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To make comparison easy, let us take the example of B737 and A320, both similar aircrafts with retractable landing gear.

As seen on the photo below, the B737 doesn't have gear doors for the wheels,

737 Underside View

but the A320 does

A320 Planform View

Obviously, Boeing and Airbus made different choices for the same feature and both seem reasonable. Both aircraft models are well accepted by the airlines.

What are the advantages and disadvantages of both designs?

EDIT: I put links to other images where both aircraft are retracting landing gear after take off. The difference is obvious as we can see the landing gear doors operated for the A320 and no doors for the B737's wheel.

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  • $\begingroup$ here is a better image of the lack of gear doors $\endgroup$ – ratchet freak Oct 13 '14 at 11:25
  • $\begingroup$ @ratchetfreak I prefer the images I originally linked (before the question was edited) because they show both aircrafts while the gears is moving and there is no ambiguity on whether the B737 has wheel doors or of it is a painting on the aircrafts body. $\endgroup$ – Manu H Oct 13 '14 at 17:14
  • $\begingroup$ I've read in an Air & Cosmos magazine (french) that Boeing decided to go without gear doors (737-100/200) because of the design : ground clearance and belly volume. But I'm sorry, I've dumped the magazine long ago :/ Wether the Air & Cosmos article were correct or not, I don't know, sometimes they aren't. That's why this is a comment but NOT an answer at all. $\endgroup$ – Karl Stephen Oct 13 '14 at 23:35
  • $\begingroup$ The 737 does have gear doors. It's just that those doors happen to be the wheels themselves. $\endgroup$ – Sean Feb 8 at 4:36
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Benefits of gear doors

  • With gear doors, there are fewer constraints on the retracted gear position. This is especially important in larger aircraft with larger and more complex landing gear. It doesn't need to be flush with fuselage or streamlined (737 also uses hubcaps).
  • The gear doors provide a good aerodynamic surface
  • Opening to gear bay can be larger and easier to access

Drawbacks of gear doors

  • Extra weight for the door and the mechanisms
  • Extra complexity of opening another door (possibility for things to go wrong)
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  • $\begingroup$ "gear can be in the best position when retracted" what is "the best position" $\endgroup$ – rbp Oct 14 '14 at 13:01
  • $\begingroup$ @rbp I tried to word that one better $\endgroup$ – fooot Oct 14 '14 at 14:21
  • $\begingroup$ How is fewer constraints necessarily a good thing, and why is it important in larger aircraft? $\endgroup$ – rbp Oct 14 '14 at 14:49
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    $\begingroup$ @rbp everything in an aircraft is limited by space and many other factors. The fewer constraints on a system, the easier it is to address other requirements. And as gear gets larger, it will have more components and take up more space, making the problem more complicated. $\endgroup$ – fooot Oct 14 '14 at 14:56
  • $\begingroup$ By saying "everything," you lose specificity, which makes that part of the answer nearly meaningless. You could have just as made it a drawback: "because space is required for gear doors under the fuselage, there is less space for cargo, etc." $\endgroup$ – rbp Oct 14 '14 at 15:00
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Doors add weight and complexity to the aircraft, and need to be inspected and maintained. But doors also provide a smooth surface for the air to travel over, so they would have less drag. The 737 does have doors, but for the gear struts, but the wheels lay fairly flat against the fuselage, so drag is minimized.

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    $\begingroup$ Also note the exhaust from the packs looks like they come our right ahead of the tires, so they are at least putting drag sources in one place. $\endgroup$ – casey Oct 13 '14 at 15:11
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Also, in Boeing's case. it was decided to eliminate doors because they add COST. When they were designing the 737, it was a relatively small, budget aircraft being built for regional service.

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    $\begingroup$ Could you expand a little? Somehow I find it difficult to believe that cost would be a major driving factor in this case. $\endgroup$ – Federico Sep 21 '17 at 6:44
  • $\begingroup$ You may enhance your answer by explaining why it differs between the A320 and the B737 $\endgroup$ – Manu H Sep 21 '17 at 9:42

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