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THE HIGH SPEED
FRONTIER
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- Chapter 3: Transonic Wind
Tunnel Development (1940 -1950)
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- HIGH-SPEED RESEARCH
AIRPLANES
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- [88] On a spring
morning in 1940, Stack and I left the office and drove to the
remote beach at the easternmost tip of the Virginia Peninsula to
watch the first attempt to obtain supercritical aerodynamic data
on an airplane in free flight. A Navy fighter, the Brewster
XF2A-2, was to be dived vertically over Chesapeake Bay to its
terminal velocity, about 575 mph, and then make a pullup at its
design load factor. The Brewster had been instrumented to measure
the pressure distribution at an inboard wing station by the
Langley Flight Division. We were most apprehensive as we watched
the dive through binoculars. This was before the possible
consequences of compressibility effects on the buffeting and
control of diving airplanes had been highlighted by the P-38
tragedy of 1941; nevertheless, our knowledge of shock-stalled
flows in the wind tunnel left little doubt about the dangers of
this dive. Happily, the flight was completed successfully without
any undue difficulties for the Navy pilot, but we were both left
with the strong feeling that a diving airplane operating close to
its structural limits was not an acceptable way to acquire
high-speed research information. This experience undoubtedly
contributed to Stack's later advocacy of a special research
airplane capable of supercritical speeds in level flight.
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- Tests of the NACA 230-series section used
on the Brewster were made in the 4 x 18-inch high-speed tunnel and
the results arc compared with the flight data in fig. 21. The principal differences (in shock location) were
due primarily to irregularities in the airplane wing, some of them
distortions under air loads (ref. 101). In general, we were satisfied that the wind
tunnel had been validated at least up to Mach 0.75, but we could
see that future flight testing would be much more valuable if the
surface distortions could be eliminated by use of thicker
skins.
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- By 1942, it was apparent that the diving
speeds of advanced fighters would penetrate more deeply into the
supercritical region, equalling or exceeding the choking speeds of
the wind tunnel test configurations then in use. We considered it
unlikely at that time that the wind tunnel could ever be used
at speeds beyond about Mach 0.8, and we therefore increasingly leaned
toward the idea of a specially configured and instrumented test
airplane capable of safe operation in this speed range.
The....
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FIGURE 21.-Comparisom of flight and wind tunnel pressure
distribution measurements for Cn = 0.4, airplane
wing , NACA 230-series section 141/3 percent thick, wind tunnel
model, NACA 23015 section.
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- [90] ...best
recollection of those of us who were involved is that this idea
did not appear full-blown as a visionary new proposal of any
single individual. Rather it took form gradually, manipulated and
developed in innumerable lunchroom conversations and other
contacts. Stack was a central figure in these discussions, and
became the chief Langley promoter of the idea, but he was vague in
regard to a specific origin. In a talk given in 1965 at a history
session of the AIAA devoted mostly to the X-1 research airplane
(ref.
10), he said:
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- After some deliberation, free flight with
a manned instrumented airplane seemed the best and most direct
way. Now, of all the people who contributed to this effort, it
seems to me the two most noteworthy were General Arnold of the Air
Force and Dr. Lewis of NACA. And as I noted, it was just about 23
years ago to the day (the summer of 1942] when word was given that
we ought to go on something like this, with the caution that we
couldn't spare many men because there was a war on.
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- This verbal authorization to "go" by no
means implied general approval to design and procure a research
airplane; it was simply permission for a limited preliminary study
of the problems and desirable design features. Milton Davidson and
Harold Turner, Jr., were logical choices to make preliminary
layouts and performance estimates because they had done some work
of this kind for Jacobs. Under Stack's direction, Davidson and
Turner first concentrated on designs capable of high subsonic
speeds up to about Mach 0.9. It is important to note here that
Stack, in that period, did not consider or advocate pushing
through Mach 1 to supersonic flight speeds. My firm recollection
on this point is substantiated by those of several others
including Soule (ref. 110). It is also supported by documentation (e.g.,
ref.
107) which gives Mach 0.8 to 1.0 as
the range of NACA interest for a research airplane. The possible
performance of prospective turbojets was uncertain at that time
but it appeared likely that an engine would emerge which might
marginally provide Mach 0.9 in a small airplane. The idea of
rocket propulsion was quite beyond NACA thinking at that time;
however, the Army with its background of JATO rocket development
was willing to consider it. The first Army proposal for a
high-speed research airplane by E. Kotcher in 1939 listed rocket
propulsion as an alternative, and in the Army study of the "Mach
0.999" research airplane in early 1944 a principal objective was
to compare performance of rocket and turbojet versions
(ref.
111). It was [91] obvious that the
only hope at that time for pushing through Mach 1 in level flight
lay in rocket propulsion.
