| The idea of using an air-cushion as
a means or aid to acceleration and reduction in (hydrodynamic) drag
was first explored by Sir John Thornycroft, a British engineer,
who, in the 1870's built some experimental models on the basis
of an air cushion system that would reduce the drag of water on
boats and ships.
In 1877 he successfully patented the idea and his theory was
that if a ship's hull was given a concave bottom, which could be
filled - and replenished - with air, it would create significant
additional lift. And so the air cushion effect was born. Decades later scientists and
inventors were still busy with his ideas but without any practical
applications. With the coming of the airplane however, it was noticed
that additional lift was obtained if the plane flew closer to
land or water, creating a "funnel effect", a cushion
of air. The air lift that
this funnel effect created differed with the type of wing
and its height above ground. The effect was strongest if this
height was between one half and one third of the (average)
front-to-rear breadth of the wing. Also known as "chord".
The next two decades saw little interest in air cushion
development. World War 1 saw a huge increase in the development
of the airplane as a military weapon and even though the ground
effect was known, airplanes were designed for speed and
manoeuverability in the air rather than close to the ground.
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But because of the many
difficulties these models were developed as flying boats rather
than air cushion vechicles or ground effect flyers, using the
ground effect for take-off from the water. No real hovercraft
came along until the 1950's.
Flying boats, however, are an integral part of aviation and
transport history becoming popular and practical in an era when
landing strips and airports were still sparse. They are still
used today and many of the older models that proved their worth
the world over are lovingly maintained by collectors and
enthusiasts. We highly recommend
Airline History for a complete pictorial overview of Flying
Boat history. |
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The German-built Dornier Do-X flying boat proved
the reality of Thornycroft's theory in 1929, when, during an
Atlantic crossing, it flew much closer to the ocean's surface
than was usual in order to take advantage of the air cushion
effect. The trip time was significantly reduced as a
result and the aircraft's performance that much greater.
The Do-X Flying Boat became quite a legend in
its day, especially when it started flying around the world in
1926. Despite its enormous fuel consumption - around 400 gallons
of gas per hour - it was one of the first large passenger planes
capable of 150 seats. But its engines tended to overheat and,
after its arrival in New York in 1931 it was refitted with
American water-cooled engines.
During World War 2 the Dornier factories made
several Do-X flying boats for a variety of purposes, from
transatlantic mail carriers to bombers.
(For more detailed information on Dornier's Do-X
see also
Dafkurse (D),
Aviation Posters,
EADS (D),
Avions-Legendaires (F) articles.) Many flying
boats were built during this era, and they formed the forefront
and backbone of many aviation routes across the world,
especially on the long haul destinations.
The Russian Sikorksy proved exceptionally
talented at manufacturing flying boats and many airlines ordered
their planes from his factories.
The military are always at the
forefront in taking advantage of technological developments, and
so it was with the air cushion effect.
Another decade later, for example, during World War 2,
scouting aircraft made good use of the air cushion effect to
extend their reconnaissance flights. |
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Dornier's Do-X Flying Boat (c)
Avions-Legendaires. |
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The Do-X Flying Boat loading and unloading.
(c)
Aviation Posters. |
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The Do-X at rest in Rio de Janeiro's harbour with
Sugarloaf Mountain as backdrop. (c)
EADS. |
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The successful use of the air cushion effect was not lost on
engineers after World War 2 was over and in the early 1950's
British, American and Swiss engineers started to rethink Sir
John Thornycroft's problem.
The Englishman Christopher Cockerell, commonly seen as the
father of the hovercraft, being retired from the army, settled
into boat building where he soon got captivated by Thornycroft's
problem of reducing the hydrodynamic drag on the hull of a boat
by using some kind of air cushion.
His theory was that, instead of using the plenum chamber - an
empty box with an open bottom as Thornycroft had devised - air
was instead pumped into a narrow tunnel circumnavigating the
entire bottom, it would flow towards the center and form a more
effective air cushion. This peripheral jet would cause the air
to build up enough pressure to equal the weight of the craft
and, as it would have nowhere to go, the pressure would force
the craft up, clearing it off the ground altogether.
Cockerell successfully tested his theory and filed his first
patent in 1955. The year after he formed a company called
Hovercraft Ltd. He further envisioned and partially worked out
other problems of the hovercraft principle that still have to be
fully exploited by modern hovercraft builders. One of these was
to re-use the air for greater overall efficiency.
Thinking that his air cushion vehicles would be eminently
suitable as amphibious craft he approached the British Ministry
of Supply, the government's defence equipment procurement
authority with his findings. Soon after, in 1956, the air
cushion vehicle was classified as "secret" and a construction
contract was placed with a British aircraft and seaplane
manufacturer. The result was the SR.N1 in 1959.
The first SR.N1 weighed four tons and could carry three men.
