Definition: Various arrangements of horizontal and vertical stabilizing surfaces at the rear part of an airplane.
Significance: An airplane’s tail design is important because it stabilizes and controls the airplane in both up-and-down movements of pitch and side-to-side movements of yaw.
The Parts of an Airplane’s Tail
The tail of an airplane is called by various names, such as “empennage” and “stabilizer.” The preferred term is “stabilizer,” because it is at least partially descriptive of the component’s function. However, the stabilizer provides not only stability but also some of the airplane’s control.
The tail of an airplane is designed to provide both stability and control of the airplane in pitch and yaw. There are many different forms an aircraft tail can take in meeting these dual requirements of stability and control. Most tail designs have a horizontal wing like structure and one or more vertical or near-vertical structures. Whenever practical, these structures are identified as the horizontal and vertical stabilizers, although some designs do not conveniently fit such a description.
The many types of airplane tail design include, but are by no means limited to, the conventional, T-tail, cruciform-tail, dual-tail, triple-tail, V-tail, inverted V-tail, inverted Y-tail, twin-tail, boom-tail, high boom-tail, and multiple-plane tail designs.
Conventional Tail Design
The conventional tail design is the most common form. It has one vertical stabilizer placed at the tapered tail section of the fuselage and one horizontal stabilizer divided into two parts, one on each side of the vertical stablilzer. For many airplanes, the conventional arrangement provides adequate stability and control with the lowest structural weight. About three-quarters of the airplanes in operation today, including the Airbus A300, the Boeing 777 and 747, and the Beech Bonanza A-36, use this arrangement.
The T-Tail Design
In the T-tail design, a common variation of the conventional tail, the horizontal stabilizer is positioned at the top of the vertical stabilizer. The horizontal stabilizer is then above the propeller flow, or prop wash, and the wing wake. Because the horizontal stabilizer is more efficient, it can therefore be made both smaller and lighter. The placement of the horizontal stabilizer on top of the vertical stabilizer can also make the vertical stabilizer more aerodynamically efficient. By making the vertical stabilizer more effective, its size may be reduced. However, the horizontal stabilizer in the T-tail layout imposes a bending and twisting load on the vertical stabilizer, requiring a stronger, and therefore, a heavier, structure. These loads are avoided in the conventional design. There is also the possibility that at the high pitch angle usually associated with landing the airplane, the horizontal stabilizer of the T tail will be immersed in the slower and more turbulent flow of the wing wake. In some cases, it is possible to compromise severely the control function of the horizontal tail. Nevertheless, the T tail is the second-most common tail design after the conventional.
Both major American transport plane builders, Boeing and McDonnell-Douglas, use the T-tail design. The Boeing 727, with its three fuselage-mounted engines, has a T-tail design, as do the variants of the McDonnell Douglas MD-90, formerly the Douglas DC-9. Other aircraft that employ the T-tail design are the Lockheed C-5A, the Gates Lear-jets 23 and 35A, the Cessna Citation CJ1, the Piper Lance II, and the Beech Skipper 77.
The cruciform tail is an obvious compromise between the conventional and T-tail designs. In the cruciform design, the horizontal stabilizer is moved part of the way up the vertical stabilizer. In this position, the horizontal stabilizer is moved up and away from the jet exhaust and wing wake. The lifting of the horizontal stabilizer also exposes the lower part of the vertical stabilizer, as well as the rudder, to undisturbed airflow. Undisturbed airflow on the rudder is important, particularly in the recovery from spins. A military example of the cruciform tail is the North American RockwellB-1B supersonic bomber. Other aircraft that use the cruciform-tail design are the Dessault Falcon 100 and the Commander.
The dual-tail design, in which the two vertical stabilizers are placed at the ends of the horizontal stabilizers, was at one time fairly common on large flying boats and twin-engine propeller-driven bombers such as the North AmericanB-25. In some cases, this arrangement is attractive, because it places the vertical stabilizers in the prop wash of wing-mounted propellers. The result is the maintenance of good directional control during low-speed operations. The positioning of the two vertical stabilizers at the ends of the horizontal stabilizers allows for a smaller, lighter, and more aerodynamically efficient horizontal stabilizer. However, the overall weight of a plane with a dual-tail design is greater than that of a plane with the single conventional-tail design.
The dual tail is part of the design of the Republic Fairchild A-10 ground-attack airplane, in which the plane’s two jet engines are mounted to the rear of the fuselage. When this airplane is viewed from the rear and slightly to either side, the engine exhausts, blocked by the vertical stabilizer, are not easily visible. If a heat-seeking missile is launched at a departing or escaping A-10, the main heat source, the engine exhausts, are at least partially blocked by the vertical stabilizer.
