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Permalink Reply by Keith Thomas on November 25, 2020 at 8:35am

Personally I have an issue with using RC servos on full size aircraft for autopilot. Also from there drawing to use the Xflight autopilot, you'll have to add a new trimtab to the elevator so the servo can run it, and one on the flaperons for the same thing, I don't think they can run the controls directly. It looks like the use trim tabs.

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Permalink Reply by Bob Pustell on November 25, 2020 at 6:59pm

When the hydraulic systems died in the Boeing 727 (which used hydraulically boosted flight controls) it was flown in "manual reversion" which is a system such as you describe, a small tab on each flight control that flew the flight control to the desired position. The difference from what you describe is that the pilot, through cables and pulleys, operated the tabs rather than a servo. It was a slow response, heavy effort way to fly a couple hundred thousand pounds of airplane, but it worked in a lumbering sort of way. In my earlier USAF life the C-141A also used manually operated tabs as the last resort flight control system in the event of loss of all hydraulics. That plane also was a lumbering beast to fly in "tab operable" as they called it, but it worked. In both of those planes you knew you were doing a lot of physical work by the time you got the thing on the ground.

Trivia for the day - back when men were men and sheep were afraid, the original big jet, the Boeing 707, used manual tabs as the primary control system. The later models with bigger engines had hydraulic boost added to the rudder but all variants had only manual tabs for the other controls. I only flew that plane in the simulator during a new hire interview and man it was a bear to fly. Those 707 guys must have had arms like gorillas! And then, during a full power missed approach they failed the rudder boost on me while they failed an outboard engine. Those guys must have had pretty strong legs also! I got the job, though...........

Given that limited exposure to tab operated flight controls I would be nervous of using such a system in a light plane but maybe it would work well in a plane with lighter control forces to deal with. Dunno.

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