When you ask people what the main function is of a landing gear they mostly say it is meant to land the aircraft, yet thats exactly what the words stands for. This is slightly correct, one of the three main functions is energy absorption (at landing). (1.1.1a). But when the aircraft has touched the ground it has to stop, so here is an other main function of the landing gear; braking (1.1.1b). The last step and also the last main function is for taxi control, in other words: to manoeuvre the aircraft on ground (1.1.1c).
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When an aircraft has reached its destination, it has to descend and land on a runway from an airport. To do this it is using the landing gear of the aircraft. When the aircraft hits the ground the landing gear is the interface of aircraft to ground. Depending on various factors like: weather conditions, aircraft load, aircraft configuration and aircraft balance the landing gear must absorb the impact energy and must transmit this gently to the aircraft structure. But not only at landing, when an aircraft is on ground while manoeuvring the landing gear has to bear the total weight of an aircraft and absorb the energy while braking, taxiing, take off roll, landing roll, steering and towing. With only a strut and wheels there is no chance of surviving an impact with a large aircraft. Therefore the landing gear is provided with shock absorbers. The shock absorber may be an independent element, or integrated with the landing gear strut or in side braces. The wheels of a landing gear are also used for shock absorption. Aircraft tires are much more stronger as tires for automobiles. Landing gear tyres are designed to resist high take-off and landing speeds and large shock loads upon landings.
Another challenge of a landing gear is to stop the aircraft while rolling over the runway. The landing gear can use the tires to achieve this challenge. But the tires cannot stop by itself, they need brakes to slow down. Almost all commercial aircrafts only have brakes at the main landing gear to slow down the aircraft and use at each main gear wheel multiple disc hydraulic powered brakes. Those brakes are controlled in flight deck. Pilots can choose three inputs for braking; manual brake, auto brake system and parking brake. The manual brakes are controlled by brake pedals mostly located at the top of the rudder pedals. The auto brake system provides automatic braking during take-offs and landings so the pilot can perform other tasks. The auto brake system can be set in different modes to apply correct braking configurations. To maintain position for a longer period on ground a parking brake can be used so the airplane does not need wheel chocks.
1.1.1c Taxi control
The last function of a landing gear is taxi control. To manoeuvre the aircraft on ground at low speeds the landing gear is equipped with steering facilities so the aircraft can move by itself and do not need to get moved by a tug. At modern commercial aircraft the nose landing gear is equipped with a hydro-mechanical system to move the nose gear. Steering is done by the pilot either from a tiller in the cockpit or the rudder pedals. Some aircrafts with more than two main gear units (bogies) need also to control these extra units to manoeuvre the aircraft safely.
Landing gear types
Not every aircraft has the same landing gear configuration. To understand why modern commercial aircrafts use a tricycle gear configuration (nose wheel equipped aircraft) there first will be looked to other kinds of configurations (1.1.2a) to understand the benefits. Not only the benefits will be explained but also the construction of the tricycle gear configuration (1.1.2b).
1.1.2a Landing gear configurations
For aircraft engineers, the landing gear is not their favourite part of an aircraft because it is not needed in flight but still it does influence the flight performance. Since the first aircraft engineers have tried all kind of configurations to fit best under a typical aircraft. Each configuration has its own advantages and disadvantages, to find out there will be take a look at several configurations available to engineers.
Single main gear
Ad 1 Taildragger gear
The taildragger gear also called a conventional landing gear (Figure…) consists of two main gear units (1) and a small wheel or skid to support the tail (tail unit) (2). This kind of configuration ruled the aircraft design from about 1910 till 1950. Nowadays this configuration is widely used on small propeller-driven aircrafts, because it’s simplicity. Is not only used for its simplicity, it has also some benefits.
Main gear units
Figure…: Taildragger gear as seen on a Spitfire
The two main gear units can easily be adapted to produce low drag in flight and because the tail wheel is small there is less weight. When you look at the taildragger configuration (Figure…) you will notice the nose of the aircraft is already titled, so the wings already have an larger angle of attack. This means when the aircraft rolls down the runway the wings generate more lift so the aircraft needs less length of runway. Besides the propeller of a propeller-driven aircraft has a higher level above the ground, what could save the propeller from damages.
