wires can be seen in this photo of a Blériot
The first successful airplanes were
biplane designs, with the Wright brothers' planes being the earliest
example. The biplane is an aircraft with two sets of wings, an upper set
and a lower set, separated by struts and wires and connected to the upper
and lower parts of the fuselage. The reason for this was structural.
Airplanes were initially quite fragile, built more like butterflies than
The reason that airplanes were so
fragile had to do with the difficulty of flight in those early days. For
the first decade or so of flight, because of low-powered engines and
designs that created a lot of drag (because nobody understood drag, nor
the other stresses of flight, all that well), airplanes could barely make
it into the air at all. Planes therefore were constructed of the lightest
materials, such as fabric and wood, and built in such a way that they used
as little of the heaviest materials as possible. As a result, their wings
were not capable of supporting much weight or handling wind gusts without
crumpling in flight, like an umbrella in the wind. Early airplanes had
more than one wing because this reduced the "wing loading," or amount of
weight that the wing had to support in flight. With an upper and lower set
of wings, the wings had to support less weight and the structural fittings
between them, such as struts and wires, could reinforce them. The truss
system also solved the dilemma of how to maintain structural strength
while also allowing the wing flexibility necessary for the wing warping
used in the Wright brothers' airplanes.
But a lot of people had caught the
aviation bug after the Wright brothers took flight in the early part of
the 20th century and there were hundreds of entrepreneurs trying to build
their own planes. Some of them experimented with monoplane designs—or
planes with only one set of wings that extended out from the
fuselage—because this seemed like an inherently logical design that
reduced drag. After all, a bird only has two wings, so why should an
airplane have more? But these monoplanes invariably failed. Many of them
folded up as soon as they started to lift off the ground. The first person
to succeed was Louis Blériot, a Frenchman born in 1872 who had an
engineering degree and manufactured headlamps and accessories for
Blériot started experimenting with
different airplane designs even before the Wright brothers proved powered
flight was possible, and between then and 1908 he built a series of
different airplanes. Some were biplanes; others were monoplanes. Most were
flops. By 1908, he had built his eleventh airplane design, which he dubbed
the Blériot XI. It was a monoplane, with a fuselage that was little more
than an open wooden frame covered by fabric with an engine and propeller
in the front. The pilot controlled the plane by twisting the wings, a
design feature borrowed from the Wright brothers.
Blériot attached the wings to either
side of the fuselage, but he reinforced them with wires that ran from the
wings to vertical poles that stuck out of the top and bottom of the
fuselage. The Blériot XI had two sets of wires. The flying wires went from
below the wings to the bottom of the poles and prevented the wings from
folding up when the plane was flying. The landing wires went from the
topside of the wings to the top of the poles and held the wings up,
preventing them from collapsing when the plane was on the ground and the
wings were simply dead weight. It was an awkward and rickety-looking
craft, but that was also true for most biplanes of the day. Blériot
exhibited his plane at the Exposition Intérnationale de la Locomotion
Aérienne in Paris and it created a great deal of excitement. Soon he was
manufacturing relatively large numbers of them and selling them to several
armies, including France and Great Britain, and to anybody who had the
money and the courage to fly. For a period, especially after Blériot
crossed the English Channel in 1909 in his Blériot XI, his plane became
the most popular in the world. Other monoplane designs followed.
Because of the lower drag of a
monoplane, it could fly faster than a biplane equipped with the same type
of engine. During the early days of flying, pilots earned fame and fortune
by winning races and so the Blériot XI was a popular and well-known plane
because it was speedier than most other aircraft. But by 1911 or so, the
plane was also earning an unwanted reputation for its nasty habit of
folding up its wings in flight, causing it to come fluttering to the
ground with all the grace of a sparrow tied to a brick.
Alarmed by the number of crashes, French
military aviation officials began to propose strengthening the structure
of their Blériot XI monoplanes. Blériot himself conducted structural
evaluations that consisted of turning the airplane upside down and putting
sandbags on the wings to simulate the lift, proving that the wings could
support many times the weight that they needed to fly. But the problem
with these tests was that they were both overly simplistic and also did
not simulate the kinds of stresses that affected an airplane's wings. They
were static tests that treated the wings as if they did not move and as if
the stresses on them were constant. But wings do move in flight and the
stresses on them change. Such stresses are called dynamic loads. Wings do
not simply experience
lift, they also experience forces from many directions during the
course of a flight. Wind gusts could push back on wings, and manoeuvres
could push down on them. Wings could also twist from these forces as well.
Blériot's tests did not simulate any of this.
In the wake of the increase in monoplane
crashes in 1912, Blériot proposed that the problem was not with the flying
wires underneath the wings but the landing wires above them. The wings
were failing when the planes were pulling out of a dive, when the airflow
was pushing down on the wings. Blériot proposed that increasing the
strength of the relatively weak landing wires could solve this problem.
But not everybody was convinced of his explanation and the number of
airplane crashes increased. At one point, the British army realized that
the only planes in its fleet that were crashing were the monoplanes. Sales
of monoplanes dropped.
Over the next several years, various
experts on structures in France, Britain, and elsewhere studied the
problems with the Blériot XI and other monoplane designs and gradually
developed a set of complex theories explaining how wings acted in flight.
They determined that numerous complex forces interacted on the wings.
Because it was difficult to make all of the calculations necessary to
determine how a wing would perform and how it would fail, not many
monoplanes were constructed during this time.
Taube was one of the monoplanes that served in World War I.
They were faster than most multi-wing airplanes.
By the beginning of World War I, though,
a number of monoplanes were in service. The pre-war Monocoque
Deperdussin set a speed record of 108 miles per hour (174 kilometres
per hour), winning the 1912 Gordon Bennett Cup in Chicago. Most World War
I monoplanes were designated as "fighting scouts" because of their higher
speeds compared to multi-wing airplanes. These included the graceful Taube
built by the German firm Rumpler and the Fokker Eindecker (German for
"monoplane"). Morane-Saulnier also built a successful monoplane fighter
that had the advantage of allowing the pilot to fire his machine gun
through the plane's propeller. These planes all had external bracing, but
other monoplane designers began using internal braces inside their wings.
Despite this, the vast majority of the airplanes that saw service during
World War I were biplanes or even triplanes like the JN-4 Jenny, Spad, and
Fokker Eindecker was one of the monoplanes used during World War I.
The war spurred a great deal of
innovation by airplane designers, who suddenly had more money and more
pressure forcing them to design faster and more manoeuvrable planes. This
also affected monoplanes. Designers soon found effective ways to brace the
wings internally, by fitting additional structures inside the wings that
connected them to the fuselage, a design called "cantilevered wings". They
also developed a much better understanding of the forces that affected a
wing in flight. Finally, they began building metal-skinned airplanes, and
took advantage of more powerful engines then becoming available.
In 1915, Hugo Junkers, a German
engineer, working in cooperation with two other engineers, designed the
Junkers J-1, an all-steel low-wing monoplane. The centre section of the
fuselage and the centre section of the wing were built as an integral
unit. The plane was covered with sheets of steel welded to the tubular
structure. Although it was fast for its day, its weight made it less
manoeuvrable. Some people called it the "Tin Donkey." Junkers followed
with numerous other low-wing monoplanes and over time monoplanes became