Kline–Fogleman airfoil

Lore & Background

The KF airfoil was designed by Richard Kline and Floyd Fogleman. 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. After many experiments he achieved this goal and presented the paper airplane to Floyd Fogleman, who saw it fly and resist stalling. The two men then filed for a patent on the stepped airfoil. Further development resulted in two patents and a family of airfoils known as the KF airfoil and KFm airfoils (for Kline–Fogleman modified). The patents refer to the introduction of a step on either the bottom (KFm1) or the top (KFm2) of an airfoil, or on both the top and bottom (KFm4). Variations include airfoils with two steps on the top (KFm3), or two steps on the top and one on the bottom (KFm7).

Time published an April 2, 1973 article, 'The Paper-Plane Caper,' about the paper airplane and its Kline–Fogleman airfoil. Also in 1973, CBS 60 Minutes did a 15-minute segment on the KF airfoil, which was rerun in 1976. In 1985, Kline wrote a book entitled 'The Ultimate Paper Airplane.' To publicize the book, Kline traveled to Kill Devil Hills, NC, the site where the Wright Brothers first flew. A crew from Good Morning America filmed the event, and the longest flight by Kline with his paper airplane traveled 401 feet 4 inches (122.33 m).

Reader's Guide

The Kline–Fogleman airfoil's significance lies in its contested aerodynamic claims and its niche adoption. In 1974, a NASA-funded study prompted by Kline and Fogleman's claims and national coverage found the airfoil to have a worse lift-to-drag ratio than a flat plate airfoil in wind tunnel tests. In the 1990s, after the original patents expired, researchers returned to the topic. A 1998 study by Fathi Finaish and Stephen Witherspoon at the University of Missouri tested numerous step configurations; while many made performance worse, promising results were achieved with backward-facing steps on the lower surface, showing considerable enhancement in lift without a significant drag penalty. However, they concluded that a single configuration could not be the best solution at every angle of attack and flight speed, and that vastly different configurations may be needed during a single maneuver, requiring active automated reconfiguration. A 2008 study by Fabrizio De Gregorio and Giuseppe Fraioli in Italy found that the trapped vortex formed by a step could not be held in place without active control; merely relying passively on wing shape was not enough, but when active controls were used to keep the vortex stably in place, the results were 'really encouraging.' A case study focused on the UAV RQ-2 Pioneer in a stepped-airfoil configuration aimed to identify a step schedule for its flight envelope using active flow control to enhance performance over the conventional NACA 4415 airfoil.

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. However, the KF airfoil and derivative stepped airfoils have gained a following in foam-constructed radio-controlled model aircraft, where low Reynolds numbers allow the stepped airfoils to produce significant lift for the drag incurred. The simple shape lends itself to construction in sheets of plastic foams like expanded polystyrene (EPS) or expanded polypropylene (EPP). The first man-carrying KF airfoil-based aircraft was successfully flown in 1987 by Richard Wood in Canada, tested on a Vector 600 ultralight. After discussion with inventor Dick Kline, it was decided to have two notches per wing. First flights with the notches covered produced a regular airfoil; subsequent flights with notches cut open showed an increase in top speed and a lower stall speed, though regular flight did not seem affected. The KF airfoil has also been applied to the Darrieus wind turbine using a trapped vortex, demonstrating higher static and dynamic torque with low-Reynolds applications and better performance for wind conditions lower than 0.8 m/s, seen as a potential solution for self-starting.

Did You Know?

• The KF airfoil was originally devised in the 1960s for paper airplanes.
• The KF airfoil has been applied to the Darrieus wind turbine to improve self-starting at low wind speeds.