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Latest book reviews: The Enigma of the Aerofoil: Rival Theories in Aerodynamics, 1909-1930.

Why do aircraft fly? That is the question posed in the very first sentence of The Enigma of the Aerofoil by David Moor. It is also a question that I probably first started asking myself, subconsciously or otherwise, when I first made a simple model aircraft at about the age of eight or nine.

It is a question that I have continued to ask ever since, to the extent that, over the years, I have collected a small library of books relating to flight, in an attempt to answer the more specific question for me, 'How do wings work?' I found this necessary despite the fact that I had received a thorough grounding in fluid mechanics and aerodynamics as an undergraduate in the 1960s. However, one book in particular, Understanding Flight by David Anderson and Scott Eberhardt, which 1 bought in 2001, highlighted the inconsistencies in the various explanations that were given in my other books.

Consequently, I decided to try to think through the problem logically for myself, rather than simply accepting one or other of the disparate explanations. The process took me the best part of a decade, with the inevitable distractions of home, family, work and latterly, an active retirement. Nevertheless, I persevered and finally arrived at my own conclusions during the past couple of years, albeit with some late refinements. It was, therefore, somewhat ironic that my attention was recently drawn to The Enigma of the Aerofoil, published last year, which I immediately purchased unseen.

In total contrast to Understanding Flight, which might be characterised as a 'popular science' publication, The Enigma of the Aerofoil: Rival Theories in Aerodynamics, 1909-1930, is a serious and detailed historical work. As its subtitle suggests, it records and contrasts the development of two competing, if not contradictory theories. Discontinuity theory was propounded and championed at the turn of the 20th century by various British academics and experts in fluid dynamics, with the notable exception of Frederick Lanchester, while circulation (or vortex) theory was promoted by their German counterparts.

Following the Introduction, which outlines the plan and purpose of the book, chapter one describes early British work on aerodynamics; while chapter two goes on to lay the foundations for understanding the two competing theories of lift. Chapter three introduces the discontinuity theory and describes the early British research programme on aerodynamic lift, which began to show up discrepancies between theory and practice. Chapter four discusses the circulatory or vortex theory, as first propounded by Lanchester, and describes the hostility he generated among other British experts.

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In chapter five, the author contrasts the two intellectual and academic traditions, British and German, which were attempting to develop a theory of lift. Chapters six and seven go on to discuss the development in the German universities of the circulation theory, while chapters eight and nine describe the British response in the years after the Great War, which resulted in theoretical confrontations. In chapter 10, the author concludes by giving a sociological, even philosophical commentary on the development of aerodynamic theory and the interactions, both theoretical and human. The book, with some half-tone illustrations and diagrams, together with notes, references and a bibliography, has over 500 pages, using a relatively small font. It is, I must reiterate, a serious and scholarly study of the history of the subject and is not a work aimed at the `popular science' market, nor is it a text book. Its readership is likely to be limited to those with an equally serious interest in the fundamental question as to how and why aircraft fly. However, it does highlight the fact that aeroplanes did fly in the first decades of the 20th century, largely as a result of the work of practical engineers and designers, and despite the theoreticians having an incomplete and flawed understanding of aerodynamic lift.

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But then what has changed in the past 100 years? My attention was drawn to this book by a retired and highly respected aeronautical engineer who told me that an 'equal transit time' condition must be achieved (ie, that the air flowing over the wing must arrive at the trailing edge at the same time as that flowing beneath the wing, from which it was separated at the leading edge), in order for the Kutta condition (ie, the optimum flow condition off the trailing edge), to be observed. That fallacy was disproved experimentally decades ago, using pulsed smoke flow visualisation techniques in a wind tunnel. The fact is that the air above the wing arrives at the trailing edge well before that flowing beneath.

The next question is usually, 'why?' The conventional answer is, typically, that the flow must accord with Bernoulli's Principle: however, this would imply the need for equal transit time. Even at the turn of the 21st century, an argument has persisted between those supporting Bernoulli and those supporting Newton, not dissimilar to the Lilliputian argument in Swift's Gulliver's Travels over which end a boiled egg should be cracked! A few years ago, the NASA Ames website showed portraits of Bernoulli and Newton, with a statement that they were both right: the current website, aimed at US high school students, cites both Bernoulli and Newton.

Nevertheless, Bernoulli has been increasingly, if not controversially 'debunked' in recent years. And what of my own theory? During the course of 2011, it finally dawned upon me that circulation theory, which I had been taught as a student, was not simply a mathematical model, but was physical fact. A wing actually generates a vortex in basically the same way as a rotating cylinder, known as a 'Flettner rotor'.

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Top left: Lanchester's account of why a spinning ball generates lift. Notice the presence of the dead air behind the ball and hence the appeal to the discontinuity theory. From Lanchester, Aerodynamics, 1907, 43.

Top centre: Diagram showing Lanchester's pictures of trailing vortices. From Lanchester, Aerodynamics, 1907, 172.

Top right: The flow that results from the combination of a uniform free stream and a vortex. The streamlines above the centre of the vortex are closer together than below it showing that above the vortex the speed is higher, and the pressure lower, than below the vortex. There will be a resultant force directed upwards, ie a lift force. From Lanchester, Aerodynamics, 1907, 164. which produces lift in a transverse airflow. The pressure gradient above the wing is, therefore, generated by the curvature of the flow, just as a pressure gradient is produced in a tornado, for example. It does not depend on the shape of the wing, which is why a flat plate at an angle to the airflow will produce lift.

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Am I alone in coming to this conclusion? I was initially gratified and then chastened when I came across an article in The Daily Telegraph earlier this year reporting on the work of Professor Holger Babinsky of the University of Cambridge. He had recently produced a video of an aerofoil with pulsed smoke flow visualisation, showing the phenomenon to which I refer in an earlier paragraph. Further research then led me to an article of his, published in 2003, in which he cited the curvature of the airflow above the wing as being the source of the pressure gradient and hence the lift force. However, it must also follow that there has to be a deflected downwash in accordance with Newton's Third Law.

Nevertheless, having read The Enigma of the Aerofoil, my final conclusion must be that, despite the existence of a conflicting theory and in the face of hostility from other 'experts', Frederick William Lanchester effectively arrived at this conclusion over a century ago. Even more to his credit and indicative of his considerable intellect, is the fact that he did so with little benefit from experimental aerodynamics and with no flow visualisation. Whilst Prandtl and others in Germany might subsequently have developed the theory, Lanchester was the originator of circulation theory and, in my view, should be more widely recognised.

* Title

The Enigma of the Aerofoil: Rival Theories in Aerodynamics, 1909-1930

* Author

David BLoor

* Publisher

The University of Chicago Press

* Publication date

December 12 2011

* Reviewer

Graham Jeffery BSc (Hons) CEng FIMchE FIED

*ISBN 78-0-226-06095-8
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Publication:Engineering Designer
Article Type:Book review
Date:Jul 1, 2012
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