# Theory of Lift 1904–26

## Rival Theories of Aerofoil, 1904–26

David Bloor

By the time of the First World War of 1914–18 the aeroplane had become established as an effective and vital piece of technology. Its practical success and potential were beyond doubt but flight posed deep scientific problems. How does a wing generate lift? The answer is by no means obvious. The flow of the air must generate forces at right angles to the direction of motion i.e., a vertical lift forcs must come from horizontal motion and, ideally, the process must be depicted and predicted using the mathematiucally sophisticated resources of hydrodynamics or fluid mechanics.

Following the work of Wilhem Kutta and Ludwig Prandtl it was widely accepted in Germany that the lift force was the product of a component of the flow that was called the "circulation." This model was developed in mathematical detail and connected with observations on model wings in wind-tunnels. In Britain, however, the approach was in many ways more empirical. In as far as a theoretical picture of the flow was developed it was quite different to that of Kutta and Prandtl. The flow was said to involve surfaces of discontinuity in the air, one surface emerging from the leading edge of the wing and one from the trailing edge, with a "wake" between them. This model, too, involved a great deal of sophisticated mathematics and it was based on Rayleigh's analysis of the effect of a flat plate held obliquely to a flow of "ideal" (that is frictionless and incompressible) fluid.

This theoretical difference persisted throughout the war years and was not resolved until around 1926 when the British accepted the superiority of the rival approach. It is interesting to ask what sustained the difference. Was it merely the good fortune of members of the one group to hit on the right idea while it was the misfortune of those in the other group not to think up a comparably good idea? If this were all that could be said it would make the issue turn on the (as yet) unpredictable mystery of creativity. However, this was not the case because the British knew of the German work (and some of Rayleigh's other contributions to fluid mechanics could have provided the clue.) They knew about it and found it unattractive and unhelpful. This suggests that rather than being random there was something systematic about the two groups of scientists themselves that must explain their divergence.

Who were the people involved? British aerodynamics was heavily dominated by Cambridge trained mathematical physicists; the German experts were mathematically sophisticated engineers coming from the Technische Hochschulen. The German effort was led by Prandtl, whom Felix Klein had brought from the Technische Hochschule at Hannover and installed at Goettingen. If the question of what sustained the divergence between the supporters of the two theories is to have an answer, it may therefore lie in the different cultures of Cambridge and (Klein's) Goettingen.

As it stands, however, this answer is in danger of being little more than a truism and would merely seems to raise the original question in another form. What is a differrence of theoretical preference and perspective if not a difference in the scientific culture of the two groups? In order to give the issue greater substance it was necessary to locate traditions of work and to ask what sustains these traditions. This meant knowling about the aims and goals of the agents and the interests that inform the institutional setting that make their respective cultures attractive and rewarding for those within them. In other words, the technical and scientific questions that were necessarily in the foreground of the problem had to be related to the broader and more general social background. The focus was on understanding the roles of the elite groups in question and the expectations present in, and inherited from, the society around them.

## Funding Institutions

Economic and Social Research Council (UK)
University of Edinburgh