During part of World War 2, my father used to service these things at No 7 Service Flying Training School at RAF Collins Bay (now Norman Rogers Airport) in Kingston, Ontario. It was part of the BCATP (British Commonwealth Air Training Plan in which trainee pilots were sent away from the war zone to Canada and Australia to learn to fly ). Many years later in the 1980's , I ended up repairing and servicing a Link trainer that had  belonged to the NZCAA and that had been given to the RNZAF Museum.  Between just two of us, we learned how it worked and got it fully operational, along with all the original instruments, the plotting table," and the <i>"remote instrument  box"</i> and the <i>"crab"</i> (both of which you can see at <a href=""https://www.youtube.com/watch?v=RJAYZgOZS08&amp;t=4m05s"">4:05</a>). The crab was an interesting thing", a triangular box with rounded corners that had three wheels on the bottom all of which (driven by servos and electric motors) steered the crab around the plotting table. The front wheel had a rubber tyre that brushed against an ink pad so that it drew the student's flight path on a map on the plotting table. <br>The Link trainer was a truly awesome thing to work on - electro-pneumatic  and electro-mechanical technology from a bygone age, and something I will never forget.