sideways cylinder is the way to go.

also moving up and down to cut the tree.

 

leaving a half circle until tree falls.

 

i don't think pushing mechanism is necessary,

as humans logging --- they push the tree at near end

of the cut, in order to let it fall at predictable direction.

 

but the machine can easily dodge it with sensors.

 

 

 

as for the energy taken....

 

rotating the cylinder.

 

will it be more than the saw? no, probably more efficient.

 

 

then. what length of the cylinder?

 

can it be replaced?

 

if the sides holding the cylinder is adjustable, yes.

 

which depends on the width of the tree.