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Why did engineers start designing planes with aluminium skin on the wings instead of fabric covering? While the force of lift acts on the skin, to my knowledge it is not subject to high stresses

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Wings and fuselages are subjected to twisting loads as well as bending so they need to be torsionally stiff. Prior to aluminum, the options were to make the wing/fuse with a frame of spars and ribs with fabric to provide the aerodynamic contours, braced with struts and/or wires to provide the torsional stiffness as well as resistance to bending, or, to do away with the struts and brace wires and make a monocoque structure (like an eggshell) with a plywood skin to provide torsional stiffness (with the spar beam inside for bending). If you wanted to go fast, you had to do away with all the bracing so plywood monocoque was the way to go.

When "duraluminum" came along after WW1, it was realized that as well as replacing sitka spruce for spar beams and ribs, it could also replace plywood for a stressed skin monocoque structure. You could build an airplane in the 1930s using plywood stressed skins bonded to wood ribs and spars, or aluminum stressed skins over aluminum spar beams, and have pretty much the same performance. You could make your egg about of plywood, or aluminum sheet, and it was pretty much the same egg.

There were so many manufacturing and durability advantages to aluminum (no glue, no wood rot etc etc), it was a no brainer to switch, once the metallurgy started to become understood well enough to use it as an engineering material, and plywood skinned wooden airplanes faded quickly.

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  • $\begingroup$ Also fabric, and particularly the fabrics available before modern synthetics, deteriorate when exposed to sun and air. Build from aluminum, and you don't have the expense of recovering it every few years. $\endgroup$ – jamesqf Aug 13 '20 at 23:38
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    $\begingroup$ Spot on! As a structural material aluminium and wood are surprisingly similar per unit of mass. $\endgroup$ – Sanchises Aug 14 '20 at 9:59
  • $\begingroup$ These bays carbohydrate fibre composite (plywood) has been replaced with carbon fibre composite,. $\endgroup$ – Jasen Aug 16 '20 at 8:21
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    $\begingroup$ You missed the intermediate step. Glass replaced cellulose in the 50s. $\endgroup$ – John K Aug 16 '20 at 12:40
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Aeroelastic flutter is an issue at higher speeds and the simplest way to overcome that is to use stiffer materials.

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  • $\begingroup$ That's not the reason. Plywood monocoque structures are just as stiff as aluminum ones. Aluminum replaced plywood in airplanes for ease of manufacture and durability. $\endgroup$ – John K Aug 13 '20 at 1:48
  • $\begingroup$ @JohnK I see what you mean but that would technically be a reason to switch from wood to aluminum rather than fabric to aluminum (as well as rot). It would mean that there is a gap in the OP's question that they were not aware of. $\endgroup$ – DKNguyen Aug 13 '20 at 1:57
  • $\begingroup$ @John K: Are there any aluminum frame planes with fabric skin? $\endgroup$ – Aaron Holmes Aug 13 '20 at 2:29
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    $\begingroup$ @DKNguyen yes the question is kind of incomplete. My bringing plywood into it was to complete the picture and to point out that what he really needed to compare is plywood monocoque vs alum monocoque when talking about the adoption of aluminum in the 1920s, not fabric covering per se. There are aluminum wing airplanes with stressed skin AND fabric covering. The Ercoupe has an alum cantiliver wing but the stressed skin only is applied around the leading edge to make a D torque box.. The aluminum rib bays behind the spar are open and covered with fabric. $\endgroup$ – John K Aug 13 '20 at 4:12
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    $\begingroup$ @Aaron Holmes Lots of light planes have wings with aluminum ribs and spars with fabric. The aluminum is just replacing the spruce plank of the spar and the wood stick ribs. The wings require bracing same as a wood one. Later models of the CItabrias have alum spar and alum ribs with strut braced wings and fabric. Early Luscome 8As were the same. The wings are considerably lighter than the later full aluminum skinned wings even though they eliminated the aft lift strut . $\endgroup$ – John K Aug 13 '20 at 4:28
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I would assert that the breakthrough occurred when aluminum became (relatively) inexpensive to extract from ore on industrial scales. There was also a psychological factor, in that the popular american football coach Knute Rockne died in the crash of a Fokker passenger plane with a wooden wing when it came apart in a storm. All-metal construction then became a selling point for airlines.

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Principally to carry greater loads. The skin on the wing forms an integral part of the wing box, which provides greater structural rigidity. It also provides better aerodynamic characteristics under loads. For instance Boeing has a particular way that it designs its wing scans, which is a proprietary secret, that causes them to flex when under load for better aerodynamic qualities.

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  • $\begingroup$ Redundant sentence is redundant. $\endgroup$ – DKNguyen Aug 13 '20 at 0:55
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    $\begingroup$ that's because it came from the department of redundancy department. $\endgroup$ – niels nielsen Aug 13 '20 at 2:28
  • $\begingroup$ "It also provides better aerodynamic characteristics 100 loads." What is "100 loads"? $\endgroup$ – FreeMan Aug 13 '20 at 10:36
  • $\begingroup$ I saw it. Goddamned autocorrect. $\endgroup$ – Carlo Felicione Aug 13 '20 at 16:07

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