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Composite-ARF YAK 55SP (3m span)
The 3mm thick plywood inner rib in the stabs
is already milled with a cutout to suit the
JR8711/881 servo, but it is mandatory that
the very powerful stab servos are screwed
into 6mm thick plywood, using the Ø 2.9
x167mm long screws provided. For this pur-
pose we have included 3mm plywood dou-
blers (2 to suit JR8711/8811 and 2 for Futaba
S9152), which must be glued securely to the
inside face of the plywood rib. If fitting the JR
servo all you need to is glue one 3mm doubler
into each stab with thick CA, aligning the
screw fixing holes carefully with some pins,
and then cut out the temporary 'spacer' bar in
the middle when the glue has cured.
If you are fitting the S9152 servo then, after
gluing the doubler into place and cutting out
the spacer bar, you will need to enlarge the
original milled cutout in the rib a little to suit
the slightly larger Futaba servo. A coarse
Permagrit needle file, with an extension han-
dle, makes this a quick job.
Servo arms
It is mandatory to use full metal servo arms for the eleva-
tor control, and we always use the SWB 1.5" double-loc
arms (shown here) which provide full deflection throws
without having to electronically reduce the end-points, to
ensure the highest torque and mechanical advantage.
These arms clamp onto the servo output shaft with no lost
movement (play) at all. They are high quality, properly engi-
neered arms, and are available from many good hobby
stores.
If using the SWB arms you may need to reduce the thick-
ness of them a little at the end so that the supplied alumini-
um clevises fit without binding, and this is easily done on a
belt sander. Drill the outer hole to 3mm diameter for the cle-
vise pin, and apply a little grease to the hole to make sure
there can be no binding of the Ø 3mm pin in the servo arm.
Whatever brand of metal servo arms you use, make sure that they are a tight fit on the splines
of the servo output shaft, with no slop or lost movement at all - otherwise you increase the risk
of flutter!
Centre both your servos using the R/C, and secure the metal servo arms tightly before installing
them into the stabs. Add a drop of Loctite to the bolt that secures the arms to the servos.
Make up the linkages as shown, using the M3 x 70mm threaded rod supplied, with the aluminum
clevise and a plain nut at the servo end, and the M3 ball-link at the elevator end. Make sure that
the threaded rod is screwed fully into the shank of the plastic ball-link. The ball-link is secured
techsupport@composite-arf.com
(above) Servo is screwed into the milled cutout
in the inner rib, which must have the included
plywood doubler glued to the inside face to
give a total 6mm thickness.
(below) JR8811 servo & SWB 1.5" servo arm.
(above) You may need to sand the
ends of the SWB arms a little for
smooth movement of the clevises.
Remember to add a little grease to
the clevise pins.
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