sectional area of the tubular passage through the cylindrical section 21 the cross sectional area of the cylindrical section is established in the conventional manner in the cylindrical section 21 the air assumes a constant velocity while fibers carried by the feed tube air stream are accelerated by frictional engagement with the air stream molecules such fibers receive a somewhat smaller acceleration than the air stream acceleration resulting from the taper of the passage through the conic section because of their inertia the fiber speed therefore is less than the air speed at the junction 23 between the conic section 20 and the cylindrical section 21 because of intermolecular friction in the accelerating air stream turbulence is created which disturbs the parallelized lay of the fibers which would result from streamline air flow consequently in order to postaccelerate the fibers to improve the lay of the fibers in the rotor 3 in accordance with the present invention the cylindrical section 21 is joined directly to the smaller end of the frustoconical section 20 as the air assumes constant velocity in the cylindrical section intermolecular friction is reduced and consequently there is little turbulence in this feed tube portion because the fibers do not achieve the same speed as the air stream in the conic accelerating section 20 the fibers are additionally accelerated in the constant air speed cylindrical section whereby they are further straightened and parallelized and such orientation.