Chapter 3: Transonic Wind Tunnel Development (1940 -1950)
[80] Not really a wind tunnel at all in the usual sense, the Annular Transonic Tunnel more properly falls in the "whirling-arm" category. It was also variously referred to as the "Rotating-Disc Transonic Research Equipment," "Special Transonic Research Equipment," "Annular-Throat Tunnel," and "Langley Transonic Tunnel." C. duP. Donaldson proposed the scheme in late 1944, thinking of it as a single-bladed axial fan rotating at transonic speeds. The single blade or test airfoil had very small tip clearance with the annular passage so that the flow could approach two-dimensionality. To avoid wake interference a low-speed axial flow was induced in the annular passage by a blower, and the boundary layers on both surfaces of the annulus were removed ahead of the rotor (fig. 18). In effect, the test airfoil would be flying in a channel of infinite depth and choking would not occur.
Of the several difficult problems of this scheme, the most formidable...

cross-sectional diagram of the multiple-pressure
   transfer device
[81] FIGURE 19.-Section through axis of rotation of the pressure-transter device of B. W. Corson, Jr.
....was the accurate determination of the forces on the whirling test airfoil. In all probability the annular tunnel would never have been attempted, and perhaps it would never have been conceived at all, had it not been for the invention, successful development, and prior use by our 16-foot tunnel group of a pressure-transfer device which made it possible to obtain accurate pressure distributions about the test airfoil.
The design of the multiple-pressure transfer device was proposed by Blake W. Corson, Jr., in 1943 (ref. 96) (see fig. 19). It was developed successfully by J. F. Runckel, R. S. Davey, and M. F. Miller, in consultation with Corson, substantially as proposed. It was used initially in a pressure distribution study on the rotating blades of a 42-inch diameter axial compressor (ref. 97). Davey later developed an improved transfer [82] device using mechanical seals which was used in the annular transonic tunnel and in the high-speed propeller program (ref. 98). The availability of the original device was at the heart of the Annular Transonic Tunnel enterprise.
Because of its close relationships to our axial compressor research at the 16-foot tunnel, the Annular Transonic Tunnel concept was placed in our group for design, development, and exploitation. Much of our electrical equipment could be used directly. A suitable cinder-block building was erected on the 16-foot grounds to house the new tunnel. As one can see in the photograph (fig. 20) the annular tunnel was a substantial new facility, considerably exceeding the other small high-speed airfoil tunnels in cost. Early in 1947 the first successful runs were made (ref. 49), and the first pressure distributions ever measured on an airfoil at Mach 1 were obtained (ref. 99).
After a 5-year life, the Annular Tunnel was decommissioned in 1952. It had major limitations: only simple airfoils could be tested; the test process was cumbersome (only 5 airfoils were tested in 5 years) ; for structural reasons speeds above Mach 1were never attempted; and when the X-1 data and semi-open-tunnel data became available, it was evident that the Annular Tunnel pressures were uniformly too high by a small but ever-present amount for which no explanation could ever be found (ref. 100). There was obviously no justification to continue the Annular Tunnel after the simple, more productive semi-open, and the more versatile slotted tunnels came into operation.
Public announcement of the Annular Transonic Tunnel was made at the opening exercises of the May 1949 Biennial Inspection of the Langley Laboratory. To heighten the emphasis, NACA called on John Stack to describe the accomplishment. What was actually happening here, beyond the revelation of an interesting new facility, was an unprecedented attempt by NACA to divert attention away from the slotted transonic tunnel developments. NACA subsequently admitted the dual intent of this announcement in a rather surprising statement found in the 40th Annual Report of 1954,

photo of 2 men inspecting the transonic tunnel
[83] FIGURE 20.-The Annular Transonic Tunnel, opened at the rotor section. R. Turner, left, and L. W. Habel.

it is to be doubted whether the NACA would have given the [ATT] device the importance implied by the [1949] public announcement except that it served to explain away rumors that a successful transonic wind tunnel had been developed. Such was in fact the case.... At the same time that the (1949] announcement of the annular-throat wind tunnel was being made, construction was being rushed towards completion of the first of the large [slotted] transonic wind tunnels.
By the time of Stack's public announcement, it was already clear that the ATT had serious deficiencies and very limited prospects; the semi-open 4 x 19-inch facility had already proved itself and had taken up the entire burden of the transonic airfoil program.
The necessity for NACA in 1954 to reveal the dual nature of the 1949 ATT announcement is obvious. They were saying in effect, "This time we are going to tell you about the real transonic tunnel." It seems to have been taken for granted that tactics of this kind were justified in the interest of national security in the environment of the early fifties.
[84] I doubt that the 1949 announcement did much to divert attention from the slotted tunnel; too many outsiders already knew about it.