project pattern
T
Redux
he last servo installed is the rudder servo. Most modern day Pattern planes use a single rudder servo (mounted close to the c.g.) with pull-pull cables for attachment to the rud- der. Pull-pull is preferred because it is light- weight, slop free, easily handles the geome- try of larger control deflections typically found on the rudder, and places the majori- ty of weight near the c.g. The rudder for Yuri uses the same com- posite skin with internal balsa reinforce- ment structure as the wings, stabs, and fuselage. As with the elevators and ailerons, a hardpoint for the rudder control horns has been preinstalled in the rudder, and can eas- ily be found by shining a bright light behind the rudder. In contrast to the live skin hinge on the elevators and ailerons, the rudder on Yuri uses a centerline “pocket” hinge with a removable hinge pin which in turn allows the rudder to be easily removed from the plane. The hinge pin terminates in a 4 mm sock- et head bolt which threads into the bottom of the fuselage. Very clever and clean in- stallation. However, I did make a small modification; I replaced the steel hinge pin and bolt with a CF rod glued into a plastic bolt, resulting in a weight savings of 5.3 grams. I also increased the internal bevel of the fin trailing edge (which overlaps the hin- geline area) slightly to allow a full 35 de- grees of rudder deflection.
Needed for the installation of the pull-pull system are control horns, servo arm, pull- pull cable material, and clevises. From the same servo arm pack (DuBro Cat No 671) used for the aileron servos, the largest dou- ble sided arm is used for the rudder servo (2 inches between the outermost holes). To achieve the correct control throw (approxi- mately 35 degrees to each side), I used a pair of F3A Unlimited 32mm (1.25 inches) car- bon fiber control horns, trimmed slightly to yield a distance between the holes of 2.125 inches when installed in the rudder. My favored cable material is 0.038-inch Kev-cord (Cat No C-07) from Aerospace
PHOTOGRAPHY: DAVE LOCKHART Replacing the stock metal rudder hinge pin and bolt with a carbon fiber rod and nylon bolt saves 5.3 grams.
Composite Products. Kev-cord is a braided Kevlar thread with a vinyl jacket, making it very durable and thermally stable (minimal change in length with temperature varia- tions). To connect the cables, nothing more than clevises are needed, as slop is not a con- cern with lightly tensioned pull-pull sys- tems. The final linkage bits are Sullivan eyebolts (SUL549), used to link the Kev-cord to the clevises.
Installing the control horns in the rudder is a relatively simple task. I cut the slots using a 3⁄32-inch carbide bit in a Dremel and glue the horns in with 30-minute epoxy or polyurethane glue (such as Gorilla glue). The slower drying glue is preferred as it will fill small voids, increasing the gluing area, and the drying time allows confirma- tion that the horns have been symmetrical- ly installed.
Very important for pull-pull systems is
the symmetrical installation of the control horns; this not only ensures equal control throw to each side, but more importantly, equal tension (or slack) in the cables when moved in each direction. When the rudder servo is stationary at center, the tension in each cable is equal. When the control horns are symmetrically installed, tension in the air will increase (equal to the opposing aero- dynamic force) in the pulling cable, and the opposing cable will go slack some amount. The degree of slack will not affect the po- sition of the rudder. While the slack can be eliminated with the proper amount of offset (Ackerman) in the servo arm (or the use of a rotary wheel at both the servo and rudder ends), there is ultimately no difference in performance.
The most difficult part of installing a pull-
pull system is cleanly installing exit holes/slots for the rudder cables. Another strong case for the practice of measuring twice and cutting once. The procedure I use to find the exit holes is essentially graphi- cal. First, I mark the height and fore/aft lo- cation of the rudder servo arm on the out- side of the fuselage (using a dry erase marker). Next, with the rudder installed, I draw a line on the outside of the fuselage be- tween the servo arm location and the rud- der horn.
Moving onto the graphical part, I draw a centerline (of the fuselage) on my work- bench with the fore/aft locations of the ser- vo arm and rudder hinge control horn at- tachment points. Then, using a set of calipers, I measure the outside dimensions of the fuselage every 4–6 inches (more fre- quently if needed to capture any detailed curves proximal to the fin area) and add those dimensions to my workbench drawing. The next step is adding the servo arm and rudder horns to the drawing, and then con- necting the “dots” between the attachment points of the servo arm and horn to show the path of the rudder cables. The thickness of the fuselage is easily added in the area of the exit slots by measuring the thickness of the fuselage sides (approximately 3mm). The points on the drawing at the cables cross the fuselage sides are the locations where the exit slots need to be cut.
To cut clean, straight exit slots, I use a large 2-inch diameter cut-off wheel on the Dremel. The length of the exit slots is typi- cally about 2 inches, but initially I cut the slots to a length of about 1 inch. The final length/position of the exit slots is completed after trial fitting the cables and checking the
by dave lockhart with algirdas ungulaitis You can reach Dave Lockhart via e-mail at
davel322@comcast.net
The “pocket” style rudder hinges (far left) pre- installed by RC-Composit yield a very clean hinge line and appearance and allow easy removal of the rudder. The rudder hinge pin terminates into a bolt (at left, center) which retains the pin securely in place, but still allows easy removal of the rudder. Rudder hinge closeup (above).
52 NOVEMBER 2012
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