Stabilator

A stabilator (stabilizer-elevator) is an aircraft control surface that combines the functions of an elevator and a horizontal stabilizer. Most fixed-wing aircraft control pitch using a hinged horizontal flap — the elevator — attached to the back of the fixed horizontal stabilizer, but some aircraft make the entire stabilizer movable.

General aviation

Because it involves a large moving surface, a stabilator can allow the pilot to generate greater pitching moment with little effort. Due to the high forces involved in tail balancing loads, stabilators are designed to pivot about their aerodynamic center (at the tail's quarter-chord). This is the point at which the pitching moment is constant regardless of the angle of attack, and thus any movement of the stabilator can be made without additional pilot effort. An airplane certified by the appropriate regulatory agency (e.g. the US Federal Aviation Administration) must show an increasing resistance to an increasing pilot input (movement), so to provide this resistance, stabilators on small aircraft contain an anti-servo tab (sometimes combined with the trim tab) that deflects in the same direction as the stabilator, thus providing an aerodynamic force resisting the pilot's input. General aviation aircraft with stabilators include the Piper Cherokee, Cessna 177 and the Schweizer SGS 2-33 sailplane.

Military

Stabilators were developed in response to the need to achieve supersonic flight, and are almost universal on modern military combat aircraft. All non-delta-winged supersonic aircraft use stabilators because with conventional control surfaces, shockwaves can form at the elevator hinge, eliminating the elevator's effectiveness as an aerodynamic control surface.

The wartime Miles M.52 project had stabilators; when the project was cancelled the information was passed to the US supersonic project and used on the Bell X-1. The North American Aviation F-86 Sabre, the first USAF aircraft which could go supersonic (although in a shallow dive) was introduced with a conventional horizontal stabilizer with elevators, which was eventually replaced with a stabilator.

Stabilators are also known in military terminology as all-moving or all-flying tailplanes. When stabilators also include the function of ailerons, as they do on many modern fighter aircraft, they become tailerons or rolling tails. A stabilator mounted in front of the main wing is a canard.

Stabilators on military aircraft have the same problem of overcontrol as general aviation aircraft. In older jet fighter aircraft, a resisting force was generated within the control system, either by springs or a resisting hydraulic force, rather than by an external anti-servo tab. For example in the F-100 Super Sabre, springs were attached to the control stick to provide increasing resistance to pilot input. In modern fighters, control inputs are moderated by computers ("fly by wire"), and there is no direct connection between the pilot's stick and the stabilator.

Airliners

Most modern airliners adjust the angle of the tailplane to trim during flight as fuel is burned and the center of gravity moves. These adjustments are handled by adjustable (in angle of attack) horizontal stabilizers. However, such adjustable stabilizers are not the same as stabilators; a stabilator is controlled by the pilot's control wheel, and an adjustable stabilizer is controlled by the trim system. One example of an airliner with a genuine stabilator used for flight control is the Lockheed L-1011.

External links

* [http://www.grc.nasa.gov/WWW/K-12/airplane/stablator.html Stabilators (NASA)] — Includes Java applet