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Aircraft landing gears
To airplane designers landing gears are a menace since airplanes are designed to perform in flight than on the ground. Landing gear, also known as undercarriage, is essentially required by airplane for few reasons. We have seen airplanes other than helicopters execute a run during take off in order wing airfoil to generate lift. During landing airplane makes another run until it's landing speed is killed. Airplanes are well supported and stabilized during these high speed runs by landing gears. Also, landing gear absorbs the shock during landing and supports the airplane when it is on ground.
Landing gears comes in many options and each option has it's own advantages and disadvantages. Typically a landing gear consists of three wheels (tricycle), two main wheels which are located on either sides of the fuselage and a third wheel positioned at the front or rear of the airplane.
Tricycle Tail Wheel
The third wheel when located at the rear is known as tricycle tail wheel configuration. This arrangement is popular even today for small piston engine aircraft for it's simplicity, better propeller to ground clearance and for short landing and take off etc. It is desirable for the operation on unimproved fields. With center of gravity (CG) located behind the main landing gear controlling of aircraft becomes difficult during ground operations. Poor forward visibility is another disadvantage to tail wheel type aircraft. DC3s, also known as Dakotas, are the known successful airplane that used tail wheels. DC3s celebrated its 75th anniversary on Dec 17, 2010 since its first flight. They are successfully being flown until today in some part of the world.
Tricycle Nose Wheel
This is the most widely used application by larger jet airliners to day for many of it's advantages. Better visibility and easier taxiing capability for pilot. Passenger and cargo handling is made easier as the floor is level to the ground. Since main landing gears are attached slightly aft of the aircraft CG, better controllability of aircraft during landing and during ground operations. Full advantage of engine acceleration is achieved during take off since engines remain parallel to the ground and as a result no ground blockage particularly in the case of jet engines. However, one great disadvantage of nose wheel application that I came across in my life is tipping down of tail section of large aircraft on ground. Great care therefore is taken during weight loading of aircraft. A tail steady is given as a ground support to the aircraft during it's ground operations. Some aircraft uses a fixed small tail wheel at the rear section to take care of this situation.
Single Main Gear
This application is made to special purpose aircraft like U2 spy planes. This long, narrow wing (high aspect ratio) aircraft fly like a glider at an altitude of 70,000 ft for 6 to 7 hrs at a stretch with a range of 3000 miles. Pay load or weight of aircraft carried up is there fore critical. Large quantity of fuel is required to be carried for the aircraft to remain in the sky for such long time, heavy reconnaissance equipment need to be included for the mission, in other words it's performance high up in the air is important than being on ground. For such reasons, weight that can be sacrificed is the complicated heavy landing gears. U2 is designed to land with a single main landing gear and a small caster like wheel at the rear section both located on the fuselage center line. U2 landing is difficult for the pilot, so the ground crew on a chase car behind the aircraft guide the pilot to a safe landing. Wing tips are fitted with titanium skid plates to avoid wing damages in case of the aircraft rolls to one side. As soon as air aircraft is brought to halt by the pilot after landing, wing supports, known as pogos, are hooked to the aircraft at the run way for safe taxiing. These supports remains on the aircraft during take off run and they are dropped at end of the run way before "wheels off" the ground and ground crew collects them back.
Bicycle gears are generally found on large bombers having narrow body and wide wingspan. It has two landing gears along the center line of the aircraft, one aft and one forward of the CG. The distance between two gears to aircraft CG is almost same so they carry same load. There are two small support gears on the sides to prevent aircraft from tilting.
Quadricycle landing gear uses four gears like cars. Two in the front and two at the back closer to CG of the aircraft. so that weight of the aircraft will be almost equally distributed. Quadricycle gears are used on very heavy cargo or bomber aircraft as they are very stable on ground and during taxiing. B-52 Stratofortress bomber aircraft uses Quadricycle gear.
As the aircraft gets heavier more numbers of gears to be employed. An aircraft with multi bogey gears are very stable on ground. Large aircraft like B747 and Airbus 380 uses multi bogey gear. Compared to other gears multi-bogey is most expensive, heavy and complex to manufacture. AN 225 Russian heavy lift has 32 wheels (2x2 on the nose) + (14x2 on the main) total to support the aircraft on ground.
Retractable landing gears
Retractable landing gears are hydraulically operated commonly, but they also operate electrically or even manually. Greatest disadvantage of retractable landing gear is it's added complexity, weight and space penalties otherwise could have been used for baggage loading or for fuel storage. Aerodynamic advantages of retractable landing gears however justifies it's disadvantages by minimizing in flight drag.
Reducing drag on an aircraft in flight is important to improve aircraft speed and fuel efficiency. Landing gears are therefore folded and retracted in to a stowage space provided under the aircraft fuselage. The space is generally known as wheel well, and the well area is covered by doors once the wheels are fully retraced in.
Picture shows, left side main landing gear of Boeing -737 aircraft. It has the main shock strut, a side strut and a hydraulic actuator attached from the aircraft body to the top of shock strut hidden inside the fairing. Side strut is collapsible from the middle and it is fully extended and locked by a down lock mechanism when the whole landing gear is fully extended.
When aircraft is taken off, freely hanging landing gear is folded by collapsing the side strut. Whole gear is then retracted in to the wheel well by operation of the hydraulic actuator. Set of doors attached to the shock strut closes the opening when the wheel is completely inside the well.