-
THE HIGH SPEED
FRONTIER
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- Chapter 2: The High-Speed
Airfoil Program
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- INCREASING THE CRITICAL SPEED
(1936-1944)
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- [21] On the morning of
August 31, 1936, I boarded a street car in Hampton, Virginia, and
traveled to Langley Field to report for duty as a junior
Aeronautical Engineer at $2000 per annum. After the usual short
indoctrination in his office, Mr. Miller escorted me to the 8-foot
high-speed tunnel and introduced me to Russel G. Robinson who
would be my boss. Robinson had been project engineer for this new
facility since its conception in 1933 at about the time the
24-inch tunnel was started. Following the usual practice of that
period, he had more or less automatically become head of the small
group of researchers who would now operate the facility. The basic
idea for this large tunnel is believed [22] to have been
first suggested by Jacobs. It was to be a "full-speed" companion
to the "full-scale" tunnel, using the same drive power (8000 hp)
to produce 500 mph-plus in an 8-foot throat as the full-scale
tunnel used for its 100 mph-plus speed in a 30- x 60-foot throat.
The name was later changed to the less vague "500-mph tunnel," and
finally to the "8-foot high-speed tunnel." The very large power
input in this closed-circuit tunnel had introduced an
unprecedented heating problem which Robinson had solved by an
ingenious air exchanger in which part of the hot air was
continuously and efficiently replaced with cool outside air
without the need for any auxiliary pumping or air cooling
equipment.
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- We spent the rest of the morning examining
the new tunnel and then walked down to the lunchroom on the second
floor of the administration building. The entire professional
staff and some of the support people, except for a few
"brown-baggers," assembled here everyday for a simple but
excellent plate lunch costing 25 or 30 cents (35 cents on steak
days). Walter Reiser, in charge of "Maintenance," and also head of
the employee's organization which operated the lunchroom, the
Langley Exchange, personally marked down everyone's charges as
they passed through the line and once each month collected
payment. The lunch tables had white marble tops, a feature which was a
great boon to technical discussions. One could draw curves,
sketches, equations, etc., directly on the table, and easily erase
it all with a hand or napkin. This great unintentional aid to
communication was lost in later years when the lunchroom was
replaced with a much larger modern cafeteria.
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- It was exciting and inspiring for a young
new arrival to sit down in the crowded lunchroom and find himself
surrounded by the well-known engineers who had authored the NACA
papers he had been studying as a student. I well remember that
first day at a table that included Starr Truscott, Ed Hartmann, and Abe
Silverstein. There were no formal personnel development or
training programs in those days, but I realize now that these
daily lunchroom contacts provided not only an intimate view of a
fascinating variety of live career models, but also an unsurpassed
source of stimulation, advice, ideas, and amusement. An
interesting consequence of these daily exchanges and discussions
was that often no one originator of an important new research
undertaking could be identified. The idea had gradually taken form
from many discussions and in truth it [23] was a product of
the group. At the same time there were undoubtedly instances where
perceptive individuals picked up new ideas from someone else's
off-hand remarks and went on to develop them successfully, perhaps
not remembering where the initial stimulation had come
from.
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- Frequent references to these lunchroom
contacts can be found. R. T. Jones tells of his first
indoctrinations into the mysteries of supersonic flow by Jacobs
and Arthur Kantrowitz in 1935 in "lunchroom conversations"
(ref.
29).
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- After lunch that first day, Robinson took
me on a tour of the various sections. I have a vivid memory of the
24-inch high-speed tunnel office. Stack and Lindsey were working
up some test data which Stack discussed with characteristic
intensity and impressive profanity.
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- The following morning Robinson outlined
the NACA outlook at that time for high-speed aeronautics, what was
expected of the 8-foot high-speed tunnel, and what part he wanted
me to play. He said that it had been determined that about 550 mph
was the probable upper limit of airplane speeds. Beyond this speed
the occurrence of the compressibility burble would cause the drag
to increase prohibitively "like throwing out an anchor." Our first
job with the new tunnel would be to determine in detail what the
high-speed aerodynamic characteristics for components and complete
configurations actually were. Our next goal would be to develop
improved shapes with higher critical speeds so that aircraft could
approach as closely as possible to the ultimate limiting speed,
perhaps even a bit higher than 550 mph. We would not invent
advanced aircraft but would provide designers with accurate
high-speed component data.
