# Why do some fighter jets have movable horizontal stabilizer instead of elevators installed on the stabilizers?

I have noticed that the horizontal stabilizers on some fighter jets move up and down to increase and decrease the angle of attack, but on some airliners they use elevators to adjust the angle of attack. Why isn't this kind of system used on airliners?

• Not all fighters use moving stabilsers. Not all airliners use elevators. You might be better rephrasing the question? Sep 21 '15 at 22:12
• Peter Kampf's answer (as usual) is correct. However, I'd like to stress the first point he makes about the benefits of full-flying tail surfaces: the full-flying stabilizer was a solution to loss-of-control issues (google "compressibility problem") when airplanes enter transonic and supersonic speeds. Civilian planes don't go supersonic and so can use the cheaper elevators. Sep 22 '15 at 16:49

All parts of an airliner's horizontal surface move, not just the rear part. The rear part, called an elevator, can move much faster and is for maneuvering. The forward part, called a (trimmable) stabilizer, is for trimming and moves slowly. It is moved in response to changes in loading, speed or flap settings and positions the tail surface such that only small continuous elevator deflections are needed. It doesn't need to move fast - high load factors would upset the passengers and overload the structure.

A330 port tailplane root (picture source). Note the markings which show the range of incidence angles covered by the trimmable stabilizer.

## Benefits of a stabilizer-elevator configuration:

• Camber: The elevator deflection changes the camber of the airfoil of the tail surface and makes the production of the intended lift change more efficient. If the elevator deflection is supposed to create a downforce, negative camber is produced and vice versa. This reduces the drag which is created in order to maneuver the aircraft.
• Lower control forces: By moving a smaller surface, less hydraulics power, and on small aircraft, less muscle power is needed for the same moment change than with a full-flying tail or all-moving surface. Makes sense, right?
• Better tailoring of the hinge moment derivatives: Two effects are important for getting the control forces right: The change in hinge moment over deflection angle ($c_{r\eta}$), and the change in hinge moment over angle of attack ($c_{r\alpha}$). With tabs, the right elevator hinge position, nose shape and control horns, both can be tailored individually, while a full-flying tail or all-moving surface will give the designer less freedom to manipulate both independently. Note that full-flying tails only emerged with hydraulic controls, because only those systems can manage the control forces of all-moving control surfaces at high speed.

Horn balance and overhang balance on control surfaces (picture source)

## Benefits of a full-flying tail surface

• By avoiding a contour break due to a flap deflection, all-moving control surfaces can avoid the shocks which would otherwise occur at high subsonic speed. This is their main benefit for combat aircraft.
• At supersonic speed, the uncambered surface will produce less drag, so for supersonic speed the all-moving control surface is more efficient.
• By moving all of the tail surface at high speed, an all-moving control surface will produce the highest rate of moment change over time possible. To produce also the highest sustained moment change, however, it needs to be larger than a comparable stabilizer-elevator combination because it forfeits the benefits of variable camber.
• The all-moving control surface has no gaps which could add radar reflections, allowing for a more stealthy design.
• Lower mechanical complexity. This is somewhat offset by the need for much beefier hydraulics.

The all movable horizontal tailplane is usually called a stabilator (stabilizer + elevator) or all moving tail plane or in some cases tail slab. This is mostly used in combat aircraft for a few reasons:

• Enhanced Maneuverability. The all moving tailplane offers higher maneuverability compared to the stabilizer + elevator found in civil aircraft. Basically, the stabilator gives wider range of pitch control over a larger range of speeds. Also, the stabilator is also hinged about the aerodynamic center, requiring comparatively lesser control input from the pilot for operation.

"F 22 Raptor Tail Feathers photo D Ramey Logan" by WPPilot - Own work. Licensed under CC BY-SA 4.0 via Wikimedia Commons.

• Weight and Drag. The stabilator construction is simple compared to the stabilizer + elevator, having practically only a plate, and having no control linkages inside. This makes it lighter and produces lesser drag.
• Stability. The stabilator design helps in eliminating the Mach tuck. Mach tuck is nose down tendency due to a change in the position of the centre of pressure. This is caused by a rearward movement of the shock wave which occurs in an aircraft in transonic flight. As the aircraft accelerates beyond its limiting Mach number or critical Mach number (the maximum Mach number it can operate), the elevator authority required to control the aircraft (to prevent a dive) increases beyond that of the elevator. The stabilator design prevents this.
• Stealth. The stabilator design, with its clean lines and no discontinuities, offers more stealth compared to the stabilizer and elevator design. The design can also be optimised for stealth with edges etc.

Some GA aircraft do use all moving tail planes for e.g. Piper Cherokee. However, there are a couple of reasons this system is not usually used in civil aircraft:

• Control Power. As already noted, the stabilator is hinged about the aerodynamic center, which means that the moment (and the pilot power) required is constant. In case of civil aircraft, the airplane must show an increasing resistance to an increasing pilot input. When they are used, the stabilators in GA aircraft have an anti-servo tab to provide more resistance.

Source: avstop.com

• Requirements. There is no requirement in civil aircrafts for higher maneuverability or supersonic flight. In case of Concorde, the trim problem in transonic range was overcome by shifting fuel between the fuel tanks, thereby changing the center of gravity of the aircraft.