Tailplane

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The tailplane of this P-51 is shown in pink. Mustang p-51d arp.jpg
The tailplane of this P-51 is shown in pink.

A tailplane, also 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 as well as other non-fixed-wing aircraft such as helicopters and gyroplanes. Not all fixed-wing aircraft have tailplanes. Canards, tailless and flying wing aircraft have no separate tailplane, while in V-tail aircraft the vertical stabiliser, rudder, and the tail-plane and elevator are combined to form two diagonal surfaces in a V layout.

Contents

The function of the tailplane is to provide stability and control. In particular, the tailplane helps adjust for changes in position of the centre of pressure or centre of gravity caused by changes in speed and attitude, fuel consumption, or dropping cargo or payload.

Tailplane types

The tailplane comprises the tail-mounted fixed horizontal stabiliser and movable elevator. Besides its planform, it is characterised by:

Some locations have been given special names:

Tail fuselage mounted.svg
Fuselage mounted
Tail cruciform.svg
Cruciform
Tail T.svg
T-tail
Tail plane flying.svg
Flying tailplane

Stability

Tailplane (in shadow) of an easyJet Airbus A319 Tailplane.JPG
Tailplane (in shadow) of an easyJet Airbus A319

A wing with a conventional aerofoil profile makes a negative contribution to longitudinal stability. This means that any disturbance (such as a gust) which raises the nose produces a nose-up pitching moment which tends to raise the nose further. With the same disturbance, the presence of a tailplane produces a restoring nose-down pitching moment, which may counteract the natural instability of the wing and make the aircraft longitudinally stable (in much the same way a weather vane always points into the wind).

The longitudinal stability of an aircraft may change when it is flown "hands-off"; i.e. when the flight controls are subject to aerodynamic forces but not pilot input forces.

Damping

In addition to giving a restoring force (which on its own would cause oscillatory motion) a tailplane gives damping. This is caused by the relative wind seen by the tail as the aircraft rotates around the centre of gravity. For example, when the aircraft is oscillating, but is momentarily aligned with the overall vehicle's motion, the tailplane still sees a relative wind that is opposing the oscillation.

Lift

Depending on the aircraft design and flight regime, its tailplane may create positive lift or negative lift (downforce). It is sometimes assumed that on a stable aircraft this will always be a net down force, but this is untrue. [2]

On some pioneer designs, such as the Bleriot XI, the centre of gravity was between the neutral point and the tailplane, which also provided positive lift. However this arrangement can be unstable and these designs often had severe handling issues. The requirements for stability were not understood until shortly before World War I - the era within which the British Bristol Scout light biplane was designed for civilian use, with an airfoiled lifting tail throughout its production run into the early World War I years and British military service from 1914-1916 — when it was realised that moving the centre of gravity further forwards allowed the use of a non-lifting tailplane in which the lift is nominally neither positive nor negative but zero, which leads to more stable behaviour. [3] Later examples of aircraft from World War I and onwards into the interwar years that had positive lift tailplanes include, chronologically, the Sopwith Camel, Charles Lindbergh's Spirit of St. Louis, the Gee Bee Model R Racer - all aircraft with a reputation for being difficult to fly, and the easier-to-fly Fleet Finch two-seat Canadian trainer biplane, itself possessing a flat-bottom airfoiled tailplane unit not unlike the earlier Bristol Scout. But with care a lifting tailplane can be made stable. An example is provided by the Bachem Ba 349 Natter VTOL rocket-powered interceptor, which had a lifting tail and was both stable and controllable in flight. [4]

In many modern conventional aircraft, the centre of gravity is placed ahead of the neutral point.[ citation needed ] The wing lift then exerts a pitch-down moment around the centre of gravity, which must be balanced by a pitch-up moment (implying negative lift) from the tailplane. A disadvantage is that it generates trim drag.

Active stability

Using a computer to control the elevator allows aerodynamically unstable aircraft to be flown in the same manner.

Aircraft such as the F-16 are flown with artificial stability. The advantage of this is a significant reduction in drag caused by the tailplane, and improved maneuverability.

Mach tuck

At transonic speeds, an aircraft can experience a shift rearwards in the center of pressure due to the buildup and movement of shockwaves. This causes a nose-down pitching moment called Mach tuck. Significant trim force may be needed to maintain equilibrium, and this is most often provided using the whole tailplane in the form of an all-flying tailplane or stabilator.

Control

A tailplane usually has some means allowing the pilot to control the amount of lift produced by the tailplane. This in turn causes a nose-up or nose-down pitching moment on the aircraft, which is used to control the aircraft in pitch.

