Vertical stabilizer

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The vertical tail of an Airbus A320 airliner

A vertical stabilizer or fin[1][2] is the static part of the vertical tail of an aircraft.[1] Its role is to provide control and stability in yaw (also known as directional or weathercock stability). It is part of the aircraft empennage, specifically of its stabilizers.

The vertical tail is typically mounted on top of the rear end of the fuselage, with the horizontal stabilizers mounted in a similar manner (a configuration termed "conventional tail"). Other configurations, such as T-tail or twin tail, are sometimes used instead.

Vertical stabilizers have occasionally been used in motor sports, with for example in Le Mans Prototype racing.

Function[]

Principle[]

Control surfaces at the tail of a conventional aircraft

The vertical tail of an aircraft typically consists of a vertical stabilizer (fixed surface) on which a rudder (movable surface) is mounted. Together, their role role is to enable trim in the yaw direction (compensate moments in yaw generated by any asymmetry in thrust or drag), enable the aircraft to be controlled in yaw (for example, to initiate side slip during a crosswind landing), as well as provide stability in yaw (weathercock or directional stability).[3]

The greater its position away from the center of gravity, and the more effective the vertical tail can be. Thus, shorter aircraft typically feature larger vertical tails; for example, the vertical tail of the short Airbus A318 is larger than that of its longer counterparts in the A320 family. The vertical tail’s effectiveness is typically expressed using the vertical tail volume coefficient[4] (also called volume ratio[5]), which non-dimensionalizes its area and arm with the dimensions of the main wing:

(where the indices v and w stand for vertical tail and wing respectively, S stands for area, and L_w is typically the mean aerodynamic chord). Values for the vertical tail coefficient vary only mildly from aircraft one type of aircraft to another, with extreme values ranging from 0.02 (sailplane) to 0.09 (jet aircraft transport).[4]

Trim and control in yaw[]

Like all aerodynamic surfaces, the vertical tail effectiveness increases with equivalent air speed. When sizing it, the determining factor is often either the case for engine failure on takeoff, which determines the minimum ground control speed VMCG, or the case for crosswind landing.[3] Floatplanes and tail dragger aircraft, because they start their take-off run with a high propeller angle of attack, experience large moments in yaw generated by the propeller. Those must be compensated with the vertical tail and rudder.[5]

Yaw stability[]

The vertical tail plays a determining role in yaw stability, providing most of the required restoring moment about the center of gravity when the aircraft slips. Yaw stability is typically quantified using the derivative of moment coefficient with respect to yaw angle.[5]

The airflow over the vertical tail is often influenced by the fuselage, wings and engines of the aircraft, both in magnitude and direction.[5] The main wing and the horizontal stabilizer, if they are highly swept, can contribute significantly to the yaw stability; wings swept backwards tend to increase yaw stability.  Sweep in the wing and horizontal tail of a conventional airplane, however, does not affect airplane trim in yaw.[5]

Dihedral in the main wing and horizontal tail can also have a small effect on the static yaw stability. This effect is complex and coupled with the effect of wing sweep and flow about the fuselage.[5]

Propellers, especially when they are advancing so that their axis makes an angle to the freestream velocity, can affect the static stability of an airplane in yaw.[5]

Coupling with roll[]

The vertical tail affects the behavior of the aircraft in roll, since the aerodynamic center of the stabilizer typically lies far above the center of gravity of the aircraft.[1] When the aircraft slips to the right, the relative wind and side force on the stabilizer translate into an anti-clockwise moment in roll.[5]

Supersonic flight[]

In supersonic flight, the fin becomes progressively less effective until the loss of stability may no longer be acceptable.[citation needed] The fin may be enlarged, or extra area may be added by installing ventral fins (such as on the Vought F-8 Crusader) or folding-down wingtips (such as on the North American XB-70 Valkyrie). If more surface area is not acceptable due to the increased drag which comes with it, as in the case of the Avro Arrow[citation needed], automatic flight controls which incorporate a yaw damper may instead provide the required stability.[citation needed]

Stall of the vertical tail[]

A dorsal fin is visible at the base of the vertical tail of this Embraer 195

The vertical tail sometimes features a fillet or dorsal fin at its forward base, which helps to increase the stall angle of the vertical surface (resulting in vortex lift), and in this way prevent a phenomenon called rudder lock or rudder reversal. Rudder lock occurs when the force on a deflected rudder (e.g. in a steady sideslip) suddenly reverses as the vertical stabilizer stalls. This may leave the rudder stuck at full deflection with the pilot unable to recenter it.[6] The dorsal fin was introduced in the 1940s, for example on the 1942 Douglas DC-4, predating the wing strakes of the fighter aircraft developed in the 1970s, such as the F-16.[7]

Configurations[]

Single stabilizer[]

The conventional vertical tail of an MD-11 trijet airliner, featuring the center engine

