Prerequisite knowledge:
An aircraft's primary flight controls are essential for maneuvering the aircraft, at the basic level. Generally speaking, an aircraft's flight controls can be split into three categories, based on the axis of rotation they control.
Axis of Rotation
An aircraft can rotate along three different axis:
An aircraft's primary flight controls are essential for maneuvering the aircraft, at the basic level. Generally speaking, an aircraft's flight controls can be split into three categories, based on the axis of rotation they control.
Axis of Rotation
An aircraft can rotate along three different axis:
- Lateral (pitch) axis: Nose up or down
- Longitudinal (roll) axis: Bank left or right
- Vertical (yaw) axis: Nose left or right
Most aircraft have three basic sets of flight controls, with each one being responsible for moving the aircraft along a certain axis of rotation. Collectively, these are known as the primary flight controls.
Control Surfaces
The flight controls are moveable surfaces mounted at various points on the aircraft. As the pilot moves their controls in the cockpit, the respective control surface is moved, which can change the direction in which the air is deflected. By deflecting air in a certain direction, a force is created which rotates the aircraft along a certain axis of rotation.
Specifically, the control surfaces change the angle of attack of the surface it's attached to. In previous articles, the chord line of an aerofoil was defined as a line drawn from its leading edge to trailing edge. The control surfaces are generally mounted to the trailing edge of an aerofoil, and therefore the chord line of the entire aerofoil depends on the position of the control surface. Since the angle of the chord line depends on the position of the control surface, the angle of attack varies with the position of the control surface. By moving the control surface, the angle of attack of the aerofoil can be increased or decreased, generating a force in a certain direction (like how a wing generates lift).
On the left is a normal aerofoil, with the control surface on its trailing edge in a neutral position. The chord line and relative wind lines are drawn, as well as the angle of attack. On the right is the same aerofoil, in the same orientation, except with its control surface deflected down. This shifts the trailing edge of the entire aerofoil down, which moves the chord line and increases the angle of attack. In both illustrations, the orientation of the aerofoil is the same, and the relative wind is assumed to be horizontal, but the shifting of the chord line due to the movement of the control surface is what changes the angle of attack.
There are three main types of control surfaces: elevators, the rudder, and ailerons.
There are three main types of control surfaces: elevators, the rudder, and ailerons.
Lateral (Pitch) Axis
The pitch axis is controlled by elevators on the tail of the aircraft. The elevators are mounted to a surface known as the horizontal stabilizer, which is an upside down aerofoil (the reason for it being upside down will be discussed in later articles). The horizontal stabilizer is generally fixed, although sometimes it is moveable; this will also be discussed in later articles. The elevator, located on the rear of the horizontal stabilizer, is what pivots up and down. As the elevator pivots up or down, it changes the angle of attack of the entire horizontal stabilizer, which creates a force that either raises or lowers the tail, rotating the aircraft along its pitch axis.
The pitch axis is controlled by elevators on the tail of the aircraft. The elevators are mounted to a surface known as the horizontal stabilizer, which is an upside down aerofoil (the reason for it being upside down will be discussed in later articles). The horizontal stabilizer is generally fixed, although sometimes it is moveable; this will also be discussed in later articles. The elevator, located on the rear of the horizontal stabilizer, is what pivots up and down. As the elevator pivots up or down, it changes the angle of attack of the entire horizontal stabilizer, which creates a force that either raises or lowers the tail, rotating the aircraft along its pitch axis.
The elevators are manipulated by moving the control column forward or backward. Moving the control column forwards deflects the elevators down, which pushes the nose down. Pulling back on the control column deflects the elevators up, which raises the nose.
Vertical (Yaw) Axis
The yaw axis is controlled by the rudder on the tail. The rudder is mounted to a fixed surface known as the vertical stabilizer. Unlike the horizontal stabilizer, which is moveable on some aircraft, the vertical stabilizer is almost always fixed. The horizontal stabilizer itself is a symmetrical aerofoil, and it relies on the rudder to change its angle of attack in either direction, which then creates a force that yaws the airplane left or right. Since the aerofoil is symmetrical, it generates no force when the rudder is centered.
