variable pitch propellers
A
variable-pitch propeller will change its angle of attack as its pitch
changes.
Some innovative aspects of aircraft were
first proposed long before airplanes were actually flying. One of these is
the variable-pitch propeller, which, as its name suggests, changes the
pitch of the propeller blades (the angle at which they cut through the
air) in order to produce more thrust.
The first proposal for a variable-pitch
propeller was made in 1871 by a Frenchman named J. Croce-Spinelli. Croce-Spinelli
proposed a design for a propeller whose pitch was changed by hydraulic
pressure (forcing oil through a tube). Croce-Spinelli claimed that this
would be most useful during takeoff, when an airplane needed the most
power (Croce-Spinelli was one of two men who died in 1875 when they went
too high in a balloon). Another Frenchman, Alphonse Pénaud, also proposed
using a variable-pitch propeller in his patent on airplane design in 1876.
Many of the people who worked on
airplane design for the next several decades, both before and after the
Wright brothers took flight in 1903, recognized the obvious advantages of
the variable-pitch propeller, which could change the engine thrust without
having to change the engine power and the speed of the propeller. But
nobody could make it work until 1910, when the first variable-pitch
propellers were used on some airships. Airships used the propellers to
reverse thrust, so that they could slow down rapidly and even back up. But
these early designs were not safe enough to use on airplanes.
One reason why variable-pitch propellers
were not developed quickly was that although they could improve
performance, they could not improve performance significantly during this
early period, so there was little demand for them. Airplanes still flew
relatively slowly and at low altitudes, and a variable-pitch propeller was
most useful at higher speeds (more than 200 miles per hour [322 kilometres
per hour]) and higher altitudes. Before World War I, a fixed-pitch
propeller could work well enough both during takeoff and at top speed and
maximum altitude so that a more complicated device proved unnecessary. But
airplane designers knew that as engines became more powerful and planes
flew higher, a variable-pitch propeller would be useful to improve
performance.
Propeller blades are twisted. The blade angle changes from the hub to the
tip with the
greatest angle of incidence, or highest pitch, at the hub (closest to the
plane) and the smallest at the tip.
World War I led to much aircraft
innovation and experimentation and propellers benefited from this boom. In
1917 the British Royal Aircraft Factory built and tested a variable-pitch
propeller on a single-engine plane. In 1918, the German R-30 bomber took
flight with variable-pitch propellers, becoming the first multi-engine
plane to do so.
Most of these early attempts by
companies in the United States, Britain and elsewhere, involved
mechanically controlled propellers. The German Garuda company developed a
much more advanced approach involving centrifugal weights and a hydraulic
servo-motor (a small motor used to control the position of the propeller),
but the company disappeared after the war and nobody benefited from its
work.
The biggest problem with all of the
early mechanically actuated designs was wear and tear. Although they could
work in flight, they did not work for long, and the bigger and more
powerful the engine, the faster they wore out. Designers thus had to limit
these to relatively small engines. But by the 1920s, designers in the
United States, Great Britain, and Canada had abandoned the mechanical
approach for a variable-pitch propeller that used either hydraulics or
electric motors to change pitch.
An American engineer, F.W. Caldwell,
conducted research on hydraulically controlled propellers in the late
1920s. He did some of his work on his own, but ultimately went to work for
the Hamilton-Standard division of United Aircraft. He built a test
propeller in 1929-1930 and tested it on an airplane. His plan was to build
a propeller that automatically adjusted its pitch according to the needs
of the airplane, a so-called "constant speed" design. This was an
ambitious goal, but by this time, there was a great demand for
variable-pitch propellers. Planes were operating at speeds and altitudes
where fixed propellers were very inefficient. Because of this demand,
while at Hamilton-Standard, Caldwell compromised with his design. He made
a simpler two-position propeller that could be set at one position for
takeoff and another for cruising. He then continued working on the
constant-speed variable-pitch propeller.
Hamilton-Standard soon began selling its
first two-position variable-pitch propellers to engine manufacturers in
1932. American aircraft designers quickly incorporated them into several
aircraft, such as the B-10 bomber. They also incorporated them into the
Boeing 247 commercial transport plane. The new propellers reduced the
airplane's takeoff run by an amazing 20 percent and increased its climbing
rate by 22 percent and its cruising speed by 5.5 percent. These figures
were so impressive that very quickly all high-performance aircraft were
redesigned to have variable-pitch propellers and Hamilton-Standard
licensed the propellers to several foreign manufacturers.
While Caldwell was doing his research in
the United States, other engineers in other countries were also working on
variable-pitch propellers. W.R. Turnbull, a Canadian, first proposed using
an electric motor to vary the pitch of the propeller. The advantage to
this design was that it did not require any modifications to the engine
itself to provide the oil to power a hydraulic mechanism. He designed his
first system in 1925 and tested it in 1927. The tests were successful and
the American company Curtiss-Wright licensed the design and began to
modify it, but it took several years before the company began to
incorporate the new propeller into its Navy and Army Air Force aircraft.
The Curtiss-Wright propeller soon became a rival for the Hamilton-Standard
propellers.
German and French firms also developed
electrically driven variable-pitch propellers during the 1930s and most
German airplanes during World War II flew with automatically adjusting
propellers produced by the firm VDM. The British had been the first to
start work on hydraulically driven variable-pitch propellers, but took the
longest to develop them. Ultimately, this forced the British aircraft
manufacturer de Havilland to license the American Hamilton-Standard
propeller instead of using a British propeller.
Today, variable-pitch propellers are
common to virtually all propeller aircraft. It is not unusual to see
commuter prop planes at an airport parked with their propellers rotated
forward, "feathered" so that the pilot can start spinning the propellers
on the ground without generating thrust. This barely noticeable, but
challenging technological innovation plays a major role in improving the
aircraft's performance.
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