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THE HIGH SPEED
FRONTIER
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- Chapter 5
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- High-speed Cowlings, Air Inlets
and Outlets, and Internal-Flow Systems
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- [139] These high-speed
programs were superimposed on the low-speed foundations built up
in the 1926-1936 decade. An understanding of the earlier work is
important in reading this section and, therefore, the pertinent
background will be reviewed briefly.
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- The exposed radial engine had been in use
only a short time before designers began to be concerned about its
drag and cooling problems. The most rudimentary knowledge of
aerodynamics suggested some kind of rounded fairing to cover the
engine. The first "cowling" of this kind was designed in 1922 by
Col. V. E. Clark of "Clark Y" airfoil fame, for the Dayton-Wright
XPS-1 airplane which was powered by the first Lawrance radial
engine. Cooling problems were encountered and the lack of
understanding of how the cowl worked and how much drag it saved
discouraged others from using it (ref. 43). A successful foreign application of a cowling
which completely covered a 50-hp air-cooled engine was made by one
Piero Magni in 1926 (ref. 164). This well-shaped cowling employed a
"blower-spinner," which provided satisfactory cooling. The design
must have had very low drag but no data were given. The
"blower-spinner" was rather obviously too great a complication to
be considered for the large radial engines of the twenties.
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- By 1927 the drag problem of radial engines
was widely perceived' as very serious. Several requests were made
by attendees at the 1927 Engineering Conference to make a
full-scale engine/cowling investigation the first work to be
undertaken in the new PRT, which was then nearing completion. NACA
fully concurred, having already in hand a request from the
military for cowling work (ref. 165). Fred E. Weick, who had been hand-picked by G. W.
Lewis to design and manage the PRT, laid [140] out a
tentative test program and diplomatically submitted it to industry
for comment and suggestions. As finally agreed upon, the program
reflected the great concern of the time that cowlings might
seriously inhibit engine cooling. Five of the seven shapes to be
tested on a representative cabin fuselage with a Wright J-5 engine
did not completely cover the cylinders; only one complete exterior
cowling was designed, and it was tested with two inner-body shapes
(ref.
166).
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- Perhaps the most important single test was
the first run with the fully exposed J-5 engine. The simple scales
of the PRT registered 85 pounds increase in drag due to the engine
at 100 mph. For typical J-5 powered airplanes of that day this
meant that up to as much as 30 percent of the engine power and
fuel was being expended simply to provide cooling. There was now
the strongest possible motivation to find an effective
cowling.
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- Only the complete exterior cowling
produced a really dramatic reduction in drag. Weick's test
procedure was to make cut-and-try changes until the engine
temperatures approached those of the fully exposed engine. Only
two important changes were made in the original complete cowl-the
exit area was increased severalfold by cutting 3 inches from the
skirt and the inlet diameter was increased from 24 to 28 inches.
Although the cooling was finally judged adequate, the barrel
temperatures were still some 600 F hotter than for
the exposed engine. After these changes to favor cooling had been
made the drag was measured carefully and found to be 60 percent
lower than that of the uncowled installation (ref. 166).
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- In retrospect there was nothing very
remarkable about the cowling itself-an arbitrary, rounded external
fairing tailored by straightforward cut-and-try changes to favor
engine cooling. The underlying achievement was NACA's creation of
the PRT, which made it possible for the first time to work with a
full-scale engine/ propeller/ cowling in a wind tunnel and measure
precisely the drag and cooling data. NACA was not the first to lay
out the cowling and did not "discover" it in the usual sense of
that word. They did discover its enormous drag-saving potential.
The basic internal flow processes and the cooling mechanisms of
the radial engine still remained obscure at the conclusion of the
J-5 installation testing in the PRT, in spite of the fact that
tolerable cooling had been achieved [141] by crude methods
involving what was later recognized as a large excess of internal
airflow and internal drag. Some of the first industrial
applications of the cowling were less successful in solving the
cooling problem, and employed very large exit openings to try to
encourage cooling flow. In several cases the external drag
advantage was apparently nullified by the huge internal drag of
the large cooling flow. Lack of understanding of these internal
drag effects caused much puzzlement. Rex B. Beisel of the Vought
Company described the situation in his classic 1935 paper
(ref.
167). "The first five years of
industrial experience with the NACA cowling has brought forth a
maze of contradictory data.....leading to confusion and
suspicion."
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- Added to the real problems was at least
one largely illusory one, the belief that the
cowling would seriously
impair the pilot's vision. The
surviving pilot of an Army midair collision had claimed that the
cowling had blocked his view. A consequence was the Anny's
adoption of the Townend Ring
for its P-12 and P-26 airplanes
(ref.
41), and this in turn led to NACA's
decision to flight-test several truncated cowlings on a Curtiss
XF7C-1 airplane. It was clear in NACA's tests that the visibility
issue had been overplayed and would not exist at all in many cases
(ref.
168). Aside from this useful result
these flight tests illustrated the primitive state of knowledge in
1929: the cowls were tested and compared with different exit areas
and thus different internal drags. Furthermore, shutters were used
in front of the engine to control part of the cooling flow,
subsequently known to be one of the least efficient flow-control
techniques.
