Equipment that supports an aircraft on the ground, allows it to maneuver between runways and parking places, and supports the aircraft during takeoffs and landings.
The weight of an airplane in flight is supported by the lift force on its wings. However, the airplane must pass through two transitional stages: takeoff, when the airplane leaves the ground, and landing, when it returns to the ground. The demands upon the landing gear during takeoffs differ from those during landings. During takeoffs, the airplane may accelerate to a speed of more than 140 miles per hour in a runway distance of less than 5,000 feet. Should the pilot stop the airplane during its takeoff run, the tires and brakes must sustain heavy mechanical friction loads without failure. During a routine takeoff, the landing gear must not only support the airplane but also respond to the pilot’s directional commands. During landing, the wheels must absorb the descent speed of the airplane as it makes contact with the runway. The tires, on first contact with the runway, spin to a rotational speed that matches the airplane’s landing speed. The brakes contained in the landing gear must then bring the airplane to a stop.
In the routine landing of heavy commercial airplanes, reverse thrust is obtained from the engines, whether propeller or jet. However, in an emergency, the brakes must be capable of stopping the airplane without any engine assistance.
Airplanes have landing gear for three reasons: to maneuver the airplane along the ground, to support and control the direction of the airplane during takeoff until the lift on the wings is able to support the weight of the airplane, and to support the weight of the airplane during landing as the wings gradually lose lift. The wheels and the connecting structure must be able to absorb the vertical or descending speed of the airplane at the instant of touchdown. During the critical landing phase, the pilot must have sufficient skill to keep the descent rate within a small enough magnitude to prevent damage to the landing gear and the rest of the airplane. During takeoff and landing, the pilot must be able to control the airplane during both routine conditions and emergency conditions, such as tire blowouts.
Airplanes that operate from an aircraft carrier must have very strong and resilient landing gear. The relative velocity between the wheels and carrier deck might be much higher than that experienced by a land-based airplane and the course is not entirely under the control of the pilot.
In addition to the requirements of landing, takeoff, and ground maneuvering, some landing gear must be retracted into the airplane’s wings or fuselage. Except for low-performance general aviation airplanes, retractable landing gear is a feature of nearly all modern airplanes. The reason for retracting the landing gear is to reduce aerodynamic drag that would otherwise be caused by the extended gear.
Because the space in either the wings or fuselage is limited, there is an incentive to limit the diameter of the wheels. To meet the airplane’s takeoff, landing, and maneuvering requirements, the tire pressure can be as high as 200 pounds per square inch for typical military airplanes; the tires in commercial airplanes might be as high as 140 pounds per square inch. An important part of the design of an airplane’s landing gear is the selection of the proper tire, and a significant part of the routine maintenance of an airplane is the regular inspection and replacement of tires.
The most obvious part of the landing gear is where and how the wheels are attached to the airplane. There are many common arrangements. In the so-called conventional arrangement, two main wheels are placed near the front of the airplane and well ahead of the airplane’s center of gravity. A much smaller tail wheel is placed at the rear, just under the elevator. For the first four decades of powered flight, nearly all airplanes, both civil and military, used this arrangement. Except in some limited-production aerobatic, sport, or homebuilt airplanes, this wheel arrangement is no longer in use. The increasingly inappropriate term “conventional” has been replaced by the more descriptive term “tail-dragger.”
The tricycle arrangement has become the most common form of landing gear. In the tricycle gear, there is a wheel and strut placed forward, with the main wheel of the tail-dragger moved back past the center of gravity of the airplane. The tail-dragger arrangement, nevertheless, has certain advantages over the tricycle arrangement, one of which is that the presence of two rather than three wheels means less drag in flight. The tail-dragger arrangement also provides for better propeller clearance when the aircraft is on the ground. Because the tail-dragger lands at a higher angle relative to the wind, it can use more lift in the wing and consequently land at a lower speed and therefore require a shorter runway. Because of its lower landing speed, the tail-dragger might be better suited to rough-field landings.
The tail-dragger’s disadvantage is the location of the center of gravity behind the main wheels, an inherently unstable condition. Unless quickly corrected, the response of a tail-dragger to a slight side motion, or drift, at landing is the ground loop, a maneuver in which the airplane turns suddenly to one side, rolling the airplane to touch down the opposite wingtip. Damage to airplane from a ground-loop can include a crushed wingtip or a collapsed landing gear. The tail-dragger pilot must have sufficient skill to keep the airplane completely aligned with the runway during landings, even at low speeds.
The advantage of the tricycle gear, used in most airplanes except heavily loaded transport aircraft or sailplanes, is the reduced likelihood of ground loops, as the center of gravity is ahead of the two main wheels. In addition, the pilot has better visibility on the ground. The cabin floor is horizontal on the ground, facilitating the loading of passengers and cargo.
The bicycle, or tandem-wheel, arrangement is a specialized arrangement occasionally used on military airplanes and common on sailplanes. The advantage is the reduced weight of a third wheel. Weight reduction is especially critical on aircraft intended for vertical takeoff, such as the Harrier jet.
Large transport airplanes often employ multiple-wheel arrangements to distribute the weight of the aircraft on the runway. The C-5A aircraft has a double wheel at the nose. In the rear, there are four sets of double bogies. A bogie is wheel arrangement in which the wheels are mounted one at each of the four corners of a cart. The center of the cart is strut-connected to the airplane.
An airplane’s landing gear permits it to take off, land, and maneuver on the ground. The landing gear also allows control of the airplane during the critical landing and takeoff operations and provides brake force as needed in emergency conditions. Although there are several types of landing gear arrangements depending upon the performance and weight of the airplane, the tricycle landing gear remains the most common.
Raymer, Daniel P. Aircraft Design: A Conceptual Approach. 3d ed. Reston, Va.: American Institute of Aeronautics and Astronautics, 1999. A comprehensive and up-to-date book on the design of airplanes directed at the engineering student, with many sections where the discussion is without complex mathematics. Chapter 11 on landing-gear design and implementation requires little more than high-school algebra. Stinton, Darrel, The Design of the Airplane. New York: Van Nostrand-Reinhold, 1985. An excellent introduction to landing-gear design, especially for general aviation airplanes. Taylor, John W. R. The Lore of Flight. New York: Crescent Books, 1974. A massive, well-illustrated, oversized book featuring nontechnical descriptions of airplanes and spacecraft, and covering controls and cockpit instruments.
Airplanes
Flight control systems
Landing procedures
Pilots and copilots
Taxiing procedures
Landing gear supports the plane on the ground, absorbs friction, and provides maneuverability during takeoff and landing.