Airframe Structures
Have you ever stopped and really thought about the structure
of an airframe? I mean, we all know how beautiful airplanes are to look at,
either on the ground or, especially, in flight. But, did you ever wonder what
sort of engineering is required to manufacture an aircraft that will hold up to
all of the stresses of flight? If you have, this article will provide you with
some insight into the engineering wonders that we so often take for granted.
The History of
Airframe Design
What we see today in modern airframes began back in 1903,
when a wooden biplane built by Orville and Wilbur Wright showed America the
potential of fixed-wing aircraft design. During World War I, military needs
spurred on further airframe developments, and a hybrid between wood and metal
structures became very popular, especially with the effectiveness of Dutch
designer Anthony Fokker’s combat aircraft for the German Empire’s Luftstreitkräfte and the U.S. Curtiss
flying boats.
During the latter part of 1915, Hugo Junkers pioneered the
use of an all-metal airframe with the Junkers J 1 and then used lighter weight
duralumin in the airframe of the Junkers D.I. of 1918. All-metal designs
continued through the 20s and 30s with some diversions into wooden composite
airframes during World War II. For commercial aircraft design, though, the
focus remained on all-metal designs, for the most part.
After World War II, newly developed aluminum alloys were
critical to the designs of turboprops and jets, since these aircraft flew at
higher speeds and under greater stresses than previous wooden or metal designs
could allow for. The lighter weight but stronger strength of the aluminum
alloys paved the way for further advances in airframe design.
These aluminum alloys were the primary material used for
airframe structures until the 1980s, when composite material construction began
to be explored. Today, exploration in composite materials for airframe designs
continues, with Boeing claiming the lead in designing its new 787 aircraft with
a one-piece carbon-fiber fuselage, replacing “1,200 sheets of aluminum and
40,000 rivets.”
The Parts of an
Airframe
The structure known as the airframe is made up of a number
of different components, each with its own distinct purpose. The fuselage provides a space for the crew,
passengers, cargo, fuel, and environmental control systems. Next is the empennage, which is made up of the
vertical and horizontal stabilizers. These are used for turning and pitching
the aircraft in flight. The wing, which passes through the air, provides life
to the aircraft. Finally, the ailerons increase life on one side of the wing
while reducing it on the other side, in order to roll the airplane on its
fore-and-aft axis.
Airframe Structure Designs
Modern Airframe
Design
The design and implementation of all of these components has become a very exacting
science. Airframes are exposed to a number of internal and external loads and
stresses, such as propulsion thrust, drag, lift, and turbulence, as well as the
weight of the cargo and passengers and crew of the aircraft and helicopter, and airframes must
be rigorously tested to ensure the aircraft will perform predictably under all
sorts of stresses.
The strength capability of an airframe must be constructed in such a way
that it can withstand all of the applied loads it is exposed to with a
predictable margin of safety throughout the life of the airframe. It must
resist deformation, so the airframe requires a structural stiffness that allows
it to maintain its shape even under extreme vibrations and oscillations
experienced during flight.
At the same time, performance requirements of the aircraft
such as range, payload, speed, altitude, and landing and takeoff distances,
require that the airframe be designed and constructed to minimize its weight.
This attempt to minimize the weight of the airframe has driven much of the
exploration of building materials, and is a leading factor for the pioneering
efforts into using carbon fiber fuselages and other composite materials instead
of aluminum alloys.
Risks of Airframe
Design Exploration
With every advance in airframe design technology comes
certain risks. Rigorous testing is conducted on airframe designs before they
ever leave the ground, but sometimes not all factors can be tested. This has
resulted in several serious catastrophes that demonstrated the risks involved
with man’s efforts to take to the skies.
In 1959, a Lockheed L-188 Electra disintegrated in
mid-flight, and the cause of the accident eluded investigators for quite some
time. The investigators concluded that the breakup was caused by the loss of a
wing to “flutter,” or cycles of reverse bending caused by the self-oscillation
or vibration of the wing, but could not ascertain what had caused the flutter.
Eventually, after another L-188 Electra suffered the same tragic end,
investigators discovered this new phenomenon in which the natural oscillation
of a material matches or comes close to matching the oscillation of the
external forces on the material, a force called “harmonic coupling.” This force
causes structural weakness and, in the case of the L-188, caused the airframe
to disintegrate.
Supplier of Airframe Parts
In another example of the complexities of airframe design,
the 2001 crash on takeoff of an Airbus A300 demonstrated the ability of an
airframe to be overstressed and destabilize when certain forces are exerted. In
this case, the over usage of the rudder resulted in the vertical stabilizer
detaching from the aircraft, an event that is still contended by both Airbus
and American Airlines to be the fault of the other part. American Airlines
claims that Airbus neglected to advise them of the inability of the airframe to
handle abrupt changes in rudder force during maneuvering speed, and Airbus
claims that American Airlines failed to adequately train its pilots in the
maneuvering capabilities of the aircraft. Whatever the root cause may be,
inadequate training or a design flaw, the accident proves that airframes can be
exposed to stresses that cannot be properly predicted by the airframe
manufacturer, leaving some risk in the advances in airframe technologies.
Summary
An airframe is a very complex structure, made up of millions
of components. They require hours upon hours of engineering and testing, and
still some risk remains. Even so, the rigorous testing and regulation of
airframe design makes for a much safer method of transportation than any other,
since no other vehicle undergoes the amount of testing and design engineering
that aircraft undergo.
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