Kline–Fogleman airfoil

In the 21st century the KF airfoil has found renewed interest among hobbyist builders of radio-controlled aircraft, due to its simplicity of construction.

In the early 1960s, Richard Kline wanted to make a paper airplane that could handle strong winds, climb high, level off by itself and then enter a long downwards glide.

[3] To publicize the book Kline traveled to Kill Devil Hills, NC, the site where the Wright Brothers first had flown where their first manned powered flight, of 122 feet (37 m).

A 1998 study by Fathi Finaish and Stephen Witherspoon at the University of Missouri tested numerous step configurations in a wind tunnel.

[6] The model aerofoils used in their wind tunnel tests were equipped with numerous small holes through which air could be blown or sucked in an active way.

Merely relying passively on wing shape wasn't enough – the vortex would detach, possibly yielding worse characteristics than the original unstepped airfoil.

The main objective of the case study was to identify and outline a step schedule for the flight envelope of the UAV Pioneer using a stepped airfoil configuration at the same time applying active flow control to obtain enhanced aerodynamic performance over conventional NACA 4415 airfoil originally used and hence improve the flight performance characteristics like Range and Endurance of the aircraft.

[7] Poor lift-to-drag ratio performance in wind tunnel testing has meant that to date the KF airfoil has not been used on any full size aircraft.

The low Reynolds numbers allow for the stepped airfoils to produce a significant amount of lift for the drag incurred, making them increasingly popular among RC hobbyists.

Experiments have found the KF rotor demonstrates a higher static and dynamic torque with low Reynolds applications and better performance for wind conditions lower than 0.8 m/s[clarification needed].

The first man carrying flight of the Kline Fogelman airfoil was July 7, 1987. in Essex Ontario Canada by Richard (Dick) Wood.