Flight Control: Understanding Ailerons and Axes

Flight Control

• Directional control of a fixed-wing aircraft takes place around the lateral, longitudinal, and vertical axes using flight control surfaces.
• There are two major groups of flight control surfaces: primary (main) and secondary (auxiliary).

Primary Flight Control

• Ailerons are attached to the outboard trailing edge of both wings and rotate the aircraft around the longitudinal axis (roll) when moved.
• Elevator is attached to the trailing edge of the horizontal stabilizer and rotates the aircraft around the lateral axis (pitch) when moved.
• Rudder is attached to the trailing edge of the vertical stabilizer and rotates the aircraft around the vertical axis (yaw) when moved.

Lateral Control

• Moving the control stick or yoke to the left causes the right aileron to go down (left aileron goes up) and rolls the aircraft to the left.
• Aileron reversal (aeroelasticity effect) can occur in high dynamic pressures and wings with low torsional rigidity, causing the aircraft to roll in the opposite direction.

Longitudinal Control

• Moving the control stick forward deflects the elevator downward (tail goes up) and causes the nose to go down.
• Elevator deflection is affected by the center of gravity (CG) location:
  • CG ahead of the center of pressure (CP): requires bigger deflection of elevator, high longitudinal stability, and bigger control force.
  • CG behind CP: requires smaller deflection of elevator, smaller control force, and low longitudinal stability.

Directional Control

• Pressing the right rudder pedal deflects the rudder to the right (tail goes left) and yaws the aircraft to the right.
• Rudder is used to overcome adverse yaw at the beginning of a turn, and also during crosswind and one-engine operation.
• Rudder limiters are fitted to prevent involuntary motion of the rudder and limit its movement unless commanded from the flight deck.

Hybrid Control Surfaces

• Ruddervator (rudder and elevator) combines pitching and yawing functions.
• Elevon (elevator and aileron) combines pitching and rolling functions.

Secondary or Auxiliary Devices

• High lift devices provide a compromise between high cruising speed and low landing speed.
• Flaps are located on the inboard wing trailing edge and can be extended for takeoff and landing and retracted for cruise.
• Aileron-rudder coupling: when the pilots turn the control wheel, the rudder deflects in the correct direction for a coordinated turn.

Other Control Surfaces

• Stabilator: a movable horizontal tail section that combines the action of both the horizontal stabilizer and the elevator.
• Variable incidence stabilizer: a horizontal stabilizer with adjustable angle of incidence to minimize drag when trimming the aircraft in flight.
• Canards: a small wing or horizontal airfoil ahead of the main lifting surfaces that creates lift and holds the nose up.