-
- Our work in the 8-foot tunnel was
necessarily mostly experimental because flow problems involving
shocks held little possibility of theoretical solution. In effect
the tunnel was used as a giant analog computer producing specific
solutions to the complex flow problems posed by each test model.
Many other Langley programs generated important theoretical
advances, among them airfoil and wing theory, wing flutter,
propeller noise, nose-wheel dynamics, stability, control,
spinning, compressible flows, heat transfer and cooling, and
others. Langley's principal theoreti-cians and analysts of the
thirties included T. Theodorsen, I. E. Garrick, C. Kaplan, R. T.
Jones, B. Pinkel, A. Kantrowitz, H. J. Allen, S. Katzoff, E. E.
Lundquist, and P. Kuhn.
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- [24] FIGURE 2.-Typical
airfoil in the original 8-Foot High-Speed Tunnel,weighted to
determine deflection corrections. J. V. Becker in photo,
1937.
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- At that time in 1936 "550 mph" seemed to
all of us to present enormous challenges for distant future
applications. True, 407 mph had been reached in 1931 by the last
of the Schnieder Cup racers. And more recently Stack had
calculated that an advanced racing-type airplane with increased
power, retractable gear, and skin-type radiators could reach about
525 mph in spite of some 18 percent increase in drag plus a
nominal loss in propeller efficiency due to incipient
compressibility effects. But these extreme racing vehicles were so
unlike any practically useful airplanes as seen in 1936 that they
had little impact on our outlook.
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- There was an air of pregnant expectation
about the splendid new 8-foot tunnel as I started work in
September 1936. My previous experience had been limited to the
venerable wooden tunnel at New York University which drew only 250
hp and had a speed of about 60 mph. The 8-foot tunnel, gleaming
with polished metal and fresh paint, was still undergoing
acceptance testing of its 8000-hp drive motor. The
[25]
mechanical aspects of the operation were supervised meticulously
by one Johnny Huston, a sharp-tongued veteran NACA shop mechanic
who seemed to relish catching and correcting the not-infrequent
mistakes of neophyte engineers in the mechanical operations of the
tunnel. I wondered if my talents would prove worthy of this
impressive and demanding facility.
-
- The acceptance testing had to be done late
at night when the Hampton power plant was able to provide us with
the necessary 5500 kw. Airfoil force tests and test-section flow
surveys were made concurrently with the motor tests (fig. 2). In those days the entire operation was conducted
by one engineer and one mechanic in the igloo-shaped test chamber.
(One other engineer involved in the electrical drive measurements
was present only during the acceptance tests in the drive
equipment room.) During a test, the engineer controlled the tunnel
speed, changed angle of attack, pushed the "print" button for the
scales at selected times, recorded visual data readings from the
scales, made quick slide rule calculations of the coefficients,
and plotted the results to insure that good data were being
obtained. (A recent visit to a comparable NASA tunnel during a
test run revealed a test crew of no less than two engineers and
two engineering aides plus three mechanics, for a similar type of
operation except that the preliminary coefficient plots were
produced by an automatic computer and data plotter.)
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- One night during my second week on the job
just before I closed the airlock doors at the entrance to the test
chamber for a test run, an unusual-looking stranger dressed in
hunting clothes came in and stood there watching my preparations.
Robinson had advised me not to allow visitors in the test chamber
during a high-speed run primarily because the pressure dropped
quickly to about two-thirds of an atmosphere, the equivalent of
about 12 ,000-foot altitude. Assuming that the visitor had come in
from one of the numerous duck blinds along Back River, I said
firmly, "I will have to ask you to leave now. Making no move he
said, "I am Reid," in such ponderous and authoritative tones that
I quickly realized it was Langley's Engineer-in-Charge whom I had
not yet met. No one had told me that Reid, who lived only a couple
of miles from Langley Field, often came out in the evening,
especially when tests of electrical equipment were being made (he
was an electrical engineer). [26] When I came to
know Reid better, the memory of this incident softened to proper
perspective.
