Small wings that are placed at positions forward or aft of an aircraft’s wings to provide balance in pitch and yaw during flight. On a missile, stabilizers may also be known as fins.
Early attempts to achieve gliding flight used only wings, occasionally adding the shifting of weights beneath the wing to keep the wing balanced in its motion. At any given angle of attack, there is some point on the wing where the forces are in balance, but this position, sometimes known as the “center of lift” or “center of pressure,” moves forward or aft as the wing’s angle to the flow changes. The location of this point and its distance from the center of mass or gravity of the wing or vehicle will determine its pitching moment, that is, the tendency of its nose to move up and down, rotating around the center of gravity. It is considered desirable to have the center of lift behind the center of gravity for positive stability, that is, to create a natural tendency for the vehicle to return to level flight after any disturbance. For example, in a stable aircraft, a gust-induced increase in lift will cause the airplane to rotate nose downward and automatically reduce its lift in correction. Because of this, a stable airplane will always have a tendency to rotate nose down in flight unless that rotation is counteracted by another force or moment. This correction is the purpose of the horizontal stabilizer.
The horizontal stabilizer is normally placed on the rear or tail of the fuselage, somewhat like the tail feathers of a bird. This placement requires the stabilizer to have a downward load or negative lift to counteract the nose-down moment of the wing. The common horizontal stabilizer is usually a small wing placed toward the rear of the fuselage and mounted at a negative angle of attack so that it will cause a downward force. The stabilizer is usually equipped with flaps known as elevators, which can be moved up and down to alter the force on the stabilizer, allowing the pilot to rotate the aircraft nose up or nose down in pitch. This allows control of the angle of attack of the wing and, hence, control of the lift produced by the wing. When larger control forces are needed, the whole stabilizer is designed to be moveable or to rotate about a pivot point, and it is then known as a stabilator or elevon.
Some airplane designers believe that the horizontal stabilizer should be in front of the wing, where it can correct the nose-down pitch of the stable wing with an upward force, thus increasing the lifting capability of the aircraft instead of decreasing it, as may happen with a tail-mounted stabilizer. The Wright brothers and other early aviators used this arrangement on their primitive designs but, like most others, eventually built airplanes with the horizontal stabilizers at the tail. When the horizontal stabilizer is in front of the wing, it is called a canard. There are special circumstances, such as transonic and supersonic flight, where canards may have advantages, but most analyses show that the best place for the horizontal stabilizer is near the tail of the aircraft.
The vertical stabilizer is almost always mounted above the tail of the airplane. It is designed to limit the rotation of the aircraft in yaw, operating as a sort of weathervane, much like the feathers at the aft end of an arrow. Attached to the vertical stabilizer or fin is the rudder, which acts as a flap on the winglike stabilizer to move left or right and create forces which will yaw the airplane when desired.
The vertical stabilizer on most single-engine airplanes is mounted on the fuselage at a slight angle to counteract the torque of the engine, which tends to make the fuselage try to roll in a direction opposite to the turning of the propeller. Some aircraft have two vertical stabilizers where larger control surfaces are needed or where at very high angles of attack, part of the stabilizer may be in the wake of the fuselage.
Vertical and horizontal stabilizers are placed on an airplane in many different arrangements, depending on the control needs of the design. Sometimes the horizontal stabilizer is mounted on the vertical stabilizer, either at its top in a T-tail arrangement or part of the way up in a cruciform design. Often the vertical/horizontal tail arrangement is dictated by the need to control the airplane in stall and to make sure that, in that situation, the vertical stabilizer and rudder are not in the wake of the horizontal stabilizer, where their usefulness would be very limited.
The stabilizers on a missile are often simply referred to as fins. These small wings are mounted at the tail of the missile and are often fully moveable and do not have attached flaps. These moveable fins provide both balance or stability in flight and the control forces needed to maneuver.
Barnard, R. H., and D. R. Philpott. Aircraft Flight. 2d ed. Essex, England: Addison Wesley Longman, 1995. An excellent, nonmathematical text on aeronautics. Well-done illustrations and physical descriptions, rather than equations, are used to explain virtually all aspects of flight. Docherty, Paul, ed. The Visual Dictionary of Flight. New York: Dorling Kindersley, 1992. A profusely illustrated book showing the parts and the details of construction of a wide range of airplane types, old and new. An outstanding source of information about what airplanes and their parts really look like. Stinton, Darrol. The Design of the Airplane. London: Blackwell Science, 1997. An outstanding reference on the design of all types of aircraft. Slightly technical but well written and illustrated.