The Kline–Fogleman airfoil or KF airfoil is a simple airfoil design with single or multiple steps along the length of the wing. It was originally devised around 50 years ago for paper airplanes. In the 21st century the KF airfoil has found renewed interest among hobbyist builders of radio-controlled aircraft, due to its simplicity of construction. But it has not been adopted for full-size aircraft capable of carrying a pilot, passengers, or other substantial payloads.