THE INTERNATIONAL TRIALS

After the war was over, British aeronautical engineers conceived a project for comparing and subsequently standardizing wind tunnel data. The original idea came from Director of Research Robert Brooke-Popham.23 In a letter to the Aerodynamics subcommittee of February 1920, he referred to a previous comparison between wind tunnel tests at the NPL, Eiffel’s Laboratory, and MIT. It was desirable, he believed, to conduct another set of such comparative tests at representative laboratories in Britain, France, and the United States. For this purpose, he suggested that identical airplane models, airscrews, and stream-lined bodies be tested.24

Accepted by the subcommittee members, the proposal was sent to the Main Committee. The Main Committee approved the proposal and ordered the subcommittee to direct this international project.25 At the same time, the Main Committee sent letters to the four foreign organizations mentioned by Brooke-Popham: the Aerotechnical Institute at St. Cyr, the laboratory of Gustave Eiffel, the Central Aeronautical Institute in Italy, and the NACA. Shortly afterwards, the British Committee received letters of acceptance from all the laboratories together with comments and suggestions on the proposed project.26 The NPL began to construct standard models, and the decade-long “International Trials” project started.

By the end of 1920, three other countries had joined this international project. In August of 1921, the Imperial Research Service for Aviation in the Netherlands asked to be included in the International Trials. Once it was learned from the Controller of Information that this institution was a government establishment, the Committee approved the inclusion of the Amsterdam laboratory.27 Likewise, the requests to participate from the Associate Air Research Committee of Canada and the Japanese Imperial Navy were both approved. It was decided that the models be sent to Japan after the completion of tests in Canada.28

By this time, the British Committee had become aware of the possible importance of aerodynamic research at the Gottingen Aerodynamics Institute in Germany.29 Through some fragmentary information, the British had learned that the aerodynamic research at Gottingen lay at the heart of the wartime achievements of German aeronautical research. The Royal Aircraft Establishment (RAE, the former Royal Aircraft Factory) sent two investigators, Hermann Glauert and Robert McKinnon Wood, to visit Prandtl’s laboratory.30 Members of the Aeronautical Research Committee naturally agreed on the desirability of cooperating with the Gottingen Laboratory. The Committee reported to the Air Ministry that it was prepared to ask the allied laboratories about their willingness to cooperate with Prandtl unless there were any official difficulties.31

At the next ARC meeting, however, the Committee members were informed by the Director of Research that the Air Council deemed it undesirable to approach

Prandtl to enquire about his laboratory’s participation in the International Trials. The message from the Air Council frustrated some Committee members. Glauert and Wood had just returned from their visit to Gottingen and had submitted a report on the theoretical achievements and the experimental facilities of the Gottingen Aerodynamics Institute. Wood’s report had specifically referred to the desirability of including the Gottingen team in the International Trials, since a discrepancy had been perceived between German and British testing of the same form of wing.32 ARC Chairman Glazebrook restated his belief in the scientific importance of the participation of Prandtl’s Laboratory, calling attention to the excellence of the Gottingen wind tunnel, which possessed a steadiness and uniformity of air flow comparable with the NPL wind tunnel. Despite Glazebrook’s appeal, the Director of Research insisted that the matter not be raised again with the Air Council at this time.33 Just why the Air Council was opposed to the contact between the British Committee and the German laboratories is not recorded in its minutes. A later ARC minute indicates that the Council’s opposition sprang from diplomatic reasons.34

The International Trials entailed two different tests:

1. Determination of lift, drag, and center of pressure on a standard airfoil model at various angles of attack.

2. Resistance measurement of an airship model with and without fins.35

Accordingly, the NPL and the RAE constructed an airfoil model of the type R. A.F. 15 and an airship model of the type R.33. These were first measured in two wind tunnels of the NPL and in three tunnels of the RAE.36 They were then sent to France in the spring of 1922 to be measured both at St. Cyr and Eiffel’s Laboratory. After their return from France early in 1923, they were measured again at the NPL to see if their travel or experiments had resulted in any changes. Then they were dispatched to the United States, and measured at the NACA laboratory at Langley Field and at MIT. They returned to England in September 1924. Checked once more, the models were forwarded to Italy. The same procedure was carried out for the Netherlands, Canada, and Japan. This long and cumbersome process took several years to complete.

The same model was used in every test. The model was carefully constructed and repeatedly examined in order to ensure that its size and shape had not altered. Otherwise, all detailed experimental procedures of measuring the forces and moments were left to individual experimenters. Even at the same institution, two different groups may have employed different procedures. At the NPL, for example, two groups, each consisting of two engineers, used the seven-foot wind tunnels #1 and #2, and turned in different reports on their experimental procedures. For example, the group for the #2 tunnel used a specially designed optical device to check the sensitivity of the aerodynamical balance, while the other group did not use such an instrument. Despite these minor differences, they shared the basic procedure, and applied the same corrections due to the drags of wire, the spindle, the sting, and so forth.37