What makes the needle move on the Slaved Gyro Magnetic Compass? What effect does tuning a VOR on NAV1 have? I'm trying to learn how to navigate and this bird has me stumped!
Don't confuse those old gyro compasses to an RMI. The old gyrosyn compass was a fixed background with a needle pointing to the airplane magnetic heading. The needle was driven by what was known as a flux gate - a measurement of the magnetic field that was processed through a gyro stabilizing routine to avoid compass errors caused in turning, climbing accel / decel as seen in a wet compass.
Later, RMIs were developed, wherein the background card driven by the flux gate moved to indicate magnetic heading under the 12 o'clock index (thereby eliminating the need for the needle), and included 1 or 2 needles that were driven by VOR, TACAN and /or ADF receivers to indicate the magnetic bearing to a station. If no station was received, the associated needle was typically set to indicate 180 degrees or alternatively, the 3 o'clock position on the card.
If an RMI had two needles, they were typically single and double bar-ed to distinguish them. The single bar was usually VOR or TACAN, the double bar ADF - but their were exceptions. Some setups used the needles for number 1 and number 2 VORs with a separated indicator for ADF.
Some old gyrosyn compasses were able to rotate the fixed background card with a knob on the outer casing; the pilot would rotate it to the desired heading he wanted to fly, then turn until the needle was at 12 o'clock - in other words it made it easier for him to observe and follow a desired heading by keeping the needle at 12 o'clock. Not all gyrosyns had this feature.
As an added point - a DG or "directional gyro" is not the same as a gyrosyn compass or RMI. They are only gyro stabilized, with no sensor for magnetic heading. The have to be set by the pilot on occasion by reading a magnetic compass in steady state conditions and adjusting per the wet compass calibration card. Their are two normal types of DG; the old horizontal rotating drum type as seen in say a DC-3, and the more modern upright rotating card type seen in typical light aircraft such as a Cessna 150, etc. They have no navigation needle overlays.
Slaved gyrosyns operate in a slaved or manual mode - in case the gyrosyn input appears to be erroneous the pilot can revert to the manual mode and it acts as in a DG system.
There are two gyro compass driven card instruments. The one on the right is an RMI with one needle for indicating bearing to an ADF station.
The one on the left they call an HSI, but it is a bastardized gizmo to use with the autopilot hdg and nav modes. The HDG bug sets the desired AP heading. The needle will rotate through the autopilot course window selector to indicate that set course. Selecting NAV with the autopilot, it will track that course. I have never seen something like this, but who knows.
Having flown some of these ancient indicators in my younger days (I turned 78 yesterday), I offered some thoughts on some of these types of indicators in my input below.
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