Minecraft wiki entry for Flying_machines, associated text: Materials: 13 Observers, 26 slime blocks, 8 sticky pistons, 8 regular pistons. For use: redstone torch, immovable block
Dimensions: 7x5, 4 high (or alternately 5x5x5)
Build guide:
Start with the central two slime blocks of layers 2 and 3. The lower of these needs to be at least two blocks above "the ground" -- that is, the highest ground you intend to fly over.
Place the upper sticky pistons outward, then use those to place the lower ones facing inward.
Add the downward facing observer on the top of this, then the four-high slime pillars outside the pistons, extending above and below them. Use those to place the downward-facing observers on layer 1.
Extend the slime pillars into T shapes, at the top on two opposing sides, on the bottom for the other two. (this diagram has the top T's at east and west). Place the regular pistons at the ends of the T-bars, facing into the machine.
Place the layer-2 observers against the slime pillars, then place the layer-4 observerd against the T-bars.
Note: the layer-4 observers alternately be placed atop their slime blocks (level 5), each pair facing each (powering the adjacent pistons). This placement fits the entire build in a 5x5x5 cube. However, that higher position slightly obstructs the rider's view, so the original builder prefers the 4-level build.
Usage: The player stands on the pistons surrounding the top central observer, with a redstone torch in hand. The engine can be started in any direction by placing and quickly breaking the torch on that side of the central observer. It is stopped by again placing the torch in the same place (and leaving it until the engine stops). With the machine in motion, placing the torch may be difficult, having a few spares may be wise. Note that it is entirely possible to fall through the corner holes, so step carefully, and stay on the piston you're riding while the machine is in motion.
Credits: Design from "Dylan D", Apr 9, 2020 "[https://www.youtube.com/watch?v=RaD97AWBXas Minecraft: MORE Efficient 4 Directional Flying Machine | One Take | 1.15.2+ Java Edition") YouTube