Its maximum speed was 25 knots (1 knot = 1.15 miles or 1.85
kilometres per hour) on calm water. It had a 6-inch (15 cm)
rubberized skirt to make it easier to contain the air cushion on
uneven ground.
Significant wear and tear of the skirt through friction with
the water at high speeds made it necessary to use more durable
material and a rubber and plastic mixture was developed by 1963.
The length of the skirt had also been extended to about 4 feet
(1.2 m).
By this time the Americans, the French, Japanese and Swedes
had begun to be interested in the air cushion vehicle and its
possibilities and companies sprang up the world over.
As far as experimental vehicles were concerned the Soviets,
in their race for technological supremacy, were as prolific as
the Americans in building craft of all sizes, shapes and types.
Nothing was too weird or technologically theoretical but it was
built. Especially after World War 2, during the 1950's and
1960's did experimental constructions reach extraordinary
proportions.
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Other parts were also improved on, the craft now used a gas
turbine engine, and was capable of carrying a payload of about
seven tons, achieving a maximum speed of 50 knots.
In June 1959 the SR.N1 crossed the English Channel, exactly
50 years after Louis Blériot's first flight across.
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Christopher Cockerell's SR.N1 hovercraft - 1959. |
Most of these craft were never put into production, many
didn't even work properly, and modern day collectors will pay
large amounts to add these craft to their collections.
But not only governments and their military where entranced
with the hovercraft and the air cushion vehicle, commercial
companies like Ford Motor Company even came up with a hovercar
which it called the "Glideair" in 1959. It was never made
commercially available for a number of reasons however.
Parallel invention
While Cockerell was busy designing and building his
hovercraft in the U.K., across the Atlantic ocean Colonel
Melville Beardsley was developing an air cushion vehicle of his
own in the 1950's.
The skirt around the ACV platform which contains the
compressed air and makes the ACV a stable vehicle is the
invention of Colonel Beardsley, for example, for which patent
Cockerell later paid $80,000. The modern Hovercraft would not
exist without it.
A gifted man of many inventions Colonel Beardsley continued
to work on ACV's for the US Naval laboratory across the river
from Annapolis, Maryland through the 70's, leading to the modern
amphibious military craft of today. He passed away in November,
1998, at the age of 85.
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Ram-wing craft were craft based on the principle of a wing using
the ground effect described above. They were first developed by
American aerodynamicists in the early 1960's.
In the global technological race that was going on during the
1960's an amazing array of wing-in-ground effect vehicles were
built the world over. The Russians in particular explored this
principle in a myriad of craft, like the Ekranoplan (shown at
the right) and its infamous Caspian Sea Monster.
Although few of the craft built got beyond the experimental
stage, they did show that a combination of airfoil and
air-cushion technology provided enough stationary hovering capability. This
lift was then transferred into forward speed.
Even though it was now possible to surpass the theoretical
hover speed limit of around 320 kilometers per hour (200 mph),
it became clear that the air cushion would not stay in place
above these speeds and another solution had to be found.
The ram-wing craft did prove beyond doubt that the air
cushion principle was a practical one and had advantages over
wheels and other methods of transport in certain situations.
Early hovercraft - sidewall craft The air
cushion principle found an application in the form of a sidewall
craft in 1963, reusing the plenum chamber principle of
Thornycroft. The resultant craft was not able to leave the water
because it was more of a boat but by using the plenum chamber
and its air cushion effect hydrodynamic drag was significantly
reduced.
An advantage of a sidewall air cushion vehicle over the more
amphibious hovering craft is that water propellers can be used,
effecting in greater steering control, especially at low speeds.
Another positive effect was the greater stability of the craft
as opposed to the circular skirt.
Hovercraft today and in the future Early
interest in hovercraft enjoyed a peak in the early 1960's as
everyone jumped to take advantage of this amazing vehicle.
However, by the end of the decade only the British had produced
a range of feasible and practical craft.
The problems inherent of the air cushion vehicle, such as
Cockerell and others had foreseen, regarding steering control,
noise, salt and skirt erosion, caused many countries to abandon
their hovercraft development programs in favour of other, more
established multi-function vehicles or to use different vehicles
specialised in each terrain or function.
Since the 1970's however, and especially over the last
decade, a renewed interest in the hovercraft as (passenger)
transport (see
Hovercraft in Use), military transport and weapons carrier
(see Military
hovercraft) and exploratory vehicle (see
Hovercraft news and development)
has taken ground, solving many of these problems in their
development (see
Hovercraft Construction).
Technology in general made large steps forward during the
past twentyfive years, enabling organisations and governments,
as well as many enthusiasts (see
Hovercraft Clubs) to enjoy the hovercraft vehicle in its
many forms including the very popular Remote Control model size
hovercraft! (See Model
Hovercraft.)
As far as hovercraft and their spinoff technology is
concerned the future looks ever brighter.
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