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Why do all the airplanes have to retract their landing gear, once they reach some specific height? Why can't they simply leave their landing gear deployed all through the flight?

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    $\begingroup$ It's important to note that not all aircraft have to retract their gear, as some of them have fixed (non-retractable) landing gear. $\endgroup$ – egid Jun 30 '15 at 19:34
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    $\begingroup$ The main advantage is that it reduces drag! $\endgroup$ – kepler22b Feb 6 '16 at 17:00
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In the early days of aviation it was simply easier to have the gear fixed. The landing gear adds drag, but so does a second wing and wire braces. Airplanes generally did not fly very fast or far, so the drag from the landing gear (and that other stuff) was not a big issue. The first design with retractable gear dates to 1911. Since then, retractable gear has been a design decision each airplane must make.

Retracting the gear into the airplane allows for a cleaner form, which reduces drag. However, this is at the expense of added weight. The retract system, which is usually hydraulic, must be added, and the plane must be designed to make room for the gear somewhere at least mostly inside the aircraft. The plane must also be designed to handle situations where some or all of the gear does not extend properly. For planes that need to be fast and/or efficient, retractable gear is worth the reduction in drag.

The additional drag has to be supported by the structure as well. Aircraft with retractable gear will have a maximum speed at which the gear can be extended. Even gear with proper fairings would add significant drag at the speeds airliners fly at. This would be much more critical at supersonic speeds. In these cases the weight of reinforcing the gear could be even higher than the weight of the retracting system.

Particularly smaller general aviation planes tend to not have retractable gear. The retract system adds extra weight and complexity (and therefore cost), which will be fairly significant in a small plane. Fixed gear is simple and can be designed to minimize drag as much as possible. These planes do not typically need to fly fast or have large range, so the added drag is less of an issue. Other small planes do have retractable gear, which will allow for greater speed and range. The PA-32 type shown in Dave's answer was later produced in a retractable gear version as well.

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    $\begingroup$ The reasons you mention are, of course, the primary answer. However, it might also be worth noting that the gear on some types is actually not designed to withstand being deployed at the maximum or cruise speeds of the aircraft. As an extreme example, I'd imagine the gear on a fighter would have a bad day at Mach 2 (let alone the gear on the SR-71 at Mach 3.3 or the gear on the Space Shuttle during re-entry.) $\endgroup$ – reirab Jun 30 '15 at 18:48
  • $\begingroup$ @reirab Thanks, the speed limitation is certainly important. I added a paragraph for this. $\endgroup$ – fooot Jun 30 '15 at 19:23
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    $\begingroup$ Also, the gear would have to not burn up in the case of supersonic flight. :) Making a tire that can survive at Mach 2 (let alone Mach 3+) would not be trivial, even if the weight weren't a factor. $\endgroup$ – reirab Jun 30 '15 at 19:32
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    $\begingroup$ @spectras Isn't that just the temperature of the air? Friction and vibration would cause extra heat on top of that. In case of a rubber tire that has uneven surface, mediocre aerodynamics, and is specifically designed to absorb vibration... I'd expect there would be quite a lot of that extra heating. $\endgroup$ – Ville Niemi Jul 1 '15 at 15:30
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    $\begingroup$ @Sean Sure, and in fact you'd probably have to but I was just pointing out that the properties and shape for a good tire would increase the temperature over the values for normal aerodynamic surfaces spectras was quoting. Regardless of the material. $\endgroup$ – Ville Niemi Dec 27 '18 at 7:45
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Short answer: It allows them to fly faster and further.

Long answer: Hapag-Lloyd Flight 3378 demonstrated this impressively on July 12, 2000. Destined for Hannover, they did not retract the gear after take-off in Chania, Crete. The fuel was sufficient for the planned distance plus reserves, but the extended gear increased fuel consumption so much that they ran out of fuel while approaching their diversion airfield near Vienna. Fuel consumption enroute was twice as high as it would had been with the gear retracted.

Drag is composed of a lift-related component (induced drag, blue line below) and a constant component (zero-lift drag, red line below), and both depend on the dynamic pressure, which is the product of air density and the square of airspeed. While the lift-related component goes up with reduced dynamic pressure, the constant component will go up with increased dynamic pressure. Thus, extending the gear will increase the constant drag component, and while flying slower helps to reduce its contribution, it will drive up the lift-related drag component. In the end, the drag will be higher at all speeds.