Elevator A conventional tailplane normally has a hinged aft surface called an elevator,

Stabilator or all-moving tail In transonic flight shock waves generated by the front of the tailplane render any elevator unusable. An all-moving tail was developed by the British for the Miles M.52, but first saw actual transonic flight on the Bell X-1; Bell Aircraft Corporation had included an elevator trim device that could alter the angle of attack of the entire tailplane. This saved the program from a costly and time-consuming rebuild of the aircraft.[ citation needed ]

Transonic and supersonic aircraft now have all-moving tailplanes to counteract Mach tuck and maintain maneuverability when flying faster than the critical Mach number. Normally called a stabilator, this configuration is often referred to as an "all-moving" or "all-flying" tailplane.

See also

Related Research Articles

Stall (fluid dynamics) Abrupt reduction in lift due to flow separation

In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases. This occurs when the critical angle of attack of the foil is exceeded. The critical angle of attack is typically about 15 degrees, but it may vary significantly depending on the fluid, foil, and Reynolds number.

Delta wing wing shaped in the form of a triangle

The delta wing is a wing shaped in the form of a triangle. It is named for its similarity in shape to the Greek uppercase letter delta (Δ).

Flight control surfaces surface that allows a pilot to adjust and control an aircrafts flight attitude

Aircraft flight control surfaces are aerodynamic devices allowing a pilot to adjust and control the aircraft's flight attitude.

Dihedral (aeronautics)

Dihedral angle is the upward angle from horizontal of the wings or tailplane of a fixed-wing aircraft. "Anhedral angle" is the name given to negative dihedral angle, that is, when there is a downward angle from horizontal of the wings or tailplane of a fixed-wing aircraft.

Elevon aircraft control surface that combines the functions of the elevator and the aileron

Elevons or tailerons are aircraft control surfaces that combine the functions of the elevator and the aileron, hence the name. They are frequently used on tailless aircraft such as flying wings. An elevon that is not part of the main wing, but instead is a separate tail surface, is a stabilator. The word "elevon" is a portmanteau of elevator and aileron.

Elevator (aeronautics) type of flight control surface

Elevators are flight control surfaces, usually at the rear of an aircraft, which control the aircraft's pitch, and therefore the angle of attack and the lift of the wing. The elevators are usually hinged to the tailplane or horizontal stabilizer. They may be the only pitch control surface present, and are sometimes located at the front of the aircraft or integrated into a rear "all-moving tailplane", also called a slab elevator or stabilator.

Stabilator fully movable aircraft stabilizer

A stabilator, more frequently all-moving tail or all-flying tail, is a fully movable aircraft stabilizer. It serves the usual functions of longitudinal stability, control and stick force requirements otherwise performed by the separate parts of a conventional horizontal stabilizer and elevator. Apart from a higher efficiency at high Mach number, it is a useful device for changing the aircraft balance within wide limits, and for mastering the stick forces.

Pusher configuration arrangement of propellers on an aircraft to face rearward

In an aircraft with a pusher configuration, the propeller(s) are mounted behind their respective engine(s). According to British aviation author Bill Gunston, a "pusher propeller" is one mounted behind the engine, so that the drive shaft is in compression.

Empennage Tail section of an aircraft containing stabilizers

The empennage, also known as the tail or tail assembly, is a structure at the rear of an aircraft that provides stability during flight, in a way similar to the feathers on an arrow. The term derives from the French language word empenner which means "to feather an arrow". Most aircraft feature an empennage incorporating vertical and horizontal stabilising surfaces which stabilise the flight dynamics of yaw and pitch, as well as housing control surfaces.

Flight mechanics are relevant to fixed wing and rotary wing (helicopters) aircraft. An aeroplane, is defined in ICAO Document 9110 as, "a power-driven heavier than air aircraft, deriving its lift chiefly from aerodynamic reactions on surface which remain fixed under given conditions of flight".

Mach tuck

Mach tuck is an aerodynamic effect whereby the nose of an aircraft tends to pitch downward as the airflow around the wing reaches supersonic speeds. This diving tendency is also known as "tuck under". The aircraft will first experience this effect at significantly below Mach 1.

Stabilizer (aeronautics) aircraft component

An aircraft stabilizer is an aerodynamic surface, typically including one or more movable control surfaces, that provides longitudinal (pitch) and/or directional (yaw) stability and control. A stabilizer can feature a fixed or adjustable structure on which any movable control surfaces are hinged, or it can itself be a fully movable surface such as a stabilator. Depending on the context, "stabilizer" may sometimes describe only the front part of the overall surface.

