Retractable wheels on aircraft are often called "landing gear", and some of the engine designs used in aircraft can contain a gear box. But gears as they look like in cars aren't used in aircraft.
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If you are talking about stick-shift kind of 'Gears', there's none. Although there might be gear boxes on aircraft engines you usually don't control them the way you do with cars.
The gear refers to the wheels and struts of the aircraft
Because if they don't bring them up it causes turbulence for the aircraft and reduces the aerodynamics of the aircraft.
A: It is like a computer of a mechanical analogue even gears combination to activate, move an actuator or close a valve. It was is extensively used on aircraft they still are except digital has replace analogue and gears to do the fast computations and positioning required on an aircraft moving at mach 3
The wheels along with the system is called Landing Gear. The Landing Gear can be retracted soon after take-off. Stationary landing gears slow the aircraft down.
In airlines and Air Forces, the aircraft is jacked up and the landing gear is cycled many times to test.
I was an Aircraft Electrician on 3 carriers. Tarawa, Essex(on 6 cruises)and the Saratoga. The squadron people called it :"the wire". As we caught the wire or missed the wire.
The keel beam with the longerons are the strongest sections of the airframe. The keel beam tie its weight to other aircraft parts, such as powerplants, fuel cells, and the landing gears Article Source: http://EzineArticles.com/3758400
a gear is used to transport force.
Fuselage, wings, engines, rudder, horizontal stabilizer. Aircraft are made basically the same way cars are. :D Then again they are not. Some aircraft parts are made of high-strength steel and some parts such as landing gears are steel forgings. Some aircraft use titanium---both sheet metal and forgings. A lot of the wing structure is riveted and bolted metal plates and aluminum skin.