-
- About a year later at 3:06 a.m. on October
8, 1937, I was running the tunnel at full power and had just
promised the operator at the Hampton
generating plant that I would
reduce power gradually when, without warning, there was a
sickening break in the steady roar of the 550-mph wind
(ref.
30). Acrid smoke filled the test
chamber as I pushed the red emergency stop button, no doubt
blowing the safety valves in Hampton. On entering the tunnel we
found the huge multi-bladed drive fan twisted and broken. The cast
aluminum alloy blades had failed in fatigue from vibrations
induced by their passage through the wakes of the support struts.
Operations were suspended until March 1938, and the staff was
temporarily dispersed to other sections.
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- Demonstration runs of the 8-foot tunnel
were made for the last of the NACA Annual Engineering Conferences,
held in May 1937. Naturally, we wanted to dramatize the
compressibility burble and to do so we mounted one of the worst
(lowest-critical-speed) NACA cowling shapes in the tunnel with a
static pressure orifice near the suction peak and a total-pressure
tube on the surface of the cowl afterbody which provided a
qualitative indication of drag. There was no way to actually see
the shock wave on the cowl, but at Robinson's suggestion, we set
up a large chart with a red light bulb directly behind a line of
small slots at the part of the cowl drawing where the shock was
located. During the demonstration the tunnel speed was advanced
rapidly to the critical speed, about 400 mph. At that point the
suction-pressure tube indicated local sonic conditions on the
chart. At a slightly higher speed the total pressure tube showed a
dramatic increase in drag and the red light was flashed on
(manually by the tunnel operator) showing the presence of the
shock. Runs were made for six groups of visitors on each of the
three conference days and we received many compliments. Orville
Wright and several other pioneers were among the visitors. I had
time for a chat with Alexander Klemin, my college mentor, who
perennially reported on these NACA affairs for Aero Digest.
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- The desire to dramatize compressibility
effects in that period reached its peak with our high-speed
testing of a model of the DC-3 configuration in 1938. Although
that stolid vehicle cruised at only about 160 mph, we
[27]
tested it up to 450 mph to show the speeds at which the various
components, designed without regard for compressibility, became
afflicted with shock wave problems. The tests showed the drag rise
for the engine cowls started to develop at speeds as low as about
350 mph. For the first time we noticed the adverse effects of
interference between components; the critical speeds of the cowls
and of the wing were reduced
about 20 mph by the presence of the
fuselage (ref. 31).
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- The predicted critical speeds of a large
number of existing airfoils and bodies were determined by Robinson
and Ray H. Wright from their low-speed pressure distributions as a
necessary prelude to the development of improved shapes
(ref.
32). Stack led the effort in the
period 1936-1940 to find airfoils with higher critical speeds,
aided by Robinson, Lindsey, and others. It was a relatively simple
matter to determine analytically from thin airfoil theory the
uniform-load camber lines which would give the lowest possible
induced local velocities for airfoils of zero thickness. There was
no way then, however, to calculate the optimum thickness
distribution, and a cut-and-try process had to be resorted to. A
considerable number of systematically varied thickness
distributions were analyzed by the Theodorsen method to obtain the
theoretical incompressible pressure distribution, until one giving
a nearly uniform distribution was found. Curiously, it was almost
identical to one of the NACA family of airfoils previously
defined, the 0009-45 (ref. 20). Combining this thickness distribution with the
uniform-load mean camber line gave what was called the "16-series"
family, the first of the high-critical-speed low-drag families
(ref.
33). Selected members of the family
were tested at high speeds and first reported in the general
literature in 1943 (ref. 34). (An extended and improved series of tests was
reported in 1948 (ref. 35), and in 1959 tests at transonic speeds up to Mach
1.25 were reported (ref. 36).
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- The 16-scries sections found immediate
acceptance by propeller
designers, not only because of
their high critical speeds but also because of their relatively
thick convex shape in the trailing edge region which was desirable
from the structural standpoint. A remarkable testimony to these
sections was heard at the-NASA Airfoil Conference of March 1978,
some 35 years afterward, when a spokesman for propeller
manufacturers said that the 16-series sections, still used in
modern propellers [28] in thickness
ratios from 2 to 10 percent, provided excellent
performance.
