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P O L Y T E C H . N U
English

Sfena H301 BAM kunstmatige horizon

English introduction
imageThis article is about the Sfena H301 BAM standby artificial horizon. This instruments is used in airplanes and indicates the horizon so then there is limited sight, the orientation of the plane in relation to the horizon can be seen. The pitch an roll are graphically displayed by the instrument. (For the direction indication a directional indicator is needed.)

When the power to the instrument fails, the marker flag appears to indicate a (possible) false reading. Also the default (off) position is shown at the image at the left. In the off position the instrument has an odd indication. The result is that the reading is very wrong and easily recognised by the pilot. If the horizon position was rather normal, the wrong indication may not be noticed. This is a nice safety feature.

English description
This instrument is a standby horizon. This means that there's a main artificial horizon for navigation and if that fails, this instrument is the backup instrument. The main horizon is usually a more advanced instrument. The main indicator is placed in the main instrument panel and the gyroscope is placed in the avionics bay. Both are linked together electrically. This main instrument is usually powered by 115 VAC single phase 400 Hz or 200 VAC three phase 400 Hz. The standby instrument is stand alone, so no synchro position outputs and it works on 28 VDC. it the regular electrical system fails, this indicator is still function al on a battery or Ram Air Turbine (RAT).

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English connections
I don't have the documentation for this device, so the connections are found out by reverse engineering. There's a six pin connector at the back of the instrument. Pin 'D' is the +28 VDC power input and pin 'B' is the 28VDC return (or ground). The gyro is powered by this power and the instrument illumination. There are four other pins connected to internal wires, but the function of the four pins isn't known (yet). At startup the power consumption (except for the illumination) is 2,1 Amps and 300 mA if the gyro is at full speed. The incandescent lightbulbs in my instrument are broken so I don't know the current draw of the illumination yet...

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English technical details
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English self balancing mechanism
imageThis gyroscope can drift to it's home position over time due to the self balancing weight on top of the gyro center. The drift is (luckily) very slow. If a gyroscope (with a self balancing mechanism) is placed upside down, after a while the gyro will drift to 'normal' position due to the self correcting mechanism affected by gravity. Since a plane is usually flying straight forward, the self correcting mechanism doesn't affect the reading. Well, the curvature of the earth is 'corrected' to get a good reading. If a plane would fly upside down for a very long time, the gyroscope will drift to it's 'normal' position and the reading would be eventually wrong. Under normal circumstances this wouldn't affect the usability. In fact, if there was no self correction of a long time, the horizon would be upside down eventually when flying halfway around the earth. So the curvature of the earth is a much smaller challenge...


The trick is that there's a weight on top of the gyroscope that can move around the axis for a certain angle. There's a second movable part that can limit the angle of movement of the weight. In upright position, the weight is rotating slowly and therefore the weight pulls the gyroscope in every direction. This means that the gyroscope is vertical as desired. If the gyro is not perfectly centred, the weight stays a little bit longer at the highest point. Therefore the gyro is pulled slightly to one side until the gyroscope axis is perpendicular to the gravity to the earth.

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English caging mechanism
Main artificial horizons have an automatic reset built in. The gyroscope gimbal is equipped with 'electrical spirit levels' and torque motors. When the gyroscope motor is powered, the automatic reset is also activated. The spirit levels detect the gyroscope's position and the electronic circuit activates the torque motors and 'send' the gyro core to the default vertical position. This is done automatically. But since the standby horizon is rather simple, there's no auto erect system available. The resetting has to be done manually by pulling the reset knob at the front of the instrument. By pulling the knob (gently) the gyroscope is caged. The mechanical mechanism forces the gimbals to the default position. The best way is to pull the cage knob and then start the gyroscope motor. The second best way is to pull the cage knob gently when the gyroscope motor is spinning up. It needs more force to reset as faster the gyroscope core spins. The caging lever is shown at the image below. When the bar ar the bottom is pulled to the left, the swing arm (left bottom) swings the roller to the gimbal mechanics. The roller is forced to the lowest spot on the gimbal cage. Therefore the gimbal cage is forced to the default position.

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English video

https://youtu.be/Lk9QTHG2SVA


imageJust for fun I made simple video of the artificial horizon. It had some views on youtube, not that special. But at the end of February 2024 comments and subscriptions same pouring in 'from nowhere'. Apparently Youtube suggested this video in the feed since the amount of views exploded from 20.000 views to 200.000 views in just four days! If I knew I spent more time checking the video. It's mentioned that this device needs 28 VDC singple phase, but there is no single phase DC dôh! The text 'single phase' was copied wrong. Oopsie...


English also interesting...
I made also a video of the first test with a Honeywell VG311 vertical gyroscope and a Smiths H6 artificial horizon visible. Both devices are powered by the 1A250 static inverter with 115 VAC 400 Hz. This is a setup for the main artificial horizon. The standby horizon like the Sfena H301 BAM is the back-up instrument.