Tested and working in 1.9.

This is a 2 way flying machine that does not require replacing blocks. I made 2 designs, both go at the same speed, and have the same properties. I made a 1x9x4 design (13 blocks), and a 2x7x4 design (12 blocks) I think these designs are the most compact. (RedstoneSpire's design is 3 tall, but it has more blocks, and is bigger in the other dimensions) The only other designs I have seen were made by DicoTheRedstoner & RedstoneSpire. Unlike Dico & RedstoneSpire's designs, these designs work facing any direction. However, their designs are faster. An issue with these designs is that all of the slime blocks on this design have to be pushed by a piston that is pushing 12 blocks on the 1 wide design or 11 blocks on the 2 wide design, meaning expansion of these flying machines will be hard. It might also be possible to modify this design to make it more easily expandable. There is no place for a boat on these flying machines. The best place to stand seems to be the end facing the direction the flying machine is moving.