In the conventional tail configuration, the vertical stabilizer is vertical, and the horizontal stabilizer is directly mounted to the rear fuselage This is the most common vertical stabilizer configuration. A T-tail has the horizontal stabilizer mounted at the top of the vertical stabilizer. It is commonly seen on rear-engine aircraft, such as the Bombardier CRJ, and most high-performance gliders. T-tails are often incorporated on configurations with fuselage-mounted engines to keep the horizontal stabilizer away from the engine exhaust plume. T-tail aircraft are more susceptible to pitch-up at high angles of attack. This pitch-up results from a reduction in the horizontal stabilizer's lifting capability as it passes through the wake of the wing at moderate angles of attack. This can also result in a deep stall condition. T-tails present structural challenges since loads on the horizontal stabilizer must be transmitted through the vertical tail.[citation needed]

The cruciform tail is arranged like a cross, the most common configuration having the horizontal stabilizer intersecting the vertical tail somewhere near the middle. The British Aerospace 146 uses this configuration.

Trijets (jet aircraft with three engines) have the central engine (e.g. McDonnell Douglas DC-10) or the engine inlet duct (e.g. Lockheed L-1011 Tristar) at the base of the fin.

Multiple stabilizers[]

The twin tail of an F-22 fighter jet

Twin-tail aircraft have two vertical stabilizers. Many modern military aircraft use this configuration. The F-22 Raptor and F-35 Lightning II have surfaces that are canted outward, to the point that they have some authority as horizontal control surfaces; both aircraft are designed to deflect their rudders inward during takeoff to increase pitching moment. A twin tail may be either H-tail, twin fin/rudder construction attached to a single fuselage, such as on the Avro Lancaster, or twin boom tail, the rear airframe consisting of two separate fuselages each sporting one single fin/rudder, such as Lockheed P-38 Lightning.

Rarer configurations include three vertical stabilizers, such as the 1950s Lockheed Constellation, or four, such as the Northrop Grumman E-2 Hawkeye used by the United States Navy.

On V-tail aircraft, vertical an horizontal control surfaces are merged into control surfaces known as ruddervators which control both pitch and yaw. The arrangement looks like the letter V, and is also known as a butterfly tail. The Beechcraft Bonanza Model 35 uses this configuration.

Winglets served double duty on Burt Rutan's canard pusher configuration VariEze and Long-EZ, acting as both a wingtip device and a vertical stabilizer. Several other derivatives of these and other similar aircraft use this design element.

Accidents[]

Accidents involving the loss of a vertical stabilizer include crashes involving the B-52 bomber in 1963 and 1964, as well as American Airlines Flight 587.

Automotive use[]

Ferrari F10 with large rear vertical fin sprouting out of the airbox and leading into the rear wing

Devices similar to vertical tails have been used on cars such as the 1955 Jaguar D-type or the 2013 Lamborghini Veneno. On race cars, its primary purpose is to reduce sudden high-speed yaw-induced blow-overs that would cause cars to flip due to lift when subject to extreme yaw angles during cornering or in a spin.[citation needed] Since 2011, the vertical stabilizer has become mandatory for all newly-homologated Le Mans Prototypes.[8]

Some Formula 1 teams utilized a vertical stabilizer as a way to disrupt the airflow to the rear wing reducing drag, the most radical system being the "F-duct" found in the 2010 McLaren MP4-25 and Ferrari F10. On demand by the driver, this system diverted air from a duct in the front of the car through a tunnel in the vertical fin onto the rear wing to stall it and reduce drag on the straights on which downforce was not needed.[citation needed] The system was banned for the 2011 Formula 1 season.[citation needed]

References[]

  1. ^ Jump up to: a b c Barnard, R.H.; Philpott, D.R. (2010). Aircraft Flight (4th ed.). Harlow, England: Prentice Hall. ISBN 9780273730989.
  2. ^ Kumar, Bharat (2005). An Illustrated Dictionary of Aviation. New York: McGraw Hill. p. 272. ISBN 0 07 139606 3.
  3. ^ Jump up to: a b Jenkinson, Llyod R.; Simpkin, Paul; Rhodes, Darren (1999). Civil Jet Aircraft Design. Reston, Virginia: AIAA education series. ISBN 156347350X.
  4. ^ Jump up to: a b Raymer, Daniel P. (1999). Aircraft Design: A Conceptual Approach (3rd ed.). Reston, Virginia: American Institute of Aeronautics and Astronautics. ISBN 1563472813.
  5. ^ Jump up to: a b c d e f g h Phillips, Warren F. (2010). Mechanics of Flight (2nd ed.). Hoboken, New Jersey: Wiley & Sons. ISBN 9780470539750.
  6. ^ NASA Flight Education website Archived February 27, 2009, at the Wayback Machine
  7. ^ Bjorn Fehrm (March 1, 2019). "Bjorn's Corner: Yaw stability, Part 2". Leeham News.
  8. ^ Erripis, Loannis K. (2010-12-13). "The New Audi R18 LMP1". Robotpig.net. Retrieved 2011-03-30.
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