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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.
(continued
from last week) ......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. It is filled with instruments, carried to
great height in an airplane and dropped. Falling bodies have
been available for studies through Mach number 1 range, but
it was not possible to use them effectively until the science
of instrumentation was perfected. Radar, radio telemetering,
rapid response accelerometers, pressure pickups and strain
gages are examples of such instruments. Information from falling
bodies is obtained by optical and radar tracking, and by telemetering
from instruments in the body.
Development of
gravity-propelled or free-falling bodies began at NACA in
1944 after discussions with the British. Little was known
about drag in the transonic range when the experiments were
launched but it was soon learned that velocities up-to, through
and beyond the speed of sound could be obtained consistently.
Most of the experiments have been made with models dropped
from heights of 35,000 to 40,000 feet. The body weighs about
1,000 pounds, has a cast iron shell, plus wing and tail surfaces
of solid duralumin. A principal use of the body-dropping tests
has been the determination of drag, at zero lift, of bodies
and wing-body combinations.
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| Use
of rocket-powered research models is one of several methods
pioneered by NACA to obtain high-speed control data. |
A third tool is
the rocket. In this approach to the problem, models are fired
from the ground and driven through the transonic range by
rocket power in the model. Telemetering and radar tracking
previously developed, were applied to the models. Solid-fuel
rockets were used. Rockets permitted NACA technicians to study
not only all configurations that must fly in or through the
transonic range, but also control effectiveness for automatic
stabilization. Rocket models are usually powered to achieve
a Mach number of 1.4 without a booster. With the booster,
the Mach number usually attained is 1.8. One advantage of
the rocket as a research tool, is that for systematic drag
studies of wings of varied form and section, the required
information can be obtained without any instrumentation in
the model, The basic drag information required is velocity
versus time and is supplied by Doppler radar. Another advantage
of rockets is that the useful portion of the flight path lies
between 2,000 and 15,000 feet altitude, where the air is dense,
and thus the scale effect, as measured in Reynolds numbers,
is high.
Still another method
of assembling in formation continuously through the transonic
range is the wing-flow or wind tunnel “bump” method.
A small wing is supported on top of an airplane wing, the
span of the model to be tested being perpendicular to the
surface of the plane’s wing. With such a set-up, the
airplane carrying the model is dived at altitude so that the
critical speed of the airplane wing is exceeded. The flow
over the wing surface during the dive increases through the
subsonic range, becoming supersonic. Using a suitably shaped
bump attached to the tunnel wall, this method has been applied
to high-speed wind tunnels with Mach numbers of about .9.
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| Semi-span
model on the wing of a flight research plane. Transonic
and supersonic speeds are reached in dives, although the
test plane may be traveling at only subsonic speed. |
The most spectacular tool in the attack on the transonic-supersonic
range is the research airplane. The research airplane program
was begun early in 1944, as part of the master plan of aerodynamic
studies in the transonic range where wind tunnels were not
giving accurate information. The research airplane program
has been a cooperative effort among the Air Force, the Navy,
several aircraft manufacturers and the NACA.
John Stack of NACA,
a vigorous advocate of the use of piloted aircraft, supervised
studies of possible configurations and dimensions of supersonic
planes and it was decided early in the program that the craft
would be as small as practical and carry no load other than
pilot, fuel and research instruments. First airplanes were
the Bell X-1, powered by a rocket motor, and the Douglas D-558-l,
which carried a turbo-jet engine. NACA conducted wind tunnel
tests, installed flight instruments and shares the job of
conducting a systematic flight-test program with the armed
services.
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| Bell’s
X-1 rocket-powered Air Force research plane was the first
to exceed the speed of sound in level flight (1947). |
The Navy’s
D-558-1 held the official world’s speed record for a
time in 1947, and the Air Force’s X-1 exceeded the speed
of sound in level flight at high altitude in October, 1947.
Both airplanes utilize conventional subsonic wings with out
sweepback. They were built to verify results obtained by other
test methods and to study further the behavior of conventional
planes in the transonic region.
Every flight of
these planes has yielded valuable research data and much of
it is being distributed within the industry through classified
reports and conferences. One important by-product of channeling
information to key people in the aircraft industry, has been
the dispelling of fear of unknown phenomena in the transonic
speed range.
To date, the success
of the laboratory-flight research program has served to reduce
the lag in application of lab results to full-scale airplanes,
such as the X-1. That success also has resulted in expansion
of the research airplane program to include the X-2, X-3,
X-4 and D-558-2 planes, all of which have swept- back wings.
The significant
results of the whole program are not speed records which make
headlines, but the quantitative data being built up on drag,
stability, trim changes, air loads and many other factors
at all speeds from subsonic through supersonic. Each research
tool has advantages and disadvantages, but results obtained
from each clearly defines the range of problems still to be
solved. The complementary nature of the laboratory and the
free-air or flight methods indicates that rapid progress can
be expected in the solution of most of the existing aerodynamic
problems in transonic-supersonic research.
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| Two-stage
interceptor missile (XM-570) takes off at Wallops Island
test station. |
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