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Control Surfaces

Knowing what happens when the controls are operated is the most basic skill of piloting. It is also among the most misunderstood. When an airplane is flying, it has a good deal of forward speed and airflow over all of its surfaces. Control movements must be understood in terms of this airflow and its effects.

The Elevator

The elevator controls the Angle of Attack [AOA] of the wings, and subsequently the pitch. Pulling back on the stick results in a down force on the tail (the same thing is operating here that was operating on the wings, only in a different direction). If the controls are reversed, the opposite happens.

Effects of Back Stick Movement

Backward stick movement forces the tail down and the nose up. This rotation occurs around the center of gravity of the airplane. Initially the airplane, even though its nose is up, is still headed in the same direction - the only thing that has changed is the angle of attack. But an increase in the angle of attack results in an increase in lift, so now the airplane starts to go up. Then, like an arrow, it points into the wind, increasing its pitch. This process continues, viewed from the cockpit as an increase in pitch, until the pilot moves the stick forward to a neutral position and stabilizes the pitch.

The temptation to think that the stick directly raises or lowers the nose is very strong, and most of the time, roughly correct. But if the stick is moved back when the airplane is very close to the stall the aircraft will not pitch up much, if at all. This back stick movement and increase in AOA will stall the wing, causing a loss of lift and acceleration downward: now the pitch moves opposite the stick movement.

The Ailerons

The ailerons are a much simpler control than the elevator. Located near the wing tips on the trailing edge of the wing, they are used in unison to change the amount of lift each wing is producing and roll the airplane.

When the pilot moves the stick side-to-side from center, the ailerons move in opposite directions. In a roll to the right (as viewed from the cockpit), the right aileron goes up and the left aileron goes down. Each aileron serves to change how that part of the wing deflects the air and thus increases or decreases the amount of lift produced by each wing. The down aileron forces the air down harder, resulting in an increase in lift and the up aileron decreases the downward force, resulting in a decrease in lift. In the case of a right roll, the decreased lift on the right side and increased lift on the left side result in a roll to the right.

Aileron Effects

Operating the ailerons causes an effect called adverse yaw. Adverse yaw is the result of an increase in drag on the wing with the down aileron, or "upgoing" wing. This wing, since it is forcing the air down harder than the "downgoing" wing and producing more lift, also produces more drag. The drag pulls the wing back and causes yaw. If this yaw is not corrected with rudder, the roll is said to be "uncoordinated."

The Rudder

The rudder is controlled by the "rudder pedals" located on the floor of the aircraft. They are both connected to the rudder so that when one or the other pedals is depressed, it moves the rudder in the desired direction. The rudder, connected to the vertical stabilizer, then starts to deflect air much like a wing, only the resulting force is to the side. This force causes a change in yaw. As mentioned earlier, the rudder is not used very often, but when it is needed (e.g., in a crosswind), its presence is appreciated.

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Page last modified: 07-07-2011 02:33:17 ZULU