The ornamental design for a helicopter, as shown and described.
FIG. 1 is a right side elevation view of a helicopter showing my new
design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a front elevation view thereof.
The left-hand side elevation of the helicopter is substantially the mirror
image of FIG. 1, eacept that the tail rotor is on the left-hand side, and
the anti-torque vane is on the right-hand side, and the views would differ
accordingly.