Pliable-wall air ducts with internal expanding structures are disclosed. An example air duct system includes a shaft to be disposed within an air duct, to extend in a longitudinal direction, and to be in longitudinal compression. The air duct system also includes a plurality of ribs to be coupled to the shaft and to engage an inner surface of the air duct and a spring to be disposed within the air duct, the spring to be coupled to the shaft. The spring under stress being a contributing factor in both the shaft being in longitudinal compression and the air duct being in longitudinal tension.