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Question
Dear Marc

en.wikipedia.org/wiki/Landing_gear

Are the Landing gears always retractable in all airplanes ?.

i.e.

Once the airplanes takes off, the front and rear wheels are always
folded ?.
Once the airplanes wants to Land, the front and rear wheels are unfolded ?.

What are the Pros and Cons with and without having these retractable mechanism for Landing gears in airplanes ?.

Once the commercial airplane takes off by what time the front and rear wheels are folded ?. i.e. after 1/2 min, 1 min, 5 min, 10 min etc ....

Once the commercial airplane has to land before what time the front and rear wheels are unfolded ?. i.e. within 1/2 min, 1 min, 5 min, 10 min, 30 min etc ....

Are both these timings critical i.e. After plane take of and Before Landing and are these timings fixed globally as part of airplane operations ?.

Thanks & Regards,
Prashant S Akerkar

Answer
I am begining to think all these questions are home work questions and I can reject them. But I will respond to this last one.
Simple answer Landing gears are retracted to reduce drag and therefore increase performance. Timing has nothing to do with time. Retraction is based on having enough speed get airborme and maintain it.  To explain this further. Some aircraft may have wheel well doord closed during takeoff. Opening these doors to retract the landing gear increases drag so that the airspeed must be fast  enough to over come this additionsal drag.
Extending the landing is based on several conditions. The most important is to help reduce the speed of the aircraft in preparation of the landing. Lowering the flaps is only done after the aircrafts speed is reduced to a save flap extension speed.  

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Marc Ebelini

Expertise

Basis aircraft and engine maintenance. Avionics and other electronic questions related to computers and radio communications.

Experience

FAA licensed comercial pilot, A&P mechanic, former authorized inspector, ground instructor certified in aircraft, powerplant and radio navigation. FCC commercial license.

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