Lecture 18 - Control Surfaces of an Airplane

Control Surfaces of an Airplane

The airplane’s movable control surfaces let a pilot change its motion. These controls also relate to the airplane’s stability.

Stability and maneuverability

• Stability is the tendency to return to an equilibrium flight condition after a disturbance, such as a gust.
• Maneuverability is the ability to change direction or attitude quickly.

These qualities often compete. A highly stable aircraft resists changes, so it generally takes more effort and time to maneuver. A highly maneuverable aircraft may be less inherently stable and can require active feedback control—using sensors and control surfaces—to maintain steady flight.

Passenger transport aircraft prioritize a comfortable, stable ride. Fighter aircraft prioritize maneuverability and may rely on active control systems to maintain stability.

The lecture compares aircraft stability with the stabilizing effects of a kite’s tail or a stick on a simple rocket, while noting that airplanes rely substantially on lift from their wings and tails.

The three basic motions and their controls

An airplane’s fixed lifting surfaces contribute to stability. A movable portion of a surface can alter its aerodynamic force and create a moment that changes the airplane’s motion.

Pitch — elevator

Pitch is rotation about the aircraft’s lateral axis. It changes the nose-up or nose-down attitude and is used to initiate climbs and descents.

The elevator is a movable section of the horizontal tail. Deflecting it down increases the tail’s effective camber and lift. Because the tail is behind the center of gravity, this produces a nose-down moment. Deflecting it up produces a nose-up moment, which can initiate a climb.

In the lecture’s simplified description: elevator up for a climb; elevator down for a descent.

Yaw — rudder

Yaw is rotation about the aircraft’s vertical axis and changes the direction the nose points.

The rudder is a movable section of the vertical tail. Deflecting it creates an aerodynamic side force on the tail, producing a yawing moment. Deflecting it one way yaws the aircraft one way; deflecting it the other way reverses the yaw.

The rudder helps turn the aircraft, but yaw alone can cause sideslip.

Roll — ailerons

Roll is rotation about the aircraft’s longitudinal axis.

The ailerons are movable sections of the wings. They normally move in opposite directions:

• One aileron deflects up, reducing lift on that wing.
• The other deflects down, increasing lift on the opposite wing.

This lift difference creates a rolling moment. Reversing which aileron goes up and down reverses the roll.

How aircraft turn

A practical turn involves coordinated yaw and roll rather than yaw alone. A turn creates an outward tendency associated in the lecture with centrifugal force; without coordination, the aircraft can sideslip.

The aircraft banks by rolling. In a bank, part of the total lift acts toward the center of the turn, helping provide the force needed to follow the curved path and reduce sideslip.

The lecture compares this with banked roads and racing tracks, which help vehicles negotiate turns at speed.

General control-surface principle

The wing, horizontal tail, and vertical tail contribute to stability in roll, pitch, and yaw, respectively. To control a particular motion, a movable section is added to the corresponding surface:

• Horizontal tail → elevator → pitch
• Vertical tail → rudder → yaw
• Wings → ailerons → roll

Deflecting a control surface changes the aerodynamic force on that surface. The force, acting at a distance from the center of gravity, creates a moment that changes the airplane’s attitude or direction.

The lecture also notes that control surfaces are checked for proper movement before flight.

Speakers and sources featured

• Speaker: One lecturer, unnamed in the subtitles.
• Illustrations and examples: Kites, simple bottle-launched rockets, passenger transport aircraft, fighter aircraft (including the Sukhoi-30 and its Cobra maneuver), cars, banked roads, and Formula 1 racing tracks.
• Visual material: Aircraft diagrams and animations showing elevator, rudder, and aileron deflections.