Monocoque, from
Greek for single (
mono) and French
for shell (
coque), is a construction technique that
supports structural load by using an object's external skin as
opposed to using an internal frame or
truss
that is then covered with a non-load-bearing skin. Monocoque
construction was first widely used in
aircraft in the 1930s.
Structural
skin or
stressed skin are other terms for
the same concept.
Unibody, or
unitary construction
is related to monocoque construction, where the body is integrated
with the
chassis into a single unit, except
that the body (or skin) is not stressed at all and is not part of
the load bearing chassis. A welded unit body is the predominant
automobile construction technology
today.
Aircraft
Early aircraft were constructed using internal frames, typically of
wood or
steel tubing,
which were then covered (or
skinned) with
madapolam or other fabric to provide the
aerodynamic surfaces. This fabric would usually be tautened and
stiffened using
aircraft dope and was
often required to brace the frame in tension but could provide no
strength in compression; to resist buckling, these aircraft relied
on the rigidity of the internal frame. Some early aircraft
designers began to apply
sheet metal or
plywood to highly stressed parts of the
internal frames; this skin did provide strength in
shear and
compression and could therefore be
considered as early examples of monocoque elements but the aircraft
still relied primarily on their internal frames.
In 1916
LFG introduced
their
Roland C.II using a fuselage made
of moulded
plywood, which provided both the
external skin and the main load bearing structure. This made the
plane immensely strong for its day, if a little heavy. Similar
designs were also produced by
Pfalz
Flugzeugwerke, who had originally built the Roland under
license.
By the late 1920s the price of
aluminium,
the principal ingredient of the aircraft alloy
duralumin, had dropped considerably and duralumin
was adopted extensively for internal framing members and later, the
skin. It was realised that if the skin were made thick enough it
could, theoretically, eliminate the need for any internal framing
at all but this would be heavier than an internal frame. Thin
sheet metal gauges could easily
resist tension and shear loads but buckled under bending and
compression. However, if curved, corrugated or rolled into pipe,
sheet metal could be made strong against bending and compression
loads as well. Stressed skins began to be combined with greatly
reduced internal stiffening and came to be what is now known as
semi-monocoque. For example, the
Ford Trimotor retained an internal frame of
U-shaped aluminium beams but relied on a thin skin of corrugated
aluminium sheet to brace this. The corrugations allowed the
Trimotor skin to take compression and bending loads, replacing most
of the wing ribs and fuselage stringers and could be regarded as a
stressed skin structure augmented by an internal frame or
semi-monocoque structure. The skin itself had now become a
significant structural element in its own right and it was to
become even more important when airframes were required to take
ever increasing loads.
In the 1930s huge increases in engine power, higher speeds, the
need for fuel efficiency and, post World War II, operating
altitudes that required aircraft cabins to be
pressurised demanded streamlined
airframes with stiff, strong, smooth skins; monocoque construction
was ideal for this.
Torsional
(twisting) stiffness was essential to avoid
aerolastic deformation under the rising
aerodynamic loads. An outstanding early example is the
Douglas DC-3.
World War
II was a major catalyst for aircraft development. At the
beginning of the war monocoque construction was in its infancy and
many aircraft still used mixed construction or internal frames; by
the end of the war, all high-performance planes were monocoque or
semi-monocoque.
Aerodynamic considerations of high performance aircraft began to
demand the creation of three-dimensionally curved surfaces; a dome
is a three-dimensionally curved surface. Happily, any sheet
material acquires far more strength when curved in three dimensions
as opposed to a simple two dimensional roll such as the Tri-motors
corrugated skin. Although expensive to mould, three dimensional
shells such as the
de Havilland
Mosquito moulded plywood fuselage provided immensely strong and
light airframes.
Monocoque construction does not suit all situations. After World
War II many lower-performance
general
aviation aircraft such as the
Piper
PA-20 Pacer still employed internal frames. Even external wire
bracing is still retained for aircraft that have no internal volume
such as
hang gliders or ultralight,
lightly stressed, slow aircraft for which a stressed skin would be
too heavy, such as the human powered
Gossamer Condor. This is because for very
light loads skin stiffness becomes less important than airframe
buckling and buckling can be resisted more weight-efficiently by
concentrating the compression loads into a few internal struts kept
in place by lightweight wire or fabric tension members. Bicycle
wheels and tents use tension members in the same way to brace a few
compression parts. Monocoque construction can still be more
efficient, even in lightly loaded situations, where
torsional resistance is the primary
need.
