Control Surfaces

Primary/main flight controls

Elevator: Controls the motion around the lateral axis, known as pitch/pitching.

Ailerons: Control the motion around the longitudinal axis, known as roll/rolling.

Rudder: Controls the motion about the normal/vertical axis, known as yaw/yawing.

What is the elevator, and how does it work?

The conventional elevator is a hinged control surface at the rear of the horizontal tailplane (stabilizer) that is controlled by the pilot’s control column. As the elevator control surface is deflected, the airflow and thus the aerodynamic force around the elevator (horizontal tailplane) changes. Moving the control column back deflects the elevator up, causing an increase in the airflow speed and thus reducing the static pressure on the underside of the elevator control surface. In addition, the topside of the elevator faces more into the relative airflow, which causes an increase in the dynamic pressure experienced. These effects create an aerodynamic force on the elevator (horizontal tailplane) that rotates (pitches) the aircraft about its lateral axis. That is, back control column movement moves the elevator control surface upward, producing a downward aerodynamic force that pitches the aircraft up. Thus the opposite is also true: Forward control column movement moves the elevator control surface downward, producing an upward aerodynamic force that pitches the aircraft down.

What are ailerons, and how do they work?

Ailerons are control surfaces located at the trailing edges of the wings that control the aircraft’s motion around its longitudinal axis, known as roll. The ailerons are controlled by left and right movement of the control column, which commands the ailerons in the following manner: Moving the control column to the left commands the left aileron to be raised, which reduces the lift on the wing, and the right aileron is lowered, which increases the lift generated by this wing, thereby rolling the aircraft into a banked condition, which causes a horizontal lift force (centripetal force) that turns the aircraft. The ailerons normally are powered (hydraulically) powered on heavy/fast aircraft because of the heavy operating forces experienced at high speeds.

What is the rudder and how does it work?

The rudder is a hinged control surface at the rear of the fin (vertical tailplane) that is controlled by the pilot’s rudder pedals. As the rudder control surface is deflected, the airflow and thus the aerodynamic force around the rudder (vertical tailplane) changes. Moving the left rudder pedal deflects the rudder to the left, causing an increase in the airflow speed and thus reducing the static pressure on the right-hand side of the rudder control surface. In addition, the left side of the rudder faces more into the relative airflow, which causes an increase in the dynamic pressure experienced. These effects create an aerodynamic force to the right on the rudder (vertical tailplane) that rotates (yaws) the aircraft about its vertical/normal axis at its center of gravity point to the left.

How does the effectiveness of the control surfaces vary with speed?

The control surfaces become more effective at higher speeds. This results in a requirement for large control movements at low speeds and smaller control movements at high speeds to produce the same control force.

What is elevator reversal?

Elevator reversal occurs at high speeds when the air loads/forces are large enough to cause a twisting moment on the deflected elevator surface to either a neutral or opposite position that results in sudden reversal of the aircraft’s pitch attitude.

What is adverse yaw?

Adverse yaw is a yawing motion opposite to the turning/rolling motion of the aircraft. Adverse yaw is caused by the drag on the down-going aileron being greater than that on the up-going aileron. This imbalance in drag causes the yawing motion around the normal/vertical axis. Since this yaw is adverse (i.e., in a banked turn to the left, the yaw is to the right), it is opposing the turn, which is detrimental to the aircraft’s performance. Adverse (aileron) yaw is corrected in the design by the use of either differential ailerons or Frise ailerons.

What is aileron reversal (adverse), and when is it likely to occur?

Aileron reversal occurs at high speeds when the air loads/forces are large enough that they cause an increase in lift. Because most of this lift is centered on the down-going aileron at the rear of the wing, a nosedown twisting moment will be caused. This will result in a decrease in the incidence of the wing to the extent that the loss of lift due to the twisting cancels the lift gained from the aileron. At this point the aileron causes no rolling moment, and if the wing twisting is exaggerated (which a down-going aileron can do), the rolling motion around the longitudinal axis can be reversed, hence an adverse rolling motion.

What is a yaw-induced adverse rolling motion, and when is it likely to occur?

The rudder inducing the aircraft to yaw one way can cause another form of adverse rolling motion in the opposite direction. This happens at high speeds (above VMO/MMO) because the deflected rudder experiences a sideways force that causes the aircraft to roll in the opposite direction; i.e., right rudder experiences a sideways force from right to left, causing a rolling moment to the left.

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