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- The lack of a solid framework of
understanding is evident also in other programs of the early
thirties (ref. 169). The basic cooling problem was visualized as how
to divert or deflect a part of the cooling-air flow toward hot
parts of the engine. This was consistent, of course, with the
excessive cooling flows that characterized the early
installations. In this concept a great deal of effort was expended
on "deflectors" of the type tried by Weick. The curved "shell"
baffle emerged from this work; however, it was conceptually more a
refined deflector than the ultimate tight-fitting baffle which
actually contacted the fins and formed an outer wall for the
finned heat transfer channel. As late as 1935 the loose-fitting
baffles were still being tested by NACA (ref. 170) in spite [142] of the fact that
by that time the work of Vought and Pratt and
Whitney had evolved the "pressure-baffle" concept. A NACA
investigation of a tight-baffled cylinder was finally carried out
in 1936 (ref. 171). Useful NACA work relating to fin design and fin
spacing was also carried out in the thirties.
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- The cylinder cooling work was one part of
a three-pronged program often described in agency literature in
the 1931-1934 period; the second part was aimed at finding the
best cowling shape, and the third was verification of the
concepts developed in parts 1 and 2 by tests of an actual cowled
engine in the PRT. An R-1340 Wasp engine was borrowed from the
manufacturer in 1932 for part 3. This program ran into
difficulties in each of the three areas and was never completed as
planned. Contributing factors were the loose, inefficient,
shell-type baffles employed in the engine which were obsolete
before testing was completed, the fact that only the climb
condition of engine operation could be simulated in the 100-mph
PRT, and the use of a very short nacelle on which the flow was
prone to separate from the afterbody so that much of the cowling
drag data were useless. In the report of the engine tests finally
issued in 1937 (ref. 172) the negligible differences in engine cooling in
climb with the four different cowlings is shown, but nothing is
concluded in regard to the relative drags of these cowlings, and
no reference is made to the initial ambitious objectives of the
program.
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- During the 1931-1934 period the Vought
group under R. B. Beisel conducted a program of wind tunnel and
flight investigations of cowling and cooling problems which
provided definitive enlightenment on the key issues. They
established the high cost in drag of the large excess of cooling
air that flowed through unbaffled and loosely-baffled engines, and
aided by Pratt and Whitney, they evolved the idea of "pressure
baffling" in which all flow through the engine is blocked except
through tight-fitting baffles in contact with the
after-quarters of the cylinders. They invented cowl flaps to vary
the size of the exit opening and so to regulate efficiently the
airflows to the minimum value required for cooling in each
condition of flight. The internal flow system of the NACA cowling
was at last understood and criteria for efficient engineering
design and operation were established (ref. 167).
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- Beisel's group had periodic contact with
the Langley finned-cylinder [143] and PRT work,
but they credit British studies, particularly those of Pye
(ref.
173), as a principal source of
their inspiration. In 1934, Langley was called on to test, in the
Full-Scale Tunnel, the first double-row engine installation with
Vought's pressure-baffle system. The first NACA references to this
system are found in the 1934 Annual Report. Vought was able to
demonstrate that a properly baffled cowled engine had lower
operating temperatures than the fully-exposed engine-an
accomplishment believed to be physically impossible in the early
years of cowling development.
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- In 1934, G. W. Stickle and M. J. Brevoort
who had been working on the R-1340 cowled-engine/nacelle tests in
the PRT were transferred to T. Theodorsen's group. In reviewing a
prospective report covering their PRT work, Theodorsen had pointed
to the blunt nacelle afterbody as the probable source of the drag
anomalies that had been observed, a perceptive speculation that
was later verified. Theodorsen's interest in the many problems of
cowling and cooling was now aroused and he supervised plans for a
comprehensive new investigation. NACA's unfortunate experiences
with the R-1340 tests together with Vought's recent successes
provided a framework for better planning. Following Vought's lead,
a wind tunnel model employing a dummy engine was used. However,
one cylinder was heated electrically so that cooling tests could
be made, and a propeller with a 150-hp electric drive was
included. Complete pressure distributions and smoke flow studies
were very important special features of the program.
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- This solid full-scale investigation
answered virtually all the remaining questions (ref. 174). The Vought findings were corroborated and
extended importantly. It was found that large-scale turbulence,
induced more or less automatically in the front of the cowling,
was the mechanism that cooled the front of the cylinders. The
internal flow was analyzed by evaluating the efficiency of the
cowling considered as a pump. Unfortunately, the unbaffled engine
showed the highest pump efficiency; however, it also had by far
the highest pumping power requirement due to its large air flow.
The tightly-baffled engine had the lowest pump efficiency but also
the lowest power absorption because of its low flow. The cooling
drag penalties were consistent with the Vought results.
Regrettably, no reference is made to Vought's work in the NACA
reports.
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- [144] By the end of
1936 it was obvious from the Vought and Langley investigations
that the radial-engine cowling had virtues that were unsuspected
in the beginning when it was thought of only as a device for
external drag reduction. With proper design the cowling would
enhance cooling rather than inhibit it as originally believed.
Through physical containment of the internal flow, the cowling
made the enclosed baffled engine in essence equivalent to a ducted
radiator as far as cooling was concerned. By application of basic
heat transfer principles the overall drag-power cost of cooling
the radial engine had now been reduced to low levels, comparable
to those of the liquid-cooled engine.
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- After 1937 and continuing through World
War II, there were many NACA contributions to cowling and to fin
and baffle design. The so-called "cooling correlations" were
evolved relating engine operating conditions, fin temperatures,
and cooling flow requirements. With the framework of basic
understanding now firmly established, these later contributions
were for the most part sharply focused and valuable. We will be
concerned in this chapter only with the high Mach number projects
relating to the NACA cowling. Of greater long-term significance
were the high-speed investigations of generalized inlet, outlet,
and internal flow systems, with which this chapter is chiefly
concerned.
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