An inadequate understanding of early, metal monocoque aircraft
design resulted in catastrophic
explosive decompressions of
pressurised airframes. The two 1954
De Havilland Comet disasters resulted in
a major investigation that solved the principal problems of
monocoque aircraft construction. It was found that the Comet
fuselage failures were caused by a combination of a poor
understanding of how stresses are redistributed around openings in
a stressed skin, such as the windows, and an inadequate theory of
metal fatigue crack propagation. The
lessons learnt from the Comet were incorporated directly into the
design of the Boeing 707 and all subsequent airliners. The oval
windows seen on all
pressurised
jet airliners are one legacy of this. Sound engineering and regular
inspections for metal fatigue have made aircraft airframes very
reliable. When a crack is found, all parts from that batch are
traced and checked carefully or replaced. De Havilland had
previously built thousands of wooden monocoque jet aircraft, the
Vampires, copying their very
successful
Mosquito
structure.
The use of
composite materials in
monocoque skins now allows strength, stiffness and flexibility to
be controlled in different directions. Careful design of the
direction of the grain of successive layers of materials used in
the skin coupled with the use of
carbon
fibre can produce different mechanical properties in different
directions while optimising for weight. Composite materials can be
readily built up into complex three-dimensional shapes making them
ideal for many aircraft components. They can also be built to be
flexible in useful ways, for example, helicopter blades can be made
longitudinally rigid but capable of being twisted transversally to
adjust the cyclic pitch instead of being mounted on a pivot.
Automobiles
The first automotive application of the monocoque technique was
1923's
Lancia Lambda.
Chrysler and
Citroën
built the second monocoque vehicles, both in 1934, with the
innovative
Chrysler Airflow and the
Traction Avant,
respectively, and
General Motors
followed with the
Opel Olympia in 1935.
In 1936, Lincoln introduced the Zephyr, a monocoque design which
was as strong as the Airflow yet much lighter.
Nash Motors introduced this type of
construction in 1941 with the new 600, generally credited with
being the first popular mass-produced unibody construction
automobile made in the United States. The all-welded steel with
sturdy
bridge-like
girders that arched front to rear made for greater
strength, safety, and longer life. Nash engineers claimed that
about 500 pounds of excess weight was cut out (compared to
body-on-frame automobiles) and the body's lower air
drag helped it to achieve excellent gas
mileage for its day. Prophetically, the company's 1942
news release text attached to the X-ray drawing
describes how "
... all auto bodies will be built ... as this
some day..." The
Alec Issigonis
Morris Minor of 1948 featured a
monocoque body. The
Hudson Hornet,
along with the rest of Hudson range, featured a monocoque body at
the same time.
In the post-war period the technique became more widely used. Other
automakers incorporated this type of
construction and the terms
unit body and
unibody
became more common in general use. The
Ford
Consul was the first Ford built in England using a
unibody.
In 1960, a major breakthrough in unibody construction was reached
in mass-produced Detroit vehicles with the
Chevrolet Corvair, the most successful
automobile of this type up to that time, with 1,786,243 cars being
produced between 1960 and 1969. Among its many other
forward-thinking and breakthrough technologies for its day, the
Corvair was built from uniform molds and relied on the shaping of
the glass and doors for help with structural integrity. Convertible
versions needed special supports welded underneath to compensate
for the missing shape on the top.
American Motors (AMC) continued its
engineering heritage from
Nash and
Hudson with breakthroughs
such as in 1963 of combining separate parts into single stampings.
The
Rambler Classic had "uniside"
door surrounds from a single stamping of steel that reduced weight
and assembly costs, as well as increasing structural rigidity and
improving door fitment.
Spot welded unibody construction is now
the dominant technique in
automobiles,
though some vehicles (particularly
trucks)
still use the older
body-on-frame
technique.
Some American automobiles, such as the 1967-81
Chevrolet Camaro and
Pontiac Firebird, 1968-79
Chevrolet Nova and virtually all
Chrysler Corporation automobiles from
1960 until the early 1980s, used a compromise design with a partial
monocoque combined with a
subframe carrying
the front end and powertrain. The intention was to provide some of
the rigidity and strength of a unibody while easing manufacture,
although the results were mixed, in large part because the
powertrain subframe contained the greatest single portion of the
vehicle's overall mass, and thus movement of the subframe relative
to the rest of the body could cause distortion and vibration.
Subframes or partial subframes are still sometimes employed in
otherwise monocoque construction, typically as a way of isolating
the vibration and noise of powertrain or suspension components from
the rest of the vehicle.
In automobiles, it is now common to see true monocoque frames,
where the structural members around the window and door frames are
built by folding the skin material several times. In these
situations the main concerns are spreading the load evenly, having
no holes for
corrosion to start, and
reducing the overall workload. Compared to older techniques, in
which a body is bolted to a frame, monocoque cars are less
expensive, lighter, more rigid, and can be more protective of
occupants in a crash when appropriately designed.
