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THE HIGH SPEED
FRONTIER
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- Chapter 5: High-speed Cowlings,
Air Inlets and Outlets, and Internal-Flow Systems
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- THE RAMJET INVESTIGATION
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- [161] In 1936, F.
W. Meredith pointed out that the waste heat of a piston engine
which is transferred to the cooling-air flow in a radiator is not
all lost; it produces a small thrust provided the pressure at the
exhaust of the radiator tubes is higher than the free static
pressure of flight (ref. 192). This phenomenon became known as the "Meredith
effect." Its mechanism was something of a mystery to many
engineers of that period. A common fallacious notion was that the
radial engine, because its fins were hotter than usual radiator
temperatures of liquid-cooled engines, would enjoy greater
benefits. (This mistaken notion still existed as late as 1949 and
is stated by Schlaifer to constitute an "inherent advantage of the
radial engine" (ref. 41).) The Meredith effect was so small at 1936
airspeeds that it could conveniently be neglected in performance
estimates both by those who did not understand it and by those who
doubted that such an effect really existed.
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- In our engineering analysis of the effects
of heat in internal flow systems, the conversion of heat to thrust
power was clearly the most [162] intriguing
aspect. Thinking in terms of flight speeds of 550 mph, we
calculated ideal thermal efficiencies of as much as 10 percent,
and by Mach 1.5 the heated duct would have a thermal efficiency
comparable to an internal combustion engine. Clearly, the
insignificant "Meredith effect" had the potential to become a
primary jet-propulsion system. (The term "ramjet" was not then in
general use, and we were unaware that there were several discussions
of propulsive ducts in the literature starting with Lorin in 1913
and including later treatments by Carter, V Leduc, Roy, and
others.)
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- Excited at these prospects, I arranged a
meeting with Langley's leading propulsion analyst at our Power
Plant Division, Ben Pinkel. I also talked briefly with D. T.
Williams, a young physicist whom Pinkel had recently assigned to
analyze propulsive ducts at high subsonic speeds, including the
effect of an engine-driver blower typical of the Campini system
under study by Jacobs. Neither man showed any real hope for
these systems, and Pinkel, reflecting the general attitude of most
of the propulsion community at that time, patiently explained "the
great weakness of all forms of jet propulsion-excessive fuel
consumption compared to piston engines". When Williams' work was
published about a year later (ref. 193), its primary conclusion emphasized the same point,
showing on overall propulsive efficiency at Mach 0.8 on the order
of one-sixth that of a piston-engine driving a propeller. Both men
felt that tests of a propulsive duct in the 8-foot high-speed
tunnel would be of little value. The duct and heater losses would,
they speculated, largely nullify any possibility of net thrust at
Mach 0.75.
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- In fairness to Pinkel and Williams it
should be recalled that in 1940 the aircraft industry generally
saw no possibility for supersonic aircraft. Mach 0.8 was regarded
as a rather optimistic upper limit for the future. The potential
of the turbojet for large improvements over the Campini cycle was
not recognized either, and it is not mentioned in Williams'
paper.
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- In spite of my disappointing session with
Pinkel and Williams I resolved to proceed with the propulsive duct
test. At the very least it would establish the Meredith effect as
a major design factor at high speeds. Our 8-foot, high-speed
tunnel afforded a unique tool for such an experiment. Stack
solidly supported the idea. In promoting the project....
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- [163] FIGURE 42.-
"Heat model" used in the first NACA investigation of a
propulsive-duct (ramjet) system in the 8-Foot High-Speed Tunnel in
February and March 1941. Model incorporated a 160-kw heater. Nose
B and cusped outlet from ref. 179.
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- ....we decided not to mention the jet
propulsion implications in order to avoid the negative reactions
of the propulsion people.
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- The nacelle model chosen for the tests
embodied our universal Nose B shape together with our most
effective cusped tail outlet (fig. 42). The all-metal nacelle was supported on a new thin
metal wing selected to avoid the local area of flow separation
that existed in the wing/body juncture of my inlet-outlet model.
(In reviewing my original work at the request of Mr. Miller, A. M.
Kuethe, who was employed briefly by NACA during the war, had
endorsed my findings generally but had raised questions about
possible drag interactions involving the separated flow. These
would now be answered. By comparing the inlet results from the new
model with the original data, we found no measurable effect of the
separated flow.)
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- How to add heat at a high rate was our
primary design problem. Combustion of fuel in the 8-foot tunnel
was quite out of the question for many reasons. A search of the
electrical heater catalogs with help from G. T. Strailman,
Langley's principal electrical engineer, turned up no
[164] high-output heater capable of being fitted
into our 11-inch diameter duct. Baals and I therefore became
high-capacity heater designers and produced a 160-kw, three-phase,
15000 F heat exchanger with 32 square feet of surface
area in the form of 1.5-inch-wide Nichrome ribbon woven on
reinforced asbestos millboard supports. This heater produced air
temperature rises of about 3000 F at high speeds
with very small frictional losses. The rates of heat input were
larger than those due to piston-engine cooling, but still only a
small fraction of the heat of combustion of kerosene.
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- Testing of the "heat model" started in
February 1941, the first NACA wind tunnel investigation of a
propulsive duct producing thrust. At a Mach number of about 0.5,
the propulsive effect had become equal to the internal
drag, and beyond this speed substantial net thrust was
developed by the internal flow.
At the highest test speed, Mach
0.75, the heated duct developed the respectable thermal efficiency
of some 9.5 percent, close to the ideal theoretical value. As
expected, the phenomena depended on the ratio of duct pressure to
stream pressure, and was independent of heater surface
temperatures per se. In all other respects, the careful
measurements of these tests confirmed the calculations made by our
engineering relations for analysis of this kind of internal flow
system (ref.187).
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- COMMENTARY
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- In 1941 during the period of our
propulsive-duct investigations, Stewart Way, of Westinghouse, made
an analysis of the subsonic propulsion possibilities of "open-duct
jet propulsion," his name for what was later called the ramjet. He
also apparently conducted some tests with an electrically-heated
model at about the same time of our high-speed tests in February
and March of 1941, although the experimental work was never
published (ref. 194), and we knew nothing of Way's work until years
later. In the first version of our internal-flow-system report
which was issued in September 1942 as a confidential document
(ref.
189), the propulsive duct data were
included but there was no emphasis in the title or text that the
first NACA tests of a potentially important jet-propulsion system
had [165] been made. Our "heat-model" tests rather
definitely settled once and for all the doubts and arguments about
the Meredith effect. Whether they had any impact on ramjet
development is questionable. The revelation of the British and
German turbojets shortly after our paper was issued had such an
enormous impact that all the scattered U.S. activities in jet
propulsion were in effect rendered insignificant. Almost overnight
the propulsion community reversed its attitudes. By war's end, the
ramjet was under vigorous development for missile applications.
Both the Langley and the Lewis Laboratories of NACA had organized
ramjet projects, concentrating on the prime problems of combustion
and burner design which we had not been able to deal with in our
1941 project.
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- NACA was now being severely criticized for
its prior general neglect of jet propulsion and it was clearly
desirable to highlight whatever had been done. Accordingly, our
report was reorganized to emphasize the tests of the ramjet
system, and the words "Ram-jet System" were added to the title.
The revised version is included in the 28th Annual Report of the
NACA, dated 1943 but actually issued after the war.
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