The engines look almost like the fans you’d see in a ventilation shaft, and they’re surrounded by an enclosure (or duct) that it spins within, as opposed to the way a regular propeller just spins in the open air. That design gives the aircraft a few advantages. For one, since the fan blades are enclosed, it’s quieter than a traditional unenclosed propeller would be. “That’s probably the biggest advantage,” Crossley says. This type of engine is also likely more efficient than a propeller of about the same size. Those ducts add weight, though—an important reminder that in aviation, features on a plane usually come with a trade-off.