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- Although it was originally thought that
the 16-series sections would be desirable also for high-speed wing
applications, it rather quickly was learned that they were not
suitable. The problems included: a low maximum lift coefficient, a
narrow operating range for high Mcr, tendency for
flow separation in the trailing edge region for the thicker
sections, and laminar flow characteristics inferior to the
6-series sections which also had high critical Mach numbers. It
was also found that the uniform-load camber line used in the
16-series family, while it obviously gave the highest possible
critical speed for zero thickness, did not give the highest
possible Mcr for finite thickness. Slightly higher
Mcr could be
obtained with a camber line which concentrated the lift loading
toward the rear (ref. 37), but the small advantage is obtained at the
expense of an undesirable rearward shift in center of pressure. An
interesting later attempt to develop high-critical-speed sections
with large leading-edge radii and good maximum lift
characteristics was made by Loftin (ref. 38) with some success, but unfortunately this program
was terminated in mid-course when NACA management decided to phase
out the airfoil program in the early fifties.
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- In late 1939, we undertook an unusual
project for Howard Hughes-the only privately-funded testing ever
done in the Langley 8-foot high-speed tunnel. Hughes was
represented by his aerodynamics consultant, Col. Virginius E.
Clark, an old-timer in aeronautics and designer of the well known
"Clark Y" airfoil. Carl Babberger, a former Langley engineer, was
Hughes' Chief Aerodynamicist and he was also present for the
tests. (Clark explained the simple philosophy behind the "Clark Y"
section: it was simply the thickness distribution of a Goettingen
airfoil deployed above a flat undersurface-the flat feature being
highly desirable in the manufacture and operation of propellers as
a reference surface for applying the protractor to measure or set
blade angles. An unhappy problem in using the Clark Y was the
interdependence of camber and thickness ratio.)
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- The most remarkable aspect of this Hughes
program, however, was the fact that the test models were not
actually representations of the configuration Hughes was
designing. To preserve company secrecy, the test models had been
designed to answer questions relative to nacelle placement,
[29]
etc., without revealing the real configuration to NACA
engineers.
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- The underlying theme for much of our work
in the first few years of the 8-foot high-speed tunnel was "to
provide accurate component data for designers." Often plans for a
forthcoming test program would include sketching the anticipated
data plots in advance, so that running the test seemed more a
matter of nicely filling in the data points rather than a search
for anything new. Our Chief of Aerodynamics, Mr. Miller,
encouraged this conservative philosophy, telling the staff at one
of the monthly department meetings, "Our aim is to produce good
sound research data -nothing spectacular, just good sound data." I
can provide this quote with confidence because, even in those days
when there was little thought given to R&D philosophy, agency
goals, etc., it provoked some negative reactions among the more
lively members of the staff after the meeting.
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- Dr. Lewis had a broader outlook and a
willingness to invest occasionally in speculative new ideas such
as the thrust-augmentor work which led to the induction drive
scheme for the first high-speed tunnels. A specific instance
occurred during a 1938 visit of Lewis to our office at the 8-foot
tunnel to review recent results and forthcoming test plans. He
approved our plans but advised us to "take some
shots-in-the-dark now and then."
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- The Langley of the thirties did not think
of itself as a part of the federal bureaucracy. Broadly directed
by a committee whose distinguished members served without
compensation, and managed by a minuscule Washington office, the
Langley operation was spiritually as well as physically separated
from Washington. The youthful staff had been largely handpicked in
one way or another to form an elite group unique in the
federal system. It was possible for the entire staff of this small
organization to become personally acquainted all the way up
through Lewis, and this resulted in a beneficial sense of family.
Whatever their personal foibles, the senior managers, all of whom
held career appointments, were intensely loyal to the
organization. They could be relied on for continuing interest in
and understanding of our researches, and for continuing support
and advocacy. These important intangibles are missing in large
agencies whose top managers come and go at four-year intervals
with changing presidential politics. The costly, crippling
internal friction common in today's large agencies, in the form of
[30]
voluminous paperwork, repetitious program reviews and
justifications, lengthy procurements, unending staff meetings,
etc., were virtually nonexistent in the Langley of the thirties.