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- Navy interest in a possible high-speed
research airplane also began to stir in the 1942-44 period
(ref.
112). However, no direct action
toward procurement was taken by any of the interested, parties
prior to the March 15, 1944, seminar-type meeting at Langley of
Army, Navy, and NACA personnel (ref. 113). Some significant differences of opinion relating
to the design features and goals of a transonic research airplane
surfaced at this meeting. NACA tended to think of the airplane as
a device for collecting aerodynamic data unobtainable in the wind
tunnel at high subsonic speeds. But the Army thought of it more as
a major developmental step toward higher operating speeds
extending upward through Mach 1. The Navy view inclined toward
dispelling the myth of an impenetrable barrier and providing
needed high-speed data. These differences could rather easily be
accommodated in a single vehicle concept except for the Army's
interest in demonstrating transonic and low-supersonic speeds
which led to their advocacy of a rocket engine for propulsion, a
feature which NACA considered too risky. Except for propulsion,
the configurations of the airplanes being studied by NACA and the
Army were similar; both agencies considered very simple
conventional unswept designs.
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- Undoubtedly, one of the main values of
this meeting was a stirring-up of the competitive natures of the
participants to the point where actual procurement activities
would soon be initiated. Further discussions took place with the
Army on May 15 and 16, 1944, and on July 18, 1944, the final NACA
turbojet-powered design produced by the small Langley group was
transmitted to the Army personnel who by now had declared
themselves expressly interested in funding a research airplane
(ref.
113). A critique of the NACA design
was presented by Army personnel at a Langley meeting on December
13 and 14, 1944, centering on the inadequate performance
achievable with the turbojet. NACA emphasized the supposed safety
aspects and relatively long-duration data-gathering flights
possible with the conventional power plant. Furthermore, the
turbojet would have obvious applicability to future military
aircraft while the rocket
propulsion system did not. This
apparently unreconcilable difference was easily resolved; the Army
was putting up the money [92] and they decided
to do it their way. In late December they started negotiations
with Bell Aircraft to procure a rocket airplane.
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- When it became clear at the meetings in
early 1944 with E. Kotcher
and his cohorts that the Army was
likely to be insistent on a rocket airplane, Stack renewed his
efforts to interest the Navy in procuring the kind of airplane
NACA wanted. Almost all of his contacts were by telephone,
personal visits, or through M. Davidson who had been detailed to
the Navy. Stack's view then was that the rocket approach was so
risky that the Bell airplane would probably not survive many
flights and in any event would not get enough air time to collect
much data. The Navy, in the persons of E. Conlon, W. S. Diehl, and
I. Driggs, was receptive. Nothing had been done in the Navy in the
way of research airplane studies and they were ready to accept the
NACA general guidelines. Belatedly, in September of 1944, they
started to consider details of such a vehicle within the Bureau of
Aeronautics, developing a philosophy not inconsistent with NACA's
that the aircraft should be designed with some potential for
militarily useful follow-on versions. Douglas was some potent
selected to build the airplane in early 1945. It was designated
the D-558-1, and was almost exactly the airplane Stack desired
(ref.
110). Throughout the development
period, he displayed a strong preference for the Navy airplane and
we extended ourselves in every way to assist in its
development.
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- During Stack's absence on his first
European trip, I was sent to Wright Field on March 15, 1945, to
represent NACA at the first design review of the X-1 (then
designated XS-1). Prior to leaving, I examined recent drop-body
drag data in the vicinity of Mach 1, visited the Flight Division,
and talked to Davidson to get their views on performance,
operations, and instrumentation. According to my notes, Mel Gough,
Langley's chief test pilot, condemned the rocket airplane. "No
NACA pilot will ever be permitted to fly an airplane powered by a
damned firecracker" was his ultimatum. (Ironically, it was the
turbojet-powered D-558-1 which killed a NACA pilot due to engine
failure while the X-1's had a good safety record at Edwards. The
D-558-1 barely exceeded Mach 0.83 in level flight and was limited
to Mach numbers below 1.0 in dives. With further irony, it was the
transonic and supersonic flight achievements of [93] the
rocket-powered X-1 which brought NACA and Stack a share of
the Collier Trophy for 1948.)
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- At Wright Field, I found Bell's design to
be basically similar to the simple arrangements of the Army Mach
0.999 study and the NACA studies. In general, NACA recommendations
other than power plant and speed range had been accepted (refs.
107, 114). Almost two-thirds of the takeoff gross weight was
in rocket propellants-an unheard-of fuel fraction for military
aircraft of that day. Were it not for the fact that a major part
of the propellants were used up in takeoff and climb, the X-1 as
then defined could have reached projected speeds far in excess of
Mach 1.2, the "cruise" speed required by the Army. It was apparent
that a cruise speed of Mach 1 could certainly be reached from
ground takeoff even with more conservative drag estimates based on
the body-drop data, and I pointed out that this made the airplane
acceptable from the NACA viewpoint which suggested Mach 0.8 to 1.0
as the desired region for flight research (ref. 107).
