warp threads (fabric). Threads that run the length of a piece of fabric. Threads that run across the fabric are called woof, or fill, threads.
warp tracers. Threads of a different color from the warp threads that are woven into a material to identify the direction of the warp threads.
Warren truss. A type of structural truss in which the main lengthwise members are separated by rigid diagonal members that carry both tensile and compressive loads. Most welded-steel truss fuselages are Warren truss structures.
washer (type of hardware). A flat metal disk with a hole in its center used to provide a smooth surface for a nut or the bolt head to seat against. Washers are also used to shim, or fill a space, between a nut and a surface to take up space if the grip length of the bolt is too long for the material being clamped.
wash-in (airplane rigging). A condition in airplane rigging in which a wing is twisted so its angle of incidence is greater at the tip than at the root. Washing-in a wing increases the lift to correct for a wing-heavy condition.
wash-out (airplane rigging). A condition in airplane rigging in which a wing is twisted so its angle of incidence is less at the tip than at the root. Wash-out decreases the lift the wing produces. If an airplane is left-wing-heavy, the left wing can be washed in and the right wing washed out.
wash primer (aircraft finishing system component). A type of primer used to prepare an aluminum or magnesium surface to receive a topcoat of finishing material. Wash primer contains an etchant, a material that chemically roughens the metal, to prepare the surface to accept the topcoat material.