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Pull-String Evolution

Discus launch gliders have been benefited with the advent of the pull-string system of actuating tail feathers, offering an extremely light complete system with no slop and ease of installation. By placing the system in constant tension, any play in the linkages and gears are taken up by the spring’s load, while removing the pushrods from the rear and replacing it with two lengths of Kevlar string and a small length of 0.5 mm wire for the spring helps save a gram from the rear and 4 to 5 grams in the overall all-up-weight.

But, what can we do to further improve this system?

In a standard setup, the displacement along the length of the string varies from the center of operations to the end of the travel. This is because the radius is essentially decreasing in size as the servo arms and control horns are moved and deflected from center. By using a pulley on the servo side, the radius by the servo is kept constant against the varying radius on the control surface side.

Taking the rudder as an example, we typically aim to have 15 mm maximum deflection, and it is achievable using a standard installation with the servo-arm : control-horn ratio at the ‘golden’ 1:2. With the pulley system, to achieve the same maximum deflection, we can set up the installation with 1:2.65, greatly increasing the resolution, precision, and holding power via mechanical means in the central region where it’s most important. Suddenly, your system is 33% more precise and strong where it counts!! As an added benefit, the load on the servo is now reduced by 33% to keep the servo at center, leading to longer battery times and lower servo strain.

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