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THE HIGH SPEED
FRONTIER
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- Chapter 3: Transonic Wind
Tunnel Development (1940 -1950)
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- THE BODY-DROP AND ROCKET-MODEL
TECHNIQUES
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- [85] Early agency
literature refers to these techniques as "Bomb-Drop" and
"Missile-Test," revealing their wartime origins. It is not certain
whether NACA supervisors Crowley and Thompson, who are said to
have "considered" the body-drop approach in 1942 (ref. 54) had any [86] knowledge of the
prior German use of the technique in 1941 (ref. 105). It is apparent, in any case, that the American
development was a great improvement over the German (ref. 106). The technique supplied primarily zero-lift drag
data at speeds up to about Mach 1.3. A question regarding possible
errors due to acceleration effects was raised early in the program
by von Karman, but it was proved later, by drops of identical
models of varying weight, that the effect was negligible. The
reliable drag data from the body drops were used, for example, to
estimate the drag and power requirements for the transonic
research airplanes (ref. 107). As the rocket models came into use in the latter
forties the body drops diminished but continued to be used
occasionally for special purposes. They have provided important
comparative data for evaluation of slotted tunnels (ref. 108), but otherwise there was little interaction of
this technique with the transonic wind tunnel developments.
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- The rocket-model approach started as a
missile test and development program, but it rather quickly
started to change character. Reflecting both the interests of its
NACA operators and the growing demand for transonic aerodynamic
data, it evolved into a program of general aerodynamic tests
covering the entire transonic region and beyond into the
supersonic regime. The flight data became increasingly more
accurate and more comprehensive as time went on as a result of the
impressive ingenuity brought to bear on the many challenging
aspects of this technique. Inevitably, the practitioners of the
technique tended to become as much interested in making the rocket
models do more things more accurately as they were in the research
problems. To a large degree, therefore, one finds that the
Pilotless Aircraft Research Division (PARD) reports tended to be
data reports for specific test objects rather than general or
analytical treatments of research problems.
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- One aspect of the technique caused major
interference with the wind tunnel programs: each firing required
the sacrifice of the test model,
including in many cases complex and
costly internal instrumentation. For example, in the years 1947,
1948, and 1949 no less than 386 models were expended (ref. 109). This is roughly equivalent to the requirements of
perhaps 10 major wind tunnels such as the 16-foot. Furthermore,
the wind tunnel models generally carry only pressure taps, or
[87]
house a balance which can be used repeatedly in many models.
There was a major slowdown in both wind-tunnel model and
instrumentation production as a consequence of PARD's voracious
appetite.
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- By mid-1946 transonic data reports had
been published from all of the transonic methods then in use and a
serious problem had arisen. Large discrepancies were apparent,
and, understandably, queries had been received from industry
users. The matter was discussed at the July 12, 1946, meeting of
the Langley General Aerodynamics Committee, and a special ad hoc
group was set up to study the problem. The group made its first
report on September 17, 1946. W. H. Phillips showed that in one
category, significant differences in the transonic drag of
straight wings were believed explainable on the basis of large
test Reynolds number differences. In the case of complete wing
body configurations, very large and unexplainable differences
existed; the gross trends, however, were similar. The group
recommended that a specific wing and the X-2 aircraft
configuration be tested by all of the techniques,
including the 9-inch supersonic tunnel, for comparative study.
Nine months later, on June 13, 1947, the group reported "no new
conclusions." By that time the Langley bump was in full operation
and the supply of discrepant data was growing rapidly. Cases were
found where not only the data values disagreed, but also the
trends were at variance. The low opinion of the bump data shared
by a majority of Langley aerodynamicists found expression in a
memorandum submitted to Langley's Chief of Research F. L. Thompson
by E. C. Draley of the 8-foot tunnel, and discussed at the
June 13, 1947, meeting of
the Langley General Aerodynamics
Committee. Draley was particularly concerned about the validity of
stability and control data from the bump.
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- Thompson considered this problem of
sufficient importance to take the unprecedented step of personally
presenting an introductory paper on the subject at the NACA
Conference on the Aerodynamic Problems of Transonic Airplane
Design on September 27, 1949. He gave a brief objective assessment
of all the different transonic techniques and focused special
attention on the problems of the wing-flow and bump. He said in
effect that the bump in most cases provided useful trends or
comparisons, but bump data should not be used
quantitatively.
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