TECHNICAL TOPICS

Tech Topics -- Direct Servo Drive for Control Surfaces (As told by cal to Oscar Weingart)

Cal Malinka, an old acquaintance from the MARKS club, is all excited about a method for driving a control surface with a servo that uses no control horns or pushrods. This could be a boon to Scale Modelers. This idea has been around for a while, and I seem to remember someone running ads for the needed adapters. I will try to sketch the method for you, and also describe it in words.

The servo must be mounted so that the output shaft is lined up at right angles to, and in the plane of, the hinge line. An adapter screws to the servo wheel, connecting a length of music wire that rotates axially with the wheel. The end of the wire is bent at an angle and resides in a close-fitting slot in the control surface. As the wire rotates, the bent portion within the slot forces the control surface to move. Simple enough!

Cal says that he made a slot insert of two pieces of sheet aluminum, using the same diameter of music wire, with some paper shims, to get the correct spacing. He tied the edges of the slot insert with thin wire stitching. This sandwich is then inserted in the control surface. If everything is properly lined up, it works just fine.

At neutral, the bent wire lies flat in the slot. As the servo rotates, the end of the bent wire goes either way from neutral, say up or down, taking the control surface with it. A sharper bend produces a greater angle of control surface movement. Cal says that no lubrication is needed, and that the slot must fit the wire closely, with just enough clearance to allow easy sliding. The adapter is easily made on a small lathe.

(The latest Issue of RC Report magazine contains a report by Cal on his experiments.)

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