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Flight Control Surfaces and how they Work



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When the Wright brothers first took to the air they had only two methods to control their aircraft. In place of a rudder and ailerons they used "wing warping" much the same as a bird does. In place of an elevator they shifted their body weight fore and aft. Aviation was a very dangerous game during the development of control surfaces. The three axes of modern aircraft are controlled by three primary control surfaces. Each of these are movable airfoils (wings) and follow the same aerodynamic rules that wings do.

Ailerons are located on the outer trailing edge of the wings. The pilot moves them up and down by moving the control stick or wheel left or right. This rotates the aircraft on it's longitudinal axis, causing it to "roll" left or right. Ailerons accomplish this by changing the amount of lift generated on each wing.

Moving the control stick right causes the right aileron to raise and the left side to lower. An aileron in the raised position deflects airflow upwards. According to Newton's laws, that wing must move in the opposite direction, losing lift and dropping down. Some aircraft deploy flight spoilers to reduce lift further. Since ailerons must move opposite to each other, the other side will lower. This forces airflow downwards, increasing lift and raising that wing. Bernoulli's law also comes into play as the aileron increases wing camber and the pressure differential. An unavoidable fact is that increased lift is accompanied by increased drag. To compensate, the "up" aileron must travel further than the "down" side.

The rudder is on the trailing edge of the vertical fin. The pilot moves it left and right with the rudder pedals. This rotates the aircraft on it's vertical axis, known as "yaw". If only the rudder is applied with level wings the aircraft will "skid" left or right. The rudder and ailerons are normally used together to achieve a roll and bank. Pushing the right pedal moves the rudder to the right, deflecting air to the right and, following Newton's laws, the tail must move to the left. This causes the aircraft to turn right on it's vertical axis.

The elevator is on the trailing edge of the horizontal stabilizer. The pilot moves it up and down by moving the control stick forward or backwards. This controls "pitch" and rotates the aircraft on it's lateral axis. Moving the elevator up deflects airflow upwards, causing a decrease in lift in the same way an up aileron does. As the tail loses lift and drops the nose will pitch up. With sufficient power and speed the aircraft will climb.

Each of these three primary controls has secondary controls called trim tabs and spring tabs. They are very small "wings" attached to each trailing edge. The trim tabs are adjusted by the pilot to "trim" an unbalance, allowing level, "hands off" flight. The spring tabs can be compared to power steering on a car. On some aircraft, the pilot only controls the spring tabs which, in turn, move the larger control surfaces.

Several types of auxiliary controls are used, either to augment or decrease lift. These movable surfaces can increase lift on takeoff and also allow slower speed and improved control for landing. Included in this category are leading edge and trailing edge flaps, slats and slots. They increase lift by increasing wing area and camber, improving laminar airflow and by deflecting additional airflow downwards. The de Havilland aircraft I work on take full advantage of these systems, making them well known for their STOL (short take off and landing) capabilities.

Speed brakes are relatively simple but strong devices that create drag when deployed. They reduce speed for landing and are used on military fighters for dogfighting. Spoilers are used to reduce speed and shorten landing runs. As their name implies, they "spoil" the laminar airflow and dramatically reduce lift. Some deploy automatically when a "weight on wheels" sensor in the landing gear engages.

These complex flight control surfaces enable fantastic maneuvering and handling which the Wright brothers could only have dreamed of. Ironically, NASA and Lockheed Martin are developing wing warping controls based on the original Wright brothers machines.

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