The landing gear supports the aircraft during landing and while it is on the ground. Simple aircraft that fly at low speeds generally have fixed gear. This means the gear is stationary and does not retract for flight. Faster, more complex aircraft have retractable landing gear. After takeoff, the landing gear is retracted into the fuselage or wings and out of the airstream.
This is important because extended landing gear creates significant parasite drag, which reduces aircraft performance. Parasite drag is caused by the aircraft moving through the air and includes drag generated by exposed structures such as the landing gear. It increases with speed.
On very light, slow aircraft, the extra weight and complexity associated with retractable landing gear can outweigh the drag penalty of fixed gear. Lightweight fairings and wheel pants can be used to keep drag to a minimum. Figure 1 shows examples of fixed and retractable gear.
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| Figure 1. Landing gear can be fixed (top) or retractable (bottom) |
Landing gear must be strong enough to withstand the forces of landing when the aircraft is fully loaded. In addition to strength, a major design goal is to have the gear assembly be as light as possible. To accomplish this, landing gear assemblies are made from a wide range of materials including steel, aluminum, and magnesium.
Wheels and tires are designed specifically for aviation use and have unique operating characteristics. Main wheel assemblies usually have a braking system. To aid with the potentially high impact of landing, most landing gear incorporate a means of absorbing and distributing landing loads so that the aircraft structure is not damaged.
Not all aircraft landing gear are configured with wheels. Helicopters, for example, have such high maneuverability and low landing speeds that a set of fixed skids is common and quite functional with lower maintenance. The same is true for free balloons which fly slowly and land on wood skids affixed to the floor of the gondola.
Other aircraft landing gear are equipped with pontoons or floats for operation on water. A large amount of drag accompanies this type of gear, but an aircraft that can land and take off on water can be very useful in certain environments. Even skis can be found under some aircraft for operation on snow and ice. Figure 2 shows some of these alternative landing gear, the majority of which are the fixed gear type.
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| Figure 2. Aircraft landing gear without wheels |
Amphibious aircraft are aircraft that can operate from both land and water. On some aircraft designed for such dual usage, the bottom half of the fuselage acts as a hull. Usually, it is accompanied by stabilizing floats or outriggers mounted beneath the wings near the tips to provide lateral stability during water operations. Main gear that retract into the fuselage is only extended when landing on the ground or on a runway. This type of amphibious aircraft is sometimes called a flying boat. [Figure 3]
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| Figure 3. An amphibious aircraft is sometimes called a flying boat because the fuselage doubles as a hull |
Many aircraft originally designed for land use can be fitted with floats with retractable wheels for amphibious use. [Figure 4]
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| Figure 4. Retractable wheels make this aircraft amphibious |
Typically, the wheels retract into the floats when water operations are required. Sometimes a ventral fin is added to the aft underside of the fuselage to improve directional stability during water operations. Some float-equipped aircraft also use water rudders connected to the aircraft's rudder pedals to improve directional control while taxiing on water. Skis can also be fitted with wheels that retract to allow landing on solid ground or on snow and ice.
Tailwheel Gear Configuration
There are two basic configurations of airplane landing gear: conventional gear or tailwheel gear and the tricycle gear. Tailwheel gear dominated early aviation and therefore has become known as conventional gear. In addition to its two main wheels which are positioned under most of the weight of the aircraft, the conventional gear aircraft also has a smaller wheel located at the aft end of the fuselage. [Figure 5]
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| Figure 5. An aircraft with tail wheel gear |
Often this tailwheel is able to be steered by rigging cables attached to the rudder pedals. Some conventional-gear aircraft have no tailwheel at all, using a steel tailskid beneath the aft fuselage instead. The small tailwheel or skid plate allows the fuselage to incline, thus giving clearance for the long propellers that prevailed in aviation through World War II.
It also gives greater clearance between the propeller and loose debris when operating on an unpaved runway. But the inclined fuselage blocks the straight-ahead vision of the pilot during ground operations. Until the aircraft accelerates to a speed at which the elevator becomes effective to lift the tailwheel off the ground, the pilot must lean his head out the side of the flight deck to see directly ahead of the aircraft.
The use of tailwheel gear can pose another difficulty. When landing, tailwheel aircraft can easily ground loop. A ground loop is when the tail of the aircraft swings around and comes forward of the nose of the aircraft. The reason this happens is due to the two main wheels being forward of the aircraft’s center of gravity. The tailwheel is aft of the center of gravity.
If the aircraft swerves upon landing, the tailwheel can swing out to the side of the intended path of travel. If the tail swings far enough to the side, the aircraft's center of gravity can move outside the line of travel of the main wheels, causing the aircraft to pivot around them. Once the center of gravity is no longer trailing the mains, the tail of the aircraft freely pivots around the main wheels causing the ground loop.
Conventional gear is useful and is still found on certain models of aircraft manufactured today, particularly aerobatic aircraft, crop dusters, and aircraft designed for unpaved runway use. It is typically lighter than tricycle gear which requires a stout, fully shock absorbing nose wheel assembly. The tailwheel configuration excels when operating out of unpaved runways. With the two strong main gear forward providing stability and directional control during takeoff roll, the lightweight tailwheel does little more than keep the aft end of the fuselage from striking the ground.
As mentioned, at a certain speed, the air flowing over the elevator is sufficient for it to raise the tail off the ground. As speed increases further, the two main wheels under the center of gravity are very stable.
Tricycle Gear Configuration
Tricycle gear is the most prevalent landing gear configuration in aviation. In addition to the main wheels, a shock absorbing nose wheel is at the forward end of the fuselage. In a tricycle-gear aircraft, the center of gravity is located ahead of the main landing gear. The tail remains elevated, providing the pilot with a clear forward view during ground operations. Ground looping is nearly eliminated since the center of gravity follows the directional nose wheel and remains between the mains.
Light aircraft use tricycle gear, as well as heavy aircraft. Twin nose wheels on a single forward strut and massive multistrut/multiwheel main landing gear may be found supporting the world’s largest aircraft, but the basic configuration is still tricycle. The nose wheel may be steered with the rudder pedals on small aircraft. Larger aircraft often use a dedicated nosewheel steering tiller located beside the pilot's seat. Figure 6 shows aircraft with tricycle gear.
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| Figure 6. Tricycle landing gear is the most predominant landing gear configuration in aviation |
The Aircraft Landing Gear Systems section discusses landing gear in greater detail.