I carry a backpack and a CPAP, travel frequently over many miles each ear, and hardly consider myself a "Kettle". That being said, it's best to take that baby off at the door and carry it in front of you so you don't become a menace to the folks in the aisle seats.

Smaller backpacks aren't as much of a problem; it's those behemoths that people have packed a week's worth of clothes in, a computer or two, food for their row, etc. etc. that stick out a good foot or two from their backs that I find the most dangerous.

The backpacks are almost as bad as the large handbags ladies carry on their shoulder while carrying other stuff in their hands - hitting just about every passenger they pass right in the face.

We all deserve to have sufficient space.. so if someone unknowingly flips the backpack into private spatial area.. then I would fend the backpack off