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THE HIGH SPEED
FRONTIER
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- Chapter 4
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- The High-Speed Propeller
Program
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- [119] The extensive
propeller testing at low airspeeds and high rotational speeds in
the Propeller Research Tunnel consistently showed a marked loss in
efficiency starting at tip Mach numbers of about 0.9. Clearly,
this was only a part of the compressibility problem of propellers
for 500-mph aircraft, for which high Mach numbers would exist over
the entire blade. The fact that the PRT tests showed a
considerable delay in the
onset of compressibility effects as
compared with wind-tunnel section data suggested that estimates of
high-speed propeller performance based on strip theory and section
data as understood at that time could not be relied upon. Tests of
propellers at high forward speeds were needed to provide precise
information on the attainable performance.
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- Unlike many other NACA programs which
started with little understanding of the problem, the high-speed
propeller program enjoyed a well-established basic understanding;
a substantial body of high-speed section data and criteria for
design of efficient advanced propellers had been built up over the
previous 20 years. In these circumstances, it was obviously
unnecessary to explore the problem by testing existing propellers
which would clearly prove to be inefficient at high speeds.
Instead, a family of advanced high-speed propellers embodying the
features known to be needed to favor high-speed performance was
defined at the outset. The main purpose of the test program was to
determine accurately the attainable high-speed performance as
affected by systematic changes in the principal design
variables.
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- The useful but rather uninspiring nature
of this test program, together with the tedious aspects of
high-speed dynamometer development, made it unattractive to
impatient imaginative researchers seeking higher levels
[120]
of challenge and excitement. But, like many other instances which
come to mind, the propeller program seemed to attract the type of
talent appropriate to the job-competent, practical, conservative
engineers who were willing to devote many years to the exacting
tasks of perfecting the large dynamometers and obtaining precision
data under difficult test conditions.
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- The first step taken by NACA toward
higher-speed testing was the approval in mid-1936 of plans for the
19-foot, 250-mph Pressure Tunnel, in essence a super-PRT. A new
propeller dynamometer powered by large electric motors was a major
feature of the plan. By the time this new tunnel was dedicated in
May 1939, however, the continued increase in speed of military
aircraft plus the growing war threat made it apparent that 250 mph
was inadequate for high-speed propeller testing, and a second new
Langley tunnel was undertaken-the 500-mph 16-foot High-Speed
Tunnel. This facility was intended to concentrate on full-scale
propellers and engine cowling and cooling, while the 19-foot
tunnel would become involved primarily with scale-model aircraft
testing and dynamic-loads research. Accordingly, the new propeller
dynamometer project was transferred to the 16-foot tunnel
enterprise.
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- Shortly after Stack had taken up his
duties as head of the 8-foot tunnel section in 1939, the outlook
for high-speed propeller testing in the 16-foot tunnel was
discouraging. Major delays had been encountered in design and
procurement of the new electrical equipment for the dynamometer,
and it appeared that three or four years, at least, would elapse
before testing could be expected. Stack reacted with
characteristic impatience. He was quite unhappy at the prospect of
a long delay in testing propellers incorporating the new 16-series
blade sections. The only apparent solution was to procure a
dynamometer for the 8-foot tunnel and run the tests on 4-foot
diameter propellers. This would have the advantage of smaller
(200-hp) electrical equipment, some of which was already
available, and we projected that the desired answers should be
forthcoming within about two years. Stack had little difficulty in
selling this plan, and it was called the "Emergency Propeller
Program" to answer any question of duplication with the 16-foot
tunnel plans.
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- After the repowering of the 8-foot tunnel
in 1945, propeller testing was extended to higher speeds (Mach
0.93) with an 800-hp dynamometer, [121] and this program
continued until conversion of the repowered tunnel to a slotted
throat in 1950.
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- The 16-foot tunnel program utilizing
nominally full-scale (10-foot-diameter) propellers got underway in
1945 with the 2000-hp dynamometer that had been so long in
procurement and development. As in the case of 8-foot, this was
later replaced by an improved 6000-hp dynamometer when the 16-foot
was repowered in 1950. Speeds up to Mach 1.04 were achieved in
full-scale propeller testing in the 16-foot slotted throat.
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- The Ames Laboratory embarked on a limited
program of propeller testing
in their 12-foot high-speed tunnel
in the early fifties when it
appeared that the turboprop
application required research at high subsonic speeds. Their
propeller dynamometer used 4-foot-diameter blades and 1000-hp
motors taken from the Langley program, and it incorporated several
other features from the Langley installations. Forward speeds up
to Mach 0.84 were covered in the one series of high-speed tests
made at Ames (reported in NACA TR 1336).
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- Throughout the period of the high-speed
wind-tunnel propeller programs (1938-1958), periodic propeller
testing was also done in flight on advanced fighter aircraft.
Starting with such piston-engine aircraft as the XP-42 and P-47,
the flight work ended in the mid-fifties with testing of three
propellers at speeds up to about Mach 1, using a special turboprop
engine installation in the nose of an XF-88B jet fighter.
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