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Aeronautical Engineering/Retractable mechanism for Landing gear in airplanes.

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QUESTION: Dear Jan

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 ?.

Thanks & Regards,
Prashant S Akerkar

ANSWER: No, for example some "conventional" airplanes have tail wheels that retract - though most do not.
Most tricycle aircraft with retracting gear have all three wheels retract but a few planes do not retract the nose wheel. Many airplanes do not fully retract the wheels, for example the Navion only retracts the nose wheel half way leaving half sticking out so that the airplane will slide on the wheel if the gear does not extend. The beech 18 left a few inches of the main gear sticking out for the same reason.  

---------- FOLLOW-UP ----------

QUESTION: Dear Jan

Thank you.

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: TAKEOFF
---------
There are two criteria used before the wheels are retracted.
1) The aircraft must be in a stabilized climb - pilots call this "positive rate of climb".
2) The wheels can be retracted once the aircraft does not have any usable runway in front of it.
  In other words, what good are the wheels if there is no runway to land on...

The wheels are retracted as soon as possible because they cause quite a bit of drag and the airplane will fly quite a bit better - as a rule of thumb the aircraft will fly about 10% better once the wheels are retracted. Typically the wheels are retracted 5-10 seconds after lift off.

LANDING
---------
When landing, the wheels are extended when the checklist the pilot is using instructs him to lower them. The checklist or landing may be modified for several normal reasons but usually a commercial jet will lower its gear about 10-15 miles from the airport. A small airplane will lower the gear about 1-2 miles from the airport.

On large jets, the gear is usually lowered consistently in the same way. However, smaller airplane pilots deviate regularly from "normal" because they may want to use the gear for extra drag to loose altitude or speed. If they are already low or slow, they may wait to put the gear down.

To help a small plane pilot from forgetting to put the gear down, they use an acronym called "GUMPS" on final approach.

G - check "gas" or fuel setting
U - check "undercarriage" (gear)
M - check "mixture" setting
P - check "prop" setting
S - check "speed"

---------- FOLLOW-UP ----------

QUESTION: Dear Jan

Thank you.


Is it advisable for the pilot when the plane is in motion (flying) i.e. after airplane take of that in between they can also do "Wheels down" and "Wheels up" status ?.

Are there any specific advantages or disadvantages in doing Wheels down or Wheels up status when the plane is in motion i.e. running at specific speed in miles per hour ?.

Awaiting your reply,

Thanks & Regards,
Prashant S Akerkar

Answer
Once the aircraft is in the air, there is no advantage to lowering the gear.
The one exception is that during an emergency the gear is sometimes lowered to loose altitude.
For example if an aircraft is at a high altitude and looses cabin pressure, the gear may be lowered until the aircraft descends to 15,000 ft where passengers can safely breath again. Then the gear would be retracted again.

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Jan Zumwalt

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SMALL & LARGE PLANES - Aeronautics, Aerodynamics, Designing, Fabricating, Trouble shooting, Repairs, Flight testing, Fluid & Pneumatic controls, Engines, Propellers, Sheet metal, Rivets, Electronics, Fabric, Schools, Career choices, Aircraft systems.

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FAA certified commercial pilot FAA certified (A&P) Air-frame & Power-plant mechanic Aeronautical Engineer - University of Anchorage (Alaska) High school instructor, computers, robotics and aviation Current president of EAA chapter 837 (Payette, ID) Past Director of Maintenance (two airlines) Past Continental Airlines maintenance shift supervisor (Anchorage, Alaska)

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FAA certified commercial pilot, FAA certified (A&P) Airframe & Powerplant mechanic, Aeronautical Engineer - University of Alaska (Anchorage), FCC MROP-Marine Radio Operators License, FCC GROL-General Radio Operators License, FCC RADAR endorsement, GEN FAM certificates MD aircraft, GEN FAM certificates Boeing aircraft, State Instructor License, High school instructor, computers, robotics and aviation

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