all a bit of a mystery to me!
Thanks…M
By: PeeBee - 16th October 2003 at 13:39
The venturi needs air through it, so facing it into wind will start it to operate. They need time rather than speed of airflow running through them, of course airflow is critical. However, in order for an vacuum instruments to operate correctly and reliably you need to spin the Gyros inside the instrument to their correct RPM, these are quite high and with a venturi system can take some minutes (rather than airspeed) to get to operating speed.
By: YakRider - 16th October 2003 at 12:46
Probably at about 40mph. This is when the ASI usually starts to register (via the pitot tube).
YR
By: MarkG - 16th October 2003 at 12:42
Thanks Eddie.
Another question though – if driven by a venturi, then nothing will work until the aeroplane is moving and there is sufficient airflow through the venturi to create the required vacuum. So how much speed is needed for that to happen?
Sorry this is turning a bit anoraky but curiosity has got the better of me now!
By: Eddie - 16th October 2003 at 12:22
Yep – there’s a pump. It’s engine driven if I recall correctly (I’ve got a brand new Mosquito one in a box around here, but it’s wrapped up so I can’t check!).
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October 16, 2003 at 12:18 pm