The trick to efficiency is to accelerate more mass at a lower delta v. Doesn't matter if it's rotating or oscillating. I.e. long flapping wings/tail or long, sl

The trick to efficiency is to accelerate more mass at a lower delta v. Doesn't matter if it's rotating or oscillating. I.e. long flapping wings/tail or long, slow propellor blades. One benefit of an airship is large propellors are more practical than on a small plane.