CONTROVERSY OVER SCALE EFFECT

During World War I, many scientists, including science students, were mobilized for weapons development. While Ernest Rutherford and other physicists were engaged in devising a submarine detection system, many Cambridge scholars gathered at the Royal Aircraft Factory to assist in the development of the airplane. George P. Thomson, the son of J. J. Thomson; Francis Aston, the inventor of the mass spectrograph; Geoffrey I. Taylor, a specialist in fluid mechanics; and other excellent students or fresh graduates, including Hermann Glauert and William S. Farren, participated in the war work. The Factory in Famborough thus became another prominent center of aeronautical research in Britain.

Famborough approached aeronautical problems differently than the NPL. Whereas the NPL relied on wind tunnel experiments using small-scale airplane models, the Royal Aircraft Factory performed test flights of full-scale aircraft. For example, Oxford physicist Frederick Lindemann performed dangerous spinning flight and his data were analyzed by G. P. Thomson. The Factory’s primary function was to construct full-scale airplanes and conduct flight tests on them. Cambridge scientists collaborated closely with pilots and aircraft designers in their aeronautical investigations.

Through a number of full-scale flight tests, Factory investigators became aware of discrepancies between model tests and corresponding full-scale tests. They prepared a preliminary report noting the differences in terms of values of drag and lift of the airplane.9 To discuss the problem, a subcommittee was formed in 1917 including among its members representatives from the NPL and the Factory.10 Its official name was the “Scale Effect” subcommittee. The term “scale effect” was enclosed in quotation marks, suggesting that its significance was a matter in question.

A vehement debate arose at the first meeting of the subcommittee. Bairstow, the advocate of model experiments, argued against the Factory conclusion that the discrepancies between the measurements achieved by the two methods was attributable to scale effect. In his report, he referred to various causes of error other than scale effect, including errors in full-scale tests themselves. He even pointed out that a previous Factory report was “illogical” because it neglected the effect of interference on airplane drag. He also mentioned French aeronautical research in which model tests at Eiffel’s laboratory and full-scale tests at St. Cyr showed fairly good correspondence.11

The subcommittee considered a variety of causes for the discrepancies, examining each cause extensively. For example, the full-scale measurement of the drag of an airplane depended on the value of the power of its engine and the efficiency of its propeller. The suggestion was raised at one meeting that the power of the engine measured during flight would be different from that measured on the ground.12 In this case, it appeared that the pressure distribution should be measured by both full-scale and model methods.

Among the causes of errors investigated by the subcommittee, the most notable was the effect of the propeller on full-scale data. To investigate this effect, it was suggested in 1917 that full-scale tests should be made while the airplane was gliding with its engine stopped. Farren at Famborough objected that airplanes suitable for such gliding tests were no longer available there, having all been sent to the front. Bairstow observed that the Factory should always be able to secure airplanes for experiments.13

Through the discussions and investigations, the subcommittee reduced the original differences between the two sets of test results. Yet, subcommittee members remained divided in their conclusions regarding scale effect. In preparing the subcommittee’s final report, Bairstow insisted that scale effect was not a significant factor. When a Factory report on the collection of full-scale data was circulated among subcommittee members, Bairstow severely criticized the report. Although it did not explicitly refer to the unreliability of model tests, it listed full-scale data as necessary and sufficient for the calculation of drag. This form of presentation, Bairstow contended, would leave the impression that model test results could not be readily applied to full-scale planes. He suggested that the subcommittee should take some steps to correct the “wrong” impression created by the Factory report.14 The subcommittee’s final report therefore carried a statement on the usefulness of model tests, virtually neglecting the significance of the scale effect. Bairstow was willing to support the publication of the complete data only if the final report explicitly stated that observed differences had not been found to be due to scale effect.15

The different positions on the scale effect taken by the Factory and the NPL investigators reflected the different research strategies pursued at the two research facilities. The NPL concentrated on model testing in wind tunnels only, whereas the Factory’s main focus was in full-scale testing using its own planes. Bairstow was apparently afraid that invalidation of the model test results would seriously undermine the significance of aerodynamic investigations in which he had been engaged while at the NPL.