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An aircraft stabilizer may provide longitudinal (or pitch) stability, yaw (or directional) stability . A longitudinal stabilizer is a surface that provides forces that tend to keep an aircraft flying level and with unchanging "pitch" angles relative to the airstream; the nose of aircraft is prevented from pitching up or down. A directional stabilizer tends to keep the aircraft flying straight ahead with unchanging direction relative to the airstream (yaw stabilization).

Passive horizontal and vertical stabilizers are small wing surfaces placed behind the center of mass of an aircraft, either as part of the tail empennage or outboard on aft-swept wings. In tailless and canard aircraft, the horizontal stabilizing surface is part of the main wing.

Stabilizers can be fixed structures on which movable control surfaces are mounted, or they can be movable to a small degree for trimming flight, or they can be operated as a control surface. Some types of aircraft include electronically controlled surfaces that may located anywhere needed and may serve as active stablizers or motion dampers.

In the V-tail and related configurations, each surface contributes to both vertical and horizontal stability.

Horizontal stabilizer

The horizontal stabilizer is a small horizontal surface attached to an aircraft, which acts with the other horizontal surfaces to provides longitudinal (or "pitch") stability. It can be a fixed surface which is sometimes adjustable, or fully movable. It is usually placed at the rear of the craft, forming part of the tail or empennage .

Fixed horizontal stabilizers usually have hinged elevators attached to the rear of the surface to provide pitch control for the aircraft. Most large airliners and transports use an a combination of a stabilizer with an adjustable angle of incidence and independently moving elevators. The elevators are used by the pilot for rapid maneuvers and are controlled by the pilots yoke.

An adjustable stabilizer is adjusted to trim the aircraft's pitch to keep the elevators centered during normal flight. Adjustable stabilizers are usually adjusted with a positive displacement device like a jack screw or a hydraulic ram. In aircraft with elevators, a trim tab on the trailing edge of the elevator is often used to move the elevator to alter the aircraft's pitch (see Douglas DC-3 or Cessna 172). The stabilized speed is known as the trim speed, and the trim is used to set the desired speed without having to hold the elevator out of its trimmed or faired (trail) position.

Other aircraft use an one-piece moving surface that serves the function of both a stabilizer and an elevator and is called a "stabilator" or "all moving tailplane". One example of a modern airliner with a stabilator used for flight control is the Lockheed L-1011 TriStar. Horizontal "Stabilators" are generally required for aircraft flying near or above the speed of sound because the formation of shock waves on the surface of the stabilizer reduces the effectiveness of elevators. Stabilators may also use a trim tab to control the angle of the all moving stabilator surface. (See Miles M.52) The North American F-86 Sabre initially used a fixed stabilizer and elevator with a trim tab, but later versions used a stabilator.

Tailplane

The 'tailplane is a horizontal stabilizer located at the rear of an aircraft and forms part of the empennage.

For the aircraft to be stable, the tailplane must carry less lift than the main wing, and may even exert a down force. In particular, as a fast aircraft crosses the sound barrier its centre of lift moves aft, requiring greater downforce from the tailplane in order to maintain trim.

A tailplane is affected by the passage of air over the wings ("downwash") and in some cases placed higher relative to the wing to be free of interference (placed at the top of the fin it is known as a T-tail).

Canard foreplane

When a horizontal stabiliser is placed forward of the main wing, this is the canard configuration.

Unlike a tail stabiliser, for the aircraft to be stable the canard foreplane must be more highly loaded than the main wing. This contributes to the overall lift, in theory allowing a smaller wing. This effect is countered by the need to operate the main wing below optimal lift capacity, in order to maintain a stall safety margin: if the main wing were to be overloaded and stall, the tail drops leading to a dangerous condition.

There is a view that, strictly, the aft-most surfaces of the main wing serve to stabilise the action of the canard foreplane. Nevertheless, without the foreplane the craft would typically not be stable and in practice the foreplane acts to help stabilise the craft by moving the centre of lift forward. It is common to refer to such a foreplane as a stabiliser.

Tailless

Tailless aircraft lack a separate horizontal stabilizer at the rear. Longitudinal stability in tailess aircraft is generally derived from the design of the wing to move the Aerodynamic center behind the centre of gravity - eg by varying angle of incidence along sweepback and or variation in airfoil.

Vertical stabilizer

The vertical stabilizer or fin is fixed to the aircraft and normally supports the rudder. It is used to give directional stability to the aircraft. An insufficiently stabilized aircraft would tend to turn with gusts rather than into them.

The vertical stabilizer often employs a small fillet at its forward base which helps to prevents a phenomenon called rudder lock or rudder reversal. Rudder lock occurs when the force on a deflected rudder (in a steady sideslip) suddenly reverses as the rudder stalls.This may leave the rudder stuck at full deflection with the pilot unable to recenter it. The filet is sometimes called a dorsal fin.

Tailless yaw stabilization

The vertical stabilizer is by far the most common type of yaw or directional stabilizer, however, it is possible to design aircraft with no discrete vertical directional stabilizer. Hang gliders are a commonly seen example. The leading edge of a hang glider, viewed from above, is swept back in a V shape. As one side of the V rotates forward into the airstream, the drag on the forward 1/2 wing increases while the drag on other 1/2 wing decreases. This tends to rotate the whole wing back so that it is facing straight into the airflow. (Similar to dihedral roll stabilization.)

V-tail

A V-tail is a 2-fin combined vertical-horizontal stabilizer. V-tails combine the functions of both horizontal and vertical stabilizers, and a rudder and elevator, into a slightly simpler mechanical design. A V-tail has two stabilizer fins mounted at 45 degree angles to the horizontal and vertical planes. Each has a moving surface known as a "ruddervators". There is a V-tailed version of the Beechcraft Bonanza light aircraft. The General Atomics MQ-1 Predator unmanned aircraft has a downward pointed V-tail.

See also

• Aircraft flight control systems
• Flight control surfaces
• American Airlines Flight 587, crashed after losing its stabilizers.
• Japan Airlines Flight 123 - second highest loss of life in single aicraft. Crashed after stabilzer lost.

Notes

1. A portmanteau word from "rudder" + "elevator"

References

1. ^ Garrison, P; Three's Company; Flying Magazine, Dec 2002, p.85.
2. NASA: this page is intended for college, high school, or middle school students
3. NASA: On some aircraft, the pitch stability and control is provided by a horizontal surface placed forward of the center of gravity
4. "Horizontal stabilizer - elevator", The Beginner's Guide to Aeronautics, NASA Glenn Research Center, Sep 13 2010 {{citation}}: Check date values in: |date= (help)
5. Barber, Horatio, "Chapter II - Stability and Control", The Aeroplane Speaks, Electronic Text Center, University of Virginia Library
6. NASA Flight Education website
7. Princeton Aerodynamics Lecture Series

• "Dynamic Longitudinal, Directional, and Lateral Stability", Centennial of Flight, US Centennial of Flight Commission

External links

• Aircraft terms
• Introduction to the Aerodynamics of Flight Theodore A. Talay, Langley Research Center

• Aircraft controls