Backpack vacuum cleaner

FIG. 1 is a perspective view of a backpack vacuum cleaner showing my new design;

FIG. 2 is a right side elevation view thereof, the left side being a mirror image of the right side view;

FIG. 3 is a front elevation view thereof;

FIG. 4 is a rear elevation view thereof;

FIG. 5 is a top plan view thereof; and,

FIG. 6 is a bottom plan view thereof.

The environmental structure shown in broken lines and the dash dot lines which illustrate boundary lines form no part of the claimed design.