The Ercoupe, a private light airplane developed in the late 1940′s and still seen at small airports, uses a dual tail to keep the vertical stabilizer out of the wake from the fuselage and the wing-fuselage junction. The Ercoupe is unique in that it is the only commercial light airplane ever produced with the dual-tail design. Other craft that use the dual-tail design include the Consolidated B-24, the Short Skyvan, and the Martin PBM Mariner flying boat.
The triple-tail design, with two vertical stabilizers placed at the ends of the horizontal stabilizers and one mounted on the fuselage, is attractive when the height of the vertical stabilizer must meet certain restrictions, such as hangar-door height. Certainly this was the important consideration in the design of the Lockheed Constellation, one of the most significant passenger airplanes of the late 1940′s. Another well-known example of the triple-tail design is the Grumman E-2 Hawkeye.
The V-Tail, sometimes called the “butterfly” tail, has had limited application in airplane design, the most significant of which has been by the Beech Company in the Beech-craft Bonanza V-35. Clearly, the usual definition of horizontal and vertical stabilizers has no application to the V tail. The intended advantage of the V-tail design is that two surfaces might serve the same function as the three required in the conventional tail and its variants. Removal of one surface then would reduce the drag of the tail surfaces as well as the weight of the tail region. However, wind tunnel studies by the National Advisory Committee on Aeronautics (NACA) have shown that for the V tail to achieve the same degree of stability as a conventional tail, the area of the V tail would have to be about the same size as that of the conventional tail.
Another disadvantage of the V tail has to do with turning the airplane. To turn left, for example, the pilot would press the left rudder pedal and bank the airplane with the left wing down. In V-tail aircraft, the right side of the V (as viewed from the rear) deflects upward, and the left surface deflects downward. This arrangement drives the nose to the left but also causes the airplane to roll away from the turn. Although this tendency to roll is overcome by the wing control provided by the ailerons, it is clear that one control of the airplane produces a secondary effect that opposes the primary effect of another control. This secondary effect of opposing the primary purpose of another control is called adverse coupling. Adverse coupling is one reason that the most recent Bonanza design, the A-36, uses the conventional tail.
The undesirable rolling motion caused by the V tail might be avoided by inverting the butterfly tail. However, except for a few small homemade glider-sail planes, this design has been avoided because of ground clearance problems.
Inverted Y-Tail Design
The inverted Y tail is actually a conventional tail with a noticeable droop to the horizontal stabilizers. In other words, the outer ends of the horizontal stabilizers are lower than the ends attached to the fuselage. The F-4 Phantom, originally a mainstay of the McDonnell Company, used the inverted Y tail to keep the horizontal surfaces out of the wing wake at high angles of attack. It is interesting to note that the tips of the horizontal stabilizers on the first McDonnell Navy fighter, the F-2H Banshee, were bent decidedly upward.
The twin tail is a feature of various air superiority fighters used by both the U.S. Navy (the F-14 Tomcat) and the U.S. Marine Corps (the F/A-18 Hornet). Although both the F-14 and F/A-18 designs have a superficial resemblance, they also have important differences. The tilt angle of the vertical stabilizer of the F-14 is more pronounced than that of the F-18, so much so that it approaches that of the V tail on the Beech model V-35 Bonanza. With two vertical stabilizers, the twin tail is more effective than the conventional single tail of the same height.
Boom tails are used when an aircraft’s fuselage does not extend entirely back to the horizontal stabilizer. In both the Lockheed P-38 Lightning fighter of World War II and the Fairchild C-119 cargo plane, engines were mounted on the booms. In the case of the C-119, the twin boom allowed easy access to the rear of the fuselage for loading and removing cargo. The twin boom has also been used for an airplane with engines mounted in the fuselage, with one engine, known as the tractor, in the nose of the airplane and one engine, known as the pusher, in the rear of the airplane. Because the thrust of both engines is along the centerline of the airplane, it is much easier in this arrangement to compensate for the loss of one engine than it is in the wing-mounted engine installation. Both the Cessna Skymaster and the new Adam 309 have fuselage-mounted engines. In the case of the Adam 309 the horizontal stabilizer is raised to avoid propeller wake from the pusher, or rear-mounted, engine.
Multiple-Plane Tail Design
Finally, the obsolete multiple-plane tail design has two or more horizontal stabilizers. This layout was used extensively in bombing airplanes of World War I and even in a few early British passenger and freight-carrying airplanes. It may be seen again on the recently constructed replica of the Vickers Vimy airplane.