Chapter 3: Transonic Wind Tunnel Development (1940 -1950)
[78] The Germans employed an interesting variant of the "small-model" technique to obtain two-dimensional airfoil data in their large (2.7 meter) high-speed tunnel. Test models of about 1-foot chord were mounted in the center of the tunnel between large, thin, wire-supported end plates (ref. 66). An impressive amount of systematic data was produced by this setup in 1943 and 1944 (see Chapter II).
Langley was slow to accept either the German or Italian semi-open techniques for airfoil testing. Recalling his early difficulties with the open-throat 11-inch tunnel, Stack was suspicious of the semi-open configuration and at the same time chagrined that it had not come from Langley. The best that he would say for it was, "a marked reduction in choked range" had been achieved (ref. 54).
On his arrival in 1945, Ferri had been surprised to learn that NACA [79] had not tried to develop a semi-open tunnel. However, he had had so much difficulty with flow pulsations above Mach 0.95 that he was reluctant to recommend that we become involved with one. C. duP. Donaldson was stimulated by Ferri's work to undertake in 1945 a series of tests of a small airfoil in a 1 by 3-inch jet to evaluate constriction effects in both the closed and semi-open configuration (ref. 53). At about the same time, W. F. Lindsey wrote a memo to Stack suggesting a more thorough investigation at a more adequate scale, utilizing the 4 x 18-inch tunnel equipment. Stack rejected the proposal, telling Lindsey that Ames was planning to undertake a similar study (perhaps the work of Allen and Vincenti (ref. 84) ). But about a year afterward, in 1946, Stack approved tests of a 9 x 9-inch open-throat configuration in connection with studies then in progress of various wind tunnel designs for the so-called "NACA Supersonic Center." Lindsey and Bates found....

drawing of
FIGURE 18.-Schematic drawings of the Annular Transonic and the 4 x 19-Inch tunnels.

[80]....that the flow pulsations near Mach 1in the 9 x 9-inch throat could be reduced by improved design of the diffuser entrance, but satisfactory conditions for testing at conditions for testing at low supersonic speeds could not be obtained (ref. 54).
A few weeks after becoming division chief in the summer of 1947, I called Lindsey and suggested that we convert the 4 x 18-inch airfoil tunnel into a semi-open facility to extend our airfoil testing to Mach 1. Recalling his earlier rejected proposal along these lines, Lindsey was naturally happy to proceed. In addition to applying the diffuser design criteria he had developed in the 9 x 9-inch throat work, Lindsey incorporated a very effective adjustable choking device located in the diffuser section to prevent downstream disturbances from affecting the test section (fig. 18). The rather small "open" sides of this 4 x 19-inch tunnel (17 percent open in contrast to Ferri's 43 percent) undoubtedly also contributed to the reduction of pulsations. Preliminary runs in 1948 revealed that testing up to Mach 1 was possible with negligible pulsations and transient disturbances (ref. 54). A comprehensive airfoil test program (reviewed in ref. 51) was initiated and the first results for Mach 1.0 (figs. 4, 5, 6, and refs. 54, 56) were published in 1949.