A stabilator is a fully movable aircraft horizontal stabilizer that combines the functions of both the fixed stabilizer and the movable elevator. Because the entire surface pivots from a central hinge point, it can simultaneously provide longitudinal stability and precise pitch control while generating appropriate stick force for the pilot.
By eliminating separate elevator surfaces, the stabilator reduced drag and, on high-speed aircraft, helps eliminate the Mach tuck caused by shock-wave formation behind a conventional elevator hinge line. A trimmable version, which does not move in response to control-column input, offers trimming power across the complete speed range and allows the tail downloads to be adjusted in flight to hold the tail down under varying conditions.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is a stabilator in aviation?
A stabilator is a flight control surface consisting of a hinged horizontal stabilizer. It is one movable surface that replaces the usual pair of fixed horizontal stabilizer plus hinged elevator. The term stabilator itself is a mixture of stabilizer and elevator, signalling that the entire horizontal tail surface is responsive to the pilot’s control wheel or control stick inputs. Because the whole surface pivots, altering the angle of attack of that airfoil changes the amount of lift generated, so the aircraft's pitch changes immediately. An anti-servo tab, a pitch-trim design that moves with increased travel, prevents overcontrol by the pilot while keeping control forces small.
What is the purpose of a stabilator?
The stabilator serves two purposes: to keep the aircraft stable and to control the aircraft's pitch. Its primary purpose is to allow the pilot to control a pitch movement with less force, making the pitch axis stay in trim as speed changes and as fuel is burned. Antiservo tabs are incorporated on the trailing edge to decrease sensitivity, prevent overcontrol by the pilot, and relieve control pressure while functioning as a trim device.
How does stabilator work?
A stabilator works by altering the angle of attack of the horizontal stabilizer. Stabilators work by pivoting via hinge. Their aerodynamic center is near the tail's mean quarter-chord, so hinge location keeps the hinge-moment coefficient derivative low and control forces small. When the control column is pulled back it pulls the nose of the aircraft whereas when the control column is pushed forward it pitches the nose of the aircraft down. Altering the angle of attack of an airfoil changes the amount of lift generated, so the pressure imbalance that results when the airplane tilts pushes the tail back into line.
To keep the stabilator in balance, the anti-servo tab deflects with increased travel. This aerodynamic force resists the pilot input, prevents over-controlling, and makes it easier for the pilot to control. The anti-servo tab also functions as a trim device and is linked to a trim wheel for trim adjustments. Trim tabs on elevators move opposite of the control surface, whereas the anti-servo tab moves with increased travel. A differential stabilator enhances manoeuvrability and is used to eliminate the Mach tuck caused by shock-wave formation behind the elevator hinge line of a conventional tail.
I noticed that a stabilator allows me to control the aircraft’s pitch by altering the angle of attack of the horizontal stabilizer. The anti-servo tabs makes it easier for me to control the stabilator.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
What is stabilator movement?
Stabilator movement is the rotation of the entire horizontal tail surface about a hinge near its aerodynamic center, a point close to the tail's mean quarter-chord. By altering the angle of attack of the horizontal stabilizer, this movement changes the lift generated by the stabilator and, in turn, the pitching moment of the aircraft. When the pilot pulls back on the control column the stabilator moves up and pushing forward pitches the nose down. Because the surface pivots about its aerodynamic center, hinge moments remain low and any movement of the stabilator can be made without added pilot effort.
The same motion is actuated by a jackscrew mounted on the leading edge, and at higher airspeeds the aircraft response becomes more rapid yet controls grow increasingly firm. To prevent over-control, an anti-servo tab on the trailing edge moves in the same direction as the stabilator with increased travel, providing resistance to pilot input and also functioning as a trim device to relieve control pressure. Small movement of the tab equals large movement of the elevator, so stabilator movement is very sensitive to control inputs while still allowing precise pitch control with minimal drag.
What is the span of a stabilator?
The span distance of a stabilator is measured tip-to-tip. The total length of the horizontal stabilizer on a 737-800 is 47 ft 1 in (14.35 m); dividing by 2 provides a rough estimate of each semi-length.
What are the benefits of a stabilator?
The benefits of a stabilator include that it allows the pilot to generate a pitching moment with very little effort because it involves a moving balanced surface. Control forces can be kept small and controlled with the servo tab. By combining functions into a single flight control surface, stabilators can lower the aircraft's total weight and reduce its drag. Easier adjustment of stick forces and appropriate stick force cues further reduce crew workload during long flights. Light weight, lower drag, and crisp pitch authority let the stabilator deliver the rapid nose-pointing and high-g manoeuvres demanded in air-combat. The hinge moment coefficient derivative over angle of attack is kept low, so the fighter retains stable feel even when the surface is deflected at high speed and high angle of attack.
