Hangar 9 Piper J-3 Cub 40 ARF - Manual (Page 13)

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 Step 7
Tie a wheel collar onto a piece of string. Drop the wheel
collar into the wing from the root and retrieve it from the
servo opening.
 Step 8
Tie the string onto the servo extension. Gently pull the
extension through the wing using the string. Untie the
string when the servo lead has been pulled through. Use
tape to secure the servo lead to the wing to prevent it from
falling back into the wing panel.
 Step 9
Secure the hatch using four #2 x 3/8" screws.
Section 3: Servo Installation (Aileron)