....the internally braced, cantilever type,
and the fuselage was of semimonocoque construction. A new feature,
which appeared on this aircraft, was a circular cowling
surrounding the 450-horsepower Pratt & Whitney Wasp air-cooled
engine. This cowling concept was one of NACA's early contributions
and provided substantial increases in the speed of aircraft
employing radial engines, but, at the same time, directed the
cooling air through the engine in such a way as to provide
adequate cooling. The maximum speed of the Lockheed Vega was
increased from 165 miles per hour to 190 miles per hour by the
addition of the NACA cowling. Fairings, called pants, around the
wheels of the landing gear also reduced the drag and resulted in
an increase in the speed of the aircraft. The Lockheed Vega had a
very low zero-lift drag coefficient of 0.0278, as shown by the
data in table
II. The low zero-lift drag
coefficient was obtained through careful attention to detailed
aerodynamic design of the aircraft and by the absence of
drag-producing struts, wires, and other external drag-producing
elements. The fixed landing gear, however, remained as a
significant drag-producing feature of the airplane. The maximum
lift-drag ratio of the Vega was 11.4, which was unusually high for
that time period. The Lockheed Vega was used in airline service
(six passengers) and was also employed in many record-breaking
flights. The aircraft shown in figure 4.3 is painted to represent
the famous Winnie Mae,
which Wiley Post flew solo around
[84]
the world in about 7 1/2 days in the summer of 1933. The actual
aircraft Post flew on this remarkable flight is in the National
Air and Space Museum In Washington, D.C. The Lockheed Vega was a
highly advanced and refined design for its day, and, even now, the
performance is very good for an aircraft with fixed landing
gear.
The demise of the Jenny and its
contemporaries opened the way for a new generation of general
aviation aircraft for fixed-base operators and barnstormers. Most
of these new aircraft employed a welded steel-tube fuselage and
wooden wing structure and incorporated a fabric covering over the
entire structure. The aircraft depicted in figures 4.4 and 4.5 are
typical of the classes of aircraft produced during the latter part
of the 1920's. The Curtiss Robin shown in figure 4.4 was designed
along the lines of the strut-braced monoplane formula popularized
by Lindbergh's Spirit of St. Louis.
The aircraft was ruggedly built
with a view toward operation from poorly prepared airfields or
pastures. The enclosed cabin provided seating for a pilot in the
front and two passengers in the rear seat. The aircraft was
usually equipped with either a Curtiss Challenger six-cylinder
radial engine or a Wright J6-5 five-cylinder radial engine. The
specifications given in table II are for the Challenger-powered Robin, which had 185
horsepower and was capable of a maximum speed of 115 miles per
hour. The aircraft was fitted with wheel brakes and a steerable
tail wheel or skid. The drag coefficient of the Robin was a very
high 0.0585, which probably resulted from the very large cylinders
of the exposed radial engine, the many sharp corners of the
forward-facing windshield, and the relatively unfaired junctures
between the multitude of struts supporting the wings....
Figure 4.4 - Curtiss Robin
three-place-cabin monoplane; 1929.
[Peter C. Boisseau]
[85] Figure 4.5 - Travelair 4000 three-place open-cockpit
biplane; 1928. [Peter C.
Boisseau]
....and landing gear. The zero-lift drag
coefficient of the Robin is seen to be more than 0.020 greater
than that of the Ryan Spirit of St.
Louis.
The biplane type was still popular and is
illustrated by the Travelair 4000 of 1928 shown in figure 4.5. The
aircraft was typical of a large number of three-place open
biplanes in which the pilot sat alone in the rear cockpit and two
passengers were placed forward under the wing near the center of
gravity in an open front cockpit. The aircraft is seen to employ
struts and wires for bracing the wings, but they are far fewer in
number than those used on the typical World War I biplane
represented by the DH-4 pictured in figures 2.26 and 2.27. Many
different power in the late 1920's. The venerable Curtiss OX-5
water-cooled engine of World War I fame was still available in
large numbers and formed a cheap source of power plants for new
aircraft. Engines of higher power and greater reliability, plants
were used in the various open cockpit biplanes produced such as
the Wright Whirlwind, were also available, but these engines were
considerably more costly than the surplus World War I engines. The
Travelair 4000 shown in figure 4.5 has the Wright Whirlwind
nine-cylinder radial engine. The large horn-balanced ailerons and
rudder on the Travelair are particularly noteworthy.
[86]
Balanced controls of this type were used on the World War I German
Fokker D-7, figure 2.14, and formed a distinctive identifying
feature of the aircraft. For this reason, the Travelair 4000,
which was manufactured in Wichita, Kansas, is often referred to as
the Wichita Fokker. Aircraft of the vintage of the Curtiss Robin
and the Travelair 4000 are highly prized antiques today and are
the subject of painstaking restoration. The Robin was used in the
1920's and 1930's in several recordbreaking endurance flights, and
in the late 1930's it was flown nonstop across the Atlantic by
Douglas Corrigan.
Meanwhile, the military services remained
wedded to the biplane concept for their fighters, observation
planes, bombers, and other classes of aircraft. One of the last
biplane fighters developed for the U.S. Army Air Corps, and one of
distinctly elegant design, was the Curtiss Hawk P-6E shown in
figure 4.6. This aircraft traces its lineage back to the Curtiss
Hawk P-1 of 1925, which in turn was derived, at least in part,
from the Curtiss racing aircraft of that period. The P-6E was the
last of the biplane line of Hawk fighters built for the U.S. Army
Air Corps. Various versions of the Hawk were also procured by the
U.S. Navy and a number of foreign countries. The entire Hawk
series employed tapered wings, and the model P-6E featured a low
drag, single-strut landing gear together with a carefully
streamlined installation of....
Figure 4.6 - Curtiss Hawk P-6E
fighter; 1931. [Peter C.
Boisseau]