We were also blessed in those days with relatively simple research
problems which yielded to straight-forward pragmatic research
methods. But this happy situation was soon to deteriorate in the
enormous expansion and other changes wrought by World War
II.
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- Crossing the Atlantic on the dirigible
Hindenburg in the fall of 1936, Lewis visited Germany and
Russia and saw many of their aeronautical research installations.
On his return he spoke to the Langley research staff in the large
room on the second floor of the Engine Lab building used for such
convocations. His principal impressions were of major expansions,
especially in Germany. Several large new centers for aeronautical
research were under construction, and Lewis was even more
impressed with the huge new staff, many times larger than NACA and
populated by a larger proportion of advanced-degree holders. He
had little or nothing to say, however, about any new aerodynamic
or propulsion concepts or any new research results (ref. 39). He made a second similar visit to Germany in June
1939 which further impressed him with Germany's preparations for
war. But again he learned little of their advanced programs. (The
Heinkel He 178, the world's first turbojet-powered airplane, was
then being readied for its first flight which occurred on August
27, 1939.) These Lewis visits to Germany together with those of
Lindbergh provided the justifications needed for major expansions
of facilities and staff at Langley starting in 1938, and for the
establishment of two major new NACA centers at Cleveland, Ohio,
and Sunnyvale, California, well before December 7, 1941.
Significantly, however, there
was little effect of any of these
visits on the nature of our research programs or the problems
being tackled prior to the actual start of the war. We were
increasingly conscious that a war was coming, but considered all
of our existing programs apropos to the improvement of military
aircraft.
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- Although there was considerable advocacy
of "military preparedness" in the press at that time there was
little pressure on us by NACA management to do anything different
in character from what we had been doing. There was no real sense
of emergency or war peril to motivate [31] a search for
radical new weapons or bold new concepts in aircraft. Aviation had
been making rapid progress and the NACA contributions had been
substantial. Although there was a minority group of vocal
detractors, the majority opinion was clearly that the United
States led the world in technical development. NACA believed that
continued supremacy could be assured by expansion of its existing
programs through increases in manpower and conventional test
facilities. Most NACA veterans, believe that it would have been
quite impossible in the pre-war period to have obtained any major
support from the military, industry, or from Congress for research
and development aimed at such radical concepts as the turbojet,
the rocket engine, or transonic and supersonic aircraft
(ref.
40).
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- A noteworthy exception to the generally
conservative pattern was
E. N. jacobs' investigation of a
full-scale Campini system of jet propulsion in the 1939-1943
period. Initially, Jacobs was motivated more by his penchant for
new ideas than by a sense of war emergency. A great deal of effort
went into this project, but like many hybrid concepts it had major
limitations, and it fell by the wayside in 1943, yielding to the
pure turbojet. The Jacobs group harbored a misconception in this
project which was shared by the American engine companies at that
time; they believed the gas turbine (turbojet) engine would be
impractical for aircraft because of prohibitive structural weight
(refs.
41, 42).
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- Not really in the same category as the
Campini effort but worthy of special note because of its important
implications for turbojet development was the axial-flow
compressor designed and tested in 1938 by E. W. Wasielewski and E.
N. Jacobs. Intended for the piston-engine supercharger
application, this machine, designed on the basis of airfoil
theory, developed an efficiency of 87 percent at a pressure ratio
of 3.4, a convincing early demonstration of the high performance
potential of this type. This result is believed to have later
influenced American turbojet designers to favor the axial over the
centrifugal compressor (ref. 41). Interestingly, Jacobs himself was left with
serious doubts about the axial design when the blades of the test
machine were destroyed during a run in which the compressor
stalled. He believed this might be an inherent weakness preventing
practical applications (ref. 42). It is significant that both this early
misconception and the one relating to [32] excessive
turbojet weight involved structural considerations outside the
field of expertise of the Jacobs group.
