Flight Control Surfaces of an Airplane

Some of you may be wondering how an airplane maneuvers in the sky. This is brought about by flight control surfaces which I will talk about here.

Aircraft flight control surfaces are essential components that allow pilots to adjust and control an aircraft’s flight attitude. They are crucial in enabling safe and stable flight, and without them, early aircraft designs were often uncontrollable, leading to disastrous results.

Fixed-wing aircraft of conventional design typically use a set of flight control surfaces to control their motion along three axes of rotation: pitch, roll, and yaw. The elevators, ailerons, and rudder are the primary control surfaces used for pitch, roll, and yaw, respectively. These surfaces are operated by aircraft flight control systems, which allow the pilot to manipulate the aircraft’s movements.

It’s worth noting that while the control surfaces on rotary-wing aircraft (helicopters) accomplish the same motions about the three axes of rotation, they operate in a completely different manner compared to fixed-wing aircraft.

Three Axes of Rotation

The three axes of rotation are the transverse axis, longitudinal axis, and vertical axis. The transverse axis, also known as the lateral axis, runs from wingtip to wingtip and controls pitch. Pitch changes the vertical direction that the aircraft’s nose is pointing. The longitudinal axis runs from nose to tail and controls roll. The ailerons are the primary control of bank, which is the angular displacement about this axis. Finally, the vertical axis runs from top to bottom and controls yaw. The rudder is the primary control of yaw, while the ailerons have a secondary effect on it.

It’s essential to note that these axes move with the aircraft and change relative to the earth as the aircraft moves. Understanding the basics of these axes and the control surfaces that manipulate them is crucial to become a competent pilot.

Main Control Surfaces

The main control surfaces of a fixed-wing aircraft are attached to the airframe on hinges or tracks so that they may move and deflect the air stream passing over them. This redirection of the air stream generates an unbalanced force to rotate the plane about the associated axis.

Ailerons

Let’s start with ailerons. Ailerons are mounted on the trailing edge of each wing near the wingtips and move in opposite directions. When the pilot moves the stick left, or turns the wheel counter-clockwise, the left aileron goes up, and the right aileron goes down. This causes the aircraft to roll to the left and begin to turn to the left. Centering the stick returns the ailerons to neutral, maintaining the bank angle, and the aircraft will continue to turn until opposite aileron motion returns the bank angle to zero to fly straight.

Elevator

Next up is the elevator, which is a movable part of the horizontal stabilizer, hinged to the back of the fixed part of the horizontal tail. When the pilot pulls the stick backward, the elevators go up, and pushing the stick forward causes the elevators to go down. Raised elevators push down on the tail and cause the nose to pitch up. Centering the stick returns the elevators to neutral and stops the change of pitch.

Rudder

The rudder is typically mounted on the trailing edge of the vertical stabilizer, part of the empennage. When the pilot pushes the left pedal, the rudder deflects left, and pushing the right pedal causes the rudder to deflect right. Deflecting the rudder right pushes the tail left and causes the nose to yaw to the right. Centering the rudder pedals returns the rudder to neutral and stops the yaw.

Using ailerons causes adverse yaw, meaning the nose of the aircraft yaws in a direction opposite to the aileron application. Adverse yaw is more pronounced for light aircraft with long wings, such as gliders. It is counteracted by the pilot with the rudder. The rudder may also be called upon to counter-act the adverse yaw produced by the roll-control surfaces.

To maintain level flight requires increased positive (up) elevator to increase the angle of attack, increase the total lift generated, and keep the vertical component of lift equal with the weight of the aircraft. If the total lift is kept constant, the vertical component of lift will decrease, and as the weight of the aircraft is unchanged, this would result in the aircraft descending if not countered.

Secondary Control Surfaces

In addition to the main control surfaces, such as the ailerons, elevator, and rudder, there are also secondary control surfaces that a pilot can use to maintain control over the aircraft. Let’s take a closer look at some of these control surfaces.

Spoilers

Spoilers are used to disrupt airflow over the wing and reduce lift. This allows a pilot to lose altitude without gaining excessive airspeed. Spoilers that can be used asymmetrically are called spoilerons and can affect an aircraft’s roll.

Flaps

Flaps are mounted on the trailing edge of the wing near the wing roots. They are deflected down to increase the effective curvature of the wing. Flaps raise the maximum lift coefficient of the aircraft and therefore reduce its stalling speed. They are used during low speed, high angle of attack flight, including take-off and descent for landing.

Slats

Slats, also known as leading edge devices, are extensions to the front of a wing for lift augmentation. They are intended to reduce the stalling speed by altering the airflow over the wing. Slats may be fixed or retractable.

Air brakes

Air brakes are used to increase drag. Air brakes are usually surfaces that deflect outwards from the fuselage into the airstream to increase form-drag. As they are located elsewhere on the aircraft, they do not directly affect the lift generated by the wing. Their purpose is to slow down the aircraft. They are particularly useful when a high rate of descent is required.

Trimming controls

Trimming controls allow a pilot to balance the lift and drag being produced by the wings and control surfaces over a wide range of load and airspeed. This reduces the effort required to adjust or maintain a desired flight attitude.

Control horn

A control horn is a section of control surface that projects ahead of the pivot point. It generates a force that tends to increase the surface’s deflection, thus reducing the control pressure experienced by the pilot. Control horns may also incorporate a counterweight, which helps to balance the control and prevent it from fluttering in the airstream.

Spring trim

In the simplest arrangement, trimming is done by a mechanical spring (or bungee) that adds appropriate force to augment the pilot’s control input. The spring is usually connected to an elevator trim lever to allow the pilot to set the spring force applied.

Rudder and aileron trim

Most fixed-wing aircraft have a trimming control surface on the elevator, but larger aircraft also have a trim control for the rudder and another for the ailerons. The rudder trim is to counter any asymmetric thrust from the engines, while the aileron trim is to counter the effects of the center of gravity being displaced from the aircraft centerline.

Understanding the primary and secondary control surfaces of an aircraft can help you appreciate the complexities of flying and feel more confident in the safety measures taken by airlines. Whether it’s the ailerons that control the roll of the aircraft, the flaps that aid in takeoff and landing, or the spoilers that can be used to decrease altitude, each control surface has a specific function that contributes to the overall stability and maneuverability of the aircraft. By having a basic understanding of how these control surfaces work, passengers can feel more informed and at ease during their flights.