Pull-Pull for weak structure and hinges, but it will be heavier

27 Aug 2018

Here is an idea of a form of indirect Pull-Pull control system for weak structures and hinges. It is more complicated and it will be heavier, but it minimizes the stress forces to structures and hinges. It uses pushrods from the servo arms to operate pulleys (or pulley strings) and at the rear part of the pulley system, it can use pull strings or pushrods to move the control horns. It is always a balancing act, depending on what I need and what I am willing to forego. Afterall, a simple wire supported in a few spots with tubes might already be the control solution if the slight weight gain is acceptable.

I have done a couple of unsatisfactory pull-pull for 1s RC (WLToys' receiver servos board). Pull-Spring is not optimal because there was a constant drain due to the servos having to resist the spring. Pull-Pull was tried on that large winged stick motor glider and I had to brace it with additional tie strings because the pull strings exerted enough force to bend the long and slender fuselage boom stick. There was also that idea of converting a single horn into a pull-pull by gluing on a pulley to the front of the horns which seems pretty neat and light, except that it could still bend the fuselage and it requires better hinges than tape as all the forces are exerted on the hingelines.

At the core of the proposed indirect Pull-Pull system is a central core which supports the load of a series of pulleys. In this way, the stress is isolated and the aircraft structure's slenderness is reduced. Servo arms' pushrod could be heat-shrunk to the pulley lines, and the output could either be control pushrods similarly heat shrunk to pulley lines, or pairs of pull strings tied to pulley lines.