page_title: Kline–Fogleman airfoil
section_title: Independent scientific testing
hierarchical_section_title: Kline–Fogleman airfoil / Independent scientific testing
caption_attribution_description: English: A simple diagram showing the purported airflow around a Kline-Fogleman KFm2 airfoil.
context_page_description: 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.
context_section_description: In 1974, a NASA-funded study prompted by Kline and Fogelman's claims and the resulting national coverage found the airfoil to have worse lift-to-drag ratio than a flat plate airfoil in wind tunnel tests.
  In the 1990s, with 20 years of technical progress opening up new possibilities and with the original patents expired, researchers returned to the topic of stepped wings. A 1998 study by Fathi Finaish and Stephen Witherspoon at the University of Missouri tested numerous step configurations in a wind tunnel.  While many made wing performance worse, they got promising results with backward-facing steps on the lower surface of the wing – in some cases giving considerable enhancement in lift without much of a drag penalty. However, they concluded that a single configuration could not be best solution at every angle of attack and flight speed. Indeed, "vastly different configurations may be needed during a single maneuver."  The idea works, Finaish and Witherspoon concluded, but only with active automated re-configuration of the shape of the step(s) during flight. Further testing has shown that this airfoil is effective in low Reynolds number flow.
  A 2008 study by Fabrizio De Gregorio and Giuseppe Fraioli at CIRA and the University of Rome in Italy tried this out. 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.
  They concluded that the trapped vortex formed by a cavity or step could not be held in place without such active control. Just relying passively on wing shape wasn't enough – the vortex would detach possibly giving you worse characteristics than the original unstepped airfoil. But when active controls were used to keep the vortex stably in place they found the results "really encouraging".
  Such scientific experimental work seems to have put paid to the idea that the original KF airfoil will find a simple practical application in the world of full-size aircraft. But the basic idea of the stepped wing has mutated, and is now bound up with a new body of research into actively controlling the airflow over a wing's surface with new mechanisms unthinkable 50 years ago.
  The case study conducted as a part of this research focuses on the UAV RQ-2 Pioneer employed in a stepped airfoil configuration by comparing its aerodynamic characteristics with the conventional NACA 4415 airfoil originally used on this aircraft. 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.
caption: KFm2 airfoil showing purported laminar flow vortex
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