A tailplane, known as a horizontal stabiliser, is a small lifting surface located on the tail (empennage) behind the main lifting surfaces of a fixed-wing aircraft. It comprises the tail-mounted fixed horizontal stabiliser and movable elevator whose primary purpose is to provide stability and control. Most tailplanes fall under one of two categories: cruciform or T-tail. Canards, tailless and flying wing aircraft have no separate tailplane.
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Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is a tailplane?
A tailplane comprises a tail-mounted fixed horizontal stabilizer and movable elevator, and it is alternatively known as a horizontal stabilizer. Horizontal tail surfaces include the stabilizer and the elevator, and fixed-wing aircraft have tailplanes located behind the main lifting surfaces. Canards, tailless, and flying wing aircraft have no separate tailplane, while gyroplanes and some helicopters feature horizontal stabilizers as part of their tail assembly.
The tailplane is the horizontal airfoil of an aircraft's tail assembly. It is normally fixed and carries the hinged aft surface called the elevator. This elevator, being hinged to the horizontal airfoil, is the pilot's primary means of varying the tailplane's lift so that the whole tailplane has the means of allowing the pilot control the amount of lift produced. Because each of its two extensions is a lifting surface, the assembly also acts as a horizontal stabilizer, itself a lifting device that gives the aft fuselage longitudinal balance.
What is the purpose of the vertical tailplane of an aircraft?
The vertical tailplane, known as the vertical stabilizer, has two primary purposes: it provides directional stability and it enables the aircraft to be controlled in yaw. Acting like a weathercock, the surface keeps the aircraft's nose facing into the direction of the relative airflow and prevents the nose from yawing back and forth. When the aircraft slips, the vertical tail generates a restoring yaw moment about the centre of gravity, supplying most of the moment required to realign the fuselage with the flight path. Together with the rudder, the vertical tail forms the yaw-control system. The rudder is the movable control surface hinged to the rear of the stabilizer. Deflecting the rudder left or right creates an aerodynamic side force that yaws the aircraft, permitting deliberate sideslip during cross-wind landings and compensating for asymmetries in thrust or drag. At small angles of attack the stabilizer itself is symmetrical, so no net side force appears when the aircraft is perfectly aligned with the relative wind.
The tail lift is negative whenever the aircraft is trimmed in a condition where the centre of gravity is far aft or the wing pitching-moment is nose-up. In such trimmed flight the horizontal tailplane must push downward, producing negative lift to balance the wing pitching moment about the centre of gravity. The vertical tailplane contributes no lift in the conventional sense, but its yaw-stability derivative remains positive.
What are the types of tailplanes?
The types of tailplanes are outlined below.
- Conventional Tailplanes: A conventional tailplane is the most common arrangement in which a horizontal stabilizer is attached to the fuselage below a vertical stabilizer at the rear of the aircraft
- T-tail Tailplanes: A T-tail mounts the horizontal stabilizer on top of the fin
- Cruciform Tailplanes: A cruciform tail positions the horizontal stabilizer midway up the vertical stabilizer
- V-tail Tailplanes: A V-tail combines the functions of vertical and horizontal surfaces into two inclined panels called ruddervators
Tailplanes can be set high, mid or low. High-set examples are T-tails used on seaplanes, smaller aircraft with limited ground clearance, and high-wing transports to keep the surface clear of engine exhaust. Mid-set examples are cruciform tails selected to keep the horizontal surface out of propeller or jet wake. Low-set examples are conventional tails on low-wing aircraft where the horizontal stabilizer is attached to the fuselage below the vertical stabilizer. Adjustable tailplanes are all-moving types or stabilators that rotate as a single unit to vary camber and provide pitch control and are common on supersonic aircraft and some light singles. They are made smaller than fixed surfaces because of the larger pitching moment they generate.
What is a variable incidence tailplane?
A variable-incidence tailplane is a tailplane that moves the whole tail surface instead of carrying a hinged elevator. By altering the angle at which the aerodynamic surface is mounted to the remaining structure of the aircraft, it changes the angle of attack of the tail and hence the aerodynamic load on the tailplane. Because the angle can be altered in flight, the same surface serves both as pitch-control device and as trim device, doing away with a separate elevator.