Monocoque design is so sophisticated that
windshield and rear window glass now often make
an important contribution to the designed structural strength of
automobiles. Unfortunately, when a vehicle with a unibody design is
involved in a serious accident, it may be more difficult to repair
than a vehicle with a full frame. Rust is also more of a problem,
since the structural metal is part of the load bearing structure
making it more vulnerable, and must be repaired by cutting-out and
welding rather than by simply bolting on new parts (as would be the
case for a separate chassis). Older cars with separate chassis can
still pass vehicle inspection tests (such as the British
MoT) with quite advanced rust in the sills (rocker
panels) and pillars, whereas in more modern cars these parts are
structural and would lead to a test failure. In the United States,
in the majority of the states which require safety inspections,
vehicles will not pass inspection if rust has perforated components
such as rocker panels, floor pans, or pillars - regardless of the
type of body construction.
Some parts of the skin like the grill, the bumpers, the fenders,
front wing and rear diffuser are so far away from any load paths
that they only hold themselves. The doors and the hood can only
transfer a limited amount of load across their gaskets, hinges, and
bolts in normal driving situations. The rear door is both far away
from any load paths and separated by a gasket. The rear door is a
mini-monocoque made of the glass window and the metal frame.
Monocoque designs are favored amongst high-performance cars and
racing cars today for their overall structural integrity and the
fact that one can design a monocoque out of lightweight materials
such as carbon fiber and expect the resulting vehicle to be light,
stiff, and stable at high speeds and in tight corners. These types
of particularly advanced monocoques can even be molded to create
diffusers and ground effects which generate huge amounts of
downforce.
Architecture
Architects occasionally take advantage of the increasing
sophistication of monocoque technology in their building projects.
Using monocoque technology in buildings allows for interior spaces
without columns and load-bearing walls; this creates more spatial
and programmatic openness inside. Notable examples are
reinforced concrete shells.
Many 1950s and 1960s UK
underground protected
nuclear bunkers were constructed as reinforced concrete
monocoque structures for their inherent strength, robustness and
protective factors. Often described as "underground submarines" in
that, if they were dug up and placed in water, they would have
floated and stayed waterproof.
Media Centre at Lord's Cricket
Ground
is a semi-monocoque aluminium structure designed by
London based Future Systems in 1999
and built by Pendennis Shipyard in Falmouth
drawing on the company's boatbuilding
experience.
The
Wichita House designed by
Buckminster Fuller used monocoque
construction based on the Dymaxion design.
Architect
Neil Denari incorporates a
monocoque-like approach to the cladding and external appearances of
otherwise conventionally-structured projects.
A
geodesic dome is a hybrid design,
combining monocoque and frame elements as are
Quonset huts.
Structural Insulated Panels, or SIPs, are a type of preinsulated
modular wall system. Formerly called "stressed skin panels", they
are monocoques in and of themselves. Fastened together properly,
they can yield a monocoque housing structure.
Boats and ships
Frameless glass fibre reinforced or moulded plastic
kayaks and
canoes and reinforced
concrete yacht hulls are monocoques, larger ships tend to have
frames but may be hybrids with stressed skins over frames. A
submarine has a massive tubular pressure hull at its heart designed
primarily to withstand water pressure but because this is so strong
it essentially forms a massive stressed skin and can be regarded as
a monocoque.
Bicycles
Traditional bicycles are not monocoques, they are classic framed
structures. However, primarily due to the widespread use of
carbon fibre in bicycle frame
construction, monocoque framesets are slowly emerging in high end
competitive bicycles. The American company
Kestrel USA pioneered the use of carbon fibre
monocoques in bike frame manufacture in the 1980s and since then
the technique has become increasingly widely used due to its
stiffness and light weight. Items such as seatposts and other
components are now employing the same technique.
Motorcycles
LCR Sidecar in race paddock.
A
Grand Prix motorcycle
racing monocoque
motorcycle was
developed in
1967 by
Ossa, a Spanish motorcycle brand .
Honda also experimented with a monocoque
motorcycle in
1979 with its
NR500 .
John
Britten developed the Aero-D One with a monocoque fairing. Road
racing
sidecars in the
Formula 1 class have used monocoque chassis since
the mid 1970s, such as those manufactured by
LCR.
Scooters
Scooters produced by
Vespa and other companies
are made from a pressed
steel monocoque frame.
This innovative design lead to the light weight and high
fuel efficiency of
scooters, and contributed to their
popularity.
Rockets
First-stage view of the Falcon
I.
The
Falcon I rocket recently developed by
SpaceX uses a graduated monocoque, flight
pressure-stabilized design for its first stage. This
pressure-stabilized design was also used by the
Atlas II rocket.
See also
References
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