But this configuration does also have some cons. The handling characteristics are not that good. This is because the centre of gravity is located behind the main gear units. Which could results in a ”ground loop”. A ground loop is basically an uncontrolled turn while on ground. With the centre of gravity behind the main gear units, any turn will naturally increase in rate because of momentum. When landing and using the brakes too many at rollout, this could result in a capsizing aircraft. These examples could easier occur when one main gear unit touches down first or with heavy crosswind. To prevent, pilots must line up a approach very carefully to keep the plane on a straight path until it comes to a stop, this can be done with constantly rudder adjustments. Some aircrafts have an locked tail unit to keep rolling in a straight line during landing.
Ad 2 Bicycle gear
The bicycle gear consist of two main gear units along the centre line of the aircraft and very often sideways two small outrigger gears mounted on the wing (Figure…). This kind of configuration is not very common but is a solution for aircraft with very long and slim fuselages or high wing aircraft. This configuration is actually very limited to aircraft with wings that generate high lift at a low angle of attack, as rotation with this configuration is impossible.
Main gear units
Figure…: Bicycle gear as seen on Boeing B-47
Only one of the benefits is lower weight and drag than the taildragger or tricycle configuration.
For the pilot this configuration is challenging, because he or she has to maintain a very level attitude during take-off and landing while carefully managing airspeed. The pilot has to land the aircraft without any rolling motion because that could cause the aircraft to land on one of the outrigger gear.
Ad 3 Single main gear
The single main gear (Figure…) consist of one main gear unit (1) and a much smaller auxiliary tailwheel (2) along the centreline. For stability an outrigger gear (3) is mounted along the wing. This configuration is very similar to the bicycle gear and is practical for use on light planes like gliders. The single main gear is just like the bicycle gear lightweight and provides low drag.
Main gear unit
Figure…: Single main gear as seen on a Lockheed U-2
Ad 4 Quadricycle gear
The quadricycle gear (Figure…) consist of four main gear units (1) roughly mounted along the fuselage. This configuration is also similar to the bicycle gear and is being used sometimes on cargo aircraft. For cargo aircraft it could be ideal because the aircraft floor can get at a very low level above the ground what is easy for loading or unloading cargo.
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Just like the bicycle configuration the pilot must maintain a very level attitude during take-off and landing. A quadricycle gear is also very sensitive for roll, crosswinds, and proper line up of a approach. The Boeing B-52 has an extra crosswind gear (2) installed to make crosswind landings. A major disadvantage also is four main gear units that contain a much higher weight and drag.
Main gear units
Figure…: Quadricycle gear as seen on a Boeing B-52
1.1.2b Tricycle Gear
The tricycle gear generally consist of two main gear units and one nose gear unit. Nowadays this kind of configuration is mostly used in aviation especially for commercial aircraft. The tricycle configuration has one wheel in the front and two or more main wheels located near the centre of gravity. The main reason why it is mostly used is the ease of operating the tricycle gear aircraft on ground. This is not the only advantage the tricycle gear has a series of advantages:
stability in braking
steady touch down with no risk of aerodynamic bounce (like taildragger gear)
high pilot visibility during taxiing (because of level aircraft)
horizontal floor (passengers comfort and easy cargo loading)
low drag during take-off acceleration
The tricycle gear does have more configurations (Figure…). The most common (and most simple) has a double main landing gear unit and single nose gear unit (1). For most aircraft this is good enough to bear the aircraft load. For airliners beyond 300.000 kg of maximum take-off mass there is need for an extra landing gear unit located under the fuselage (2), or even two main landing gear units located under the wing (3). This is needed to make the surface pressure on all landing gears acceptable.
Figure…: Tricycle gear configurations
Those extra landing gear units are called bogies. When an aircraft has extra landing gear units it is called a multi-bogie gear configuration. The heavier the aircraft becomes, the more wheels are added to the bogie to spread the surface pressure on the runway pavement. The nose wheel could also get an extra wheel or even an extra nose gear unit.
Unfortunately the tricycle gear configuration has a greater weight and drag because of the large nose wheel gear unit. Aircrafts with a tricycle configuration almost always require retraction mechanisms to reduce drag. Another challenge at tricycle gear configurations is designing a properly balance load by the main gear versus the nose wheel. When you will put too little load on the main wheels, braking effectiveness will reduce. When putting too little load on the nose wheel the aircraft is losing its steering effectiveness. Loading the aircraft must be carefully done to balance the aircraft to prevent the aircraft from fall over on its tail.
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