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Inspired by this question, I would like to know what the pro's and con's of the mostly used vacuum-gyro and the less used high-pressure-gyro is.

Side-question: Are there any other ways to make the Artificial Horizon work?

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    $\begingroup$ Definitely other ways, yes. For example, the old electric spinning gyros, or the new solid state electronic gyros that are taking over the market. $\endgroup$ – J Walters May 17 '17 at 13:43
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Inspired by this question, I would like to know what the pro's and con's of the mostly used vacuum-gyro and the less used high-pressure-gyro is.

It is largely related to the instruments in your aircraft but in reality its all about air pressure and the systems are really doing the same thing in the end of the day.

Lets look at the evolution a bit, early aircraft that lacked an electrical system generally used a small external venturi to generate low pressure suction for the early gyro instruments. Note the venturi tube in front of the door about half way up the body on this early Cessna 170.

enter image description here

(source)

Since it was easy to generate low pressure this way early instruments were built as suction based. As planes got faster and external (drag educing) things were removed these systems went internal. Since the instruments were already suction based it was logical to make engine (or electrical) suction pumps to drive them.

When pumps moved into the engine compartment they came with some issues. Wet suction pumps have an oil issue that can seep into the system and cause issues with the instruments. That was solved with the advent of the dry pneumatic pump however that was a bit more susceptible to debris clogging but could provide both positive and negative pressure.

This paper outlines both systems as well as their pros and cons in depth and answers the question quite well. However the end result is that both systems provide the same outcome and in the view of the instruments there is no real difference.

Gyro Instruments Within the case of the gyro instrument the air flows through the air seals of the gimbalsʼ bearings and across the rotor, spinning the rotor on its axes. The exhausting airflow from the rotor housing applies precession forces to maintain a reference to earth in the attitude gyro and the attitude of the aircraft in the directional gyro. The mechanics of the gyro instrument remain the same, whether it is driven by the airflow in a pressure or vacuum system. The air- flow through the instrument is a result of the differential pressure between the instrumentʼs air inlet and the air outlet ports. In a vacuum system the pressure at the air outlet port is lowered below atmospheric pressure. The differential pressure between the atmosphere and the pump now forces air to flow through the instrument. In a pressure system the pressure at the air inlet port is raised above atmospheric pressure and the airflow is in the same direction.

So what are some pros and cons,

In a vacuum system the filter is upstream and air is pulled through it and it does a good job of keeping the air clean.

...filtering the air entering the instrument. This filter will remove most contaminates from the air, but moisture is not one of them. Periodic replacement of this filter will ensure clean airflow through the instruments.

While in pressure based systems the air filter is generally mounted in line but has a similar effect

In a pressure system there is an inline filter in the line between the outlet port of the pump and the inlet port of the gyro instrument. This restricts, but does not completely stop, the flow of all contaminates to the instrument.

However pressure systems have a moisture problem since the compression of air creates moisture, this may add moisture while a suction system will only draw ambient moisture.

Unfortunately when air is compressed, moisture is created. Although the pressure is extremely low (approximately 2.5 to 4.5 PSI), this system is no exception. Moisture is the No. 1 contaminate, contaminating the gyro bearings in a pressure system.

The paper also outlines that both systems are susceptible to downtime due to pump inspections and system maintenance. Which is a con of both systems.

Side-question: Are there any other ways to make the Artificial Horizon work?

Yes, electricity

enter image description here (source)

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    $\begingroup$ Another view of Venturi tubes. $\endgroup$ – mins May 17 '17 at 18:09
  • $\begingroup$ You might add that the failure mode for some vacuum pumps / compressor may send a lot of debris out with the compressed air. When that is the case, a vacuum system protects the expensive instruments better compared to a pressure system. Debris in the engine department is less expensive compared to debris in the instruments. $\endgroup$ – ghellquist Jun 19 '18 at 15:37

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