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- In 1939 R. G. Robinson became an assistant
to Lewis in Washington and Stack replaced him as head of the
8-foot high-speed tunnel section. Stack's upbringing under Jacobs,
plus his natural inclinations, relaxed and enlivened the
atmosphere at 8-foot. There was a lessening of the emphasis on
data-gathering and chore-doing for industry. There was also a
pronounced increase in the level of talk, badinage, and
practical-joke playing. Although his entire background had been in
high-speed airfoils, Stack rather quickly became interested in the
other areas of our work-high-speed cowlings, internal flow,
interference effects, and aircraft configuration problems.
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- The war period at NACA has often been
described as a time when "fundamental" or "general" research was
largely forsaken and replaced by war work for specific aircraft.
This is inaccurate. Virtually an of the general programs underway
in 1939, together with their natural extensions and many new
programs as well, were completed during the war years, subject to
occasional delays due to the specific work. Much of the burden of
specific configuration testing fell upon the horde of new
employees; extended facility test periods were obtained by
multiple shifts and the 48-hour week. The involvement in general
research undoubtedly declined on a per capita basis, but in
absolute terms my belief is that it increased.
-
- The long exhausting hours which NACA
employees generally are said to have put in during the war is
another myth. Only a small minority at Langley worked more than
the 48-hour week except for infrequent stints of additional
overtime. Of course there were
some
notable exceptions, one of the more
interesting occurring in E. N. Jacobs' section. About a year
before U.S. entry into the war Jacobs unilaterally imposed a
48-hour week on his men, with no increase in pay, in order to
expedite their growing programs. He also let it be known that
leave requests were not likely to be approved unless the applicant
had put in considerably more than the 48-hour minimum.
Surprisingly, there were only a few protests. The fact that a
strong section head could get away with a high-handed move of this
kind implies both patriotic motivations in the staff and relaxed
flexibility in Langley
personnel [33] operations at
that time. Such a move would be unthinkable by any federal agency
in today's world.
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- The influx of thousands of new employees
during the war period caused irreversible changes. The selective
standards which had provided the exceptional talent of the
twenties and thirties had to be abandoned. Both the quality and
the per capita yearly output of reports declined. Not a few of the
newcomers hinted openly that immunity from the draft was the
reason they had come. The increased wind-tunnel testing of
specific military designs provided convenient undemanding
assignments for the less-talented new engineers. The term
"wind-tunnel jockey" was coined during the war and is still used
to describe inveterate tunnel operators.
-
- A distinctly pleasant aspect of the large
expansion of the 8-foot tunnel staff was the addition of a female
computing group. They not only took over most of the slide-rule
work and curve-plotting formerly done by the engineers, but also
added an interesting social dimension.
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- The staff relaxed through all of the usual
sports and social events with little apparent effect of wartime
pressures. Five of us had formed an informal golfing group
consisting of Donald Baals, Henry Fedzuik, Carl Kaplan, Stack, and
myself. Stack called it the "Greater Hampton Roads Improvement
Society and Better Golfing Association." I well recall the first
afternoon we played at the Yorktown course. Stack had never played
before and had no clubs of his own, but we offered to loan him an
old bag with a broken strap and some of our spare clubs. Fedzuik,
who was the chief humorist of the group, had often been the butt
of Stack's practical jokes and saw here a welcome chance to turn
the tables. With enthusiastic help from some of the rest of us he
lined the bottom of Stack's bag with some 10 pounds of sheet lead.
We also made sure the bag had a full complement of clubs, and we
told Stack that caddies were used only by the rich and decrepit.
By the start of the back nine, with a score card showing well over
a hundred in spite of considerable cheating, Stack was seen to
start dragging the bag along behind him, his expletives becoming
louder and more colorful, and a short time later he discovered
what had been done. Understandably, he always examined his
equipment very suspiciously at subsequent sessions.
