engineering

Kline–Fogleman airfoil

Kline–Fogleman airfoil is a engineering topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Kline–Fogleman airfoil rather than just read about it. In short: The Kline–Fogleman airfoil or KF airfoil is a simple airfoil design with single or multiple steps along the length of the wing. The purpose of the step, it is claimed, is to allow some of the displaced air to fall into a pocket behind the step and become part of the airfoil shape as a trapped vortex or vortex attachment.

Key takeaways

Kline–Fogleman airfoil belongs to engineering; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Kline–Fogleman airfoil to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Kline–Fogleman airfoil from memory before moving on to harder problems.

Reference excerpt

The Kline–Fogleman airfoil or KF airfoil is a simple airfoil design with single or multiple steps along the length of the wing. The purpose of the step, it is claimed, is to allow some of the displaced air to fall into a pocket behind the step and become part of the airfoil shape as a trapped vortex or vortex attachment. This purportedly prevents separation and maintains airflow over the surface of the airfoil. The KF airfoil was originally devised in the 1960s for paper airplanes. In the 21st century it has found renewed interest among hobbyist builders of radio-controlled aircraft, due to its simplicity of construction. It has not been adopted for full-size aircraft capable of carrying a pilot, passengers, or other substantial payloads.

History 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 was able to achieve this goal. He 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 two patents, US Patents #3,706,430 and #4,046,338, 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). The purpose of the step, it is claimed, is to allow some of the displaced air to fall into a pocket behind the step and become part of the airfoil shape as a trapped vortex or vortex attachment. This purportedly prevents separation and maintains airflow over the surface of the airfoil.

Reception 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. CBS reran the show 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 had flown where their first manned powered flight, of 122 feet (37 m). A crew from Good Morning America filmed the event. The longest flight by Kline with his paper airplane traveled 401 feet 4 inches (122.33 m).

Independent scientific testing

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, after 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 step configurations made wing performance worse, promising results were achieved with backward-facing steps on the lower surface of the wing, in some cases showing considerable enhancement in lift without a significant drag penalty. However, the researchers found that a single configuration could not be the best solution at every angle of attack and flight speed; instead, they concluded that "vastly different configurations may be needed during a single maneuver." The idea works, Finaish and Witherspoon concluded, but only with active automated reconfiguration of the shape of the steps during flight. A 2008 study by Fabrizio De Gregorio and Giuseppe Fraioli at CIRA and the University of Rome in Italy pursued this idea further. The model airfoils 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. Merely relying passively on wing shape was not enough – the vortex would detach, possibly yielding 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". The case study conducted as a part of this research focused 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.

Applications of the KF airfoil today

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Illustrations

Worked examples

Example 1 — a first encounter with Kline–Fogleman airfoil

Start with the simplest possible case. Write down what Kline–Fogleman airfoil claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Kline–Fogleman airfoil before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Kline–Fogleman airfoil ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Kline–Fogleman airfoil

In research: Kline–Fogleman airfoil appears in engineering research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Kline–Fogleman airfoil in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Kline–Fogleman airfoil is common in secondary-school and first-year university syllabi. It links to neighbouring topics Aerodynamics, Aircraft wing design, so understanding it makes those chapters shorter.
In everyday life: Look for Kline–Fogleman airfoil outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Kline–Fogleman airfoil in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Kline–Fogleman airfoil means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Kline–Fogleman airfoil out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Kline–Fogleman airfoil in simple terms?

Why does Kline–Fogleman airfoil matter?

Because it connects several engineering ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Kline–Fogleman airfoil?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Kline–Fogleman airfoil.