A chopper has a housing centered on an axis and having a radially open input and a radially open output spaced angularly therefrom as well as a pair of axially oppositely open intakes, and a cutter drum rotatable in the housing about the axis and having a pair of end plates transverse of the axis and having axially oppositely directed faces confronting the respective intakes and a plurality of blades secured to and between the end plates and having cutting edges defining on rotation of the drum an orbit centered on the axis. Respective pluralities of vanes at the end plates can be secured either to the respective end plates in in-service positions extending generally radially thereon so that rotation of the drum moves the vanes to draw air in the axial intakes and expel it through the radial output or in an out-of-service position in which they do not move appreciable air. The vanes can be secured in the out-of-service position also to the respective end plates but extending generally only angularly. In particular the vanes form a radially closed cylindrical annulus in the out-of-service position. This annulus generally blocks flow from the intake to the output in the out-of-service position. The vanes can also be secured in the out-of service position so as not to rotate with the drum.
