Vertical stabilizer
A vertical stabilizer or tail fin is the static part of the vertical tail of an aircraft. The term is commonly applied to the assembly of both this fixed surface and one or more movable rudders hinged to it. Their role is to provide control, stability and trim in yaw. It is part of the aircraft empennage, specifically of its stabilizers.
The vertical tail is typically mounted on top of the rear fuselage, with the horizontal stabilizers mounted on the side of the fuselage. 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
The vertical tail of an aircraft typically consists of a fixed vertical stabilizer or fin on which a movable rudder is mounted. A trim tab may similarly be mounted on the rudder. Together, their role is to enable trim in the yaw direction, enable the aircraft to be controlled in yaw, as well as provide stability in yaw.The greater its position away from the center of gravity, 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 effectiveness of the vertical tail depends on its efficiency and the vertical tail volume coefficient, which non-dimensionalizes its area and arm with the dimensions of the main wing:
. Values for the vertical tail coefficient vary only mildly from aircraft one type of aircraft to another, with extreme values ranging from 0.02 to 0.09.
The tail efficiency is the ratio of the dynamic pressure at the tail to that in the freestream. The tail has its maximum capability when immersed in the free stream with an efficiency of one. When partially immersed in a wake its effectiveness is reduced because the wake has a lower dynamic pressure than the free stream. The fin height may need to be increased to restore its required effectiveness in certain flight conditions. The Panavia Tornado had a tall fin for directional stability at high angles of incidence.
Trim and control in yaw
The rudder is the directional control surface and is usually hinged to the fin or vertical stabilizer. Moving it allows the pilot to control yaw about the vertical axis, i.e., change the horizontal direction in which the nose is pointing.Maximum rudder deflection is usually controlled by a rudder travel limiter. The largest achievable angle of a rudder at a particular flight condition is called its blowdown limit. It represents a balance between the aerodynamic forces on the rudder and the mechanical forces from the actuating mechanism.
Multi-engined aircraft, especially those with wing-mounted engines, have large powerful rudders. They are required to provide sufficient control after an engine failure on take-off at maximum weight and cross wind limit and cross-wind capability on normal take-off and landing.
For taxiing and during the beginning of the take-off, aircraft are steered by a combination of rudder input as well as turning the nosewheel or tailwheel. At slow speeds the nosewheel or tailwheel has the most control authority, but as the speed increases the aerodynamic effects of the rudder increases, thereby making the rudder more and more important for yaw control. In some aircraft both of these mechanisms are controlled by the rudder pedals so there is no difference to the pilot. In other aircraft there is a special tiller controlling the wheel steering and the pedals control the rudder, and a limited amount of wheel steering. For these aircraft the pilots stop using the tiller after lining up with the runway prior to take-off, and begin using it after landing before turning off the runway, to prevent over correcting with the sensitive tiller at high speeds. The pedals may also be used for small corrections while taxiing in a straight line, or leading in or out of a turn, before applying the tiller, to keep the turn smooth.
With the controls in the neutral position, a plane may still gently yaw to one side. This is corrected through the setting of a trim surface, often a separate trim tab mounted on the rudder but sometimes the rudder itself, to counteract the yaw and ensure the plane flies in a straight line.
Changing the setting of a trim tab adjusts the neutral or resting position of a control surface. As the desired position of a control surface changes, an adjustable trim tab will allow the operator to reduce the manual force required to maintain that position—to zero, if used correctly. Thus the trim tab acts as a servo tab. Because the center of pressure of the trim tab is further away from the axis of rotation of the control surface than the center of pressure of the control surface, the movement generated by the tab can match the movement generated by the control surface. The position of the control surface on its axis will change until the torque from the control surface and the trim surface balance each other.
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.The airflow over the vertical tail is often influenced by the fuselage, wings and engines of the aircraft, both in magnitude and direction. 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.
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.
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.
Coupling with roll
The vertical tail affects the behavior of the aircraft in roll, since its aerodynamic center typically lies far above the center of gravity of the aircraft. When the aircraft slips to the right, the relative wind and side force on the vertical tail translate into an anti-clockwise moment in roll.Supersonic flight
In supersonic flight, the vertical tail becomes progressively less effective with increasing Mach number until the loss of stability may no longer be acceptable. The stability is reduced because the lift, or side force, generated by the tail reduces with speed for each degree of sideslip angle. This results from the very different pressure distribution, with shock waves and expansion waves, compared to subsonic. To achieve the required stability at the maximum operating speed of the aircraft the vertical tail may be enlarged, such as on the North American F-100 Super Sabre. Extra area may be added by installing ventral fins, or folding-down wingtips. If a bigger tail is not acceptable automatic rudder deflections may be used to increase the tail side force and restore directional stability. This method was used on the Avro Arrow.Stall of the vertical tail
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, and in this way prevent a phenomenon called rudder lock or rudder reversal. Rudder lock occurs when the force on a deflected rudder suddenly reverses as the vertical tail stalls. This may leave the rudder stuck at full deflection with the pilot unable to recenter it. 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.Structural considerations
The rudder and fin on a large, or fast, aircraft are each subject to a considerable force which increases with rudder deflection. An extreme case occurs with a departure from controlled flight, known as an upset, which in the context of fin and rudder is excessive sideslip. For large transport aircraft the stabilizing moment necessary for recovery comes from the fin with little requirement for rudder deflection. These aircraft do not have a requirement to withstand near-full rudder deflections in these circumstances because the structural weight required to prevent structural failure would make them commercially unviable. Loss of the complete fin and rudder assembly occurred on American Airlines Flight 587 when the pilot used full rudder deflections while following in the wake of a very large jet.Clear-air turbulence caused the failure of the complete fin and rudder assembly on a Boeing B-52 Stratofortress after which the pilots made a successful landing. B-52 bombers instrumented for gust and maneuver loads recorded gusts from clear-air turbulence considerably more than the design limit with highest loads at 34,000 feet.
The English Electric Lightning T4 prototype fin failure was caused by inertial roll coupling while doing high-rate rolls. The fin was enlarged, strengthened and roll-rate limitations were imposed. However, the first T5 also had a fin failure while doing rapid rolling trials with rocket pack extended.
A Lightning lost its fin due to interaction between aircraft in close proximity at low level when flying in formation at M 0.97, an aerobatic display routine. Limitations were imposed including separation between aircraft when in formation.
Fin buffeting is a critical issue for fighter aircraft with twin or single fins because the fatigue life of the fin structure is reduced by the fluctuating loads caused by burst vortices impinging on the fin. The single fin on the Eurofighter Typhoon experiences buffet loads caused by burst vortices which originate from the canard and wing leading edges at high angles of attack. The sides of the top-mounted airbrake, when deflected, also shed vortices which impinge, after bursting, on the fin. Buffeting from the extended airbrake is highest when the airbrake effective angle of attack is greatest, which for a fully-extended airbrake is greatest at low aircraft angle of attack and least when maneuvering. The McDonnell Douglas F/A-18 Hornet twin fins are subject to buffeting from the breakdown or bursting of the leading-edge extension vortex in front of the tail. The addition of a LEX fence significantly reduces the buffeting and increases fin fatigue life.