The yaw axis is controlled by the rudder on the tail. The rudder is mounted to a fixed surface known as the vertical stabilizer. Unlike the horizontal stabilizer, which is moveable on some aircraft, the vertical stabilizer is almost always fixed. The horizontal stabilizer itself is a symmetrical aerofoil, and it relies on the rudder to change its angle of attack in either direction, which then creates a force that yaws the airplane left or right. Since the aerofoil is symmetrical, it generates no force when the rudder is centered.
Coordinated Flight
The rudder is generally used to keep the aircraft coordinated. In uncoordinated flight, the aircraft is said to be moving sideways through the air, instead of pointing in the direction it is travelling. Uncoordinated flight is most common during turns, although it can also happen in level flight. There are two type of uncoordinated flight: skids and slips. A skid (also known as a skidding turn) is a condition where the aircraft's nose is pointed too far towards the direction of an aircraft's roll. A slip is where the aircraft's nose is pointed too far away from the direction of an aircraft's roll.
The rudder is generally used to keep the aircraft coordinated. In uncoordinated flight, the aircraft is said to be moving sideways through the air, instead of pointing in the direction it is travelling. Uncoordinated flight is most common during turns, although it can also happen in level flight. There are two type of uncoordinated flight: skids and slips. A skid (also known as a skidding turn) is a condition where the aircraft's nose is pointed too far towards the direction of an aircraft's roll. A slip is where the aircraft's nose is pointed too far away from the direction of an aircraft's roll.
To correct a slip, rudder must be applied towards the direction of the turn. To correct a skid, rudder must be applied away from the direction of the turn.
The rudder is controlled by pedals on the cockpit floor. To yaw in a certain direction, the pedal in that direction is pushed forward (e.g. pushing on the right pedal yaws the aircraft to the right).
The rudder is controlled by pedals on the cockpit floor. To yaw in a certain direction, the pedal in that direction is pushed forward (e.g. pushing on the right pedal yaws the aircraft to the right).
Longitudinal (Roll) Axis
The roll axis is the most complicated, which is why we saved it for last. The ailerons are a pair of surfaces mounted to the wings that are used to roll the aircraft left or right. The ailerons always move opposite of each other to produce roll (when one aileron on one wing moves down, the other aileron on the other wing moves up). The ailerons are controlled by moving the control column left or right, moving it in one direction causes the aileron on that wing to move up, and the aileron on the opposite wing to move down. The wing with the downgoing aileron will have a higher AoA, and will generate more lift, which will raise that wing. At the same time, the wing with the upgoing aileron will have a lower AoA, and will generate less left, causing that wing to drop.
The roll axis is the most complicated, which is why we saved it for last. The ailerons are a pair of surfaces mounted to the wings that are used to roll the aircraft left or right. The ailerons always move opposite of each other to produce roll (when one aileron on one wing moves down, the other aileron on the other wing moves up). The ailerons are controlled by moving the control column left or right, moving it in one direction causes the aileron on that wing to move up, and the aileron on the opposite wing to move down. The wing with the downgoing aileron will have a higher AoA, and will generate more lift, which will raise that wing. At the same time, the wing with the upgoing aileron will have a lower AoA, and will generate less left, causing that wing to drop.
For an example, look at the above image. The pilot moves his control column to the right. This makes the right aileron move up, and the left aileron move down. The right wing, with the upgoing aileron, now has a lower angle of attack, and generates less lift, causing the right wing to drop. The left wing, with the downgoing aileron, has a higher angle of attack, and generates more lift, causing left wing to rise. The rising left wing combined with the dropping right wing is what creates the rolling motion to the right.
Adverse Yaw
Unfortunately, ailerons aren't perfect, and they are accompanied by a phenomenon known as adverse yaw. Adverse yaw is an undesirable effect of using the ailerons to roll the aircraft left or right.
