Primary Flight Controls: Ailerons, Elevator, and Rudder

5.primary-controls-elevator-rudder-aileron. Primary Flight Controls: Ailerons, Elevator, and Rudder

The airplane's primary flight controls are the ailerons, elevator (or stabilator), and rudder. These surfaces enable the pilot to maneuver the airplane about its three axes of rotation—longitudinal, lateral, and vertical—each of which passes through the airplane's center of gravity (CG). Movement of any primary control surface changes the airflow and pressure distribution over and around the airfoil, generating an aerodynamic force that rotates the airplane about the corresponding axis.

Ailerons — Roll About the Longitudinal Axis

The ailerons are hinged surfaces on the outboard trailing edge of each wing. They are connected to the control yoke (or stick) by cables, bellcranks, pulleys, and/or push-pull tubes so that they move differentially: when the yoke is rotated left, the left aileron deflects up and the right aileron deflects down. The down-deflected aileron increases the camber and angle of attack of that wing, increasing lift; the up-deflected aileron decreases lift on the opposite wing. The lift differential rolls the airplane about the longitudinal axis (nose-to-tail).

Because the wing producing more lift also produces more induced drag, the airplane tends to yaw away from the intended direction of roll—a phenomenon called adverse yaw. Manufacturers reduce adverse yaw using:

• Differential ailerons — the up-going aileron deflects more than the down-going aileron, increasing parasite drag on the descending wing to counter induced drag on the ascending wing.
• Frise-type ailerons — the up-deflected aileron pivots so its leading edge protrudes below the wing into the airstream, producing parasite drag on the descending wing.
• Coupled ailerons and rudder — interconnect springs deflect the rudder proportionally with aileron input.

In most light airplanes the pilot must still apply coordinated rudder with aileron to keep the slip/skid ball centered during entry to and rollout from a turn.

Elevator (or Stabilator) — Pitch About the Lateral Axis

The elevator controls rotation about the lateral axis (wingtip-to-wingtip), known as pitch. It is the movable trailing-edge portion of a fixed horizontal stabilizer. Aft (back) yoke pressure deflects the elevator upward, which decreases camber and angle of attack on the underside of the tail, producing a downward tail force; the airplane rotates nose-up about the CG. Forward yoke pressure produces the opposite effect.

It is important to remember that the elevator does not directly control airspeed or altitude—it controls angle of attack. Pitch attitude, combined with power, determines performance.

Several variations exist:

• Stabilator — a one-piece horizontal surface (entire stabilizer-elevator combination) that pivots about a central hinge. Stabilators are very effective and typically incorporate an antiservo tab that moves in the same direction as the stabilator to provide control feel and prevent overcontrol; the antiservo tab also functions as the trim tab.
• T-tail — places the horizontal stabilizer on top of the vertical fin, out of the propeller wash and wing wake. Pilots will notice less pitch authority at low airspeeds and high angles of attack, requiring different rotation and flare techniques.
• Canard — a horizontal surface forward of the wing that produces upward lift and stalls before the main wing.

Rudder — Yaw About the Vertical Axis

The rudder is the movable trailing edge of the vertical stabilizer. It is controlled by foot pedals. Pressing the right pedal deflects the rudder right; the airflow strikes the deflected surface, pushing the tail to the left and yawing the nose to the right about the vertical axis.

The rudder does not turn the airplane—the horizontal component of lift from the banked wings does that. The rudder's job is to coordinate the turn by counteracting adverse yaw, to maintain directional control during takeoff and landing roll, and to compensate for asymmetric thrust effects (P-factor, slipstream, torque). Rudder is also essential for slips, crosswind landings, and—critically—stall and spin recovery.

Summary of Axes and Controls

All three primary controls operate on the same principle: deflection of a hinged airfoil section changes the effective camber and angle of attack of that surface, generating an aerodynamic force that creates a moment about the CG. The control surfaces are typically built with the same airfoil construction techniques as the wings—aluminum or composite skins over ribs and spars—and may be balanced statically (mass-balanced to prevent flutter) and aerodynamically (with horns or overhang ahead of the hinge line to reduce control forces).

At low airspeeds, control surfaces are less effective because aerodynamic force varies with the square of velocity (F ∝ V²); a control deflection that produces a brisk response at cruise may produce a sluggish one near stall. This is why coordinated, smooth, and progressively larger inputs are required as airspeed decreases—particularly during takeoff, landing, and slow flight.