What are Flight Control Surfaces?

One of the most interesting subjects in any aeronautical engineering course is that of flight mechanics where students learn bout the principles of flight. In this article, we will identify the main components on an aircraft that are responsible for manoeuvring and aircraft within flight, which are commonly referred to as flight control surfaces.

Flight control surfaces are simply physical devices that the pilot can control and adjust to change the roll, pitch, and yaw of an aircraft. These are terms that relate to movement around the three primary axes that define aircraft movement.

Figure 1: Aircraft Controls on 3 Axes

In figure 1, we can see that there are three primary axes which are the longitudinal axis, lateral axis, and vertical axis. Sometimes these can be referred to as x, y, and z axes.

• Roll is the movement around the longitudinal axis. A positive roll involves lifting the left wing and lowering the right.
• Pitch is rotation around the lateral axis and movement that results in an aircraft nose up is positive pitch.
• Yaw is rotation around the vertical axis. Positive yaw is when the nose of the aircraft points to the right and the aircraft is said to be turning right

Aircraft Flight Control Surfaces

Now that we have defined the types of motion for aircraft, we can begin to identify how the pilot creates the motion provided by the control surfaces.

Fuselage, Wings and Tail Section

We can begin by discussing the body of the aircraft, which is referred to as the fuselage. Simply speaking the fuselage has the function of transporting cargo and passengers to a destination. It must accomplish this in often extreme conditions at high altitudes and so must remain pressurized and sufficiently heated.

Aircraft Fuselage Structure

The fuselage is hollow to reduce weight and to accommodate its payload. As with most other parts of the airplane, the shape of the fuselage is normally determined by the mission of the aircraft. A supersonic fighter plane has a very slender, streamlined fuselage to reduce the drag associated with high-speed flight. An airliner has a wider fuselage to carry the maximum number of passengers.

Typical Aerofoil (wing cross section)

Typical Aircraft Tailplane Components

The wings of an aircraft are responsible for providing the lift force required for flight to take place. This is perhaps their primary function, and it is accomplished through specific designs in their shape, or profile.

In addition to providing lift, the wings also contain other moving control surfaces such as the spoilers, flaps, slats, and ailerons. In most modern commercial aircraft, the engines are also situated on the underside of the wing. They store the fuel of an aircraft and maintain balance.

The tail section pictured above refers to the aft (rear) portion of the aircraft usually consisting of the horizontal and vertical stabiliser which provide stability in these directions. The rudder is situated on the vertical fin and the elevators on the horizontal.

Flaps and Slats

One possible way of increasing the lift generated from a wing section is to increase the wing area and change the profile of the aerofoil section. To achieve this, aircraft have moving parts at the front and rear of the wing section. Moving sections at the rear of the aerofoil are called flaps, they are positioned on the inner wing section, closer to the fuselage on the trailing edge of the wing. Slats on the other hand are positioned on the leading edge of the wing.

Flaps are referred to as high lift devices, and when required, they can extend fully to increase the camber and overall wing area. This has the effect of increasing the maximum lift coefficient and hence the lift the aircraft can produce. As the aircraft is producing more lift for a given speed this has the effect of reducing the stall speed. The increase in camber also increases the wing drag, which can be beneficial during approach and landing.

Roll Control – Ailerons

Whilst flaps and slats are primarily used to increase lift and drag for take-off and landing, Ailerons are used to orient the aircraft during flight. They are situated in an outboard position on the leading edge of the wing, as shown below.

Ailerons work in pairs meaning that when one aileron is raised the other will be lowered, they are not raised or lowered at the same time.

The pilot will control the ailerons by movement of the control stick left or right. If the pilot was to move the control stick or yoke to the right the aircraft would bank to the right or move with a positive roll around the longitudinal axis. This has the effect of increasing the lift on the left-hand wing (like how flaps perform) but destroying the lift on the right wing (like a spoiler). This has the physical effect of lowering the right wing and raising the left causing the aircraft to bank, or roll. Figure 3 demonstrates this process.

Yaw Control – Rudder

Rudders are situated at the rear of the vertical fin, or vertical stabiliser, section as depicted in figure 2. The pilot will control these devices often with a pedal in the cockpit. The vertical stabiliser prevents side-to-side, or yawing motion of the aircraft nose. Because the rudder moves, it varies the amount of force generated by the tail surface and is used to generate and control the yawing motion of the aircraft.

The rudder works by changing the effective shape of the aerofoil section on the stabiliser, if we consider it as a normal wing rotated 90 degrees, then moving the rudder to the left will increase the force going to the right. Deflection of a rudder pedal causes a corresponding rudder deflection in the same direction; that is, pushing the left rudder pedal will result in a rudder deflection to the left. This, in turn, causes the rotation about the vertical axis moving the aircraft nose to the left, as shown in figure 4.

In Summary:

The primary yaw control surface is the rudder.

Deployment induces a change in camber and lift and thus a yawing moment as it is positioned aft of the centre of gravity.

Pitch Control – Elevator

The elevators are situated on the trailing edge of the horizontal tail section. They are hinged control surfaces controlled by hydraulic actuators. Opposite to the ailerons, these devices will work together by deflecting in the same direction on the right- and left-hand side.

The pilot will control the ailerons by pulling back on the stick or pushing it forward. Pulling back on the stick has the effect of raising the elevators. Once the elevators are raised this will decrease the lift generated by the horizontal tail section and cause the aircraft to rotate around the centre of gravity along the lateral axis.

In summary, deflection of the elevator produces:

• Increased camber on the tailplane
• Nose-down pitching moment about centre of gravity (CG)
• Reduced lift coefficient on main wing
• Less lift on main wing
• Drop in aircraft altitude

Spoilers

Aircraft Spoilers

Spoilers are devices that extend upwards from the wing surface via hydraulic actuators. Spoiler controls can be used for roll control or descent control. Some aircraft use spoilers in combination with or in lieu of ailerons for roll control, primarily to reduce adverse yaw when rudder input is limited by higher speeds.

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