Ice protection system

Ice protection systems are designed to keep atmospheric ice from accumulating on aircraft surfaces, such as wings, propellers, rotor blades, engine intakes, and environmental control intakes. If ice is allowed to build up to a significant thickness it can change the shape of airfoils and flight control surfaces, degrading the performance, control or handling characteristics of the aircraft. An ice protection system either prevents formation of ice, or enables the aircraft to shed the ice before it can grow to a dangerous thickness.

Canard (aeronautics) aircraft wing configuration with a small wing ahead of the main wing, or such a forewing

A canard is an aeronautical arrangement wherein a small forewing or foreplane is placed forward of the main wing of a fixed-wing aircraft. The term "canard" may be used to describe the aircraft itself, the wing configuration, or the foreplane.

If an aircraft in flight suffers a disturbance in pitch that causes an increase in angle of attack, it is desirable that the aerodynamic forces on the aircraft cause a decrease in angle of attack so that the disturbance does not cause a continuous increase in angle of attack. This is longitudinal static stability.

Tailless aircraft

A tailless aircraft has no tail assembly and no other horizontal surface besides its main wing. The aerodynamic control and stabilisation functions in both pitch and roll are incorporated into the main wing. A tailless type may still have a conventional vertical fin and rudder.

In flight dynamics, longitudinal static stability is the stability of an aircraft in the longitudinal, or pitching, plane under steady flight conditions. This characteristic is important in determining whether a human pilot will be able to control the aircraft in the pitching plane without requiring excessive attention or excessive strength.

Supermaneuverability is the capability of fighter aircraft to execute tactical maneuvers that are not possible with purely aerodynamic mechanisms. Such maneuvers can use controlled side-slipping and angles of attack beyond maximum lift.

Wing configuration Describes the general shape and layout of an aircraft wing

The wing configuration of a fixed-wing aircraft is its arrangement of lifting and related surfaces.

Three-surface aircraft fixed-wing aircraft with a main central wing plus fore and aft surfaces

A three-surface aircraft or sometimes three-lifting-surface aircraft has a foreplane, a central wing and a tailplane. The central wing surface always provides lift and is usually the largest, while the functions of the fore and aft planes may vary between types and may include lift, control and/or stability.

References

  1. Anderson, John D., Introduction to Flight, 5th ed, p 517
  2. Burns, BRA (23 February 1985), "Canards: Design with Care", Flight International, pp. 19–21, It is a misconception that tailed aeroplanes always carry tailplane downloads. They usually do, with flaps down and at forward c.g. positions, but with flaps up at the c.g. aft, tail loads at high lift are frequently positive (up), although the tail's maximum lifting capability is rarely approached..p.19 p.20 p.21
  3. Answers to correspondents, Flight, 2 November 1916, Page 962; "A "lifting tail" is one which normally carries a certain amount of load, and which is therefore often cambered in order to make it more efficient. For instance, the tail planes of the old Farman biplanes were "lifting tail planes," and were, as a matter of fact, rather heavily cambered. By a non-lifting tail plane is meant one which does not, in the normal flying attitude, carry any portion of the load, but is merely "floating." This type of plane is usually, although not invariably, made of symmetrical section—i.e., it is either a perfectly flat plane, built up of a framework of steel tubes, or it is constructed of spars and ribs after the fashion of the main planes, but symmetrical in section and convex on both sides. The object of the latter form of section is, of course, to provide a good "streamline" shape which will offer a minimum of resistance. During flight it constantly occurs that such a tail plane is momentarily loaded, the load being either upwards or downwards according to circumstances, and then, of course, the tail plane is no longer, strictly speaking, " non-lifting." ... a non-lifting tail plane is not invariably symmetrical in section. Some designers favour a section in which the upper surface is convex, while the lower surface is perfectly flat. The reasons usually advanced for the employment of such a section are that, as the tail planes may-—and, indeed, frequently do—work in the down draught from the main planes, a tail plane set parallel to the path of the machine, or, in other words, parallel to the propeller shaft, is virtually subject to a load acting in a downward direction. Now, an unsymmetrical tail plane like that referred to above is still giving a certain amount of lift a to angle of incidence, whereas the symmetrical .section would, of course, give no lift when the incidence was zero. The plano-convex section therefore tends, owing to the slight lift at no angle of incidence, to counteract the effect of the down draught from the wings, and may therefore be said to be equivalent to a flat or streamline plane set at a slight angle to the propeller shaft. The tail plane of the B.E.2C, as is the case on the majority of modern machines, is of the non-lifting type."
  4. Green, W.; Warplanes of the Third Reich, Macdonald and Jane's, 1970.