Flight control systems Summary

  • Last updated on November 10, 2022

Electric, mechanical, and hydraulic systems that help to move an aircraft while flying.

Early History

The early experimenters and inventors who preceded Orville and Wilbur Wright, who made their first flight in 1903, did not fully appreciate the necessity for positive control of the machine. Prior to 1903, the prevailing ideas about aircraft control were that the airplane must have some kind of inherent stability and that the pilot’s only function was to make small directional changes. Much of inventors’ efforts prior to beginning of the nineteenth century were directed at obtaining a lightweight engine. The Wright brothers realized that a proper engine was a necessary ingredient in mechanical flight. However, they appreciated the importance of control and the fact that the pilot must be an active participant in the control of the airplane. By 1909, a control system had evolved consisting of ailerons, a rudder, and an elevator, which, in its essentials, remains in use today.

Types of Controls

Modern aerodynamic flight control systems, as opposed to engine controls, are essentially the same for all airplanes. Flight controls can be separated into two categories: primary and secondary controls. The primary controls change the angles that the airplane makes relative to the ground. The secondary controls are flaps that control the lift of the airplane, especially at low speeds, and tabs that reduce or eliminate the forces the pilot must exert on the controls in the cockpit.

All controls, whether primary or secondary, have three important subdivisions. The first are external moveable surfaces on the airplane, such as the rudder, aileron, and elevator. The second are the cockpit controls, which are moved by the pilot to change the direction of the airplane. The third are the links between the cockpit controls and the external surfaces of the airplane. These connections might be cables, electrical-conducting wires, electrical motors and computers, hydraulic lines, and hydroid motors.

Primary Controls

There are three categories of primary controls. Category A refers to the three hinged panels that are rotated about their hinge line to change the angular attitude of the airplane.

Category B controls are those which the pilot moves to change the direction of the aircraft and, to a limited extent, the speed of the aircraft, particularly the descent rate. These controls consist of the stick or wheel, which is moved to pitch and roll the airplane, and the rudder, which is moved to yaw the airplane. These controls have not changed significantly since 1915, during the second decade of mechanical flight.

Category C controls vary the most widely between different types of airplanes. These types of controls have also evolved most radically over the history of mechanical flight. A small, low-cost training plane connects the pilot’s control to the aerodynamic controls with cables or push rods; hydraulic lines and associated motors perform the same function in high-speed commercial airliners. Electrical conducting wire or even fiber optic lines might be used to carry the control signal from the cockpit to an electrical motor at the surfaces in other commercial airplanes or high performance military airplanes.

The airplane responds to the movement of the primary category A cockpit controls in a number of ways. The elevator is deflected to change the pitch angle of the airplane: When the trailing edge of the elevator is moved upward, a down force is generated on the horizontal stabilizer. The result is that the nose of the airplane pitches upward. The airplane will pitch in the opposite direction if the trailing edge of the elevator is moved downward.

When the rudder is moved to the left side of the airplane, from the point of view of the pilot, a side force to the right is applied to the vertical stabilizer. This force swings, or yaws, the nose of the airplane to the left. Reversing the direction of the rudder movement will reverse the yaw direction.

Finally, movement of the ailerons causes the airplane to roll. The ailerons move differentially; when one aileron moves upward, the other moves downward. On the wing with the downward aileron, there is a slight increase in the lift. On the wing with the upward aileron, there is a slight decrease in lift. The unbalance in lift between the two wings causes the airplane to roll.

Cockpit controls are connected to the airplane’s external controls. Moving the stick back brings up the elevator trailing edge, placing a down force on the horizontal stabilizer. The horizontal stabilizer and tail goes down while the nose goes up. Reversing the direction of the stick movement reverses the motion of the pitch of the airplane.

The rudder is moved by pushing on the rudder pedals located on the floor of the airplane. Pressing the left pedal causes the trailing edge of the elevator to move to the left, resulting in the application of a side force to the right on the vertical stabilizer. The tail of the airplane moves to the right, and the nose moves to the left. Moving the nose left or right is called yawing the airplane left or right.

Finally, the ailerons are moved by either sideways motion of the stick or rotation of the control wheel. To roll the airplane to the right, for example, the stick is moved to the right, lowering the left aileron and raising the right aileron.

Secondary Controls

The secondary aerodynamic controls are the tabs and the flaps, both of which can be operated by the pilot from the cockpit. The tab is a small elevator hinged to the trailing edge of the elevator. To hold the nose up for a prolonged period of time, the pilot must continually apply a backward force on the stick to keep the elevator in the up position. By moving the tab downward, in this case, a small force through leverage balances the much larger force on the elevator, with the result that there is no or little stick force required of the pilot to keep the elevator trailing edge upward. Tabs are found also on the rudder and aileron. On an airplane with two wing-mounted engines, a rudder tab is nearly essential in helping the pilot set and hold the extreme rudder deflection required for single-engine flight.

Flaps deflect in unison, unlike ailerons, which move differentially. Flaps help maintain lift, especially during low-speed flight. The pilot can control the deflection angle of the flap. Flaps are deflected at maximum deflection for landing and at a small angle for takeoff.

There are three basic flap designs. The split flap, the simplest but least effective, consists of a small plate that comes down from the lower surface of the wing. Because this flap does not change the contour of the wing, it primarily produces drag. The plain flap changes the shape of the wing and therefore produces lift as well as drag. The slotted flap is derived from the plain flap with special attention given to the junction of the flap and the wing. The design of this junction is crucial to the flap’s effectiveness. The Fowler flap is the most effective and the most mechanically complicated flap. When deflected, it changes not only the shape but also the area of the wing.

There are two types of control used on airplanes: primary and secondary. Primary controls are the elevator, rudder and ailerons, and the primary cockpit controls are the stick and rudder, located in the cockpit. The secondary controls are tabs and flaps. The flaps allow the airplane to fly at lower speeds than would otherwise be possible. The tab allows the pilot to remove any forces required to hold control deflections. Tabs are usually located on the elevator and can also be found on the rudder and ailerons.

Bibliography
  • Hubin, W. N. The Science of Flight: Pilot-Oriented Aerodynamics. Ames: Iowa State University Press, 1992. A book requiring some familiarity with high school algebra, but there are many sections that are entirely descriptive. There is much basic information about the control of airplane flight.
  • Illamn, Paul E. The Pilot’s Hand Book of Aeronautical Knowledge. Rev. ed. Blue Ridge Summit, Pa.: TAB Books, 1991. A description of airplane control mostly from a pilot’s point of view, requiring only basic arithmetic.
  • Raymer, Daniel P. Aircraft Design: A Conceptual Approach. 3d ed. Reston, Va.: American Institute of Aeronautics and Astronautics, 1999. A highly recommended, comprehensive, and up-to-date book on airplane design, directed at the engineering student, but featuring many sections requiring little more than high school algebra.
  • Stinton, Darrel. The Design of the Airplane. New York: Van Nostrand-Reinhold, 1985. An excellent introduction to airplane design.
  • Taylor, John W. R. The Lore of Flight. New York: Crescent Books, 1974. A massive, well-illustrated, oversized book featuring nontechnical descriptions of airplanes and spacecraft, and covering controls and cockpit instruments.

Ailerons and flaps

Airplanes

Cockpit

Landing procedures

Rudders

Stabilizers

Tail designs

Takeoff procedures

Wing designs

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