As previously discussed, the ailerons change the angle of attack of the wing they are mounted to, which causes one wing to generate more lift and the other to generate less. This imbalance of lift is what creates the rolling motion, however his lift imbalance also comes with another consequence. The wing with the downgoing aileron will produce more lift, and this increase in lift is accompanied by an increase in induced drag (remember, induced drag is drag from lift generating surfaces, more lift means more induced drag). This means that one wing will experience more drag than the other wing, which will create a yawing motion against the direction of the roll.
For example, in a left turn, the pilot moves their control column to the left. This raises the left aileron, which reduces lift on the left wing, and lowers the right aileron, which increases lift on the right wing. The right wing experiences more induced drag since it is now generating more lift, and this increase in drag on the right wing forces the aircraft to yaw to the right. If uncorrected, the aircraft will fly in a slip, with the wings rolled left, but the aircraft yawed to the right.
Adverse Yaw
Unfortunately, ailerons aren't perfect, and they are accompanied by a phenomenon known as adverse yaw. Adverse yaw is an undesirable effect of using the ailerons to roll the aircraft left or right.
As previously discussed, the ailerons change the angle of attack of the wing they are mounted to, which causes one wing to generate more lift and the other to generate less. This imbalance of lift is what creates the rolling motion, however his lift imbalance also comes with another consequence. The wing with the downgoing aileron will produce more lift, and this increase in lift is accompanied by an increase in induced drag (remember, induced drag is drag from lift generating surfaces, more lift means more induced drag). This means that one wing will experience more drag than the other wing, which will create a yawing motion against the direction of the roll.
For example, in a left turn, the pilot moves their control column to the left. This raises the left aileron, which reduces lift on the left wing, and lowers the right aileron, which increases lift on the right wing. The right wing experiences more induced drag since it is now generating more lift, and this increase in drag on the right wing forces the aircraft to yaw to the right. If uncorrected, the aircraft will fly in a slip, with the wings rolled left, but the aircraft yawed to the right.
In order to counter adverse yaw, and keep the aircraft flying in a coordinated fashion, rudder should always be used in the direction of the turn. When turning left, adverse yaw will pull the nose to the right, which is then countered by adding left rudder to pull the nose back to the left. In a real aircraft, you should always apply a little bit of rudder pressure in the direction of the aileron application in order to counter adverse yaw (e.g. when rolling to the right, a little pressure on the right pedal should also be added; when rolling to the left, pressure on the left pedal should be added.). This coordination of hands on the yoke and feet on the pedals becomes instinctual in experienced pilots.
Aircraft manufacturers also use different types of ailerons to help counter adverse yaw. There are two main types of ailerons that can be used to reduce the effects of adverse yaw:
Aircraft manufacturers also use different types of ailerons to help counter adverse yaw. There are two main types of ailerons that can be used to reduce the effects of adverse yaw:
Differential ailerons: With a normal aileron, the angle of deflection between the upgoing and downgoing ailerons is around the same. That means that if the aileron on one side deflects up 20 degrees, the aileron on the other side will deflect down 20 degrees. However, this is not true of differential ailerons. With differential ailerons, the aileron does not deflect as far down as it deflects up. This means that when an aircraft is rolling, the upgoing aileron deflects further than the downgoing aileron, creating more profile drag to help counter out the induced drag from the downgoing aileron. Remember, adverse yaw is due to induced drag from the downgoing aileron, so drag must be added to the oppoiste upgoing aileron to cancel it out.
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Frise ailerons: Frise ailerons instead rely on their structure to counter adverse yaw, rather than their deflection angles. A frise aileron generally pivots at point close to the trailing edge, and the aileron itself generally has a protrusion of some sort from the bottom. This means that when the aileron is deflected upwards, it generates large amounts of profile drag, which counteract the induced drag from the opposite downgoing aileron.
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That's it for primary flight controls! The next article will cover stalls.