Aircraft Controls

10.1 Primary Flight Controls

• Aircraft are maneuvered around their three axes using primary flight controls. These are
• typically "flap-type" surfaces positioned at the extremities of the aircraft to provide the
• longest possible moment arm from the CG.
•  Elevator: Controls pitch around the lateral axis (longitudinal control). Located on the
• horizontal stabilizer.
•  Ailerons: Control roll around the longitudinal axis (lateral control). Located on the outboard
• trailing edge of the wings.
•  Rudder: Controls yaw around the normal axis (directional control). Located on the vertical
• stabilizer.

10.2 Control Balancing and Feel

• At high speeds, the aerodynamic forces on control surfaces can become very large, resulting
• in heavy stick forces for the pilot. Several methods are used to reduce these forces.
•  Aerodynamic Balance: These methods use airflow to help move the control surface,
• reducing the pilot's effort.
• Inset Hinge: Setting the hinge line back from the leading edge of the control surface creates
• a balancing force.
• Horn Balance: A portion of the control surface extends forward of the hinge line.
• Internal Balance: An aerodynamic panel inside the wing moves with the control surface to
• use pressure differences for balancing.
•  Tabs: These are small, secondary control surfaces hinged to the trailing edge of the main
• surface.
• Balance Tab: Moves in the opposite direction to the main control surface. The airflow on
• the tab creates a force that helps move the main surface.
• Anti-Balance Tab: Moves in the same direction as the main control surface. It increases the
• stick force and control effectiveness.
• Trim Tab: An adjustable tab that the pilot can set to hold the control surface in a specific
• position, relieving the need to maintain constant pressure on the controls ("trims" the
• aircraft).
• Servo Tab: The pilot's controls are connected only to the tab. Moving the tab causes an
• aerodynamic force that then moves the main control surface.

10.3 Secondary Effects of Controls

• Using one control surface often produces a secondary, unintended motion around another
• axis.
•  Adverse Aileron Yaw: When ailerons are deflected to initiate a roll, the down-going aileron
• (on the rising wing) creates more lift but also significantly more induced drag than the up-
• going aileron. This difference in drag causes the aircraft to yaw in the
• pposite direction of the turn. It is corrected by using rudder in the direction of the turn or
• by design features like
• differential ailerons or Frise-type ailerons.
•  Rolling Moment due to Yaw: Deflecting the rudder causes the aircraft to yaw. The outside
• wing travels faster through the air than the inside wing, generating more lift and causing the
• aircraft to roll in the direction of the yaw.

10.4 High-Speed Controls

•  Spoilers: Panels on the upper surface of the wing that can be raised into the airflow. They
• "spoil" the lift and increase drag. They can be used for:
• Roll Control: By raising the spoiler on one wing only (to assist or replace ailerons). They do
• not cause adverse yaw. 247247247247
• Speed Brakes: By raising the spoilers symmetrically on both wings to increase drag and
• allow for a steeper descent without increasing speed.
• Ground Spoilers (Lift Dumpers): Deployed automatically on landing to dump all lift from
• the wings, placing the aircraft's full weight on the wheels for more effective braking.
•  All-Moving Tailplane (Stabilator): For pitch control on high-speed aircraft, the entire
• horizontal stabilizer pivots, rather than just a hinged elevator. This is much more effective at
• supersonic speeds.