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Landing gear systems must be able to sustain and manoeuver the aircraft on ground, absorb the kinetic energy of landing, and provide proper clearance for aircraft components on ground level. Depending on each aircraft model, the landing gear systems must be designed and installed at specific locations on the aircraft to fulfill the main landing gear functions without sacrificing aerodynamic characteristics. The B747-400 applies one of these designs, the multi-bogey gear configuration (2.1), with the use of nose, body, and wing gear systems. The configurations, with its components, must be placed at specific locations relative to the aircraft Center of Gravity (2.2), which is of importance for the structural design (2.3).
2.1 - Landing gear initial design
The multi-bogey gear configuration (figure X1), designed for larger aircraft corresponding the B747-400, and the Airbus A380, is a basic concept that consist of nose (1), body (2), and wing (3) gear systems with multiple wheels per strut to sustain larger weights and forces. The multiple wheels are employed in tandem, which is connected to a component at the end of the strut referred to as the bogey. The B747-400 consists of two nose gear wheels, two sets of four body and main gear wheels. The multi-bogey gear system is the product of many attempts at designing the ideal landing gear configuration and modifying former landing gear configurations. The multi-bogey gear configuration is the successor to the tricycle gear configuration for large aircraft with large wheel loading. The wheel loading is the static load on each wheel of the landing gear at aircraft take-off weight, and is decreased per wheel using bogeys. The multi-bogey gear configuration ensures a better view of area, and stability for ground operation guaranteeing a greater safety factor. However, the multi-bogey is the most complicated and expensive landing gear configuration, and also increases the aircraft turn radius, thus causing maneuvering complications. The multi-bogey gear configuration often is used on aircraft with weights greater than 90000 kilograms, where the mentioned will usually use arrangements with two wheels per bogey.
Nose gear system [1x2]
Body gear system [2x4]
Wing gear system [2x4]
Figure X1 - B747-400 Multi-bogey configuration
2.2 - Landing gear advanced design
The landing gear advanced design is the preparation for the structural design, in which the forces will be taken into account. These forces will be spread over each wheel of the aircraft. The wheel load can be calculated with the aid of using the wheelbase and wheel track distances (figure X2). The position of the aircraft Center of Gravity (1) is the most important consideration during the layout process, as it influences the geometric parameters of the landing gear. The wheel base (2) distance is the total distance between the front and rear wheel axles in the vertical plane of symmetry. The wheel track (3) distance is the maximum distance between the main wheels in the lateral plane of the aircraft. The wheel base and wheel track distances determine the aircraft turning radius. The Center of Gravity position relative to the wheel base and wheel track determines the aircraft over-turn characteristics. The B747-400 has a wheel base of 25,62 meters, a wheel track of 11 meters, and a distance of 17,02 meters between the aircraft Center of Gravity and the nose gear system.
Center of gravity
Figure X2 - Landing gear positioning relative to the Center of Gravity
2.3 - Landing gear structural design
3 - Landing gear control system
3.3 - Brakes
3.3.5 - Anti-skid protection system
Available through both the normal and alternate brake system, the anti-skid protection system is installed to prevent aircraft from skidding during breaking operations. The brakes can be activated within pressure limits, of which the sent pressure must accord and differ per circumstance to properly brake the wheels. However, if the brake pedals do not send enough or too much pressure, the brakes will act incorrectly. When the brakes are used incorrectly, it will damage the tires, and can cause the aircraft to skid. The anti-skid protection system maximizes the braking efficiency, minimizes tire damage, and minimizes the loss of aircraft control. Skidding is the phenomenon that occurs when the aircraft wheels do not share the same rotation velocity.
The electrical anti-skid protection system is applied on the B747-400, as conventional mechanical anti-skid protection systems are used for older aircraft. The anti-skid system compares the velocity of each landing gear wheel, and will adjust the brake pressure in each wheel if the velocities do not match. Each main gear wheel is equipped with an individual anti-skid transducer that measures the rotation velocity of the wheel, and sends signals to the Brake System Control Unit if the wheel rotation speed of the wheels does not match. The Brake System Control Unit will process and send the signal to the associated anti-skid valves to release and reapply the brake pressure to prevent or take control of the skidding phenomenon. Skid protection control is available when the wheel rotation speed is greater than eight knots. During a skid, the anti-skid system will override the pressure from the braking pedals, and reduce brake pressure by dumping hydraulic pressure through an anti-skid control valve, allowing the wheel to spin up. When the slip is reduced, the controller allows brake pressure to be reapplied. The anti-skid system can be activated with the switch of a button, which is located directly above the auto-brake selector.
The anti-skid system provides protection under normal, dry, and wet conditions. The anti-skid system also provides skid protection at runways with ice or snow contamination. During wet conditions layers of water will build up between the tires of the aircraft and the runway surface, leading to traction loss and it will prevent the vehicle from responding to control inputs. Hydroplane and touchdown protection is only applied for the back aircraft wheels. This protection prevents the hydroplane phenomenon, and prevents the wheels from locking back during touchdown. The touchdown protection system prevents brake pressure from reaching the wheel before touchdown, whether intentionally or not.
Currey, Norman S.
Aircraft Landing Gear Design: Principles and Practices
Lockheed Aeronautical Systems Company
Marietta, Georgia 1988
Boeing 747 landing gear operational manual
Oxford Aviation Academy
Aircraft general knowledge I Airframes - Systems
ATPL Ground training series
Shoreham, England 2011
Kumar Kundu, Ajoy
Cambridge aerospace series
Cambridge, England 2010
List of sources from publications.
Landing gear layout design
Consulted on: 12-09-12
Boeing 747 manual
Consulted on: 12-09-12
Anti-skid protection system
Consulted on: 13-09-12
List of terms and abbreviations