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- Later Bell's considerations of safety and
performance with a less energetic propulsion system led in May
1945 to a major change from ground takeoff to air launch. NACA
strongly opposed air launch. Not only did it violate the NACA
notion that a research airplane should operate as conventionally
as possible, but it also meant that in all probability the
airplane could never be operated out of Langley Field. Langley
managers thus feared they would lose control of an air-launched
X-1 flight program (ref. 110). The NACA protests were of no avail because air
launch was now the only remaining option if low supersonic speeds
were to be achieved as required by the Army.
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- Concurrently with, but unrelated to, the
X-1 and D-558-1 research airplane activities of 1944 and 1945, M.
C. Ellis and C. E. Brown of Langley's 9-inch supersonic tunnel
section studied the feasibility of a small supersonic airplane
powered by a hypothetical ramjet engine at Mach 1.4. As was
appropriate in a rough preliminary assessment of this kind, their
airplane was a primitive assemblage of basic elements-straight
sharp-edged wings and tail, and simple propulsive-duct fuselage
with the pilot sitting in a small enclosure in the middle of the
duct (fig.
22). The results showed that a
ramjet of practical proportions could....
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FIGURE 22.-Ramjet-powered configuration analyzed by Ellis and
Brown, 1944-1945. From a Langley Conference chart.
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- ....indeed provide the necessary cruise
propulsion for a 60-mile range at Mach 1.4; however, other means
of acceleration through the transonic region (rockets) would be
required, and airplane tow (later, air launch) was envisioned for
takeoff (ref. 115). At Stack's instigation, there was brief local
consideration of this vehicle as a possible addition to the X-1
and D-558-1 stable of research airplanes. However, because of the
lack of any ramjet engine, the problems of acceleration, and more
particularly the fact that transonic flight testing of the simpler
X-1 and D-558-1 was still several years away, it was quite obvious
that such a vehicle [95] could not
logically be undertaken at that time, and Stack wisely did not
attempt to mount a real crusade for it.
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- COMMENTARY
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- It is unlikely that the many innovations
and rapid progress in the transonic ground facilities would have
happened as they did without the stimulus and focus provided by
the X-1 and D-558. Clearly there was a most important two-way flow
of benefits: stimulated by the problems of the research airplanes,
new ground facilities and techniques were developed which, in
turn, produced vitally needed new data in time for the design and
safe operation of the aircraft.
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- The primary postulate justifying the
transonic research airplanes was the supposed impossibility of
useful wind tunnel operations in the speed range above Mach 0.8.
And yet before the research airplanes were operated transonically
the fallacy of this justification had been demonstrated; the wind
tunnel choking problem had been circumvented in a variety of ways.
Thus the early concept of research aircraft providing unique new
data otherwise unobtainable became obsolete. Instead, a principal
value of the transonic flights lay in evaluation and validation of
the ground-based techniques. The fact that the first transonic
flights showed no unexpected occurrences was also of great value.
The most basic value, however, was the liberation of researchers
and aircraft designers from their fears and inhibitions relative
to the "sonic barrier." The awesome transonic zone had been
reduced to ordinary proportions, and aeronautical engineers could
now proceed with the design of supersonic aircraft with
confidence.
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- During the course of this review of the
first research airplanes, I turned up a number of apparent
misconceptions and inaccuracies in the records which are worth
noting. One should expect, of course, that the offhand and
undocumented remarks recorded in interviews of NACA old-timers
will contain inadvertent inaccuracies, distortions, and
oversimplifications. I am concerned here with larger issues, in
which questionable NACA party-line versions of what happened seem
to have gained general acceptance, establishing a sort of agency
mythology or folklore.
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- Myth:
That NACA deliberately planned for two complementary
[96]
vehicles, one (the X-1) to be a unique special design for pushing
through Mach 1, and the other (the D-558-1) to be representative
of advanced military service types with turbojet propulsion for
studying flight problems in the Mach range up to about 0.95. This
view is specifically stated in certain of the interviews conducted
by Bonney in the early seventies, and both Keller and Hallion
gained the same impression from their interviews (refs.
112, 116). Actually, as previously documented, NACA had
argued strongly against a rocket vehicle like the X-1, and even
after it was in procurement NACA stated that the subsonic speed
range from 0.8 to 1.0 was the area they desired to explore (refs.
107, 114). The D-558-1 was the research airplane NACA wanted
(ref.
110).
