Bad Piggies Flight in the Night Level 4-6 is three starred with a single contraption. Use three boxes, the helicopter propeller dead center, and a lemon-lime soda bottle on one side. The unbalanced contraption will dip off the side of the butte when the propeller is toggled on, then trigger the soda to push the falling contraption back toward the finish line.
If you have a different strategy for completing this level feel free to leave a comment below.
It´s much more easier if you just switch the soda bottle around so that it tips over the right side instead of the left one.