HOVERCRAFT
WHAT
ARE HOVERCRAFTS??
Vehicles designed to travel close to
but above ground or water. These vehicles are supported in various ways. Some
of them have a specially designed wing that will lift them just off the surface
over which they travel when they have reached a sufficient horizontal speed
(the ground effect). Hovercrafts are usually supported by fans that force air
down under the vehicle to create lift, Air propellers, water propellers, or
water jets usually provide forward propulsion. Air-cushion vehicles can attain
higher speeds than can either ships or most land vehicles and use much less
power than helicopters of the same weight. Air-cushion suspension has also been
applied to other forms of transportation, in particular trains, such as the
French Aero train and the British hover train.
Creation
of Hovercrafts
When
building a hovercraft it is imperative that you are sure you have a firm grasp
of the important concepts and principles involved. An elementary knowledge of
physics is required, but higher level math, physics, fluid dynamics, etc. is
not necessary. Ease of use, cost, availability and safety are all significant
considerations when building a hovercraft. Care must be taken in selecting a
motor and propeller for the proper function and stability of the hovercraft and
to meet your needs for thrust and lift. A good skirt design is essential for
stability and of course, body designs must be well thought-out in order to meet
your needs for speed and stability. Finally, the rudders must be well weighed
out in order to avoid weighing down your hovercraft and also well shaped in
order to move air as efficiently as possible.
How Does a Hovercraft work
Hovercrafts work on the two main principles of lift and propulsion. When
dealing with a hovercraft, the existence of lift is imperative for the proper
function of the vehicle. lift is an essential factor because it is that which
allows the craft to ride on a cushion of air several inches off the ground.
This process, the process of attaining lift begins by directing airflow under
the craft. In order to quarantine the air under the air cushion, a skirt is
required. This is done in order to create pressure under the hovercraft which
forces the vehicle off the ground. Attaining the proper amount of airflow is
imperative for the maintenance of the craft’s stability. If too much airflow is
directed under the craft, it will then hover too high above the ground,
resulting in the hovercraft to tip. Not enough lift will cause the craft to
remain on the ground which defeats the very purpose of the hovercraft
altogether. The source of the airflow which propels the craft of the ground is
a fan. The fan can be used for lift and thrust. It can be dedicated to lift or
thrust or even both simultaneously. In either case the passage where the air
flows through to reach the air cushion affects the stability of the hovercraft.
This passage is a hole located on the base of the craft. Another vital
component is the motor. The motor is usually located in the rear of the vehicle
and is the heaviest of the components. Due to the weight of the motor, extra
pressure is required under the area where the motor is positioned in order to
attain hovering capabilities.
That which makes hovercrafts so
efficient and different from other vehicles of its category is that very little
force is required for it to move. Propulsion is that which makes the craft
move. The source of this effect is the fan, which is used to move the air for
propulsion. However odd as it may seem, the fan produces more than enough force
for the hovercraft to move. This is achieved through the existence of another
major factor:
friction, or better yet, the elimination
of friction. Hovercrafts have no contact with the ground, therefore any
resistance the ground may produce under other circumstances is now non-existent for the craft. As
explained above, the propulsion of the craft requires a fan but a normal fan is
not sufficient. This is because a normal fan does not blow air straight back.
Instead it spins the air in a spiral shape. Therefore engineers decided to use
turbines or stationary blades, that un-spin the air. When air does not spin
more of its kinetic energy can be used for translation and less is required for
rotation.
The shape of the body also affects
the stability of the hovercraft. The larger the area of the base, the more
stable it will be. Wider base=greater stability. Longer and narrower shapes
increase speed but decrease stability. Most hovercrafts have rounded ends, and
offer both stability and speed.
The skirt is another vital component.
The common skirt is known as a bag skirt. It is comprised of a bag that covers
the bottom of the base and has holes in it to allow air to escape and push the
craft off the ground. Each part of the skirt inflates independently which makes
repairs much easier and improves stability. Unfortunately, the more stable a
skirt, the slower it will go.
When the hovercraft is finally able
to move it will most definitely require steering capabilities. This is achieved
through the use of rudders. These rudders can be controlled by a variety of devices
including computers. Rudders cannot be too heavy otherwise they will weigh down
the craft because they are located very close to the motor. The shape of the
rudder dictates how well it will be able to move air.
