Theory and Experiment

I now move from the context to the content of the Advisory Committee’s work to see how it carried out the research program it had originally set itself. Mr. Asquith assured the House of Commons on May 20, 1909, that the new committee would pursue the problems of aeronautics “by the application of both theoretical and experimental methods of research.”83 No significance

should be attached to the word order, placing theory before experiment, be­cause both found vigorous expression, although the relation between theory and experiment assumed very different forms in the different areas of the committee’s work.

Important tests on full-scale aircraft were carried out at Farnborough, but the main arena in which theory and experiment confronted one another was the wind channel (and sometimes the water channel) in which fluid flow over model wings and model aircraft could be observed and measured. The National Physical Laboratory already had a small water channel, and even a small vertical air channel, but the first task of the ACA was to oversee the construction of a better and more modern horizontal air channel to match those already known to be in use in Paris and Gottingen. By the end of the first year they were able to report on their plans to build a 4 X 4 X 20-foot channel with a draught of nearly 50 feet per second produced by a fan of 6 feet in diameter.84

Difficulty was experienced getting a steady flow, but by keeping the veloc­ity down to 30 feet per second, the flow was found to be “satisfactorily uni­form.” The measuring apparatus for registering the aerodynamic forces on various plates and models was also ready. It was now possible to measure the force component perpendicular to the flow (the lift) and that in the direction of the air current (the “drag” or “drift”). The apparatus could also be set up to determine centers of pressure, and the model could be adjusted to be at any angle with the current without stopping the flow.85

How was the apparatus to be used? Would it be employed to study the behavior of wings and other models in a purely empirical manner to build up an inductive knowledge of the regularities in their behavior? Or would it be used in a theory-testing manner for work that started not with the observable facts but with some theoretical conjecture? If the latter, what theories would be tested and where would they be found? The answer is that both strategies were present in the empirical work. Many of the measurements on model wings involved the highly empirical, and essentially inductive, engineering method of “parameter variation,” that is, systematically altering one factor at a time.86 For example, in one of the studies of a model biplane, the procedure involved keeping the sections, spans, chords, and the distance between the wings constant while altering the angle of stagger in order to try to isolate its effect on lift.87 But there were also bodies of important and sophisticated theoretical work waiting to be explored. The provenance of this theoretical work lay almost exclusively in the achievements of Cambridge mathemati­cal physics. Predictably, the orientation toward the fundamental, theoretical problems of aerodynamics was swept aside in 1914 by the demands of the war, which gave precedence to short-term, practical investigations. Before the cataclysm, however, in the period between 1909 and 1914, theory testing provided the focus for much of the research.

The theories in question concerned two general areas: (1) stability and control and (2) lift and drag. They therefore lay in two quite distinct areas of physics—one being grounded in rigid-body mechanics, the other in fluid dynamics. I consider them in turn, beginning, in this chapter, with the work on stability and, in the next chapter, moving to the fluid dynamics under­lying the theory of lift and drag.