The magnetic wheels used in the SFENA HSI are in itself a pretty neat thing. Prior to the use of magnetic wheels digit drums had been driven by gear linkages and motors. Magnetic wheels work by rotating a drum with a magnet and digits printed on the outside around 10 positions that can be individually energized. By doing the former, the display can show single digits ... one at a time and, they are perfectly silent.
The magnetic wheel allowed for a much more simplified way of showing digital data without the need to create complex motor mechanisms. Below is a video that shows the 10 position magnetic wheels in the SFENA HSI instrument in more detail. Each digit in the 5 digit distance-to-go is a unique magnetic wheel:
Both ground speed as well as distance-to-go use magnetic wheel technology. Of course, later on magnetic wheels got replaced with 7 segment LED displays.
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