A stabilizer is an aerodynamic control surface that provides longitudinal and/or direction stability and control. A stabilizer can feature a fixed or adjustable structure on which any movable control surfaces are hinged, or can be a fully movable surface such as a stabilator. Depending on the configuration, the stabilizer may only refer to the front part of the control surface. There are three primary types of stabilizers: Horizontal stabilizers, vertical stabilizers, and combined longitudinal-directional stabilizers. This blog will explain each type, their differences, and their functions.
1. The Horizontal Stabilizers
Horizontal stabilizers are used to maintain the aircraft’s longitudinal balance known as trim. The stabilizer exerts a vertical force at a distance so the summation of pitch moments (the rotational equivalent of linear force) at the center of gravity is zero. The vertical force exerted by the stabilizer varies with flight conditions particularly relative to the aircraft lift coefficient and wing flaps deflection which affect both the center of pressure and the position of the aircraft center of gravity. Transonic flight (near or at the speed of sound) places tough requirements on the horizontal stabilizer. For instance, when the local speed of the air over the wing reaches the speed of sound (343 meters per second), there is a sudden move aft of the center of pressure.
There are four configurations of horizontal stabilizers. These are the conventional tailplane, three-surface aircraft, canard aircraft, and tailless aircraft configurations. In a conventional tailplane configuration, the horizontal stabilizer is a tail or tailplane located at the rear of the aircraft. As its name suggests, this is the most common configuration. In three-surface aircraft, there are actually two horizontal stabilizers: a tailplane at the rear, and a foreplane at the front. On a canard aircraft, a foreplane is located on the front of the plane. This provides foreplane stability but does not necessarily aid in pitch stability. Tailless aircraft do not feature a dedicated horizontal stabilizer. Instead, the stabilizing surface is part of the main wing.
2. The Vertical Stabilizers
Another type of stabilizer, the vertical stabilizer, is used to provide direction stability. Vertical stabilizers usually comprise a fixed fin and movable control rudder affixed to its rear edge with a hinge. Though uncommon, some configurations feature no hinge but rather a fin surface pivoted for both stability and control. When an aircraft encounters a horizontal gust of wind, directional stability from the vertical stabilizer causes the aircraft to turn into the wind, rather than the same direction as the wind. The size of the stabilizer is affected by factors such as aircraft fuselage geometry, engine nacelles, and rotating propellers. Without a vertical stabilizer, an aircraft is nearly impossible to maneuver.
3. The Combined Longitudinal-directional Stabilizer
The third type of stabilizer, the combined longitudinal-directional stabilizer, features an all-in-one horizontal and vertical stabilizer called a V-tail. V-tails act as both a yaw and pitch stabilizer. In this configuration, two stabilizers (fins and rudders) are mounted at 90 and 120 degrees respectively, providing a larger horizontal projected area than vertical projected area, just as many conventional tails do. Other configurations of combined stabilizers do exist, such as the inverted V-tail and Y-tail. For example, the MQ-1 Predator unmanned aerial vehicle features an inverted V-tail in which the tail is affixed to the bottom of the fuselage and faces the ground.
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