When riding a hovercraft the natural
state of motion is easily seen to be constant vector velocity with a constant
rate of rotation. A sloping floor will definitely change your velocity vector
without changing your rate of rotation. In addition to Newton’s three laws of
motion it will become obvious that to avoid spinning or tilting the hovercraft
you must apply the forces in line with the center of mass of the combination of
the craft and your body.
Lifting Fan
Firstly the volume of
air needed is very large and a propeller is designed to be most efficient in
open air like on an aircraft. Also the fan needs to force air into the chamber
below the craft so creating a specific pressure under the craft. Propellers again
are not efficient in applications when an air backpressure will be applied to
the propeller blades as they rotate. Because of this the lifting fan on most
Hovercraft uses what is known as a centrifugal fan. This is a fan in which two
discs and fitted together and looks rather like a doughnut with angled slats at
their edges.
When the
assembly is rotated at high-speed air is sucked into the center hole in the fan
and the slats force it out at the edges. The advantages of the fan are two
fold. They operate efficiently in an environment when backpressure is high and
they will move larger volumes of air for a given rotation speed than a
propeller with the same speed and power input.
The lifting fan is coupled via a gearbox to the engine. The engine also
drives the propeller on the craft, which provides thrust for forward motion of
the Hovercraft.
Thrust Propellers
The propeller used to drive the
hovercraft along is usually an aircraft type with variable pitch blades. Its
speed of rotation must remain fixed to that of the engine and the lift fan.
This is because the
amount of lift air required dictates the engine speed to drives the lift fan.
In turn the amount of propulsion, which the propellers provide, must be
obtained by varying the propeller pitch and not its rate of rotation. This
system is termed 'integrated lift/propulsion'. A Hovercraft having more than
one lift fan and propeller generally has a separate engine for each fan-and
propeller unit.
The propellers used
on hovercraft can vary from four-bladed versions and about nine feet in
diameter on the smaller craft to the four propellers on the SRN4 cross-Channel
hovercraft. These are four-bladed and nineteen feet in diameter! On the SRN 4
the pylons on which they are mounted can be rotated to change the direction of
thrust. On smaller craft, rudders like on aircraft, are used for direction
control.
Momentum Curtain
When early models were built and
analysis was done on the airflow using the plenum chamber type of hovercraft it
showed that there were problems with stability. In addition the craft would
require enormous power to maintain a reasonable hover height.
Stability of the
hovercraft on its cushion of air remained a real problem despite some design
efforts and a new approach was needed. To solve these problems, a plenum
chamber with a momentum curtain was developed by Sir Christopher Cockerall.
Hovercraft Skirt
Despite the momentum curtain being very
effective the hover height was still too low unless great, and uneconomical,
power was used. Simple obstacles such as small waves, or tide-formed ridges of
shingle on a beach, could prove to be too much for the hover height of the craft.
These problems led to the development of the 'skirt'.
The skirt is a
shaped, flexible strip fitted below the bottom edges of the plenum chamber
slot. As the hovercraft lifts, the skirt extends below it to retain a much
deeper cushion of air. The development of the skirt enables a hovercraft to
maintain its normal operating speed through large waves and also allows it to
pass over rocks, ridges and gullies.
The skirt of a
hovercraft is one of its most design sensitive parts. The design must be just
right, or an uncomfortable ride for passengers or damage to the craft and the
skirts results. Also, excessive wear of the skirt can occur if its edges are
flapping up and down on the surface of the water. The skirt material has to be
light flexible and durable all at the same time.
For the skirt to meet
all of its requirements the design and use of new materials has slowly evolved.
The current skirts use ‘fingers at the lower edge of the skirt envelope which
can be unbolted and replaced. By doing this there is a quick and easy way to
counter the effects of wear without having to replace the whole skirt
structure. A shocking example of the costs is the replacement of the skirt
assembly on the SRN 4’s which used to cross the English Channel from the UK to
France. The replacement cost for a set of skirts for this craft is over 5
million US Dollars.
The Engine
The SRN 1 and other
early hovercrafts used piston type engines. As models like the SRN 4 and SRN 6
were brought into service they tended to favor the use of gas turbines. This
type of engine is smaller and lighter for a given horsepower and has been used
extensively in turbo prop aircraft.
