Control Horns, Servos, and Pushrod Z-Bends

Precise aircraft control depends on tight, accurate mechanical linkages. Even the highest-end radio transmitter cannot compensate for slipping servo horns, flexing pushrods, or misaligned control horns.

Crucial Rule: Center Servos Electrically First!

Never screw on a servo arm before electrically centering the servo motor.

1. Connect your 9g servo to the receiver or a servo tester.
2. Power on the transmitter, set all trim sliders and sub-trims to 0.
3. Power on the receiver. The servo will jump to its exact 90° neutral center.
4. Now push the servo horn onto the splines so it is perpendicular (90 degrees) to the servo body.
5. Secure the arm with the small center retaining screw.

Step 1: Aligning the Control Horn Hinge Pivot

The holes of the plastic control horn must be positioned directly above the hinge line of the control surface:

• Correct: The pushrod attachment hole aligns directly over the hinge axis. This produces equal control deflection in both directions (linear throw).
• Incorrect (Forward of hinge line): Causes differential throw and mechanical binding.
• Incorrect (Behind hinge line): Causes severe non-linear throws and excessive servo strain.

Secure the horn through the foam using backing plates and a drop of medium CA or hot glue.

Step 2: Mechanical Advantage vs. Throw Sensitivity

Where you plug the pushrod determines sensitivity and torque:

• Outer hole on servo arm + Inner hole on control horn: Maximum deflection throw, minimum torque (ideal for 3D aerobatic planes like VT-Sport Aerobatic).
• Inner hole on servo arm + Outer hole on control horn: High torque, gentle gentle movements, zero jitter (ideal for beginners on VT-Simple Trainer).

Step 3: Making Perfect Pushrod Z-Bends

A standard 1.2mm or 1.5mm steel piano wire pushrod requires a clean Z-bend to lock into the servo arm without slipping:

1. Grip the wire with round-nose or needle-nose pliers at the designated measurement point.
2. Bend the wire 90 degrees upward.
3. Move the pliers 2mm down the wire and bend 90 degrees in the original direction, creating an offset step.
4. Thread the Z-bend into the servo horn hole. It cannot slip out under flight loads!

Step 4: Preventing Pushrod Flex with Guide Sleeves

On long fuselages (such as the VT-Super Cub or VT-Trainer), steel pushrods will flex under compression when pushing the elevator up.

• Slide a plastic sleeve tube over the pushrod.
• Glue the sleeve to the inside wall of the fuselage at the halfway point.
• This prevents buckling and guarantees crisp elevator authority.