Some light airplanes use a "hollow" structure in the inner panels (cored skins and ribs) for fuel stowage and/or landing gear stowage and equipment bays, but transition to massive core for the outboard panels, which tends to be lighter than the "hollow" structure at the modest wing chords we use in homebuilts. Many sailplanes use four-piece wings to make for easier transport and assembly, with the inner panels attaching like in a two-piece wing, and then with the outer panels attached to the inner panels like in a three-piece wing. In some airplanes, you may find four- or five-piece wings. This also allows the homebuilder to build shorter wing segments at any one time, facilitating the build process and requiring less precision in build per amount of precision in finished project. The biggest practical advantages com in allowing considerable airplane structures to be built into the fuselage, such as landing gear, fuel tanks, and hardpoints for connections. This scheme too has practical advantages. At same strength, this adds more weight over the Two-Piece and One-Piece wings through: More hard points Duplication of shear webs, duplication of spar caps. The center section spar must be every bit as beefy as in the other versions, but then we either have the outer panel spars overlapping major portions of the center section spar or have very beefy fittings at the breaks in the wing to transmit the shear and bending moments between outer panel spars and the center section. The center section of the wing is either entirely within the fuselage or the break occurs outboard of the fuselage on the wing. At a cost of a little more than doubling the shear web, the build, transport, and storage advantages combined with closer approach to symmetry may be considered appropriate. The big advantages are that the separate wings are more easily built, transported and stored, particularly in sailplane length wings, and the individual wings can be built separately using many of the same fixturing pieces in first one wing, and then in the other for symmetry assurance. The adverse weight impact is relatively small as the shear webs are usually quite a bit lighter than the caps. The amount of material in the spar caps is approximately the same as for the one-piece, but now you do have two shear web sets and both are carrying shear between the pin locations. Two Piece Wing - The nominal way to do a two-piece is with the same pin locations as for a one-piece wing, but now you have two separate wings with two spars that overlap each other through the fuselage and both engage both pins, so the spars have to be slightly different for port and starboard wings. For a homebuilders, it becomes rather difficult to achieve. For production aircraft, this is doable and has advantages. This requires a long very true build table, an ability to execute all of the pieces for both wing and their tooling with excellent symmetry, and then be able to finish it symmetric too. The one-piece wing has the disadvantage of having to build the entire wing from tip to tip in one piece. This is a minimum material requirement for the spar designed to any shear and bending moment. Pins loads are only as high as the one-half the wing lift. This requires all of the spar cap size required to the pin location, and usually results in the shear web from just outside the pins being carried between the pins as well. Between pinned locations, the shear in the main beam is essentially zero while the moment is equal to the moment just outboard of the pin location and constant. The drag spar typically is picked up by a fitting on the fuselage that reacts off torsion in the wing the results from pitching moment generated by the wing. The main beam accumulates all of the lift outboard of the any spot in question, culminating in the pinned location. It usually mounts with two pins very near the fuselage walls. One-Piece Wing - The spar is one continuous piece from somewhere near the tip on one end to the same spot on the other end. Before we do that, let's understand that the wing outboard of the connection points is the same for all three options I will present below: Sometimes you can save enough weight in other places while accepting more weight in one set of parts to overall have a better and lighter plane. This is a more sophisticated question than you may realize, because just building pieces to their min weight does not neccessarily mean we will have the lightest airplane. IDEALLY, we always design to be adequately strong for the mission, then calculate what the total airplane weight impact will be for each option, then choose the option gives us the lowest weight airplane. So, if I were to define "best" (because you have not) I would say that best is the lightest spar set within a material set for a given airplane.
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