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- In mid-1944, Stack was notified that he
had been chosen to present [34] the Wright
Brothers. Lecture of the Institute of Aeronautical Sciences for
that year, the first of many honors he was to receive. He was now
recognized not only as NACA's leading expert in high-speed
aerodynamics but also as an unusually colorful character. This
lecture (ref. 20) was in essence an updated broader version of
Jacobs' Volta paper. The compressibility burble phenomena were
illustrated and discussed in full detail, results of the
systematic efforts to obtain improved components with high
critical speeds were reviewed, and the stability and control
problems of advanced aircraft in dives through shock stall were
discussed for the first time in the open literature. Several of
us, particularly W. F. Lindsey, participated in making new flow
photographs and a schlieren movie of shock-stalled flows in the 4
x 18-inch tunnel which had been
placed in operation in 1939,
superseding the 11-inch high-speed tunnel. It had higher choking
Mach numbers and the 4-inch width made it better suited for
airfoil flow photography. The movie proved to be the highlight of
the lecture. H. L. Dryden, commenting on the talk 20 years after
his pioneering high-speed tests, said, "We did not understand
these [high-speed flow-breakdown] results at the time [1925]. The
lecturer and his associates have now given us a complete
interprepation.. . . The direct shock loss is much smaller than
the loss due to [shock-induced] separation (ref. 20)."
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- In the course of producing these pictures
a mysterious oscillating shock structure was observed in the wake
of a circular cylinder which engendered much discussion. Stack
dubbed the apparition "Yehudi" but this appellation was
edited out of the text. (Among other names he coined were
"Reichenschmutz" for a ducted propulsion scheme, and
"Rumblegutwhiz" for an unsuccessful noise-making device considered
by the Army during the war; it was to be attached to diving
airplanes in the hope of terrifying the enemy.)
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- A figure was included in the lecture
emphasizing the inadequacy of the critical Mach number as an index
of force break-either the Mach numbers of force break or the
severity of the subsequent changes. A long discussion of
"supercritical flows" was included but unfortunately this covered
only the speed range up to about 0.83, the highest speed at which
reliable results could be obtained with the 4 x 18-inch tunnel.
Interest in the entire transonic range up to low supersonic speeds
was [35] only starting to build up at this time, as a
consequence of the exciting new propulsion possibilities opened up
by the turbojet and rocket engines. Stack's Wright Brothers
Lecture brought to an end the period in which the true nature of
the shock stall had been exposed in detail and the concept of
designing for the highest possible critical speeds to avoid shocks
had been fully exploited. For the next decade or more, the
emphasis would be on developing airfoils and wings capable of
efficient performance through the entire transonic speed range.
Only the threshold phenomena had been treated so far, and what
happened beyond shock stall in the transonic zone from about Mach
0.8 to up to 1.2 was still unrevealed.
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- COMMENTARY
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- In the light of later events, NACA's 1936
vision of the "550-mph" propeller-driven piston engine airplane as
the ultimate goal of high-speed aeronautical research was
obviously too shortsighted and restrictive. Focusing the effort
totally on the immediate problem of increasing the critical Mach
number of conventional aircraft components denied consideration of
the broader and far more important "barrier" problem areas of
transonic flight, including new propulsion concepts, radical
configurations, transonic facilities, etc. A small cadre of the
more imaginative thinkers could have been separated from the main
effort to provide high-critical-speed data for industry, and
encouraged to look beyond the speed range of the existing
high-speed tunnels at these "barrier" problems. Even in 1936, it
was predictable with certainty that within a few years the
approach of improving the critical speed would reach a point of
zero return, leaving the barriers still to be reckoned
with.
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- The 550-mph airplane was achieved in the
early forties by the Germans in the form of the turbojet-propelled
Me 262, which went into service about the time of Stack's Wright
Brothers lecture in 1944. In January 1945 every airplane in a
12-plane American bomber squadron was destroyed by Me 262's. Only
the German failure to produce them in large numbers made possible
continued Allied bombing (ref. 41). The Germans were also applying variable-sweep
(with outboard pivot locations) to more advanced aircraft as the
war terminated (ref. 43). These [36] shocking
developments together with the German long-range rocket missiles
produced in NACA a "large loss of prestige. Never had NACA
relations with the industry, Congress, and the scientific
community sunk so low" (ref. 40).
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- NACA's prestige was largely recovered
during the next five years, not by the usual research services
which were continued as necessary, but by several bold new
ventures, the most noteworthy of which were the transonic research
airplanes, the impressive rocket-model testing at Wallops Island,
and the transonic wind tunnels. However, the unique esprit de
corps and effectiveness of the NACA organization of the twenties
and thirties was never fully regained.
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