| [32] Figure 2.13 depicts a stubby but graceful-looking
biplane with wings of equal chord and span, configured with no stagger and
relatively small gap. The small gap in combination with the center cutout
of the upper wing gave the pilot excellent visibility over the top of the
wing. The design appears to be that of a double-bay biplane; however, the
inner struts served only to stabilize the rather long wing-bracing wires
and prevent their flapping and chaffing (ref. 22). The wires themselves
consisted of round cables. The cockpit was close behind the engine with
the pilot's feet and part of his legs located in aluminum tunnels beneath
the engine (ref. 110). The landing gear was positioned well forward, ahead
of the center of gravity, to minimize the risk of a nose-over on landing.
Ailerons were on the upper wing only, and, as with the other aircraft
described, no means of longitudinal trim was provided.
The SPAD XIII was powered with the Hispano-Suiza 8BA engine of 220
horsepower. The engine had eight water-cooled cylinders in two banks of
four arranged in a V-type configuration, much like that of many modern
automobile engines. The distinctive round radiator, equipped with manually
operated (from the cockpit) shutters for controlling the cooling airflow,
may be seen in figure 2.13. Long exhaust pipes ran on either side of the
fuselage and terminated behind the pilot's cockpit. This arrangement
resulted in a relatively quiet environment for the pilot (ref. 110). In an
interesting survey of aircraft piston engine development, Taylor (ref.
111) credits the Hispano-Suiza with being one of the best and most
advanced engines of World War I, as well as one that served as a sort of
progenitor for a long line of Curtiss and Rolls-Royce liquid-cooled
engines that culminated in the Rolls-Royce Merlin of World War II.
The data in table
I indicate that the SPAD XIII had the most favorable power loading of
any of the aircraft considered and a high (for its day) wing loading.
These characteristics coupled with a relatively low zero-lift drag
coefficient and low drag area gave the SPAD the highest speed of any of
the aircraft listed in the table. As shown by the data in figure
2.18, the climb characteristics of the SPAD were bettered only by
three of the Fokker aircraft.
The reference literature suggests that the SPAD XIII was not as
maneuverable as some of the other fighters, but its high performance,
great strength, and multigun armament made it a highly effective weapon.
Its ability to dive steeply for prolonged periods of time without fear of
structural failure is emphasized in all the reference material. [33]
Piloting the aircraft required care, particularly at low speeds, and the
use of moderate amounts of power was recommended in landing.
Although the SPAD XIII incorporated no new technical innovations, it
synergistically combined an airframe of relatively high aerodynamic
efficiency and great structural strength with an excellent engine to
produce an outstanding aircraft. It may be regarded as representative of
the top of the state of the art of a 1918 fighter aircraft equipped with
thin, strut-and-wire-braced wings. The SPAD was so highly regarded that a
number of countries maintained the aircraft as part of their active air
force inventory for several years following the war. A total of 8472 SPAD
XIII aircraft were manufactured. |
22. Andrews, C. F.: The SPAD XIII C.I,
no.17 (Surrey, England: land: Profile Publishers, c.1965)
110. Tallman, Frank: Flying the Old Planes (Doubleday & Co., 1973).
111. Taylor, C. Fayette: Aircraft Propulsion, a Review of the Evolution of
Aircraft Piston Engines (Washington: Smithsonian Institution, 1971).
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