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- If the D-558-1 could have been promoted in
the early forties, it would have been timely. But coming into the
flight picture as it did in 1947, it was unnecessary. Contemporary
service airplanes with equal or better performance became
operational in the same period and they could have been
instrumented and used for most of the work conducted by the
D-5581. For example, the F-86 Sabre began to exceed Mach 1
regularly in dives in the summer of 1948, some time before the
D-558-1 inadvertently slightly exceeded Mach 1 for the only time
on September 29, 1948. The world's speed record of 650 mph briefly
held by the D-558-1 also fell to the F-86 on September 15, 1948,
when 671 mph was recorded.
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- Nevertheless, it was the D-558-l's and not
the advanced service aircraft that were used for extensive flight
research at high subsonic speeds by NACA, complementing coverage
of the higher transonic speeds by the X-l's. It is quite
understandable how some NACA managers by hindsight can see a logic
in the way these two vehicles were used that did not really exist
when they were promoted in 1944 and 1945.
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- NACA always chose to emphasize the
positive factors of the program as it finally evolved, passing
over early controversies. An example is seen in Stack's 1951 paper
(ref.
54) in which he said, "The research
airplane program has been a cooperative venture from the start....
The extent of the cooperation is best illustrated by the facts
that the X-1, sponsored by the Air Force, is powered with a
Navy-sponsored rocket engine, and the D-558-1, sponsored by the
Navy, is powered with an Air Force-sponsored turbojet
engine."
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- [97] Myth: That a lack of
knowledge or misunderstanding of the effects of wing thickness
ratio on transonic performance led to the major differences of
opinion in NACA as to what thickness ratio should be used on the
research airplanes. Actually, from the earliest works of Dryden
and the NACA high-speed airfoil group, a major conclusion was that
the severity of shock-stall effects could be minimized by using
thin sections. Ferri's airfoil work (ref. 45), extending to Mach 0.94, edited in report form in
January 1945 and published in June 1945, listed as a primary
conclusion: "Airfoils of large thickness ratio should not be used
at high Mach numbers because of radical adverse changes in their
characteristics at supercritical speeds. "Gilruth's secret
wing-flow data of 1945 extended the test speed range beyond Mach
1and it served to underscore the existing understanding of the
problem. It did not provide pivotal new revelations of the
advantages of thin wings as has been implied (ref. 112).
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- The real argument was over whether the
research airplanes should be designed deliberately to encounter
severe shock stalls well below Mach 1 for correlation with the
wind tunnel data. Stack argued vociferously for a 12-percent-thick
wing (an "average" rather than a "thick" wing according
to 1945 practice) which would start to encounter flow
changes at Mach numbers of about 0.75. This was one of the first
major crusades into which he put the full force of his unusual
talents. The main thrust of his argument was that there would be
far less risk with this over-strength airplane with a 12-percent
wing in level flight than Army test pilots had accepted repeatedly
in pullups from high-speed dives. Gilruth, however, took the more
conservative view that the first aircraft to penetrate deeply into
the supercritical zone should have every known feature which would
contribute to a safe operation-and a thin wing was indisputably
one of the most important features for minimizing supercritical
buffeting, lift loss, and control problems. Thompson sided with
Gilruth. The first X-1 was flown with an 8-percent-thick wing of
very low camber. However, the pressure distribution measurements,
which were of prime importance for comparison with the wind
tunnels, were made on a 10-percent-thick wing-not much thinner
than Stack had wanted.
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- At it turned out, the most important
region for comparison of flight and tunnels was from Mach 0.9 to
1.1, and the thinner wings served as well as a thicker one would
have. The region of deep shock stall, Mach [98] 0.75 to 0.9,
which Stack advocated, proved relatively unimportant from the
correlation standpoint. Twenty years later, accepting the
teachings of in history, Stack acknowledged the correctness of the
thin-wing decision in remarks made at the AIAA history meeting of
1965 (ref.
10) where he said, "We knew it
should have a thin wing."
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- Myth:
That NACA made a substantial effort to promote a supersonic
ramjet-powered research airplane in 1945. The unusual emphasis
with which Stack recalled the exploratory study of Ellis and Brown
in his 1965 history talk and interviews with Hallion and others
(refs. 10, 112) seems to have created an exaggerated historical
view of the importance of this concept in the research airplane
picture of 1945. There was no ramjet engine then in existence to
power such a vehicle; the X-1 and D-558 were still in the early
stages of procurement; rather obviously, any proposal for such a
vehicle was premature and had virtually no chance of support.
Neither Ellis nor I have any record of the proposal. H. A. Soule
believes he recalls a Stack memorandum which was either lost or
withdrawn (ref. 110). In any case Stack's effort was brief and in no
way comparable to his vigorous and long-standing promotions of the
transonic airplanes. There is no doubt that Stack had a strong
personal interest in supersonic flight in 1945-in addition to his
better-known interest in flight research at high subsonic speeds.
Perhaps this is the point he wished to make in his talks with the
historians.
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