The engine has a main
shaft on which is mounted a compressor and a turbine. A starter motor is
connected to one end of the shaft and the other end is connected to the lift
fan and propeller gearboxes. Both compressor and turbine look like fans with a
large number of blades.
When the engine is
started, the compressor compresses air from the engine intakes and pushes it
into combustion chambers mounted around the engine. Fuel is squirted into the
combustion chambers and ignited. The compressed air then rapidly expands as it
is heated and forces its way out through the turbine to the exhaust. As the gas
pressure rises, the turbine speeds up, thereby driving the compressor faster.
The engine speed increases until it reaches the engine's normal operating
speed.
However the use of
these engines results in a very high level of engine noise outside the craft.
In the SRN 6 this meant that it was possible to hear the craft traveling across
the Solent between Portsmouth and the Isle of Wight in the UK several miles away.
With the newer generation of craft close attention was paid to engine
noise and fuel efficiency. The current AP188craft that runs on the old SRN6
routes has now moved back towards piston engines and uses marine diesel engines
that are much quieter and fuel efficient.
Airbox
The box-like structure at the rear of
the hovercraft, right behind the propeller, The box-like structure is called an
airbox. The airbox takes about 10% of the air being pushed backward by the
propeller and forces it downward, underneath the hovercraft. There are three
small ducts cut into the base of the hovercraft, underneath the airbox. Two of
these ducts lead into the skirt, which is basically a bag that goes all the way
around the perimeter of the craft, while the third duct leads directly
underneath the hovercraft.
Hovering Power
Take a
hovercraft which, complete with crew, fuel and load, weighs 2,000 pounds
(lbs.), and is 15 feet (ft.) long and 7 ft. wide. Its area would be
15
ft x 7 ft. = 105 square (sq.) ft.
If the craft is to
hover, the pressure of air forming the cushion must be 2,000 pounds or
greater. This represents 19 pounds. per sq. ft. Yes, only 19 pounds. per sq.
ft.is required to lift the hovercraft which seems much smaller than you might
imagine!
From existing designs
of Hovercraft that have been developed, it is possible to make some simple
estimate of the power needed to lift a Hovercraft. Using 19 pounds per square
foot it is estimated 4 horsepower for each sq. ft. of curtain or skirt area can
maintain that hover.
Curtain area is its
length times its height. A hovercraft 15 ft. long by 7 ft. wide would have a
curtain length of 44 ft.-twice the length plus twice the width.
If we want it
to hover one foot high we would need sufficient power to provide a curtain of
44 x 1 sq. ft. At 4 horsepower per sq, ft. we would need 176 horsepower Just to
lift the craft up to hover one foot above the ground. Don't forget we now need
to push the craft along as well so that engine is the minimum size we can use.
Hovercraft Operation
Piloting a hovercraft is an interesting
proposition. Since very little of it actually touches the ground, there isn't
much friction, making it very difficult to steer and also very susceptible to
strong winds. Imagine trying to drive around on top of an air-hockey puck!
We've discovered that the best way to drive it is treat it like a jetski, i.e.
leaning back and forth and steering very carefully. It is also possible to do a
360-degree turn without stopping, which is quite a sight!
Aerodynamics
Aerodynamics is
defined as the branch of fluid physics that studies the forces exerted by air
or other gases in motion. Examples include the airflow around bodies moving at
speed through the atmosphere (such as land vehicles, bullets, rockets, and
aircraft), the behavior of gas in engines and furnaces, air conditioning of
buildings, the deposition of snow, the operation of air-cushion vehicles
(hovercraft), wind loads on buildings and bridges, bird and insect flight,
musical wind instruments, and meteorology. For maximum efficiency, the aim is
usually to design the shape of an object to produce a streamlined flow, with a
minimum of turbulence in the moving air. The behavior of aerosols or the
pollution of the atmosphere by foreign particles are other aspects of
aerodynamics.
Conclusion
Hovercrafts are generally simple
mechanisms in theory. Yet the process from theory to manifestation is not as
easy as it may seem. A plethora of problems exist and must be faced in order to
attain a well functioning hovercraft. The plans and designs must be flawless.
One must take under consideration the weight and the shape of each component in
order to avoid problems such as instability and dysfunction. One thing is
certain; when building a hovercraft, be well aware of the demands of
construction. Be prepared and willing to embrace failure for it is the only way
to success. Only after failed attempts will you be able to finally design an
effective hovercraft.
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