N.A.C.A. Cowling
Most propeller aircraft in the 1920s had radial engines. In this configuration, the engine cylinders were mounted in a circular pattern. Most of these engines were air cooled by air flowing back through the propeller and over the cylinder heads (the tops of the cylinders sticking out from the engine). But this presented a problem, for the cylinder heads stuck out from the aircraft's skin and created drag. In the 1920s, the National Advisory Committee for Aeronautics (NACA) developed a covering, or cowling, for the engine that not only reduced drag but also improved cooling of the engine. It was a major technological breakthrough achieved through careful, systematic research and testing.
Existing air-cooled engine designs of the 1920s called for finned cylinders which projected into the external airstream for cooling. While this effectively did the job, the trade off was the introduction of induced drag on the aircraft. Interest in designing a cowling, or cover, for the engines soon became a major area of engineering concern.
One of the first designs which proved successful in dealing with the problem
was the Townend ring cowl. Designed by British engineer B. Melvill Jones, this
cowling was designed to reduce cooling drag, not for improved cooling but for increased
airspeed.
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| Townend Ring Cowl installed on a Boeing XP-15, a never-produced single-wing version of the P-12. |
The Townend Ring Cowl (also called called a Speed Ring), when installed properly, reduced
drag by as much as 11%. However, the Townend ring impeded visibility while taxiing
on the ground. Engineers and researchers began to explore improved designs that
would address the issues of drag and visibility. In 1928, Fred Weick, an engineer
from the National Advisory Committee for Aeronautics led the development of what was to
become known as the The NACA low-drag engine cowling.
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| A NACA Cowl installed on a Curtiss P-3 Hawk in 1929. |
NACA's Solution
Drag can present a major problem for aircraft and many of Langley's early research was
focused upon reducing aircraft drag. One method was to place a cowling or covering
over the engine cylinder heads, much like the hood over the engine of a car. The
open enclosure added an effective 80+ horsepower to a 400 h.p. engine with improved
cooling and no increase in fuel use or engine weight. By 1937 the same cowling
developments proved that cowling an engine was more effective in cooling than leaving it
exposed to the relative wind.
The idea for the NACA low-drag cowling originated in 1926 during a
demonstration of the new Propeller Research Tunnel (PRT) at Langley Memorial Aeronautical
Laboratory (LMAL) in Virginia near the Atlantic coast. The PRT was then the largest
wind tunnel in the world and capable of holding a full-size airplane engine and propeller
and the fuselage behind it (but not the wings). Several people witnessed a test of a
Sperry Messenger airplane and immediately wondered if it was possible to cover the
cylinder heads in order to reduce drag. The problem was that covering the cylinder heads
was likely to reduce the cooling of the engine, so any engine covering would have to be
carefully designed.
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| A Langley researcher ponders the future, in mid-1927, of the Sperry M-1 Messenger, the first full-scale airplane tested in the Propeller Research Tunnel. Standing in the exit cone is Elton W. Miller, Max M. Munk's successor as chief of aerodynamics. |
Researchers at Langley started an effort to explore engine cowlings under the direction of
Fred Weick, a young Chicago-born engineer. Weick designed ten different cowlings, from a
partial covering to a complete covering of the engine. These were fitted to a Wright
Apache biplane with a J-5 Whirlwind air-cooled engine. The goal was to produce a
cowling that reduced drag but still cooled the engine as much as an uncovered engine.
The ten cowlings were tested on the engine in Langley's PRT. Engineers measured
drag, propulsive efficiency of the propeller (in other words, how efficient was the
propeller at pushing air past the cowlings), and the engine temperature. The best
versions of the cowlings were then modified, with the addition of vents and with changes
in their shape. This was all done in a methodical manner. Eventually, the engineers
decided upon a cowling design they designated "No. 10," which completely covered
the engine and its protruding cylinder heads, letting in air at the front. The air
was then directed over the hottest parts of the engine and out the sides along the
fuselage. What they also learned was that the shape of the airplane behind the
cowling was important to understanding how to design the cowling in the first
place—the cowling had to smoothly connect to the fuselage so that the air flowing
over it was not disrupted.
NACA Cowling No.10 being tested at Langley Field
The No. 10 cowling reduced drag by a factor of almost three. This was such an
impressive improvement in performance that Weick chose to make the results public
immediately so that industry could take advantage of them. In November 1928, Weick
wrote Technical Note 301, directed at airplane manufacturers, which described the cowling.
Weick stated that using a cowling that completely covered the engine was practical,
but warned that "it must be carefully designed to cool properly." The
NACA's Washington office announced that a cowling could be installed for about $25 per
airplane and that the possible overall savings from the industry's use of the cowling was
at least $5 million (which was more than had been spent on the NACA since its
establishment in 1915).
NACA drawing depicting the difference between the airflow over an exposed radial
engine (l) and original NACA cowling (r)
Langley engineers then mounted cowling No. 10 onto a borrowed Curtiss Hawk AT-5A biplane
that used the same Wright Whirlwind J-5 engine that had been tested in the PRT. The
tests showed that the airplane's maximum speed increased from 118 to 137 miles per hour
(190 to 220 kilometers per hour) with the cowling.
The engine cowling was an important technological development. But its importance was not so much developing a thing as it was improving understanding of aeronautical design techniques. Airplane designers could not simply stick an "engine cowling"—a piece of tin—on any aircraft and have it work. They had to specially design a cowling for each aircraft. But once they understood the principles behind this device, and once they understood the method for developing it, they could design engine cowlings for existing and new aircraft and improve their efficiency. This then led to further research on the proper placement and cooling of propeller engines, particularly on large multi-engine craft, such as bombers. For instance, most multi-engine craft before the mid-1930s, such as the Ford Trimotor, had the engines mounted below the wings in pods (or "nacelles"). But further research at the NACA, using the same methods developed for the low-drag cowling development, demonstrated that the best place to mount the engines was directly in front of the wing, blended into it. Thereafter, planes like the B-17 bomber had engines that were fitted into the wing itself and achieved greater efficiency.
Lockheed Air Express flown by Frank Hawks with new NACA cowling installed
On February 4-5, 1929, Frank Hawks, a barnstorming pilot, flew a Lockheed Air Express
equipped with a NACA low-drag cowling from Los Angeles to New York nonstop and established
a new record. Hawks flew this distance in 18 hours and 13 minutes in an airplane
whose top speed had been increased from 157 to 177 miles per hour (253 to 285 kilometers
per hour). Gerry Vultee, of Lockheed, sent the NACA a telegram that stated:
"Cooling carefully checked and OK. Record impossible without new cowling. All credit due NACA for painstaking and accurate research."
Soon many new speed records were being established with aircraft equipped with
versions of the NACA low-drag cowling.
Historical Note:
There is some discrepencies as to which was the first production aircraft designed with
the new NACA cowling. According to some sources it was the Lockheed Vega...
Lockheed Vega before the introduction of the NACA cowling
Lockheed Vega ("Winnie Mae") which shows the new lines of the NACA
cowled airplane
But, some sources claim it was the Stearman Speedmail...
Stearman Speedmail with NACA cowling
Either way, the introduction and adoption of the new design forever changed aircraft
engineering and led to ever faster airplane designs.
In January 1930, the National Aeronautic Association awarded the Collier Trophy to NACA for their work achievement.
