Hasn’t the outermost part of an aircrafts structure always been referred to as a “skin”?
I tried to find some reputable sources to back up this claim. I hope these will suffice.
These from the RAAF website describing the de Havilland Mosquito: “Plywood ribs and skins also formed the mainplane leading edges and flaps.”
“Sheet metal skins enclosed the engines.”
The Smithsonian National Air and Space Museum describes an aircraft construction technique as, “Steel tubing airframe, aluminum and fabric skin.”
The USAF Museum website when describing a restoration stated, “The complex compound curvatures of the skin, meant a tough job for the sheet metal workers.”
So, the actual plane itself?… Yes, that is a skin.