Variable-incidence tails are a feature of essentially all jet transport airplanes and are commonly used on large airplanes. The Westland Lysander tailplane had a variable incidence tailplane that was required to be at different angles for take-off and landing. The DH89 Rapide had a variable incidence tailplane for trim, as did the final version of the Lysander.
What is the difference between a tailplane and a fin?
A tailplane and a fin serve different stabilizing purposes on an aircraft. The tailplane, commonly called the horizontal stabilizer, controls pitch. Its elevator allows the pilot to adjust pitch attitude. The fin, called the vertical stabilizer, is a symmetrical aerofoil set at incidence that experiences a side force to the right whenever the aircraft yaws. It includes the rudder, a primary control surface that governs yaw.
In a conventional arrangement the horizontal stabilizer is bolted to the rear fuselage beneath the fin, whereas in a T-tail the tailplane is mounted on top of the fin. This top-mount position makes the T-tail heavier, because the vertical tailplane must now support the horizontal tailplane as well as carry its own aerodynamic loads.
What is the difference between tailplane and empennage?
The difference is that the tailplane forms part of the empennage, whereas the empennage is the complete rear-section unit that also includes the vertical stabiliser, rudder, elevator and trim tabs. The empennage is the entire tail assembly, the aft-most skinned structure that comprises both fixed and movable surfaces. The tailplane has only one element inside that assembly - the horizontal stabiliser.
What is the tailplane design?
In the conventional tail design, a horizontal stabiliser is attached directly to the fuselage and accounts for roughly 70% of aircraft worldwide because it is lightweight, easy to manufacture and simpler to maintain. The T-tail, popular on aircraft with aft-fuselage-mounted engines, removes the tail from the exhaust blast and prop-wash, improves elevator flow and looks like the capital letter T when viewed from behind, but it increases structural weight of the vertical fin. The V-tail replaces the horizontal stabiliser and vertical fin with a pair of surfaces mounted at a high dihedral angle. These surfaces generate force normal to their plane, can be lighter, yet need a larger total area to equal the control power and damping of a conventional tail and are more prone to flutter.
Shape and camber of the tailplane aerofoil are dictated by the required lift direction. Many horizontal tail airfoils have negative camber so that they can efficiently produce downforce at small negative angles of attack; reflexed airfoils are used for the same purpose. A cambered aerofoil set at negative incidence (declage) relative to the wing, or a linearly cambered section, increases downforce at low speed. A symmetrical airfoil is chosen when bi-directional lift is needed or when the designer wishes the surface to carry zero lift in trim; a positively cambered section is used on a lifting tail mounted ahead of the wing.
Setting angle is expressed as incidence or declage relative to the wing reference line. Negative incidence (declage) ensures that the tailplane produces downward force in level flight whereas positive incidence (incage) is used only on specialist lifting-tail layouts. The aspect ratio of the horizontal tail is always lower than that of the main wing to keep structural weight down, yet it must be large enough for the tail-volume coefficient to guarantee adequate stabilising influence. Thus size, aspect ratio, camber, incidence and configuration are traded so that the surface supplies consistent control movements throughout most flight regimes while keeping weight, drag and structural complexity to a minimum.
What is tailplane sizing?
Tailplane sizing is the iterative, largely empirical process of choosing the horizontal-tail area (SHT) and the vertical-tail area (SVT) so that, throughout the certified centre-of-gravity range, the aircraft remains statically stable, can be trimmed in steady flight, and possesses enough control authority for engine-out or low-speed conditions. The activity is driven by conflicting requirements: a smaller tail reduces drag and stick forces but also lowers static stability, whereas a larger tail improves stability and control yet increases wetted area and overall aircraft drag. The designer therefore seeks the smallest tail that still satisfies every stability and control criterion. The tailplane is re-sized repeatedly as the wing position, centre of gravity and fuselage length evolve, each loop checking equilibrium of pitching and yawing moments until the smallest adequate surface is obtained.