Glider drag components

Typical drag contributions over speed for a glider. The physics for airliners is the same, only the numbers are bigger. In cruise, all airliners try to fly as close to the minimum drag as possible.

The only reason for a fixed gear are cost, weight and simplicity. Performance will always suffer from it.

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    $\begingroup$ Maybe good to mention an approximate point at which it pays off. Roskam (Airplane Design: Layout of landing gear and systems) mentions: "Experience indicates that airplanes with cruise speeds above 150 kts tend to use retractable landing gears because of gear drag penalty" $\endgroup$ – ROIMaison Jul 1 '15 at 7:32
  • $\begingroup$ @ROIMaison ... and just about any glider going over 50kts, even if "retractable" kind of overdescribes a wheel peeking out of the fuselage ;-) $\endgroup$ – yankeekilo Jul 1 '15 at 9:38
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    $\begingroup$ You're correct, I should have mentioned that Roskam was focusing on powered aircraft. The decision changes quite a bit if you can make a light and simple retraction system such as in gliders $\endgroup$ – ROIMaison Jul 1 '15 at 9:42
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Because its a drag to leave it down(pun intended). Some smaller GA planes (and big planes too) can extend the landing gear through a wide range of their operating speeds and use them as a pretty effective speed break. The DA42 comes to mind in this regard.

For what its worth there are many general aviation planes that have fixed gear. enter image description here

Retracting gear in a small plane has always been an interesting topic. Some older piston singles have manual retracting gear like the early Mooney M20's which used a Johnson bar to retract the gear. This some what dated system is, by some very sought after for its simplicity.

(the big silver bar in the middle is the gear lever) enter image description here

Folding gear also has little to do with aircraft size. The Mooney M-18 "Mite" which is a tiny plane by anyones standards had retractable gear (also Johnson bar operated). enter image description here

One way to reduce drag on a fixed gear plane is through the use of gear fairings. while they do help with speed they can also cause an issue if the plane touches down hard. In some cases (if the wheel is improperly inflated) a hard touchdown or bounce will cause the wheel to balloon out while also spinning which could chafe or seriously damage the fairing. enter image description here

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    $\begingroup$ Fairings - also known as a cowl or boot. $\endgroup$ – KeithS Jun 30 '15 at 15:25
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    $\begingroup$ @KeithS, or spats, or wheel pants! :) $\endgroup$ – egid Jun 30 '15 at 19:35
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    $\begingroup$ For the last word you used "fearing" instead of "fairing". $\endgroup$ – ThomasW Jul 1 '15 at 6:01
  • $\begingroup$ You are correct, a change has been made. $\endgroup$ – Dave Jul 1 '15 at 11:57
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Why do all the airplanes have to retract their landing gear, once they reach some specific height?
Why can't they simply leave their landing gear deployed all through the flight?

Both questions are false on their premise:

  • All airplanes do not have to retract their gear; many planes are fixed-gear, and cannot retract gear. Some planes retract their gear.

  • Planes can leave their gear deployed all through flight, provided they remain slow enough that the extra airflow drag doesn't damage the airplane and landing gear.

Planes that have retractable gear generally retract their gear for improved performance. Not having wheels and struts hanging down streamlines the plane, improves fuel efficiency, enables them to fly faster and higher, and makes for a quieter, more comfortable ride inside the cabin. So, retracting gear is generally desirable.

But, as your question suggested, it is not required, and it is not based on a specific height. (if anything, it is based on an airspeed)

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Simply retractable landing gear is made so as to reduce extra drag which created by the fixed landing gear and hence heavier than air aircraft(airplane) can improve in speed and travel at high speed at higher altitude

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  • $\begingroup$ I ask if anyone with some questions concerning with any part in aviation,please send to me $\endgroup$ – Denis Charles Feb 6 '16 at 11:05
  • $\begingroup$ Welcome ! What does this comment mean: "Any part in aviation" and "send to me"? $\endgroup$ – mins Feb 6 '16 at 11:32
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    $\begingroup$ This answer doesn't add new elements compared to the answer selected 7 months ago, right? $\endgroup$ – mins Feb 6 '16 at 11:34

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