18 U.S. Code § 31(a)(1) says,
The term “aircraft” means a civil, military, or public contrivance invented, used, or designed to navigate, fly, or travel in the air.
What exactly is an aircraft? Airplanes, helicopters, sailplanes, drones, etc are obviously aircraft, and I am quite certain that bullets and arrows are not aircraft (although I guess they do technically meet that definition?). What about something like a paper airplane, that does glide using aerodynamic effects but is not controlled or propelled after launch? Or what about an RC glider launched with a bungee (technically not very different from an arrow, except it's controllable)?