Instrument Air - suction?
 

Good afternoon, aviation enthusiasts!

I am a new user, finally registered on the forums but I've been looking for the countless interesting articles and topics that PPrune presents us :)

I have one question that I've been searching but I don't have a proper answer so far.

In what concerns the analog instruments on small planes where air is used, my vision on its working principle is that:

We have a directed flow of air that hits the detents on the edge of the gyroscopes themselves which makes them spin. The "box" where the respective instrument is in, would, in return, be filled with the air getting in, creating some pressure.

Now, where does the suction air enter in these scheme? Is it, instead, suction the force that makes the directed air come and hit the edges of the gyros, instead of pressure upstreams of the instruments?

I hope my question wasn't too much confusing!

Best regards,

André

Merry Christmas André,

You are right; the 'box' or instrument casing is connected via a hose to the suction pump. The air is sucked out of the instrument and there is therefore a flow of 'replacement' air into the instrument. This arrives from the ambient cockpit air via a filter built in to the instrument case and is then forced through the nozzles that impinge on the 'buckets' that are carved into the edge of the rotor-wheel. The reaction to the airflow causes the rotor to spin.

So, as you say, there is not really any high pressure air upstream of the instruments but rather a low pressure downstream, causing the air to flow into the instrument.

Thank you very much for your answer, that was actually my doubt but now I'm clarified that it is vaccum downstream of the cases of the instruments instead of "high-pressure" upstream that creates the airflow!

Merry Christmas eckhard!

Yes - the case of the instrument contains a partial vacuum. In small aircraft - 100 hp or less - there may be no vacuum pump on the engine, but rather a small venturi tube on the fuselage skin which creates a vacuum.

Many thanks again for the answers!

Much more clarified now about this issue I've been wondering about some months now!