| Transonic-Supersonic
Research Tools |
Editors
Note: If you are using this article to learn aspects of supersonic
flight, take note; Once you are flying much faster than the
speed of sound, aircraft control is consistant. The challange
of contol is in the transonic range, passing through the speed
of sound.
SCORES of articles
on the spectacular subject of supersonic flight may have convinced
the reading public that soon we’ll all be able to whiz
through the wild blue yonder at speeds of 750 to 1,000 mph.
Commercial travel and military tactical operations in that
speed range may become commonplace in our lifetime, but the
time is not now. Now is the time of intensive research to
determine answers to a thousand and one questions in the realm
of transonic-supersonic flight and to build a backlog of basic
design data for men who some day will evolve practical faster-than-sound
aircraft.
 |
| Bell’s
X-1 rocket-powered Air Force research plane was the first
to exceed the speed of sound in level flight (1947). |
It is a matter
of record, of course, that a piloted airplane built for the
Air Force has exceeded the speed of sound many times. Its
design, however, is unsuitable for military operations. It
is a research tool for transonic-supersonic exploration and
only one of the several tools in that highly specialized field
of aeronautical science. This is the story of those tools.
While the Sunday
supplement writers are concerned almost entirely with supersonic
flight, researchers are centering much study on the transonic
zone, where the transition from subsonic to super sonic flow
occurs. Parts of the airflow have subsonic characteristics
and parts have supersonic characteristics and abrupt changes
take place in both. Such changes have pronounced and varying
effects on control surfaces and can lead to uncontrollable
airplanes.
Much has been learned
about the transonic range through studies with major research
tools—wind tunnels, free-falling bodies, rockets and
airplanes—but what is not known about it is the basis
of a broad-scale investigation. Until there is a vast background
of knowledge of the effects of the transonic flows on airplanes,
there can be no really practical supersonic aircraft.
The most
important tool in aeronautical research is the human mind.
The minds of men conceived the tools now used in the transonic
and supersonic research program and are constantly examining
and evaluating those tools and inventing new ones. This critical
examination of methods and procedures is slowly widening frontiers
for the research workers.
 |
| Model with
63-degree swept wing mounted in the 40 foot x 80 foot
tunnel at Ames Laboratory. |
One recent example
of the value of making test methods, themselves objects of
research, concerned a basic tool, the wind tunnel. In studies
of speeds approaching and exceeding the speed of sound, it
was discovered that the wind tunnel suffered from limitations
produced by the phenomenon known as “choking”
or “choke-out.”
This choking effect
manifests itself by appearance of a shock wave in the tunnel
test section, which effectively blocks a further increase
in the tunnel speed. Supports or mounts for the wind tunnel
models also have caused trouble, creating flow deviations
and distortions. For a time, the “choke” range
extended from Mach number 0.8 to Mach number 1.3, the transonic
zone. (Mach number 1 is the speed of sound, which varies according
to temperature of the air). Supersonic wind tunnels were not
then effective be low Mach number 1.3. The wind tunnel, a
primary tool, thus had a blind spot in the very region researchers
were most concerned about.
Experts at the
Langley and Ames laboratories of the National Advisory Committee
for Aeronautics, where most of the basic and applied research
in transonic-supersonic studies has been carried on, realized
that the choked-out range could be reduced by cutting the
model size. At the same time, they knew that reduction in
model size alone was not sufficient. Conventional types of
wind tunnel supports created choking or constriction of flow.
New sting-type supports have been designed and their use in
the Langley 8-foot tunnel has made possible successful tests
of models up to Mach number .95. Wings with sharp sweep-back
have been tested to .97. The work was carried further by the
introduction of a supersonic throat for the 8-foot tunnel,
permitting tests at 1.2 on the Mach number scale.
 |
| Swept-wing
model in supersonic tunnel. Photographs of airflow patterns
are made through glass plates by optical cameras. |
Two recent improvements
in tunnel theory and technique have further importantly narrowed
the speed range wherein reliable information cannot be obtained.
One is a new tunnel at Langley, which differs greatly from
conventional tunnels. In it, the model is mounted on the rim
of a disk about five feet in diameter, and great speed is
obtained by whirling this disk carrying the model. Extraneous
interference flow is eliminated by a large fairing flush with
the rim of the disk and enclosing all drives and instrumentation.
Downstream of the model, a single-stage fan draws air through
the ringed space at relatively low speeds, moving the wake
of the model down stream and out of the model path, and also
providing a means of changing the angle of attack of the model.
Complete pressure-distribution measurements are made and the
results checked closely with information obtained by other
research techniques.
Other research
information has been obtained in the Mach number 1 range through
use of a “half-open” tunnel, obtained by removal
of the floor and ceiling of a conventional tunnel. Above Mach
number 1, it is felt that this “half-open” tunnel
technique may contain discrepancies, but it is also clear
that at least over the subsonic range to Mach number 1, this
type of tunnel will permit a very significant reduction of
the choked range. The blind spot in tunnels did not, however,
prevent NACA engineers from ferreting out reliable information
through Mach number 1. It was obvious from the beginning that
no one tool could accomplish all purposes, even in the most
skilled hands. Knowing the limitations of tunnels, researchers
continued to improve them but also sought other tools and
put them to work.
One such tool is
the falling body........(continued next week)
 |
| Triangular,
or delta, high-speed wing mounted in tunnel. Tufts on
wing show direction and kind of airflow at various angles. |
 |
| View of
4x4-foot supersonic tunnel. Photos are taken of pressure
readings on the model under various speed conditions. |
 |
| John Stack,
noted expert in the field of supersonic research, examines
tunnel model of faster-than-sound plane at Langley. |
 |
| An aerodynamics
engineer setting up model in 9-inch tunnel. It is capable
of speeds up to 2.4 times the speed of sound. |
 |
| Schlieren
photograph of NACA research airplane model being tested
at Mach number 1.90 in Langley’s supersonic tunnel. |
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