Excepted from:

Model Aviation Magazine, February,2002

Radio Control Soaring Column

By Mike Garton


Walt Dimick, maker of the Little Big Winch, is manufacturing heavy-duty RDS components. These components are for large and/or fast airplanes.

Regular RDS couplers are nylon; these are machined from aluminum. They are predrilled and tapped to accept two setscrews in each coupler. The torque rods are type O-1 hardened and tempered tool steel (hardened after bending). Pre-made pockets are included. The systems are available with 3/32 or 1/8 inch torque rods.

I have talked with some pilots who have reservations about installing RDS in large, fast airplanes. They argue that the torque rod can function as a torque spring and contribute to flutter.

One feature of Blaine’s design spreadsheet (Blaine Rawdon is the author of an Excel spreadsheet that calculates linkage throws and forces, mentioned earlier in Mike’s column. http://members.home.net/evdesign/ If this link does not work, you can try another link to another technical page at: http://www.proptwisters.org/rds2/RDS_Kinematics/RDS%20Kinematics.html) is the ability to calculate stiffness of RDS linkages. A 2X30 inch flap deflected 5 degrees at 100mph bends a 90 degree bent 1/8X3 inch steel torque rod a total of 0.013 degrees. From the analysis spreadsheet, I can also see that approximately 18 ounce-inches of torque would be transmitted back to the servo. My servo deflection vs. load data (from a detailed comparison of digital and conventional servos earlier in Mike’s column) shows that the “stiffest” servo I tested will deflect roughly 0.4 degrees from its target position with 18 ounce-inches of torque on it. So this would mean that a 1/8 inch RDS linkage deflects 1/30 as much as the stiffest digital servo I tested (the JR DS33010). The RDS linkage would deflect approximately 1/150 as much as the average servo in my test data.

Yes, the RDS torque rod does act as a torque spring, but a torque spring that is much stiffer than the servos in the system.

Besides 90 degree bent torque rods for flaps, Walt sells rods with 45 and 32 degree bends for ailerons (custom bent angles available on special order). Lower bend angles effectively gear down the servo much like a shorter control horn does on a conventional linkage. Here again the spreadsheet tells me that using a 32-degree bent torque rod in the above example would decrease the required servo torque by 30 percent.

Most pilots will choose 90-degree rods for flaps and 45 degree for ailerons. Keep in mind that the maximum control surface travel available is limited to the value of the torque rod bend angle. More angle more travel.

I am currently installing the heavy-duty RDS system in my 11-pound cross-country racing model.

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